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Johnson Matthey’s international journal of research exploring science and technology in industrial applications

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Johnson Matthey’s international journal of research exploring science and technology in industrial applications

Contents Volume 65, Issue 1, January 2021

2 Guest Editorial: Platinum Group Metals for a Greener Future By Stewart Brown 4 Professor Robert D. Gillard: Transition Metal Chemist 1936–2013: Part I By John Burgess and Martyn V. Twigg 23 Professor Robert D. Gillard: Transition Metal Chemist 1936–2013: Part II By John Burgess and Martyn V. Twigg 44 Optimising Metal Content in Platinum Group Metal Ammonia Oxidation Catalysts By Julie Ashcroft 54 A Re-assessment of the Thermodynamic Properties of Osmium By John W. Arblaster 64 Comprehensive Review on High Hydrogen Permselectivity of Palladium Based Membranes: Part I By Hasan Mohd Faizal, Bemgba B. Nyakuma, Mohd Rosdzimin Abdul Rahman, Md. Mizanur Rahman, N. B. Kamaruzaman and S. Syahrullail 77 Comprehensive Review on High Hydrogen Permselectivity of Palladium Based Membranes: Part II By Hasan Mohd Faizal, Bemgba B. Nyakuma, Mohd Rosdzimin Abdul Rahman, Md. Mizanur Rahman, N. B. Kamaruzaman and S. Syahrullail 87 Lattice Dynamical Study of Platinum by use of Van der Waals Three Body Force Shell Model By U. C. Srivastava 94 Electrodeposition of Iridium-Nickel Thin Films on Copper Foam: Effects of Loading and Solution Temperature on Hydrogen Evolution Reaction Performance of Electrocatalyst in Alkaline Water By Jianwen Liu, Wangping Wu, Xiang Wang and Yi Zhang 112 Different Deformation Behaviour Between Zirconia and Yttria Particles in Dispersion Strengthened Platinum-20% Rhodium Alloys By Ziyang Wang, Xi Wang, Futao Liu, Faping Hu, Hao Chen, Guobin Wei, Weiting Liu and Weidong Xie 120 On Deformation Behaviour of Polycrystalline Iridium at Room Temperature By Peter Panfilov, Irina Milenina, Dmitry Zaytsev and Alexander Yermakov 127 Platinum Group Metals Recovery Using Secondary Raw Materials (PLATIRUS): Project Overview with a Focus on Processing Spent Autocatalyst By Giovanna Nicol, Emma Goosey, Deniz Şanlı Yıldız, Elaine Loving, Viet Tu Nguyen, Sofía Riaño, Iakovos Yakoumis, Ana Maria Martinez, Amal Siriwardana et al. 148 Johnson Matthey Highlights 151 BIORECOVER: New Bio-based Technologies for Recapture of Critical Raw Materials By Annette Alcasabas, Felicity Massingberd-Mundy, Barbara Breeze, Maite Ruiz Pérez and Cristina Martínez García https://doi.org/10.1595/205651321X16045877200573 Johnson Matthey Technol. Rev., 2021, 65, (1), 2–3

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Guest Editorial Platinum Group Metals for a Greener Future

Clustered together in the centre of the Periodic locations. Demand is generally price inelastic, Table lie six remarkable elements, six metals meaning that consumption volumes are often without which the world would be a completely relatively insensitive to underlying metal prices different place. Think about the food you eat, (1). Importantly, the pgms are widely recovered your computer, your car, your mobile phone or and recycled; for example, through the recovery even the clothes you wear. At some stage during of catalytic convertors from end of life vehicles their production one or more of these six rare or through a closed-loop system where the metals has been utilised, whether as a catalyst catalyst that is installed in a chemical plant is or perhaps in the end product itself. The platinum recovered, sent for refining and ultimately reused. group metals (pgms) play an essential role in our Sustainability not only of the metals themselves modern lifestyles. but also with regard to their end uses is why the Platinum, palladium, rhodium, ruthenium, pgms are so important, as described in two of the iridium and osmium are rare, expensive and have a articles – European projects BIORECOVER and unique combination of incredibly useful properties. PLATIRUS. For example, high thermal stability, corrosion and The use of platinum, palladium and rhodium is oxidation resistance and the ability to catalyse dominated by the automotive sector where for a broad range of chemical reactions make them several decades they have been a vital component indispensable in processes such as petroleum in emission control catalysts. These three metals refining, nitric acid, bulk chemical production and have been fundamental in removing carbon glass manufacture. They are also to be found in monoxide, hydrocarbons and nitrogen oxides a diverse range of products such as the hard disk from gasoline and diesel engine exhausts to drives in computers and data storage centres, the dramatically improve air quality across the world, airbag in your car or the jet engine that carries as detailed in several publications by one of our you to your holiday destination. Apart from their authors, Martyn Twigg, who in his long career chemical properties the pgms and platinum and was at the forefront of autocatalyst development palladium in particular have found favour in both (2–4). the jewellery and investment markets. Platinum In this special edition, Twigg and Emeritus Reader has for many years been marketed as a premium John Burgess from the University of Leicester, jewellery metal, rarer and more precious even UK, have written a two-part commemoration than gold. of the late Professor Bob Gillard, discussing his remarkable life, work and contribution to the Science and Industrial Applications understanding of transition metal chemistry, particularly the chemistry of rhodium and other But it is not for the pgms’ aesthetic or investment platinum group metal complexes. value that this collection of papers has been The use of pgms by the chemical industry collated, rather to highlight the fascinating science is of vital importance to a huge range of bulk of these incredible metals and their wide range of and speciality products. One example of this is industrial uses. This special edition of the Johnson the oxidation of ammonia to produce nitric acid Matthey Technology Review will examine both the which has used platinum and rhodium in catalyst fundamental properties of these metals and their gauze for over 100 years (5). The latest work use in a variety of applications and fields. in this field will be discussed by Ashcroft in this The pgms are rare elements, occurring in special edition. Interestingly, the use of pgm for economic quantities in only a few geographical chemical catalysts has remained one of the more

2 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16045877200573 Johnson Matthey Technol. Rev., 2021, 65, (1) robust areas of demand during the coronavirus of suitable purity makes use of one of the key disease (COVID-19) pandemic. Nitric acid is used properties of palladium. Palladium has an intrinsic to manufacture both fertilisers for global crop selectivity for hydrogen, which makes it an ideal production and explosives for the mining industry, choice for purification membrane technology. In which are essential for the supply of metals such as this issue, Faizal et al. discuss the use of palladium nickel and platinum and will be central to the future in this vital application for the growing hydrogen electrification of the automotive fleet. economy. The minor pgms ruthenium, iridium and osmium The pgms: six of the rarest elements in the can often appear somewhat neglected despite their Periodic Table that have and continue to change the use in a huge number of applications. Iridium is world around us. Metals that are driving forward prized for its high melting point which makes it ideal sustainable technology and the move towards net for use in crucibles to produce high purity metal zero, metals that will help drive the clean energy oxide single crystals, used in medical scanners, revolution, provide food to billions and facilitate light-emitting diode production and surface acoustic wave filters, amongst others. The behaviour and communication and data sharing and storage across properties of iridium are the subject of two papers the globe to enable a more connected society. in this edition. Osmium is perhaps the least known of the metals given its more limited applications. However, Arblaster has remedied that with a paper STEWART BROWN discussing the thermodynamic properties of the Johnson Matthey, Orchard Road, Royston, densest element in the Periodic Table. Hertfordshire, SG8 5HE, UK Email: [email protected] The Most Useful Elements References The pgms are among the most invaluable elements discovered. To sum up all their useful properties in 1. A. Cowley, “Pgm Market Report”, Johnson Matthey, one key attribute is that they enable the world to be London, UK, May, 2020, 43 pp a more sustainable place. Globally we are starting 2. M. V. Twigg, Platinum Metals Rev., 1999, 43, (4), to undergo a monumental change in energy use 168 and production, away from reliance on fossil fuels 3. M. V. Twigg, Catal. Today, 2011, 163, (1), 33 towards a cleaner, greener, more sustainable model 4. M. V. Twigg, Platinum Metals Rev., 2010, 54, (3), (6). The move to hydrogen as an energy source is 180 vital in the move to a net zero economy, a key example of which is the fuel cell vehicle. Johnson 5. M. V. Twigg, Johnson Matthey Technol. Rev., 2017, Matthey actually provided the platinum to William 61, (3), 183 Grove when he demonstrated the first fuel cell in 6. A. Walker, Johnson Matthey Technol. Rev., 2020, 1839 (7). Aside from the use of pgm in the catalyst 64, (3), 234 of the vehicle itself, the production of hydrogen 7. W. R. Grove, Phil. Mag., 1839, 14, (86–87), 127

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Professor Robert D. Gillard: Transition Metal Chemist 1936–2013: Part I From early life to the University of Kent at Canterbury

John Burgess His subsequent time at Cardiff and then into Department of Chemistry, University of retirement will be covered in the second part of Leicester, Leicester LE1 7RH, UK this commemoration.

Martyn V. Twigg* 1. Introduction Twigg Scientific & Technical Ltd, Caxton, Cambridge CB23 3PQ, UK With the death of Professor Robert D. Gillard on 4th June 2013 in Cardiff, UK, Chemistry lost one of its *Correspondence may be sent via the more charismatic and energetic figures whose work Editorial Team: [email protected] on transition metal coordination complexes and particularly chiral metal complexes spanned more than four decades. His chemical research began This first part of a two-part commemoration of with a student Part II Project at the University the life and work of Robert D. Gillard begins with of Oxford supervised by Dr H. M. N. H. Irving a biographical outline which provides a context on the formation of optically active and racemic for his chemical achievements. He was awarded isomers of 1,2-propanediamine tetraacetate (pdta) a State Scholarship and after his National complexes with alkaline earth metals. In his typical Service in the Royal Air Force he went up to St fashion, Gillard expanded greatly the scope of Edmund Hall, Oxford, to read Chemistry. There the project to include transition metal complexes. follows a chronological account of his career in This experience kindled a life-long interest in the Chemistry starting with his undergraduate days optical activity of metal complexes and inorganic in Oxford, where a Part II project with Dr Harry coordination chemistry in general. He developed Irving on alkaline earth and cobalt complexes a particular interest in compounds and complexes proved seminal. His PhD research at Imperial of the platinum group metals (pgms), publishing College, London in the Geoffrey Wilkinson substantial amounts of research on rhodium (over group broadened his experience into the then 60 articles) and platinum (over 30 articles), with poorly developed chemistry of rhodium and smaller contributions on ruthenium and iridium other platinum group metal complexes. Gillard complexes (12 and 9 articles respectively). He thus next went to Sheffield University as a Lecturer made a significant contribution to the renascence where he developed independent research while of interest in these elements at the time. continuing to work on earlier topics. There The present article provides some biographical followed a move to Canterbury as a Reader at the background before Gillard’s published research University of Kent. In his particularly productive contributions are reviewed selectively in seven years there with a large research group chronological order. After leaving Oxford and he widened his experience further, expanding his following a short break from academia Gillard interests in such areas as the optical properties worked for a PhD (1961–1964) with Geoffrey of transition metal complexes, considering Wilkinson at Imperial College, London. He then biological and medical relevance, and increasing went to the University of Sheffield as a Lecturer the range of metals and ligands he investigated. (1964–1966) after which he moved to the University

4 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040160 Johnson Matthey Technol. Rev., 2021, 65, (1) of Kent at Canterbury as a Reader (1966–1973). which are thoughtful and cogent. From my conversations with him I know that he reads Gillard then moved to Cardiff University to take the widely and with zest. He takes, of course, a Chair in Inorganic Chemistry. The second part of very full part in school life – plays for the first this commemoration covers his time there and his XV, the second XI, and swims for the school. He work into retirement. works hard in the school choir and has taken leading parts in the school opera production. 2. Biographical and General He would have been in the cast of this term’s Background Shaw production if it had not clashed with the date of the examination” 2.1 Parents, Schools and Military Service He went on to say “Gillard is a joint Vice-Captain of the school. He Robert David Gillard was born on 23rd August 1936 was a strong candidate for the Captaincy, but I in Balham in the London Borough of Wandsworth. finally chose a boy who was a year his senior. His mother, Eva Margery Gillard (née Arthur As a Prefect he is loyal and co-operative.” 1914–1985) and father (who at the time was a bus conductor for London Transport) were married It is not clear why Gillard studied Chemistry rather in Wandsworth in 1933. Gillard’s father Thomas than Pure Mathematics at Oxford, perhaps he had Gillard (1904–1983), who referred to himself as developed a strong interest in the subject towards Thomas Patrick Gillard, was born in Nottingham the end of his time at school? At first, as in the and the family moved to Bradford where in 1911 Royal Air Force, he participated in sport activities his sister Elizabeth was born. Gillard’s grandfather, and for a while he represented St Edmund Hall at also Thomas Gillard, was born in 1873 in Newcastle- rugby and rowing in his first year (1956–1957). under-Lyme where he worked in iron works there. He rowed in the 5th VIII, in seat 5, and he also Gillard’s older brother, Thomas R. Gillard, was also led a particularly active social life. However, these born in South London. Gillard did well at school activities had to be curtailed as he only managed and he won a Surrey County Council scholarship to obtain a minimal Pass in the Chemistry Part I to Mitcham Grammar School where he gained examinations in the School of Natural Science, and distinctions in all his O-level and in four A-level increasing academic demands led to St Edmunds examinations. He won a State Scholarship in Pure Hall’s Principal writing on 3rd June 1957 to Surrey Mathematics to Oxford University but before going County Council, the Ministry of Education, and up to Oxford he did National Service in the Royal Gillard’s parents about him being seriously in Air Force (1954–1956), becoming a Pilot Officer. dereliction of his studies and, unless there was “an Part of his training was undertaken in Canada. improvement by the end of this term we shall be Much later he told stories illustrating his navigation obliged to discontinue his residence.” But even by skills were not always perfect. For instance, he 27th June 1957 the Principal was able to write to once said (as related to MVT by Dr J. G. Jones in the Ministry of Education (who were funding his 2015) that when flying from Libya to Malta his plane State Scholarship) stating overshot and landed in Cyprus! After his time in “since the warning I gave him about the middle the RAF Gillard went up to Oxford University for his of term, there has been a marked improvement undergraduate studies, not for Pure Mathematics in Mr Gillard’s work. His attendances at the but rather to read Chemistry and the reasons for laboratory are reported to be the best of all this change are unclear to the present authors. the students in his year, and in other respects he appears to be working quite satisfactorily. 2.2 The Oxford Years (1956–1960) In view of this I am not proposing to take any further action beyond warning him that this On leaving the RAF Gillard started undergraduate improvement must be maintained.” studies at St Edmund Hall in Oxford, where his elder brother had been earlier. His headmaster at Clearly Gillard maintained his improved academic Mitcham Grammar School (Mr G. I. P. Courtney) in standing and in 1960 he was awarded his BA with a letter to St Edmund Hall said First Class honours. His subsequent Part II research project was supervised by Dr Harry Irving (1–4). “I have made it my business to observe Gillard His project began with the formation of optically in various activities. He does essays for me active and racemic isomers of pdta complexes with

5 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040160 Johnson Matthey Technol. Rev., 2021, 65, (1) alkaline earth cations and highlighted to Gillard in 1958 in Oxford, Andrew born in 1964 in Kent and the chirality some metal complexes can have. Duncan born in 1966 in Sheffield. Tragically Duncan Gillard expanded the project to include transition was killed in a car crash in Devon in 1998. Isabelle metals, and collaboration with Irving in this area studied Law at the University of Birmingham and continued over almost a decade and resulted has a very successful career as a lawyer, while her in several joint publications. His BSc thesis was brother Andrew studied mathematics at University entitled ‘Conformational Factors and the Stability College London and later worked in digital of Some Metal Complexes’; his BSc was awarded information technology areas. in September 1961 (Figure 1). This research experience strongly influenced the directions of 2.3 The Commercial Year (1960– much of his subsequent academic work. 1961) Gillard had indeed done very well at Oxford and on 13th July 1960 the St Edmund Hall Principal After leaving Oxford Gillard joined the Agricultural wrote to him saying Department of Burt, Boulton and Haywood Ltd at Belvedere in Kent as a European Liaison Chemist. “I was delighted to hear that you had obtained Burt, Boulton and Haywood Ltd (BBH) was the a First. This is a splendid achievement and it does you every credit. I know that at times oldest timber preservation company in the world, your tutor thought you might not manage to established in 1848. By the time Gillard joined the pull it off, but you have confounded your critics business they were importing pine for their major in a manner which must be really satisfying activity, making and treating wooden telephone and and pleasant to look back upon.” electricity poles, from Iivari Mononen in Finland. The Iivari Mononen Group eventually absorbed BBH, Gillard did not look back – he moved forward with whose pole and fence post treatment operations vigour! are now based in Newport, South Wales. Gillard In 1957, while an undergraduate at Oxford, found work in industry tedious and frustrating, so Gillard met Diane Laslett, a trainee nurse at wanted to go back into academia. This return was Hammersmith Hospital, at the Hurlingham Club in soon accomplished, in a manner determined by a London through a mutual friend (who happened to coincidental turn of events. be at Imperial College). Later that year they were married at St Luke’s Church in Ramsgate, on 28th 2.4 Imperial College and Sheffield December. They had three children: Isabelle born University (1961–1966)

Unknown to Gillard Professor Irving, who had left Oxford and taken a chair at Leeds University, wrote to him offering him a place to do a PhD at Leeds. But the offer letter did not reach Gillard for some time because his address had changed. By the time Irving’s letter reached Gillard he had secured a position with Professor Geoffrey Wilkinson (5, 6) to work for a PhD at Imperial College supported by being an Assistant Lecturer, and later a Lecturer. This came about serendipitously by talking with an acquaintance, Dr Ron Ashby, who was a Senior Lecturer in Engineering at Imperial College. He arranged for Gillard to meet Dr A. J. E. Welch, an inorganic chemist at Imperial College who was involved in editing later editions of the important “Thorpe’s Dictionary of Applied Chemistry”. Welch introduced Gillard to Professor Geoffrey Wilkinson Fig. 1. Graduation photograph of Robert D. Gillard who there and then offered him an Assistant at the University of Oxford. He completed his Part Lecturer position; he also registered for a PhD II research project with Dr Irving in 1961 and was supervised by Wilkinson! It appears Wilkinson’s duly awarded his BSc (Courtesy of Isabelle Gillard and Fiona Hammett) haste was caused by Tony Poë (7, 8), who had a similar position to that offered to Gillard, leaving

Imperial College to spend a year in America at North Western University at Evanston in Illinois. Gillard started at Imperial College in late 1961 and in 1963 he was promoted to Lecturer. In 1964 he was awarded a PhD degree with a thesis entitled ‘Spectroscopic Studies of Transition Metal Compounds’ (9, 10). During this period Gillard worked industriously, mainly at preparing and characterising rhodium(III) complexes that are usually kinetically inert although their reactions and preparation can often be catalysed by the presence of small amounts of more reactive reduced rhodium species that can often be conveniently produced via reaction with a small amount of ethanol. After completing his PhD Gillard moved to the University of Sheffield as a Lecturer in Chemistry. For two years Gillard prepared and delivered chemistry lectures at the University of Sheffield, continued Fig. 2. Photograph of Robert D. Gillard just after he research and maintained his prolific publication arrived at the University of Kent at Canterbury in 1966 (Courtesy of Tony Fassom) rate of research papers. Some of his papers at this time were based on work done or started at Imperial College and also extensions to some of his Oxford research involving collaboration with the effects of inorganic species on biological H. M. N. H. Irving. Research was also initiated with systems, though they never looked at interactions, postgraduate students at Sheffield. for example of platinum or rhodium complexes Gillard said his first PhD student was J. H. Dunlop, with DNA, at the molecular level. who had worked with him at Imperial College and Gillard’s time at Canterbury was very productive then moved to Sheffield with him (11). Other with about 100 published papers during his time PhD students rapidly followed. At Sheffield these there (some 15 per year) although this was rather included Nicholas C. Payne and Keith Garbett. less than the halcyon years at Imperial College and They were followed by increasing numbers of the University of Sheffield (more than 20 papers graduate students and postdoctoral workers later were published in 1966). However, the period at at Canterbury and then at Cardiff, and perhaps Canterbury was brought to a close in 1973 by a there were a hundred in all throughout his career. move to the Chair of Inorganic Chemistry at the In 1966 Gillard’s prolific research output was now University of Cardiff. Sometime later the recognised by the award of the Chemical Society’s Department of Chemistry at Canterbury closed Meldola Medal for 1965 for having “conducted (1990 – albeit to be re-opened several years later), the most meritorious and promising original and one can but wonder what might have happened investigations in chemistry and published the had Gillard’s successful dynamism been retained results of those investigations”. at Canterbury by him being appointed to a Chair there rather than him having to seek promotion 2.5 Canterbury and Cardiff (1966– elsewhere. As Professor of Inorganic Chemistry at Cardiff 2013) Gillard’s chemical interests broadened and In the year following the award of the Meldola included a continued fascination with the potential Medal, Gillard (1966) was promoted to Reader hydration of C=N bonds in diimine ligands activated in Chemistry at the recently established (1965) by coordination to metal centres by analogy with University of Kent at Canterbury (Figure 2). This the reactions of some alkylated aromatic nitrogen provided opportunities for considerably enlarging organic compounds. He published widely on this his Research Group, facilitated in 1967 by the topic that was generally referred to as ‘covalent founding of the Medical Research Council Research hydration’. His interests also extended into Group on Biological Inorganic Chemistry, of which several new areas including geological inorganic he was Director until 1975. This brought into his chemistry, and a copper nickel hydroxyl chloride

Group people skilled in bio-techniques who studied mineral (Cu3NiCl2(OH)6) discovered in Western

conferences and events. They had two children, Fiona born in 1983 and Thomas born in 1985 both in South Glamorgan. Later Fiona studied law at Oxford University and became a lawyer. Her brother Thomas works in the construction design industry in London. Gillard’s retirement from the Chair of Inorganic Chemistry at Cardiff in 1998 was impelled by ill health and he underwent successful triple bypass heart surgery. However, complications resulting from his Type 2 diabetes condition eventually led to serious health problems, especially those associated with kidney function. Ultimately renal failure coupled with a stroke led to his death in Fig. 3. Gillardite, Cu3NiCl2(OH)6, a mineral June 2013. from the 132 North deposit, Widgiemooltha, Western Australia named in 2007 (see (12)). Its zinc analogue is Herbertsmithite also from 2.6 Travel and Collaborations Widgiemooltha Gillard had an enthusiastic lecturing style, possibly influenced by that of his PhD supervisor Geoffrey Australia was named ‘Gillardite’ after him by Wilkinson at Imperial College, and throughout his his former colleague at Cardiff, Peter Williams, academic career he travelled and lectured widely. He in recognition of his contributions to inorganic established many professional friendships around chemistry (Figure 3) (12). the world that often stimulated new directions Music had an important role in Gillard’s life. He for his research. For instance, in 1966 he spent played the piano and was a good oboe player, the summer in Italy in the laboratory of Professor continuing lessons in Canterbury, and it is said Lamberto Malatesta in Milan. During 1971 he was he played semi-professionally when visiting the a Visiting Professor at the University of Minnesota University of Minnesota. During his school days in the USA, and there he consulted with the 3M he enjoyed singing – he had parts in some school Company in St Paul, a relationship that continued operas. At school and later in life he sang in several for more than two decades (despite there being choirs. When in Cardiff Gillard took great delight in no published outcome, such as patents). While at being a member of the Second Tenors section in the the University of Minnesota in 1971 he was shot long-established Cwmbach Male Voice Choir. The though not seriously injured (13, 14). choir’s history, printed in 2001, covered the years In 1973 Gillard spent a semester in Germany of the choir’s existence from 1921 to 2001 and at the Friedrich-Alexander University Erlangen- shows on average there were around 12 concerts Nürnberg in the laboratory of Professor Klaus a year between 1970 and 2000. Gillard sang in Brodersen (15). Later, in 1981–1982, he spent a the choir for almost 20 years (July 1976–1995). sabbatical year travelling and he visited universities The choir visited many towns and cities around in New Zealand (where he was an Erskine the UK, and sang memorably at several chemistry Visiting Professor at the University of Canterbury, conferences. During this period the choir was Christchurch), Australia, the Netherlands successful in several competitions, winning prizes (Leiden), and Germany (Freiburg-im-Breisgau), in Wales and Ireland, and had considerable success as well as Barbados. He was a Visiting Professor at in the Bela Bartok Choral Festival in Hungary in the Lajos Kossuth University, Debrecen, Hungary 1986. Gillard apparently was present on the tour of in 1983, and acted in an advisory capacity to Hungary which followed this competition. the University of Brunei from 1985 to 1994. While at Cardiff Gillard married his second wife Gillard collaborated extensively with a number Anne Howard in Leicester in 1982. His Best Man of Portuguese colleagues, and as a result during was Malcolm Pilbrow, who had been one of his the period 1967 to 2004 he often visited Portugal. research students at Canterbury and was co-author Collaboration started with Júlio Domingos Pedrosa of several publications on platinum complexes da Luz de Jesus, who obtained his first degree between 1969 and 1984. Anne had worked for from the University of Coimbra, came to Cardiff as the Royal Society of Chemistry organising various a research student (16) in 1974 and was awarded

8 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040160 Johnson Matthey Technol. Rev., 2021, 65, (1) his PhD in 1978 (17). Gillard’s other notably productive collaboration with Portuguese chemists centered on the Instituto Superior Técnico (IST) in Lisbon, involving also the Universidade Nova de Lisboa (S. M. Luz, R. Duarte and J. J. G. Moura). Gillard published nearly 30 papers jointly with João Costa Pessoa (18) of the Centro de Química Estrutural of the IST over the period 1988–2004, almost exclusively dealing with oxovanadium(IV) complexes of amino acids, Schiff bases, and related ligands. Several coworkers were involved, in particular Maria Teresa Duarte, co-author of ten of the papers (19, 20), and Luis Vilas Boas (21). The list of co-authors and their addresses for his final paper (see the end of Section 2, Part Fig. 4. Robert D. Gillard working in his fourth floor office in the University of Kent at Canterbury II (22)) includes nine Portuguese scientists from in 1969 where he was a Reader in Inorganic five locations (Lisbon, Aveiro, Faro, Savacém and Chemistry until 1973 (Courtesy of Professor A. W. Oeiras), well illustrating his extensive association Addison) with chemistry in Portugal. Gillard had several important collaborations with members of staff in the universities in which he worked. For example at Sheffield there were above, he introduced geological inorganic publications with Professor Ron Mason and the chemistry to Gillard. Australians E. Don McKenzie and his good friend Brice Bosnich (‘Bos’) who was then at University 2.7 General Publishing Interests College, London, later a Professor at the University of Toronto and afterwards until his retirement at In addition to the preparation of original research the University of Chicago (23, 24). papers and reviews Gillard had a range of other At Canterbury Gillard’s Group was of a publishing interests. With Professor R. F. Hudson substantial size and occupied most of the top (University of Kent at Canterbury) and Professor (fourth) floor of the new Chemistry Department J. N. Bradley (University of Essex) Gillard was a building (Figure 4), so much of his time was cofounding editor of Essays in Chemistry (25) spent organising their work and writing up their published by Academic Press which appeared research results for publication. There were regularly from 1970 to 1977, and he was on however collaborations with other people in the the founding Editorial Board of the international Department including with the lecturer Brian journal Transitional Metal Chemistry, first Heaton (later Professor at the University of published by Chapman and Hall in 1975 and now Liverpool) on the preparation and characterisation by Springer. He was on the Editorial Board of the (especially by nuclear magnetic resonance (NMR) Journal of Coordination Chemistry (26, 27). With techniques) of a variety of rhodium coordination his former PhD Supervisor Professor Sir Geoffrey complexes, and with the vibrational spectroscopist Wilkinson and his friend from Imperial College days Alan Creighton on rhodium complex salts of Professor Jon A. McCleverty (then at the University - the acid anion [H(NO3)2] , now known to play a of Birmingham, later at the University of Bristol) role in atmospheric chemistry. At Cardiff there he edited the seven volumes of Comprehensive were fruitful collaborations with the Australians Coordination Chemistry published by Pergamon Leon Kane-Maguire and Peter Williams. Kane- Press in 1987 (28–30). In retirement Gillard’s Maguire had experience of determining the writing activities were diverted to aspects of life kinetic parameters of relatively slow reactions in in the Edwardian and Victorian eras (see the last solution and with Gillard he did work on reactions paragraph of Section 3, Part II (22)). of metal complexes that perhaps involved covalently hydrated intermediates. There was 3. Research Activities extensive collaboration with Williams (over 40 papers), especially on ‘Equilibria in Complexes There is no doubt chemistry was particularly of N-Heterocyclic Molecules’, and, as mentioned important in Gillard’s life. His love of making

9 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040160 Johnson Matthey Technol. Rev., 2021, 65, (1) chemical discoveries in the laboratory and rapidly 5th November 1960. Gillard was the sole author rationalising the observations he made was (see entry A in Table I); when submitted for infectious. Clearly whatever caused him to turn publication he was working at Burt, Boulton and to chemistry rather than mathematics at Oxford Haywood Ltd in their Agricultural Department. was a most profound decision for him and later In this short note Gillard pointed out that metal the chemical community as a whole. He published complexes containing only one optically active research papers at every university he was at: his diamine ligand should have a similar conformation Part II work at Oxford, his doctoral work at Imperial to the diamine (32). This idea originated from his college, as a Lecturer at Sheffield University, as a Oxford work with Irving, and subsequently they had Reader at the University of Kent at Canterbury and several joint publications. The first of these was a as a Professor at Cardiff University. note on the optical resolution of propylene diamine by the stereospecific reaction with lævorotatory III – 3.1 University of Oxford: Part II [Co -(+)-pdta] (pdta = propylene-1,2-diamine- tetra-acetate) that appeared in the December 1960 Research Project with Dr H. M. N. H. issue of the Journal of the Chemical Society (B in Irving Table I) (33). Again this was based on his Part At about the time Gillard began his Part II research II research project, and a full paper with Irving at Oxford J. C. Bailar in the USA was undertaking entitled ‘A New Stereospecific Reaction’ submitted research on the optical activity of transition metal for publication in November 1960 (Table I, C) complexes that paralleled his studies, illustrating continued the theme. This paper (34) highlighted the topical nature of the project set by his supervisor the stereochemistry of the reaction shown in Dr Irving – similar work had very recently been Equation (i) (en = ethane-1,2-diamine). reported from Bailar’s laboratory (31). Gillard’s (–)-[CoIII-(+)-pdta]– + 3 en → first publication to appear (but see footnote ato III 3+ 4– (+)-[Co (en)3] + (+)-pdta (i) Table I) in the chemical literature was a Letter entitled ‘The 1,2-Propylenediaminetetra-acetato- Gillard’s work with Irving on the optical activity cobaltate(III) Anion’ that appeared in Nature on of metal complexes was the start of one of his

Table I Gillard’s Publications With or Based on Research With Professor Irving Title Reference Notes The 1,2-Propylenediaminetetra-acetato- A R. D. Gillard, Nature, 1960, 188, 487 a cobaltate(III) Anion Resolution of (±)-Propylenediamine by a H. Irving and R. D. Gillard, J. Chem. Soc., B a Stereospecific Reaction 1960, 5266–5267 H. Irving and R. D. Gillard, J. Chem. Soc., C A New Stereospecific Reaction 1961, 2249 R. D. Gillard, H. M. Irving, R. Parkins, N. The Stabilities of the Metal Complexes of D C. Payne and L. D. Pettit, Chem. Commun. Optically Active Amino-acids (London), 1965, (5), 81–82 R. D. Gillard and H. M. Irving, Chem. Rev., E Conformational Aspects of Chelate Rings 1965, 65, (5), 603–616 R. D. Gillard, H. M. Irving, R. M. Parkins, N. C. The Isomers of Complexes of α-Amino-acids F Payne and L. D. Pettit, J. Chem. Soc. A, 1966, b with Copper(II) 1159–1164 Stability Constants of Copper(II) Complexes of R. D. Gillard, H. M. Irving and L. D. Pettit, J. G c Optically Active α-Amino-acids Chem. Soc. A, 1968, 673–674 a It is not altogether clear which of these two articles should be regarded as Gillard’s first publication. In a detailed research CV compiled in 1988 he gives the Note co-authored with Irving as his first publication. This corresponds to the apparent order of completion and submission of the respective manuscripts – the Note to J. Chem. Soc. was received by the Editor on 30th May 1960, while the Letter to Nature appears to have been submitted after Gillard had left Oxford. The particularly rapid refereeing → acceptance → publication sequence characteristic of Letters to Nature in that era may well have resulted in a manuscript posted by Gillard after he had left Oxford overtaking the Note submitted to the Chemical Society – the Letter to Nature was published on 5th November 1960, the Note to J. Chem. Soc. appeared in the last (December) issue of 1960

b The authors’ addresses are given as R. D. G. and N. C. P. at Sheffield, H. M. I., R. M. P. and L. D. P. at Leeds

c The author indicated for correspondence is Pettit. By this time Gillard was at Canterbury

10 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040160 Johnson Matthey Technol. Rev., 2021, 65, (1) major research themes during his career, and their His research at Imperial College was the start of collaboration continued to when Irving was Professor Gillard’s lifelong interest in rhodium chemistry and at Leeds University and Gillard was at the University the relevant publications from this time are listed of Kent at Canterbury. In 1965 when Gillard was at in Table II. There was a large amount of exciting Sheffield they submitted for publication Chemicala rhodium chemistry being done that was a part of Communication (Table I, D) about the stabilities Wilkinson’s fascination with the rapidly evolving of the metal complexes of optically active amino chemistry of some of the less common metals acids, and a year later, in 1966, a full paper entitled like rhodium as their reactions were explored that ‘The Isomers of Complexes of α-Amino-acids with often involved then-novel hydride ligands. A huge Copper(II)’ was published (Table I, F). Before that number of new rhodium compounds were being a short (14 pages) but significant review article prepared and characterised which ranged from that had its origins with background literature robust coordination complexes to lower oxidation work done for his Part II thesis, was published state organometallic compounds with alkyl moieties in Chemical Reviews in 1965 (Table I, E). The with remarkable catalytic properties. Gillard was last of Gillard’s publications with Irving appeared in the midst of all of this excitement and he was in 1968 when Gillard was at Canterbury. That involved in clarifying the oxidation state of hydrido paper reported further results from their work on rhodium(III) complexes previously thought to copper(II) complexes of optically active α-amino be rhodium(II) species (35), and he extended acids and included their formation constants some of the early studies on catalytic approaches (Table I, G) – Irving was very well known for the to preparing rhodium(III) complexes via labile determination of formation (stability) constants. lower oxidation state rhodium species (36). To complement this rhodium(II) to rhodium(III) 3.2 Imperial College: Research with oxidation state reassignment, he later showed that most rhodium complexes earlier believed to contain Professor Geoffrey Wilkinson rhodium(IV) or rhodium(V), such as Claus’s Blue Gillard must have had a significant amount of (see several mentions below, also Table S6), were lecturing commitments at Imperial College yet he also compounds of rhodium(III) whose oxidising was able to do much research himself. One of the properties resided in such ligands as superoxo main areas of chemistry Wilkinson introduced him rather than in the metal centre. to was preparative rhodium coordination chemistry At Imperial College Gillard also worked with a which was then topical. A wide range of new rhodium variety of other metal complexes as well as those complexes was being prepared and characterised of rhodium, and he investigated the nature of so- in Wilkinson’s laboratory, with a particular interest called ‘platinum blues’ (37) that are a group of in those with hydride ligands that were amenable polynuclear mixed-valence, metal-metal-bonded, to study by NMR techniques. The involvement of ligand-bridged polymeric complexes, often catalytic amounts of rhodium species in a growing containing chains of Pt(NH3)2 units with various number of homogeneous organic reactions was bridging ligands. In the original platinum blue another important area of intensive research in (Platinblau) of Hofmann (38) acetamide was the Wilkinson’s Group. Gillard’s career-long fascination bridging ligand, and its intense colour, Gillard and with the chemistry of rhodium is reflected in the Wilkinson said, was consistent with the presence of number of publications – over 60, from 1963 to mixed oxidation states, while its dichroism, red/blue

2000 – containing ‘rhodium’ or ‘rhodate’ in their like [Pt(dmgH)2] (dmgH = dimethylglyoxime), was titles. Platinum was another metal of continuing consistent with a linear chain structure. Later there interest, with over 30 publications (1964–2001) was a resurgence of interest in platinum blue(s), featuring its compounds in their titles. stemming from observations that incubation of Including later collaborations with Wilkinson, the hydrolysed anticancer drug cisplatin (cis-

Gillard published 19 papers with him (and one [Pt(NH3)2Cl2]) with pyrimidines or polynucleotides with J. H. Dunlop, Gillard’s first student, without often resulted in deep blue solutions from which Wilkinson). The first 11 were written and submitted blue solids with high antitumour activity and low for publication while Gillard was at Imperial College renal toxicity were obtained (39–41). However, and these and the others are listed in Table II. claims for significant anticancer activity were often Six of these papers have only Gillard and Wilkinson found to be exaggerated or non-reproducible. as authors, reflecting the tremendous amount of Difficulties in characterising these blue materials energy Gillard put into his graduate research. led to the synthesis of a range of analogues, such

Table II Gillard’s Rhodium Chemistry Publications with Professor Geoffrey Wilkinson Title Citation Triethylenetetramine Complexes of Cobalt(III) and R. D. Gillard and G. Wilkinson, J. Chem. Soc., 1963, A Rhodium(III) 3193–3200 Hydrido-Complexes of Rhodium(III)- containing R. D. Gillard and G. Wilkinson, J. Chem. Soc., 1963, B Nitrogen Ligands 3594–3599 The Coordination of Ethylenediaminetetraacetate in R. D. Gillard and G. Wilkinson, J. Chem. Soc., 1963, C Complexes of Cobalt(III) and Rhodium(III) 4271–4272 R. D. Gillard and G. Wilkinson, J. Chem. Soc., 1964, D Aquation of the Trisoxalatorhodate(III) Ion 870–873 Complexes of Rhodium(III) with Chloride and R. D. Gillard and G. Wilkinson, J. Chem. Soc., 1964, E Pyridine 1224–1228 Absolute Configurations of Somed 6 Complex Ions R. D. Gillard and G. Wilkinson, J. Chem. Soc., 1964, F of Cobalt, Rhodium, Iridium, and Platinum, and of 1368–1372 Complex Ions of Chromium(III) Adducts of Protonic Acids with Co-ordination R. D. Gillard and G. Wilkinson, J. Chem. Soc., 1964, G Compounds a 1640–1646 J. H. Dunlop, R. D. Gillard and G. Wilkinson, J. H Configurations of Trisdiamine Complexes b Chem. Soc., 1964, 3160–3163 d-d Transitions in Hydrido-complexes. The Position J. A. Osborn, R. D. Gillard and G. Wilkinson, J. I of the Hydride Ion in the Spectrochemical Series Chem. Soc., 1964, 3168–3173 Activation of Molecular Hydrogen by Complexes of R. D. Gillard, J. A. Osborn, P. B. Stockwell and G. J Rhodium-(III) Wilkinson, Proc. Chem. Soc., 1964, 284–285 Complexes of Ruthenium, Rhodium, Iridium, and J. F. Young, R. D. Gillard and G. Wilkinson, J. Chem. K Platinum with Tin(II) Chloride Soc., 1964, 5176–5189 The Action of Reducing Agents on Pyridine B. N. Figgis, R. D. Gillard, R. S. Nyholm and G. L Complexes of Rhodium(III) Wilkinson, J. Chem. Soc., 1964, 5189–5193 Catalytic Approaches to Complex Compounds of R. D. Gillard, J. A. Osborn and G. Wilkinson, J. M Rhodium(III) Chem. Soc., 1965, 1951–1965 Polarographic Reduction of Complexes of R. D. Gillard, J. A. Osborn and G. Wilkinson, J. N Rhodium(III) Chem. Soc., 1965, 4107–4110 R. D. Gillard and G. Wilkinson, Inorg. Synth., 1967, O trans-Dichlorotetrakis(pyridine)rhodium(III) salts 10, 64–67 a + Includes hydroxonium salts of trans-[Rh(LL)2X2] with LL = en or bipy, X = Cl or Br b 3+ 3+ Includes investigation and discussion of circular dichroism and absolute configuration of [Rh(en)3] and [Rh(pn)3]

as the series of platinum pyrimidine blues prepared Platinum and of Complex Ions of Chromium(III)’ from a variety of nucleosides or nucleotides (42). (46) so the chiral properties of cobalt(III) Later Gillard worked on Claus’s Blue, obtained by complexes first undertaken at Oxford continued at oxidising rhodium(III) compounds – see Table S6. Imperial College. Another important area Gillard was involved Some dimeric copper(II) complexes were in at Imperial College was the development of prepared and characterised at Imperial College tin(II) chloride as a ligand in platinum, ruthenium, (47), and some years later Gillard returned to rhodium and iridium complexes that later became copper(II) complexes after leaving Imperial important in some catalytic processes (43). College (see Table S3). Later papers on dimeric Work within the Wilkinson Group did not exclude complexes involved Co(III) (1969) or VO2+ (1973) research on cobalt(III) complexes that Gillard had rather than Cu(II). He maintained an interest in gained familiarity with at Oxford. Indeed Gillard’s copper for many years, the last paper devoted to first publication at Imperial College with Wilkinson this metal appearing in 1995 (48). Here, and in was entitled ‘Triethylenetetramine Complexes of several publications from the period 1977 to 1980 Cobalt(III) and Rhodium(III)’ (44) which was soon (by which time Gillard was well established in followed by a paper entitled ‘Hydrogen Bonding in Cardiff), the ligands were various amino acids. Complexes of Dimethylglyoxime with Cobalt(III)’ Gillard did a huge amount of research across (45), and then ‘Absolute Configurations of some a wide range of inorganic transitional metal d6 Complex Ions of Cobalt, Rhodium, Iridium and chemistry at Imperial College, and it is clear his

serendipitous move to do research with Professor their Raman study of KNO3 in HNO3 indicated the – Wilkinson was a major event that stimulated and presence of a species [(HNO3)2NO3] at 268 K shaped much of his later professional research (64). Hedin (65) prepared the first nitric acid career. adduct of a transition metal complex [Pt(py)4Cl2]

(NO3)2·2HNO3·2H2O and Poulenc later in the 3– 3.3 University of Sheffield: course of his study of reactions of [RhBr6] with pyridine (66), isolated shiny yellow triangular Independent Research prisms which analysed as [Rh(py)4Br2]NO3·HNO3. During his time at Imperial College much of Gillard’s However, it was not until the mid-1960s that research followed directions set by Professor his product was re-formulated, mainly on the Wilkinson and topics pursued within his group, but basis of infrared spectroscopy and conductivity Gillard also had considerable independence that measurements, as a hydrogen dinitrate salt, trans- of course increased immensely when he moved to [Rh(py)4Br2][O2NO·H·ONO2] (67, 68). At the time the University of Sheffield in 1964. Here he was of his reformulation of Poulenc’s salt Gillard also able to concentrate more on his personal research showed that the vibrational spectrum of [Ph4As] interests and establish his own research agenda NO3·HNO3 was consistent with its containing – while continuing to publish with both Irving and symmetrically H-bonded [H(NO3)2] (69). In 1967

Wilkinson. He turned enthusiastically to an area of single crystal X-ray studies of trans-[Rh(py)4Br2] coordination chemistry he worked on at Oxford: NO3·HNO3(70) and of [Ph4As]NO3·HNO3 (71) – the optical activity properties of metal complexes confirmed the presence of the [H(ONO2)2] anion and soon he expanded on what he had done in both salts, albeit in different configurations previously. There were several joint papers with (72–75). The last two members of Gillard’s Professor Irving (see Section 3.1 and Table I), ‘Adducts of Coordination Compounds’ series report and his independent work included the first papers the isolation and characterisation of several more in series (49, 50) such as ‘Adducts of Coordination hydrogen dinitrate salts of complexes of pgms, for + Compounds’ (Table S1) and ‘Optically-active example of trans-[Ru(py)4Cl2] (76) and of trans- + Coordination Compounds’ (Table S2). The former [IrL4X2)] (L = pyridine, perdeuteriopyridine or ran to 15 parts and continued until 1990; while 4-methylpyridine; X = Cl or Br) (77–79). Since the latter, his most extensive series of papers, Gillard’s investigation of hydrogen dinitrates comprised 51 parts, of which the first 37 appeared there has been sporadic interest in compounds at frequent intervals between 1965 and 1975, the containing this anion. For example, an iron- next 13 roughly annually until 1989, and the final substituted polyoxometalate/hydrogen dinitrate III 5− part in 1995–1996. There was also a short review system – specifically [Fe W11O39·H(ONO2)2] entitled ‘Optically-Active Coordination Compounds’ – was in 2006 reported to be a highly reactive published in 1967 (51–53) that followed the catalyst for aerobic oxidation of thioethers (80). publication of his more substantial review ‘The In recent years the hydrogen dinitrate anion, as Cotton Effect in Coordination Compounds’ (54). one of a number of species in nitrate clusters Some papers published or submitted for publication with nitric acid and water (81) has become of while Gillard was at Sheffield University are listed considerable interest in relation to chemistry of in Tables S1 and S2. the atmosphere. Several of the papers in the ‘Adducts of The first paper in the ‘Optically Active Co-ordination Coordination Compounds’ series (Table S1) Compounds’ series (Table S2) was on the optical relate to the once-elusive hydrogen dinitrate configurations of bisethylenediamine complexes of – anion [H(ONO2)2] . Schultz may have made a few cobalt(III) with his PhD student Keith Garbett who species containing this anion in his investigation, must have worked quickly to have produced such briefly reported (55) in 1869, of solubilities of a volume of results in such a short period of time nitrates in nitric acid. Ditte prepared several acid (82). The very small number of papers from this nitrates M(NO3)·xHNO3 (M = K, Rb, Tl, NH4) in series detailed in Table S2 illustrate the range of the course of an extensive study of reactions of complexes and topics covered in these 51 papers, a range of nitrates with fuming nitric acid; Ditte’s published over a period of some 30 years. Thus the results, published (56–60) in 1879, were largely nine citations in Table S2 include such favourite confirmed by Wells and Metzger (1901) (61) and subjects as rhodium complexes, copper complexes by Chédin, Leclerc and Fénéant (1947) (62, 63). (especially with amino acids), circular dichroism Chédin and Fénéant subsequently reported that and reaction mechanisms.

Work on what became the series ‘Isomers of However, there is rather little supporting evidence 2+ α-Aminoacids with Copper(II)’ (Table S3) and from similar systems. [Co(NH3)5(NO2)] is not ‘Reactions of Complex Compounds of Cobalt’ aquated directly in an acetate buffer, but it does (Table S4) was probably initiated while Gillard react to give cobalt in the +2 oxidation state (90– was at Sheffield, and the paper entitled ‘Stability 92). This presumably results from initial electron Constants of Copper(II) Complexes of Optically transfer to the cobalt; the electron probably comes Active α-Amino-acids’ (83) forms a link back to his from the anion rather than from the cationic ligands, research at Oxford. as in base hydrolysis, or thermal decomposition 3+ 2+ of [Co(NH3)6] and [Co(NH3)5X] salts (93). 3.4 University of Kent at Canterbury: Indeed hydroxide can be reducing when coupled to strongly oxidising systems, as in the mechanism Continued Independent Research n+ suggested for base hydrolysis of [M(diimine)3] {M Gillard’s move in 1966 to the almost completed = Fe, Ru, Os; diimine = phen or bipy; n = 3} (94) Chemistry Department at the University of Kent (cf. the covalent hydration discussion in Section at Canterbury (84) with new laboratories, new 2.1.1, Part II (22)). As with so many of his novel equipment and good funding coupled with a growing suggestions, this proposal attracted criticism, with number of youthful enthusiastic researchers from Endicott stating that “the Gillard mechanism can a variety of universities gave him a marvellous be ruled out on both kinetic and energetic grounds” opportunity to work productively with vigour, to – and indeed was “outrageous” (95). expand his research group, and to enhance his By Part 4 (96) of the series ‘Reactions of already considerable personal reputation. His main Complex Compounds of Cobalt’ (Table S4) research areas at Canterbury included cobalt and Gillard’s penchant for looking into long-published some copper chemistry, much rhodium chemistry as (but also long-ignored!) oddities had led him well as bioinorganic chemistry which was facilitated to investigate the green products, described in by the Medical Research Council funding his the 1920s, which had been obtained (97, 98) Research Group on Biological Inorganic Chemistry. on reacting cobalt(III) salicylatotetrammine or Early in his time at Canterbury Gillard began his salicylatobisethylenediamine complexes with series of papers entitled ‘Reactions of Complex nitric acid. He suggested, largely on the basis of Compounds of Cobalt’ that are summarised in electronic spectra and circular dichroism data, Table S4. Most of the work for the opening that the colour was due to ligand-oxidation and paper, presumably done at Sheffield, gave a that the green complexes were cobalt(III) species novel explanation for the then-much-discussed containing 5-nitrososalicylate rather than the apparently ambiguous second-order rate law for cobalt(IV)-5-nitrosalicylate analogues proposed by base hydrolysis of cobalt(III)-ammine complexes the original authors (99). (85). The simplest interpretation of the established A more recent example of his interest in all-but- rate law was rate-determining attack by hydroxide forgotten puzzles involved the so-called ‘Tipper’s at the metal centre (86). However, both in the light Compound’. In 1955 C. F. H. Tipper described of the difficulty of access of the hydroxide to the low- the reaction between H2PtCl6 and cyclopropane, 6 spin t2g Co(III) centre and in the absence of any suggesting the formula (PtCl2·C3H6)2 for other well-established bimolecular substitutions his product (100, 101). A few years later a at this ion, an alternative SN1CB mechanism had polynuclear structure was proposed on the basis also been proposed, in which rapid reversible loss of infrared and NMR spectra (102); then in 1969 of an ammine or amine proton was followed by Gillard, on the basis of mass spectrometry and rate-limiting dissociation of the conjugate base additional infrared evidence, argued that the (87–89). Gillard’s suggestion that the rate- compound was tetrameric dichlorotrimethylene- determining step in such base hydrolyses was platinum(IV) (103). electron transfer from a hydroxyl ion to cobalt(III), Even before going to Canterbury Gillard was to give a labile cobalt(II) complex intermediate, interested in the potential pharmaceutical offered a neat way of circumventing the vexingS N2 applications of inorganic complexes. This interest versus SN1CB question. This idea is consistent both was fostered by the founding of the Medical with many observations from preparative cobalt Research Council Research Group on Biological chemistry, and with differences in kinetic behaviour Inorganic Chemistry at Canterbury; MRC funding for base hydrolyses of between cobalt(III), began very soon after he went to Canterbury. rhodium(III) and chromium(III) complexes (85). Practical work in biological areas was facilitated

14 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040160 Johnson Matthey Technol. Rev., 2021, 65, (1) by Gillard recruiting a number of microbiologists Royal Society of Chemistry (and its predecessor, such as Richard Dainty (104) and Colin Thorpe the Chemical Society) (128). Gillard’s contribution (105), together with several others who had some described some effects of rhodium complexes on experience of metal-biological interactions such as bacterial growth (129). The final part in the series Stuart Laurie (106). The result was an enthusiastic dealt with the antibacterial activity of rhodium group of ‘bioinorganic’ researchers. complexes of S-nicotine (124). Interestingly, he Gillard’s interest in this area was probably returned to these rhodium-nicotine complexes prompted by reports of the antibacterial, a decade later (130–132). This was in a paper antitumour and wound healing properties and entitled ‘Stereoselectivity in Rhodium Antibiotics’, topical infection control of stable metal complexes which reported on the concentration-dependence n+ of the type [M(diimine)3] {M = for example Fe, of the bactericidal properties of the enantiomeric Ru; diimine = phen or bipy}. Effects of this type rhodium(III)-nicotine complexes trans-[Rh(S-(–))- + + of metal-diimine complex on cells and organisms nicH )4Cl2](PF6)5, trans-[Rh(R-(+)-nicH )4Cl2] + seem to have been first reported, specifically for (PF6)5 and trans-[Rh(RS-(+)-nicH )4Cl2](PF6)5. 2+ [Fe(bipy)3] , in the late 1930s by Emilio Beccari This work was claimed to be the first to demonstrate (107–111). In the 1950s a wider-ranging study stereoselective prevention of bacterial growth by was described by a group of Australian chemists one enantiomer of a metal complex. Gillard had led by F. P. Dwyer (112–117). This work was earlier briefly reviewed the antibacterial properties later developed by several groups, including that and redox behaviour of rhodium complexes with of his younger colleague Shulman (118–120). pyridine and substituted pyridines (133). Whereas Gillard was intrigued by the dramatic elongation of several rhodium complexes have considerable growing cells brought about by low levels (a few bactericidal properties there are very few reports ppm) of platinum(IV) complexes. These abnormal of significant antitumour capability. Indeed a filamentous cells could be several hundred times recent review (134) (101 references) of the role of longer than normal cells (121, 122). pgms in cancer therapy mentions just one rhodium Gillard started publishing his 12-part series compound – a rhodium(I) complex [RhCl(COD) ‘Coordination Compounds and Micro-Organisms’ (NHC)], 1, where COD is cycloocta-1,5-diene and (Table S5) in 1969 (123); the last part appeared NHC is an N-heterocyclic carbene ligand derived in 1989 (124). His work on this series was carried from caffeine. This compound shows multiple out over more than two decades, during which anticancer properties and antibacterial activity time Gillard maintained a strong interest in the (135). Other earlier publications by Gillard in area. During this time he will have noted such this area but not included in the ‘Coordination relevant publications as the reported bacteriostatic Compounds and Micro-Organisms’ series include 2+ effects of [Ru(phen)3)] and related complexes a significant review on metal-protein interactions (125) and by the description of the cholinolytic (136) with Stuart Laurie. activity of 3,4,7,8-tetramethyl-1,10-phenanthroline Shortly after starting the series of papers on complexes of copper(II) and nickel(II) (126). He microorganisms Gillard began publishing another may well also have been influenced by the later rhodium-centred series of papers ‘Oxidants research of Margaret Farago and her group at Containing Rhodium’ (Table S6) and at the same Imperial College on the effects of added inorganic time a series of papers on metal complexes, mainly species on the growth of various aquatic plants of nickel, as probes for the structure of solvents (see below). (Table S7). The former consists of a disparate An early product of the practical work, reported in collection of papers, reviews and a conference Part 1, was the investigation of the bacteriostatic abstract, apparently gathered together during the activity of complexes of a range of Co(III), Rh(III), preparation of the manuscript for what was to be and Ir(III) complexes containing substituted labelled as Part 8. Most papers in Table S6 (and pyridines, 2,2′-bipyridine, 1,10-phenanthroline, a paper entitled ‘Oxidants Containing Rhodium’ or ethane-1,2-diamine (127). Part 10 of this but not included in the series!) deal with superoxo series derives from the 1981 conference at Bristol complexes. Their main conclusion (137) is that on the chemistry of the platinum metals – the nearly all the substances generated in water and first such on an international scale. This was the said to involve rhodium-(IV), -(V) and -(VI) species first of a series of eight triennial conferences (138, 139) are in fact superoxide complexes of on this area, one example of the long-standing rhodium(III) (140–144). Eventually, after his move collaboration between Johnson Matthey and the to Cardiff, Gillard published the last paper in the

series in which was reported X-ray photoelectron H H N spectroscopy on so-called Claus’s Blue (145–147). N O N Co This is one of several long-known blue or violet N N N Rh rhodium species obtained by chlorine, peroxoanion N N Cl Me (148, 149), or electrolytic (66, 150) oxidation of O 1 [Co(meab)] rhodium(III). This enabled the assignment of the 2 oxidation state of 3+ to the rhodium in this species, indicating that it too was a superoxo-Rh complex. The formula proposed (151) for Claus’s Blue was bigger changes in spectra. Interestingly, Gillard III III Ba[(H2O)4Rh (μ-O2)(μ-OH)Rh (OH2)4], rather switched to cobalt(III) and the rather different probe VI 2– 3+ than the [Rh O4] originally suggested by Claus. of interactions between [Co(en)3] and anions for The series of papers devoted to the use of metal the final paper in this series – published almost complexes to probe solvent structure (Table S7) 20 years later, long after he had left Canterbury. Here, reflects Gillard’s abiding interest in the development differences in the concentration-dependence of the of coordination chemistry. The nickel salt he selected circular dichroism spectra of sulfate and perchlorate for all but the last member of the series was bis-meso- ion-pairs are ascribed to a balance between ion-

(stilbenediamine) nickel(II) acetate, Ni(stien)2(OAc)2 pairing and selective solvation (171). Then, 14 years

(stien = stilbenediamine = H2NC6H4CH:CHC6H4NH2). on (in 2003), a paper on the salting-in of uncharged This salt is one of the so-called Lifschitz salts – cobalt(III), chromium(III), and copper(II) complexes prepared and investigated in the mid-20th century of natural amino acids by salts such as MgCl2 or by such worthies as Mann (152, 153), Lifschitz AgNO3 effectively provided a postscript to the series. (154–156) and Pfeiffer (157), and later by several The existence of significant complex-salt interactions other groups (158–161). Lifschitz salts may be was illustrated by the isolation of adducts such as obtained in yellow or orange diamagnetic form or in trans-[Cu(gly)2]·2AgNO3 (172). blue or violet paramagnetic form, in some cases in The seven years or so Gillard spent at Canterbury both forms, depending on the coordination number proved to be an exceptionally productive period and stereochemistry at the Ni2+ centre (coordination in terms of the diversity of research undertaken number 4 planar versus coordination number 6 – from fundamental work on classical inorganic octahedral respectively), which in turn depends complexes (173–176) through biochemical to on the natures of the diamine (generally an N,N- medically-related topics – and the number of dialkylethane-1,2-diamine or stilbenediamine), of papers published (slightly more than one hundred). the anion, and of the solvent (coordinating or non- The MRC funding of key aspects of his work very coordinating) used for their preparation. Equilibrium much helped facilitate Gillard’s industriousness between the two forms may be observed in solution that continued when he was a Professor at Cardiff, in systems where the coordinating powers of the though he had considerably fewer coworkers there. anion and solvent are appropriately balanced. Gillard used the consequent marked changes in colour with 4. Acknowledgements change of stereochemistry to monitor changes in solvent structure on adding various cosolvents – Many people provided information and alcohols, ketones, amides, or glycol ethers – to their reminiscences about Gillard and a full list aqueous solutions (162–165). Appropriate plots appears at the end of the second part of this of ratios of yellow to blue forms as a function of commemoration. Those who provided photographs solvent composition reflected changes in structure of are acknowledged in the accompanying caption. the mixed solvents analogous to those established by various techniques for alcohol-water and similar binary solvent systems (166). This approach 5. Supplementary Material represents a major advance from earlier similar work The following Tables may be found in the on solvent structure (167). This had been based Supplementary Information included with the on very small changes in visible absorption spectra online version of this article: of square-planar [Co(meab)], 2. Nevertheless Table S1 Publications in the Series “Adducts of the results, particularly in aqueous pyridine, were Coordination Compounds” deemed to be “intriguing” (168–170). Gillard and Table S2 The First Seven and Final Two Publications Sutton’s nickel probes proved better than [Co(meab)] in the Series “Optically-active Coordination in that their stereochemical change resulted in much Compounds”

Table S3 Publications in the Series “Isomers of 1973 (6); knighted in 1976 α-Amino acids with Copper(II)” 6. ‘The Nobel Prize in Chemistry 1973: Ernst Otto Table S4 Publications in the Series “Reactions of Fischer and Geoffrey Wilkinson’, “Nobel Prizes and Complex Compounds of Cobalt” Laureates”, The Nobel Foundation, Stockholm, Table S5 Publications in the Series “Coordination Sweden, 1973 Compounds and Micro-organisms” 7. Anthony J. Poë (later Professor of Chemistry at Table S6 Papers Published in the Series “Oxidants the University of Toronto) was at Imperial College Containing Rhodium” and registered for a PhD (completed in 1961). Gillard had a position at Imperial College as a Table S7 The Series “Metal Complexes as Indicators vacancy became available thanks to Poë being for Solvent Structure” invited by Professor Fred Basolo to supervise Table S8 The Series “Sulphides of the Platinum the work of his research group at North Western Group Elements” University at Evanston, Illinois while Basolo spent Table S9 The First Six and Last Four Publications, a sabbatical in Rome during the year 1961–1962 Plus Intervening PGM-Relevant Papers, of the (8) Series “Equilibria in Complexes of N-Heterocyclic 8. F. Basolo, ‘Foreign Guests Hosted: A. J. Poë’, in Molecules” “From Coello to Inorganic Chemistry: A Lifetime Table S10 Publications Concerned with Medical of Reactions”, Profiles in Inorganic Chemistry, Aspects, Mobilisation of Aluminium from Cookware Ch. 6, Springer Science and Business Media, New and Accumulation of Aluminium by Tea Plants York, USA, 2002, p. 207 Which Led to Gillard’s Article “Beware The Cups 9. This is dated July 1964; it has ix+103 pages. That Cheer” There are 95 references, the earliest dating from Table S11 The Series “Oxovanadium(IV) – Amino- 1885, several are pre-1914. This spread of dates, Acids” and the occasional esoteric source, are an early Table S12 Reports on Conference Proceedings in example of Gillard’s wide-ranging scanning of the the Journal of Inorganic Biochemistry Related to chemical literature (10) the Series “Oxovanadium(IV) – Amino-Acids” 10. R. D. Gillard, “Spectroscopic Studies on Complex Compounds of Transition Metal Compounds”, PhD Thesis, Department of Chemistry, Imperial 6. References and Notes College, London, UK, July, 1964, 112 pp 1. Harry Munroe Napier Hetherington Irving’s wide- 11. Dunlop subsequently moved to the Technische ranging research interests included structural and Hochschule, München, Germany. Payne later solution chemistry of coordination complexes. He became a Professor at the University of Western moved from Oxford (where he was in Gillard’s Ontario in Canada. Garbett later worked time Vice President of St Edmund Hall as well at Northwestern University with Professor as University Demonstrator in Chemistry) to I. M. Klotz, using Mössbauer spectra to study the chair of Inorganic and Structural Chemistry hemerythrin, before going on to work on at the University of Leeds in 1961. He retired corrosion in nuclear power plants at the UK’s in 1971, but in 1976 travelled to the University Central Electricity Generating Board (CEGB) of Cape Town (UCT), initially for a three month 12. M. E. Clissold, P. Leverett, P. A. Williams, stay. In practice he stayed on in South Africa D. E. Hibbs and E. H. Nickel, Canadian Mineral., until his death in 1993. He inaugurated the chair 2007, 45, (2), 317–320 of Analytical Science at UCT in 1979, remaining 13. The University’s official information publication in that post until his second retirement, at the reported, in issue 32 of May 25, that “Robert age of 80, in 1985 (2, 3). Gillard invoked the D. Gillard, chemistry, visiting professor from Irving-Williams Series (4) from time to time, and England, was shot Saturday in his office. He shared Williams’s interest in matters inorganic, was taken to U Hospitals, where he is listed in biochemical and medical; R. J. P. Williams was satisfactory condition. Suspect was taken into one of the examiners for Gillard’s BSc thesis custody pending investigation” (14) 2. A. T. Hutton, Trans. Roy. Soc. S. Africa, 1994, 14. University of Minnesota Brief, 1971, 1, (32), 2 49, (2), 256–258 15. A decade later, collaboration with Brodersen 3. R. J. P. Williams and R. D. Gillard, Polyhedron, was reflected in a visit to Cardiff by Brodersen’s 1987, 6, (1), 1 protégé Hans Ulrich Hummel (see the paragraph 4. H. Irving and R. J. P. Williams, J. Chem. Soc., on gravimetric analysis of [Fe(phen)2(CN)2] 1953, 3192–3210 hydrate toward the end of Section 2.1.1, Part II 5. Nobel Prize Winner, jointly with E. O. Fischer, in (22) and the endnote cited there)

16. At Cardiff Gillard supervised several graduates of 1987, pp. 453–583 Portuguese universities for MSc or PhD degrees, 30. J. D. Pedrosa de Jesus, ‘Hydroxy Acids’, in an early example being Jaime Alejandro Arce “Comprehensive Coordination Chemistry”, eds. Sagüés, whose MSc, completed in 1977, dealt G. Wilkinson, R. D. Gillard and J. A. McCleverty, with several ruthenium(III)-diimine complexes Vol. 2, ch. 15.7, Pergamon Press, Oxford, 1987, 17. On his return to Portugal he was appointed to pp. 461–486 the Chemistry Department of the University 31. E. J. Corey and J. C. Bailar, J. Am. Chem. Soc., of Aveiro, becoming a Professor in 1979 and 1959, 81, (11), 2620–2629 acting as Minister of Education 2001–2002. He 32. R. D. Gillard, Nature, 1960, 188, (4749), 487 published a dozen papers with Gillard over the years 1977 to 1990. These mainly deal with 33. H. Irving and R. D. Gillard, J. Chem. Soc., 1960, complexes of rhodium(III) and molybdenum(VI); 5266–5267 he contributed to three papers in the “Optically- 34. H. Irving and R. D. Gillard, J. Chem. Soc., 1961, Active Compounds” series 2249 18. Gillard’s postgraduate student Ray Wootton 35. B. N. Figgis, R. D. Gillard, R. S. Nyholm and worked both with Costa Pessoa and with Frausto G. Wilkinson, J. Chem. Soc., 1964, 5189–5193 da Silva in Portugal 36. R. D. Gillard, J. A. Osborn and G. Wilkinson, 19. Her contributions can be traced through the J. Chem. Soc., 1965, 1951–1965 latest Part, viz. “Preparation and characterisation 37. R. D. Gillard and G. Wilkinson, J. Chem. Soc., of new oxovanadium(IV) Schiff Base complexes 1964, 2835–2837 derived from amino acids and aromatic 38. K. A. Hofmann and G. Bugge, Ber. Dtsch. Chem. o-hydroxyaldehydes” (20) Ges., 1908, 41, (1), 312–314 20. J. Costa Pessoa, I. Cavaco, I. Correia, 39. P. J. Davidson, P. J. Faber, R. G. Fischer, S. Mansy, M. T. Duarte, R. D. Gillard, R. T. Henriques, H. J. Peresie, B. Rosenberg and L. VanCamp, F. J. Higes, C. Madeira and I. Tomaz, Inorg. Chim. Cancer Chemother. Rep., 1975, 59, (2), 287– Acta, 1999, 293, (1), 1–11 300 21. L. F. Vilas Boas worked in Gillard’s group in 40. B. Rosenberg, Cancer Chemother. Rep., 1975, Cardiff, and later was a colleague of Costa Pessoa 59, (3), 589–598 in Lisbon. Gillard published nine papers (1977– 1992) and at least three conference abstracts 41. R. J. Speer, H. Ridgeway, L. M. Hall, D. P. Stewart, with Vilas Boas K. E. Howe, D. Z. Lieberman, A. D. Newman and J. M. Hill, Cancer Chemother. Rep., 1975, 59, 22. J. Burgess and M. V. Twigg, Johnson Matthey (3), 629–641 Technol. Rev., 2021, 65, (1), 23–43 23. The Australian Professor Brice Bosnich was 42. T. V. O’Halloran, P. K. Mascharak, I. D. Williams, elected a Fellow of The Royal Society in 2000, and M. M. Roberts and S. J. Lippard, Inorg. Chem., after retirement at the age of 70 he left Chicago 1987, 26, (8), 1261–1270 and returned to Australia where he questioned 43. J. F. Young, R. D. Gillard and G. Wilkinson, the validity of some global warming claims. He J. Chem. Soc., 1964, 5176–5189 died in 2015 (24) 44. R. D. Gillard and G. Wilkinson, J. Chem. Soc., 24. J. D. Crowley, W. G. Jackson and S. B. Wild, Aust. 1963, 3193–3200 J. Chem., 2016, 69, (5), 485–488 45. R. D. Gillard and G. Wilkinson, J. Chem. Soc., 25. The seven volumes published consisted of 1963, 6041–6044 short reviews aimed at undergraduates and 46. R. D. Gillard and G. Wilkinson, J. Chem. Soc., postgraduates 1964, 1368–1372 26. J. D. Atwood, Department of Chemistry, 47. R. D. Gillard, D. M. Harris and G. Wilkinson, University of Buffalo, New York, USA, February, J. Chem. Soc., 1964, 2838–2840 2017, personal correspondence 48. H. O. Davies, R. D. Gillard, M. B. Hursthouse and 27. J. Coord. Chem., 1997, 41, (3), a A. Karaulov, J. Chem. Soc. Dalton Trans., 1995, 28. Gillard’s strong Portuguese connections are (14), 2333–2336 reflected in the inclusion of two substantial 49. The vagaries of research and publication chapters by three of his collaborators from that occasionally led to irregularities in numbering country (29, 30) Parts of series. Thus, for instance, there are (a 29. L. F. Vilas Boas and J. Costa Pessoa, ‘Vanadium’, very few) papers included in two series (see, in “Comprehensive Coordination Chemistry”, eds. e.g., the footnotes to Table S5) and several G. Wilkinson, R. D. Gillard and J. A. McCleverty, Parts of the series “Equilibria in Complexes of Vol. 3, ch. 33, Pergamon Press, Oxford, UK, N-Heterocyclic Molecules” are missing – though

this series has two Part 50s. Moreover there are or ½(bipyridine) and X = CI or Br several publications which were not assigned to 68. R. D. Gillard and R. Ugo, J. Chem. Soc. A, 1966, appropriate series despite their being central to 549–552 the subjects of the respective series, for example 69. B. D. Faithful, R. D. Gillard, D. G. Tuck and R. Ugo, “Direct Evidence for Existence of a Covalently J. Chem. Soc. A, 1966, 1185–1188 Hydrated Coordination Compound” (50) 70. G. C. Dobinson, R. Mason and D. R. Russell, 50. R. D. Gillard, L. A. P. Kane-Maguire and Chem. Commun. (London), 1967, (2), 62–63 P. A. Williams, Transition Met. Chem., 1976, 1, 247 71. B. D. Faithful and S. C. Wallwork, Chem. Commun. (London), 1967, (23), 1211 51. A preliminary heavily abbreviated version of Gillard’s Meldola Lecture (52), followed by a short 72. Disappointingly, infrared-Raman spectroscopic, article on Raphael Meldola himself, appeared X-ray diffraction (73) and neutron-scattering earlier that year (53) (74) studies of Cs[H(ONO2)2] failed to establish the position of the proton. However, a subsequent 52. R. D. Gillard, Chem. Brit., 1967, 3, (5), 205–211 neutron diffraction study of trans-[Rh(py)4Cl2] 53. R. D. Gillard, Chem. Brit., 1967, 3, (1), 1–2 NO3·HNO3 highlighted the disorder that probably 54. R. D. Gillard, ‘The Cotton Effect in Coordination explains the difficulty in locating the exact Compounds’, in “Progress in Inorganic Chemistry”, location of the H atom in the hydrogen dinitrate ed. F. A. Cotton, Vol. 7, John Wiley and Sons Inc, anion (75) New York, USA, 1966, pp. 215–276 73. J. M. Williams, N. Dowling, R. Gunde, D. Hadži 55. C. Schultz, Z. Chem., 1869, 5, 531–532 and B. Orel, J. Am. Chem. Soc., 1976, 98, (6), 56. This paper (57) was summarised (58, 59) and the 1581–1582 results included in (60). See page 293 of (60), 74. J. Rozière and C. V. Berney, J. Am. Chem. Soc., “Deuxième Partie: Sels Ternaires Oxygénés”, 1976, 98, (6), 1582–1583 where it is specifically stated that the nitrates of 75. J. Rozière, M. S. Lehmann and J. Potier, Acta rubidium and of caesium, like those of potassium Crystallogr., Sect. B: Struct. Sci., 1979, B35, and of ammonium, dissolve in fuming nitric acid (5), 1099–1102 to give “acid nitrates” 76. N. S. Al-Zamil, E. H. M. Evans, R. D. Gillard, 57. A. Ditte, Ann. Chim. Phys., Series 5, 1879, 18, D. W. James, T. E. Jenkins, R. J. Lancashire and 320–345 P. A. Williams, Polyhedron, 1982, 1, (6), 525–534 58. A. Ditte, Comptes Rendus, 1879, 89, 576–579 77. (78); many years earlier trans-[Ir(py)4Cl2)]

59. A. Ditte, Comptes Rendus, 1879, 89, 641–643 [H(ONO2)2] had been mentioned as an aside 60. A. Ditte, “Étude Générale des Sels: Deuxieme in a short paper on synthesis of iridium(III) Partie: Sels Ternaires Oxygénés”, Leçons complexes (79) Professées à la Faculté des Sciences de Paris, 78. R. D. Gillard and S. H. Mitchell, Polyhedron, H. Dunod et E. Pinat, Paris, France, 1906, 382 1987, 6, (10), 1885–1899 pp, p. 293 79. R. D. Gillard and B. T. Heaton, Chem. Commun. 61. H. L. Wells and F. J. Metzger, Am. Chem. J., 1901, (London), 1968, (2), 75 26, 271–275 80. N. M. Okun, J. C. Tarr, D. A. Hilleshiem, L. Zhang, 62. J. Chédin and S. Fénéant, Comptes Rendus, K. I. Hardcastle, C. L. Hill, J. Mol. Catal. A: 1947, 224, 930–932 Chem., 2006, 246, (1–2), 11–17 63. J. Chédin, R. Leclerc and R. Vandoni, Comptes 81. N. Heine, T. I. Yacovitch, F. Schubert, C. Brieger, Rendus, 1947, 225, 734–736 D. M. Neumark and K. R. Asmis, J. Phys. Chem. 64. J. Chédin and S. Fénéant, Comptes Rendus, A, 2014, 118, (35), 7613–7622 1949, 228, 242–244 82. K. Garbett and R. D. Gillard, J. Chem. Soc., 1965, 65. S. G. Hedin, Acta Univ. Lund, Sect. 2, 1886, 22, 6084–6100 1–6 83. R. D. Gillard, H. M. Irving and L. D. Pettit, 66. P. Poulenc, Ann. Chim., 1935, 11, (4), 567–657 J. Chem. Soc. A, 1968, 673–674 67. Gillard suggested this reformulation at the 84. At the University of Kent at Canterbury Autumn Meeting of the Royal Society of Chemistry chemists were first housed in a temporary at Nottingham in September 1965 (Abstract ‘hut’ near Eliot College, and they moved into B4), then published it the following year (68). the new Chemistry Building well before it was This paper also reports the preparation of a formerly opened by the Chancellor Her Royal

number of similar compounds, trans-[MA4X2] Highness Princess Marina Duchess of Kent on

(O2NO·H·ONO2), with M = Co or Rh, A = pyridine 20th October 1967 when most of the offices

and laboratories were, or in the process of 101. C. F. H. Tipper, J. Chem. Soc., 1955, 2045–2046 being, occupied. The chemistry building had 102. D. M. Adams, J. Chatt, R. G. Guy and N. Sheppard, four floors for Radiochemistry (Professor J. Chem. Soc., 1961, 738–742 G. Martin, later Dean of Natural Sciences and 103. S. E. Binns, R. H. Cragg, R. D. Gillard, B. T. Heaton Deputy Vice-Chancellor), Physical Chemistry and M. F. Pilbrow, J. Chem. Soc. A, 1969, 1227– (Professor E. F. Caldin), Organic Chemistry 1231 (Professor R. F. Hudson) and the top floor Inorganic Chemistry (Dr R. D. Gillard). 104. R. H. Dainty later did extensive work on the Spacious teaching laboratories were in a single effects of bacteria on meats at the Norwegian story annex. Today the building is known as Food Research Institute the Ingram Building 105. C. Thorpe went on to work at Department of 85. R. D. Gillard, J. Chem. Soc. A, 1967, 917–922 Chemistry and Biochemistry in the University of Delaware 86. F. Basolo and R. G. Pearson, “Mechanisms of Inorganic Reactions: A Study of Metal Complexes 106. S. H. Laurie, whose PhD work at Aberystwyth and Solutions”, 2nd Edn., John Wiley and Sons, with C. B. Monk contributed experience in the New York, USA, 1967, p. 177 field of interactions of ligands with metal ions; 87. Comparison of H/D exchange rates with those for subsequently he worked at Leicester Polytechnic (later De Montfort University) until his retirement base hydrolysis lends strong support to the SN1 CB mechanism for the latter. See for example 107. E. Beccari, Boll. Soc. Ital. Biol. Sper., 1938, 13, (88, 89) 6–8 88. C. K. Poon and M. L. Tobe, Chem. Commun. 108. E. Beccari, Boll. Soc. Ital. Biol. Sper., 1938, 13, (London), 1968, (3), 156–158 8–11 89. M. L. Tobe and J. Burgess, “Inorganic Reaction 109. E. Beccari, Boll. Soc. Ital. Biol. Sper., 1941, 16, Mechanisms”, Addison-Wesley Longman, Harlow, 214–216 UK, 1999, p. 158 110. E. Beccari, Boll. Soc. Ital. Biol. Sper., 1941, 16, 90. D. Banerjea, J. Inorg. Nuclear Chem., 1967, 29, 216–218 (11), 2795–2805 111. E. Beccari, Arch. Sci. Biol. (Bologna), 1941, 27, 91. Redox catalysis of substitution at analogous 204–246 chromium(III) complexes is well-known, as for 112. Francis Patrick John (Frank) Dwyer (1910–1962) example in the facile preparation of tris(diamine) was an important and influential Australian chromium(III) salts (see for example (92)). chemist, one of the pioneers of bioinorganic Gillard also used zinc reduction (though chemistry. He mentored most of the following preparatively rather than catalytically) of generation of Australian inorganic chemists, ruthenium trichloride in his preparation of trans- including e.g. R. S. Nyholm, A. M. Sargeson [Ru(py)4Cl2][H(ONO2)2] and B. Bosnich. The last-named provides a 92. R. D. Gillard and P. R. Mitchell, ‘Tris(diamine) link between Dwyer’s seminal work on metal chromium(III) Salts’, in “Inorganic Syntheses”, complexes in bio-systems with Gillard’s research ed. F. A. Cotton, Vol. 13, McGraw-Hill Inc, New on such topics as bacterial growth (cf. the series York, USA, 1972, p. 184–186 “Coordination Compounds and Micro-Organisms” 93. N. Tanaka and K. Nagase, Bull. Chem. Soc. – see Table S5) (113–115). Dwyer’s contribution Japan, 1967, 40, (3), 546–550 to this field was outlined many years later in 94. N. Serpone and F. Bolletta, Inorg. Chim. Acta, (116) and subsequently detailed in (117) 1983, 75, 189–192 113. F. P. Dwyer, E. C. Gyarfas, W. P. Rogers and 95. D. P. Rillema, J. F. Endicott and J. R. Barber, J. H. Koch, Nature, 1952, 170, (4318), 190–191 J. Am. Chem. Soc., 1973, 95, (21), 6987–6992 114. W. W. Brandt, F. P. Dwyer and E. C. Gyarfas, 96. A. G. Beaumont and R. D. Gillard, J. Chem. Soc. Chem. Rev., 1954, 54, (6), 959–1017 A, 1968, 2400–2403 115. F. P. Dwyer, E. C. Gyarfas, R. D. Wright and 97. G. T. Morgan and J. D. Main-Smith, J. Chem. A. Shulman, Nature, 1957, 179, (4556), Soc., 1922, 121, 1956–1971 425–426 98. G. T. Morgan and J. D. Main-Smith, J. Chem. 116. E. Meggers, Curr. Opin. Chem. Biol., 2007, 11, Soc., 1924, 125, 1996–2004 (3), 287–292 99. Y. Yamamoto, K. Ito, H. Yoneda and M. Mori, Bull. 117. N. L. Kilah and E. Meggers, Aust. J. Chem., 2012, Chem. Soc. Japan, 1967, 40, (11), 2580–2583 65, (9), 1325–1332 100. C. D. Lawrence and C. F. H. Tipper, J. Chem. Soc., 118. A. Shulman, G. M. Laycock, E. J. Ariëns and 1955, 713–716 A. R. H. Wigmans, Eur. J. Pharmacol., 1970, 9,

(3), 347–357 Trans., 2016, 45, (33), 13161–13168 119. H. M. Butler, J. C. Laver, A. Shulman and 136. R. D. Gillard and S. H. Laurie, ‘Metal-Protein R. D. Wright, Med. J. Aust., 1970, 2, (7), Interactions’, in “Biochemistry of Food Proteins”, 309–314 ed. B. J. F. Hudson, Ch. 5, Springer Science and 120. A. Shulman and D. O. White, Chemico-Biol. Business Media, Dordrecht, The Netherlands, Interact., 1973, 6, (6), 407–413 1992, pp. 155–196 121. B. Rosenberg, L. Van Camp and T. Krigas, Nature, 137. I. J. Ellison and R. D. Gillard, J. Chem. Soc., 1965, 205, (4972), 698–699 Chem. Commun., 1992, (11), 851–853 138. Pages 56–69 of (139) list actual and proposed 122. B. Rosenberg, L. Van Camp, E. B. Grimley and rhodium(IV), (V) and (VI) compounds and A. J. Thomson, J. Biol. Chem., 1967, 242, (6), complexes (as of 1982); pages 60 and 64 are 1347–1352 the most relevant to Gillard’s work in this area 123. R. J. Bromfield, R. H. Dainty, R. D. Gillard and 139. D. J. Gulliver and W. Levason, Coord. Chem. B. T. Heaton, Nature, 1969, 223, (5207), 735–736 Rev., 1982, 46, 1–127 124. R. D. Gillard, J. D. Pedrosa de Jesus and 140. However, Cs RhCl is, as one might expect, a A. Y. A. Mohamed, Trans. Met. Chem., 1989, 14, 2 6 Rh(IV) compound – see (141) – as are several (4), 258–260 other M2RhX6 salts and the binary halides RhF4 125. F. P. Dwyer, I. K. Reid, A. Shulman, G. M. Laycock and RhCl4 (see (142, 143)) RhO2 is also known – and S. Dixson, Aust. J. Exp. Biol. Med. Sci., 1969, see (144) and references therein 47, (2), 203–218 141. I. J. Ellison and R. D. Gillard, Polyhedron, 1996, 126. H. Grossman, A. Shulman and C. Bell, Experientia, 15, (2), 339–348 1973, 29, (12), 1522–1524 142. W. P. Griffith, “The Chemistry of the Rarer 127. The effects of these complexes on the in vitro Platinum Metals (Os, Ru, Ir and Rh)”, Wiley growth of a range of bacteria were studied. Interscience, London, UK, 1967, pp. 317–318 Only trans-[RhL X ]+, where L = a pyridine and 4 2 143. S. A. Cotton, ‘Rhodium and Iridium: Halides and X = chloride or bromide, showed usefully high Halide Complexes: Iridium Halides’, in “Chemistry levels of antibacterial activity, with bromide of Precious Metals”, Ch. 2, Chapman and Hall, complexes being about ten times more effective London, UK, 1997, p. 80 than their chloride analogues. At very low concentrations these rhodium complexes lead to 144. E. M. Miguelez, M. A. A. Franco and J. Soria, filamentous growth of Escherichia coli, recalling J. Solid State Chem., 1983, 46, (2), 156–161 the seminal experiments of Rosenberg’s group 145. C. Claus, “Beiträge zur Chemie der Platinmetalle”,

using cis-[PtCl4(NH3)2] cf. (121, 122) Festschrift zur Jubelfeier des 50-Bestehens der 128. W. P. Griffith, Platinum Metals Rev., 2013, 57, Universität Kazan, Dorpat, 1854 (2), 110–116 146. C. Claus, J. Prakt. Chem., 1860, 80, (1), 129. R. D. Gillard, ‘Rhodium Complexes and Bacteria’, 282–317 1st International Conference on the Chemistry 147. C. Claus, Bull. Acad. Imp. Sci. Saint-Pétersbourg, of the Platinum Group Metals, 19th–24th July, 1860, 2, 158–188 1981, Bristol, UK, Royal Society of Chemistry, 148. The blue colour eventually generated by reaction London, UK, Abstract A8 with hypochlorite solution was used in the late 130. (131) describes the preparation and 19th century as a test for rhodium – see, for characterisation of enantiomers and a racemate example (149) of trans-[Rh(nicH) Cl ](PF ) , while (132) 4 2 6 5 149. E. Demarçay, Comptes Rendus, 1885, 101, reports on the biological activities of these 951–952 complexes 150. P. Poulenc and G. Ciepka, ‘Rhodium’, in “Nouveau 131. R. D. Gillard and E. Lekkas, Trans. Met. Chem., Traité de Chimie Minérale”, ed. P. Pascal, Vol. 19, 2000, 25, (6), 617–621 Masson, Paris, France, 1958, pp. 353–355 132. R. D. Gillard and E. Lekkas, Trans. Met. Chem., 151. A. N. Buckley, J. A. Busby I. J. Ellison and 2000, 25, (6), 622–625 R. D. Gillard, Polyhedron, 1993, 12, (2), 247–253 133. R. D. Gillard, Platinum Metals Rev., 1970, 14, 152. Mann studied a series of Lifschitz salts derived (2), 50–53 from unsymmetrical aliphatic diamines in an 134. U. Ndagi, N. Mhlongo and M. E. Soliman, Drug attempt to determine whether the nickel was in Des., Dev. Ther., 2017, 11, 599–616 a square-planar or a tetrahedral environment. 135. J.-J. Zhang, J. K. Muenzner, M. A. Abu El Maaty, His failure to obtain any evidence for the B. Karge, R. Schobert, S. Wölfl and I. Ott, Dalton existence of cis and trans isomers favoured

tetrahedral stereochemistry at the metal centre Commun. (London), 1967, (6), 273–274 – see (153) 167. M. J. Barcelona and G. Davies, J. Chem. Soc., 153. F. G. Mann, J. Chem. Soc., 1927, 2904–2918 Dalton Trans., 1975, (19), 1906–1909 154. I. Lifschitz, J. G. Bos and K. M. Dijkema, Z. Anorg. 168. Several years earlier Gillard had described Allg. Chem., 1939, 242, (2), 97–116 synergic solubility behaviour for the rhodium complex [RhCl(py)3(C2O4)] in water + pyridine 155. I. Lifschitz and J. G. Bos, Rec. Trav. Chim. Pays mixtures, where the complex is much more Bas, 1940, 59, (5), 407–422 soluble (up to approximately 30 times) than in 156. I. Lifschitz and K. M. Dijkema, Rec. Trav. Chim. either solvent – see (169). He revisited synergic Pays Bas, 1941, 60, (8), 581–598 solubility towards the end of his career, this time in

157. H. Glaser and P. Pfeiffer, J. Prakt. Chem., 1939, relation to the iridium analogue [IrCl(py)3(C2O4)] 153, (10–12), 300–312 again in water + pyridine mixtures (170) 158. D. M. L. Goodgame and L. M. Venanzi, J. Chem. 169. R. D. Gillard, E. D. McKenzie and M. D. Ross, Soc., 1963, 616–627 J. Inorg. Nucl. Chem., 1966, 28, (6–7), 1429– 1434 159. D. M. L. Goodgame and L. M. Venanzi, J. Chem. Soc., 1963, 5909–5916 170. N. S. A. Edwards, R. D. Gillard, M. B. Hursthouse, H. F. Lieberman and K. M. A. Malik, Polyhedron, 160. W. C. E. Higginson, S. C. Nyburg and J. S. Wood, 1993, 12, (24), 2925–2928 Inorg. Chem., 1964, 3, (4), 463–467 171. R. D. Gillard, Trans. Met. Chem., 1989, 14, (4), 161. S. C. Nyburg and J. S. Wood, Inorg. Chem., 1964, 295–297 3, (4), 468–476 172. H. O. Davies, J.-H. Park and R. D. Gillard, Inorg. 162. R. D. Gillard and H. M. Sutton, J. Chem. Soc. D., Chim. Acta, 2003, 356, 69–84 1969, (16), 937–938 173. For example, the characterisation of 163. R. D. Gillard and H. M. Sutton, J. Chem. Soc. A, Hg2[Ni(SCN)6]·H2O and Ni[Hg(SCN)4]·2H2O in 1970, 1309–1312 the nickel(II)–mercury(II)–thiocyanate system 164. R. D. Gillard and H. M. Sutton, J. Chem. Soc. A, (174), whose components had been a matter for 1970, 2172–2174 discussion since the 1860s (175, 176) 165. R. D. Gillard and H. M. Sutton, J. Chem. Soc. A, 174. R. D. Gillard and M. V. Twigg, Inorg. Chim. Acta, 1970, 2175–2176 1972, 6, 150–152 166. M. J. Blandamer, D. E. Clarke, T. A. Claxton, 175. P. T. Cleve, Öefersigt. Kongl. Veten.-Akad. M. F. Fox, N. J. Hidden, J. Oakes, M. C. R. Symons, Förhandl., 1863, 20, 9–13 G. S. P. Verma and M. J. Wootten, Chem. 176. P. T. Cleve, J. Prakt. Chem., 1864, 91, 227–228

The Authors

Martyn Twigg did inorganic reaction mechanisms graduate research in a laboratory next to Gillard’s office at Canterbury. After fellowships at Toronto and Cambridge and being headhunted into the ICI Corporate Laboratory he moved to ICI Billingham to work on industrial process catalysts. Later at Johnson Matthey he was responsible for autocatalyst development and production at Royston. After emissions control successes he retired in 2010 and continues research with universities in the UK and overseas with honorary positions at some. His catalyst development, manufacturing and consulting business is thriving with novel catalytic systems in production.

After grammar school (Queen Elizabeth’s, Barnet), National Service (Royal Artillery), and Cambridge (Sidney Sussex; MA, PhD on inorganic kinetics) John Burgess started work at Fisons Fertilizers in Suffolk. Two years later he embarked on an ICI Fellowship at the University of Leicester which led to three decades of teaching and research – ranging from mechanisms to solvatochromism to biochemistry, linked by solution chemistry of iron complexes. He is now Emeritus Reader in Inorganic Chemistry at the University of Leicester, combining the preparation of an expanded version of “Color of Metal Compounds” with gardening and pursuing his interests in music, East Anglian churches and railways.

www.technology.matthey.com

Professor Robert D. Gillard: Transition Metal Chemist 1936–2013: Part II From the University of Cardiff to retirement interests and scientific legacy

John Burgess included many years singing in the Cwmbach Male Department of Chemistry, University of Voice Choir. Leicester, Leicester LE1 7RH, UK 1. Introduction Martyn V. Twigg* Twigg Scientific & Technical Ltd, Caxton, The second part of this commemoration continues Cambridge CB23 3PQ, UK the account of Professor Robert D. Gillard’s life and work from 1973 when he took up the chair *Correspondence may be sent via the of Inorganic Chemistry in Cardiff University, then Editorial Team: [email protected] a College of the University of Wales, where he stayed until his retirement. This was impelled by serious ill health in 1998, although after The second part of this commemoration covers the heart surgery he continued publishing papers final stage of Robert Gillard’s career as Professor of for several years. At Cardiff Gillard expanded his Inorganic Chemistry at Cardiff University and his research interests and embarked more fully into time in retirement. At Cardiff he built on earlier work several areas. These included the hydration of while extending his scientific interests still further coordinated nitrogen heterocycles (often referred into mineralogical and archaeological chemistry, to as “covalent hydration”) and vanadium, copper, and even into forensic dentistry. Coordination rhodium and platinum coordination complexes – chemistry research continued and included the often with biologically relevant amino acids. These polysulfide 5 S chain as a bidentate ligand in the complexes had been given significant impetus all-inorganic cyclic PtS5 unit and the rhodium(III) at the University of Kent at Canterbury through 3– complex [Rh(S5)3] . His penchant for discussion funding from the Medical Research Council. At led him into several controversies, particularly Cardiff the interests of younger members of the over his ‘covalent hydration’ hypothesis of Department influenced additional areas of his coordinated nitrogen-carbon double bonds in metal research such as mineral chemistry (with Peter complexes which included those with platinum Williams) and reaction kinetics of substitution and 2,2’-bipyridine. He travelled widely attending inert complexes (with Leon Kane-Maguire). All international conferences and giving lectures. of this reflects the perpetual interest Gillard had Research collaborations continued throughout his in seeking new areas for exploration that went time at Cardiff and in particular he had many strong alongside his extensive investigations in his long links with Portugal, both with colleagues there and standing themes of interest such as rhodium and as supervisor of Portuguese higher degree students platinum chemistry and the optical properties of at Cardiff. His years in retirement were spent in chiral metal complexes. In these areas he published finalising his research legacy, in continuing to read several multi-paper series that were interspersed historical literature, both chemical and otherwise, with a smaller number of papers on topics that and in following his musical interests that had did not develop as broadly and saw publication as

23 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040223 Johnson Matthey Technol. Rev., 2021, 65, (1) single papers or two-part series. Gillard had much entitled ‘Equilibria in Complexes of N-Heterocyclic wider scientific interests than his published work Molecules’ – plus a few others on closely related might indicate, and he was always happy to enter matters. Table S9 documents the first few, some into vibrant enthusiastic discussion on all sorts intermediate examples relating to pgms, and the of topics with all sorts of people, and on some last few publications (7) in this series. Parts 1 to 3 occasions this would inspire him to work on topics set the stage for a series whose wide range is new to him, leading to a number of joint projects. apparent from the first few parts and is still evident Some of these were of short duration while others in the final parts 20 years later (Table S9). The involved long-term collaboration as with several metal centres that feature most prominently are Portuguese chemists. iron and ruthenium (both 2+ and 3+ oxidation states in both cases); several studies feature 2.1 University of Cardiff: Continued nickel, while complexes of platinum, iridium, cobalt, copper, silver and chromium also make occasional Expansion of Research Interests appearances. The majority of complexes were of The now Cardiff University was a University College 1,10-phenanthroline or 2,2′-bipyridine, sometimes of Wales when Gillard went there as Professor of with nitro or methyl substituents, but related ligands Inorganic Chemistry in 1973 (he was Head of the such as pyridine, 2,2′-bipyrimidine, 2,4,6-tri-(2- Chemistry Department 1983–1988). It is now pyridine)-1,3,5-triazine and caerulomycin, 1, (8, difficult to differentiate between research that was 9) also appear. Two terdentate ligands occasionally started there and research that was completed at replaced three equivalent bidentate ligands, for n+ Canterbury but published when he was at Cardiff. example [M(2,2′-6′,2′′-terpyridine)2] versus n+ Series such as ‘Coordination Compounds and [M(2,2′-bipyridine)3] , for increased complex Micro-Organisms’ continued seamlessly across the stability. Alternative routes to stabilisation (with change in location. However, as a Professor it is respect to substitution) of complexes or transient clear Gillard’s chemical interests could, and did, intermediates involved the use of ruthenium(II) continue to broaden even wider. rather than iron(II) and studying [Fe(diimine)2(CN)2] 2+ For instance he developed a great interest in the in place of [Fe(diimine)3] (10–12). Approaches long-known penta-atomic S5 chain as a ligand, used included the determination of equilibrium particularly in its behaviour as a bidentate ligand in constants, the establishment of rate laws, various the cyclic PtS5 unit that is almost unique in being forms of spectroscopy (visible and infrared a metal-complex containing an all-inorganic ring absorption, nuclear magnetic resonance (NMR), (1, 2). The short series of papers entitled ‘Sulphides electron spin resonance (ESR) (13–18) and circular of the Platinum Group Elements’ are listed in dichroism (CD) – one of Gillard’s long-established Table S8. Gillard’s contributions included an favourite techniques), and the occasional X-ray 3– extension to the rhodium(III) complex [Rh(S5)3] , investigation (19–22). While the investigations and a single-crystal structure determination (3) and reported in this series ranged over preparation, X-ray photoelectron spectroscopy (XPS) studies on characterisation and reactions of this group of the platinum complex (4). The binding energies, complexes, the main area of interest was in

4f7/2 for platinum(IV) or 3d5/2 for rhodium(III), apparent anomalies in their solution chemistry and are lower than most of those for the respective Gillard’s attempts to provide a common explanation metals bonded to oxygen or nitrogen. His brief in terms of a key role played by covalent hydration review of these compounds in Chemistry in Britain and pseudobase formation. (5) reflects his interest in the optical properties of inorganic complexes, in the history of chemistry, 2.1.1 Covalent Hydration and and in matters musical – the article includes both Pseudobases mention and an illustration of Elgar’s dabbling in practical chemistry, including that of sulfides. Gillard spent much time speculating about the Towards the end of his time in Canterbury Gillard involvement of covalent hydration and pseudobase had started to work on his major investigation into (23) intermediates in several reactions of complexes the properties and reactions of nitrogen-containing containing appropriate ligands. His proposals, first heterocyclic ligands and their metal complexes. In set out (24, 25) in 1973 (26) and 1974 (27), were 1973, around the time of his move to Cardiff, the fully detailed in a review published in 1975 (28, 29). first reports on this research appeared. Eventually He was particularly concerned about the possible some 50 papers (6) were published in a series role of covalent hydration in solution equilibria and

24 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040223 Johnson Matthey Technol. Rev., 2021, 65, (1) in the possibility of participation by pseudobases as later, Gillard himself presented evidence for a key intermediates in nucleophilic attack at low‑spin covalent hydrate of nicotine (46). The observation d6 and d8 complexes. The solution equilibrium of a Pfeiffer effect in theN -methylphenanthrolinium studies focused on platinum(II) complexes, the cation has also been taken as evidence for the mechanistic studies on diimine complexes of formation of a covalent hydrate (47–50), as has iron and of ruthenium, but results both from his the fluorescence behaviour of bipy, phen and terpy research and that of others on such centres as (51–53). Gillard also proposed a covalent hydrate Cr(III) (30, 31), Co(II), Ni(II) and Cu(II) (32–35) as a key intermediate in the oxidation of pyridine also featured in his discussions. to 2(1H)-pyridone – Equation (ii) – when heated In organic chemistry, covalently hydrated in a sealed tube with zinc or cadmium salts in air species and pseudobases are formed by addition or oxygen (54) or with CuSO4·5H2O (55). Later of a nucleophile to carbon in an electron-poor workers used a similar method – hydrothermal heteroaromatic species or addition of water oxidation by copper(II) nitrate – to generate across a C=N double bond. The phenomenon of 2,2′-bipyridin-6(1H)-one and 1,10-phenanthrolin- covalent hydration was first described by Adrien 2(1H)-one, postulating a reaction pathway Albert in 1952, who reported reversible hydration- through intermediates such as 4 (56). dehydration of 7-hydroxypteridine, 2, when treated successively with refluxing hydrochloric N N acid then boiling sodium hydroxide solution + H2O (i) (36, 37). It has been demonstrated in many bi- N NH and poly-aza-aromatic compounds (38–42). It H is rarely observed in mono-aza monocyclic rings OH (43, 44), but occurs with one-ring systems such H2O [O] + as pyrimidines. In 1967 Albert suggested that in + 2H (ii) OH aqueous solution 1,10-phenanthroline may be in N N N O H equilibrium with a hydrate (Equation (i)) (40), H H while even earlier (1935) T. M. Lowry suggested Loss of a proton from a covalent hydrate – or (45) the formation of a covalent hydrate, 3, in reaction of, for example, a nitrogen heterocycle cyclohexane-water solutions of nicotine. Much with hydroxide – gives a pseudobase. Heterocycles

O Me

MeO OMe N N N N O O OH + H2O OH HO N N 2 N CH3 N CH3 3 H H N N N 1 H OH

CH3O OC2H5 O N NO H OR 2 2 O2N NO2 O N NO – 2 2 – H N N – N N N II OH NO 5a N 5b 5c OR + + 4 Cu H 2 6

2– NO 2 HO CN NO2 N CN H H + OH N N Fe H N CNCN H – OH 7 N N N N OH 2+ 8 Ru 9 Ru2+ 10

25 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040223 Johnson Matthey Technol. Rev., 2021, 65, (1) may also react, under suitable conditions, with effect on the reactivity ofN -heterocycles caused other nucleophiles, such as cyanide or methoxide, by binding to metal ions (coordination) will be Equation (iii): similar in kind to the effects of bonding the N-heterocycles to other charge acceptors, such as alkyl or aryl groups (quaternization).” (70). – CD3O (iii) N +N N N OCD A little later (1986), in his discussion of analogies H 3 CH3 CH3 between coordination and quaternization of imines, especially N-heterocycles, he commented that “much to form similar species, such as pseudocyanides of the work done in inorganic chemistry laboratories (pseudobase analogues; a type of Reissert and institutes since the time of Werner has been intermediate) (57–61) or Meisenheimer complexes, coordination chemistry, often involving carbonaceous 5 (62–66). These pseudocyanide or Meisenheimer ligands”, emphasising the important role of organic species are also of some relevance to the following chemistry in coordination chemistry (71). discussion of substitution mechanisms for iron and Equation (iv) shows the equilibria and species for ruthenium-diimine complexes. As with the formation a complex in which the diimine ligand is activated of covalent hydrates, pseudobase formation from by coordination to a metal ion: monocyclic heterocycles with just one heteroatom 2+ 2+ is very difficult. Here, as with polyheteroatomic polycycles, reaction is much easier if one of the N N (NN) M (NN) M heteroatoms is quaternised. Thus 1,10-phenanthroline 2 2 H N N does form pseudobases when suitably activated, HO as in, for instance, pyrazino[1,2, 3,4-lmn ]-1,10- H Covalent hydrate phenanthrolinium dication, 6, (67). Formula 7 shows + (iv) the structure suggested (68) for the pseudobase form, 6. Covalent hydration and the formation of N pseudobases and pseudocyanides are all facilitated (NN)2M – N by electron-withdrawing substituents; in particular HO quaternization of N-heterocycles leads to an increase H in susceptibility to nucleophilic attack (69). Pseudobase Although generally known as ‘Meisenheimer complexes’ (62–65) such anionic σ-species As indicated at the start of this Section, and generated from aromatic compounds had been exemplified by Gillard’s initial review article on observed several times pre-Meisenheimer, initially covalent hydration and related topics (Part 3 of the with the report in 1886 (66) of a violet colour on series ‘Equilibria in Complexes of N-Heterocyclic adding alkali to a solution of 1,3-dinitrobenzene. Molecules’ – see Table S9), his interests in Meisenheimer’s key contribution was the isolation this area embraced complexes of a wide range of the potassium salt of the intermediate 5a in of elements. However, there was a marked nucleophilic aromatic substitution of ethoxide concentration on complexes of platinum(II) and of at 1,3,5-trinitro-4-methoxybenzene. Terrier iron and ruthenium, which featured in the first and documents Meisenheimer intermediates derived second parts of this series (Table S9). Complexes from azaheterocycles, including 5b, which of Gillard’s much-favoured rhodium and of the isomerises to 5c – the latter providing a model for much-studied cobalt(III) and chromium(III) make the reactive intermediates in the Gillard mechanism very few appearances, though the thermal and 3+ discussed in this Section of the text. photochemical lability (72, 73) of [Cr(bipy)3] , 3+ 3+ Gillard reasoned that coordination of a metal ion [Cr(terpy)2] and [Cr(phen)3] provided an to an N-heterocyclic ligand should have a similar early example of the anomalies (74, 75) that effect, leading to enhanced covalent hydration or prompted his covalent hydration theories, and 3+ stabilisation of pseudobases and pseudocyanides. [Cr(bipy)3] is the subject of Part 47 (31), almost In his brief overview of 1983 Gillard summarised at the end of this series (Table S9). Cobalt(III) his approach thus makes an appearance in the form of cis- 2+ [Co(en)2(benzimidazole)Cl] , whose relatively “The novel suggestion which is at the heart of rapid base hydrolysis (76) is, by implication, my research in this field is the following: The facilitated by covalent hydration (70, 77, 78).

6 Substitution-labile 2+ first-row transition metals low-spin d Fe(II) in an SN2 mechanism would be also, understandably, make very few appearances, difficult due to the full complement oft 2g electrons though nickel(II) does appear in Parts 10, 22, 28, – as is well exemplified by base hydrolysis of 46 and 50 of this series. cobalt(III)-amine complexes (cf. Section 3.4,

The covalent hydration theory was applied Part I (86)), which proceeds by the SN1CB route. to aqueous solutions of several platinum(II) Gillard therefore proposed his mechanism involving complexes. Thus, for example, the oft-reported, covalent hydration and pseudobase formation though occasionally denied (70, 79), acidity of salts (Scheme I); the key steps are detailed in Equation 2+ of [Pt(py)4Cl2] in aqueous solution was attributed (vii). The hydroxide in the intermediate conjugate to the equilibrium shown as Equation (v): base both reduces the aromaticity of the ligating ring and places it in close proximity to the iron. 2+ [Pt(py)4Cl2] + H2O The key papers for the kinetic and mechanistic [Pt(py) (pyOH)Cl ]2++ H+ (v) aspects are the reviews cited above, plus Part 31 ⇌ 3 2 of the ‘Equilibria in Complexes of N-Heterocyclic Similarly, Gillard claimed, on the basis of 1H NMR Molecules’ series (87–89). In that part, results for 2+ spectra, to have identified a covalently hydrated [Fe(LL)3] with LL = phenanthroline, bipyridyl, species derived from [Pt(bipy)(CN)2]·H2O (80, 81). Gillard’s nine publications referring to nucleophilic attack at bis-diimine-platinum(II) 2+ complexes are summarised in a paper published in the year of his retirement, which detailed and N L M compared observations on the series of bis-diazine 2 N 2+ complexes [Pt(LL)2] with LL = 2,2′-bipyridine, 2,2′-bipyrazine, 3,3′-bipyridazine, and [O]

2,2′-bipyrimidine (82). After his retirement he Ks gathered together a number of examples of cases 2+ where he had applied his covalent hydration and pseudobase theory to platinum(II) complexes of N L2M H bipy, 5,5′-Me2bipy and terpy. The evidence he +N deployed embraced equilibrium measurements, HO H – [HA] kinetics, electronic absorption spectra (including circular dichroism) and the relation of proposed KH+ KA KOH– solution species to well-known 5-coordinated K + H2O + platinum species. This review article, dedicated to 3 the eminent Croatian chemist (and fellow inorganic N N kineticist) Smiljko Ašperger, gave an overview of L2M H L M N 2 N + + – solution equilibria involving hydroxide and bis- H2O 2 HO diimine-platinum(II) or terimine ([Pt(terpy)Cl]+) H (CA) H (CB) N complexes (83). L2M + NH As stated above, the main area of interest in this HO series was in iron and, to a somewhat smaller extent, + + ruthenium. The central question was the mechanism 3 of attack by hydroxide at low-spin iron(II) complexes N N of N-heterocyclic ligands. The rate law for reaction L2M + L2M NH N of most diimine-iron(II) complexes in basic solution H2O Products HO is as given in Equation (vi):

2+ 2+ –d[Fe(LL)3 ]/dt = k1[Fe(LL)3 ] Products Products 2+ – + k2[Fe(LL)3 ][OH ] + k [Fe(LL) 2+][OH–]2 (vi) Scheme I. Gillard’s ‘covalent hydration’ mechanism 3 3 for base hydrolysis of d6 transition metal complexes (M = Fe, Ru), as set out in his 1975 – Here the k2[OH ] term dominates under most review (29) conditions (84, 85). But the approach of OH– to

27 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040223 Johnson Matthey Technol. Rev., 2021, 65, (1) bipyrimidinyl, bipyrazinyl and bipyridazinyl from The thermogravimetric behaviour of several earlier parts are collected and compared. [Fe(phen)2(CN)2]·2H2O provides another, though very different, instance where the results may 2+ 2+ be interpreted in terms of covalent hydration.

N N Whereas phen·H2O loses its water of crystallisation

L2M L2M H at 93ºC (the resultant anhydrous phen melts, N +N HO without decomposition, at 117ºC), hydrated H – [Fe(phen)2(CN)2] does not lose its water of [O] [HA] (vii) crystallisation until 323.5ºC, at which temperature + + decomposition rapidly occurs (98). The thermogravimetric study was carried out N N by Hans-Ulrich Hummel, from Brodersen’s group L2M L2M Products N in Erlangen (Section 2.6, Part I (86)). His visit to – HO N HO Cardiff gave rise to two publications related to the H (CB) subject of addition to complexes of N-heterocyclic molecules. The first joint paper discussed There is some early spectroscopic evidence said the thermogravimetric behaviour of hydrated to support the intermediacy of covalent hydration [Fe(phen)2(CN)2] (98). This appeared as Part 44 of or pseudobase formation in reactions of iron(II)- the series ‘Equilibria in Complexes of N-Heterocyclic and ruthenium(II)-diimine complexes. Thus, for Molecules’. There seems to be some uncertainty 2+ example, electronic spectra of M(5NO2phen)3 , over the hydrate(s) of [Fe(phen)2(CN)2] – Gillard II II M = Fe , Ru and 5NO2phen = 5-nitro-1,10- and Hummel, Schilt (earlier, (12)), and a Chinese phenanthroline, in basic solution were said to be group (later, (99)) all used the same method of consistent with pseudobase formation in basic preparation, but whereas Schilt, on the basis aqueous solution (89). The 1H-NMR spectrum of elemental analysis, and Gillard and Hummel of K2[Fe(phen)(CN)4] dissolved in D2O shows an (albeit without reporting elemental analysis unexpected non-equivalence of the two halves results) formulated their compound as a dihydrate, of the ligand molecule; this inequivalence was the Chinese X-ray structure analysis indicates attributed to the covalent hydrate, 8 (90–93). a trihydrate. Interestingly, Schilt had earlier Much later, extensive NMR evidence was reported characterised the 2,2′-bipyridine analogue as the for the formation (facilitated by the electron- trihydrate [Fe(bipy)2(CN)2]·3H2O (12). The second withdrawing 5-nitro-substituent) of adducts paper appeared, coincidentally also as Part 44, 2+ in reactions of [Ru(5NO2phen)2(bipy)] and in the ‘Optically-Active Coordination-Compounds’ 2+ of [Ru(5NO2phen)(bipy)2] with hydroxide or series (100), but is a complementary study of cyanide in aqueous solution (94) However, such [Fe(phen)2(CN)2] involving the addition of Lewis 2+ adducts, 9 and 10, like those proposed (95, 96) for acids, here Hg , Hg(CN)2, BF3, to the coordinated 5-nitro-1,10-phenanthroline itself with hydroxide cyanide. In contrast to the controversial subject or with methoxide, 11, have the added nucleophile of the addition of nucleophiles to coordinated 2+ in a position far removed from the metal atom, diimine ligands in complexes of the [M(diimine)3] indicating a different mechanism (of ligand type, the addition of electrophilic entities such as cleavage to give a high-spin species) from that Hg2+ to coordinated cyanide in complexes of the shown in Scheme I for the slow reaction of these [M(diimine)2(CN)2] type is a long-established compounds with hydroxide (96, 97). feature of their chemistry (see for example (101)).

Complexes of the [Ru(diimine)2(CN)2] type are Gillard’s proposals were thought contentious by particularly substitution-inert with respect to the many. Despite the possibilities for interpreting metal centre and thus particularly suitable for a variety of spectroscopic, kinetic, and seeking model intermediates. The complexes where thermogravimetric observations in terms of covalent the diimine is 1,10-phenanthroline, 2,2’-bipyridine, hydration of diimine ligands, there appears to be 5-nitro-1,10-phenanthroline or 5,5’-dimethyl- no hard evidence, such as from an X-ray structural 2,2’-bipyridine react reversibly with hydroxide demonstration, for the existence of a stable, or even or cyanide to form new species whose electronic ephemeral, covalent hydrate of the type proposed spectra were deemed consistent with their being by Gillard. As observed by Constable in his 2016 pseudobases or pseudocyanides (81). review cited below – “The Gillard hypothesis was

28 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040223 Johnson Matthey Technol. Rev., 2021, 65, (1) remarkable in being logical and reasonable but These criticisms were rebutted by the proposers in having very little unequivocal experimental (108), who also took issue with earlier comments evidence to support it.” Many of Gillard’s publications by the latter authors on the relative importance reporting experimental kinetic or spectroscopic of this area of pseudobase chemistry to organic results attracted considerable criticism from and to inorganic chemistry (109). In Nord’s 1985 several authors, including Gwyneth Nord and Ole contribution to Comments on Inorganic Chemistry Mønsted (see below) and, especially, Nick Serpone (110) she reviewed equilibrium, structural and in 1983 (78) who offered the most trenchant spectroscopic (NMR) evidence as well as kinetic. criticism. Less intemperate and more sympathetic She asserted that there was no unequivocal consideration was provided by Ed Constable, evidence (111, 112) for nucleophile (OH– or CN–)- both in 1983 (“The reactions of nucleophiles substituted compounds or transient intermediates with complexes of chelating heterocyclic imines: in this group of reactions. This article provides a A critical survey”) (102) and, more recently in useful, if admittedly biased, overview, linking the 2016 (75). Gillard published a mild response base hydrolysis mechanism of these complexes (70) to Serpone’s criticism in which he presented with the closely related topics of reduction of 3+ comparisons with analogous organic reactions; he [M(LL)3] (M = Fe, Ru, Os) and hydroxide attack was unhappy about the concentration on platinum at coordinated 1,10-phenanthroline activated by complexes in critical comments. The whole topic 5-nitro or 5-sulfonate substituents. was hotly debated in sessions at the Royal Society The concept of covalent hydration was also of Chemistry Autumn Meeting (chaired by one of applied to some systems in the solid state (cf. the present authors (MVT)) and at a meeting of the (98)). Thus, for example, Gillard proposed (113) 1 3+ Inorganic Mechanism Discussion Group (IMDG), – on the basis of H NMR spectra of [Ir(bipy)3] both held in Cardiff in 1980 (103). A facet of the in d6-DMSO solution – that [Ir(bipy)3]Cl3·4H2O pseudobase controversy published, by Gillard and was the trihydrate of a complex containing one Wademan, outside the ‘Equilibria in Complexes of the bipy ligands covalently hydrated. His of N-Heterocyclic Molecules’ series involved the proposal was countered by those preferring a 2+ trans-[Pt(py)4Cl2] cation and the possibility of structure containing one unidentate ligand (114– a pseudobase of pyridine being involved. Their 119). Gillard’s arguments in favour of covalently- proposal (104) attracted critical comments from hydrated intermediates rather than intermediates Seddon, Constable and Wernberg (105) (who also involving a unidentate bipy ligand were set out in took issue (106) with Gillard and Hughes’s proposal his discussion of the ruthenium analogue (120). (107) of a covalently hydrated form of the cis- However, the controversy was resolved by the 2+ [Ru(bipy)2(py)2] in equilibrium with this complex X-ray crystal structure determination (121–123) of 3+ in alkaline media) and from Mønsted and Nord (79). the hydrated perchlorate salt of [Ir(bipy)3] , which

OH H NO2 2 RO H – 3+ + N n

N N 11 N N IrIII N N N N Pt N N N 12b X 12a 13

N H H CO OH Fe O 3 N COCH3 14 15 16

29 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040223 Johnson Matthey Technol. Rev., 2021, 65, (1) indicated that the apparently anomalous properties oxidation of water when the catalyst is embedded of this cation were due to one of the bipy ligands in a zeolite matrix – such isolation of the complex being C,N-bonded to the metal, 12a and 12b, precludes the degradation reactions which dominate an unusual mode of attachment reminiscent of in aqueous solution. These authors state specifically orthometalation. This is a well-known reaction in that “The reaction is initiated by water attack on organometallic chemistry with many applications in the bipyridine ligand to form a covalent hydrate” syntheses of organic compounds. (133). This builds on an earlier proposal by Sutin In a link to his biochemical interests (124–126), that ligand degradation in hydroxide-dependent 3+ Gillard suggested a covalently-hydrated form of reduction of Group 8 [M(bipy)3] ions involved + trans-[Rh(py)4Cl2] in the course of a conference rate-determining attack of hydroxide on bipy- presentation on the effects of some rhodium carbon to give a transient pseudobase (134, 135). complexes on bacterial growth (see Part 10 in A similar mechanism has been suggested for water Table S5). There have been several searches oxidation catalysed by dimeric μ-oxo-bridged III 4+ for similar intermediates in reactions of binuclear ions cis,cis-{[Ru (bipy)2(OH2)}2O] or a ring- copper(II)-diimine complexes, which are related, substituted derivative (136, 137). if distantly, to the CuICuII species which figure in Density functional theory (DFT) calculations long-distance electron transfer in metalloproteins. suggest that the addition of water or hydroxide Indeed, evidence has been presented for ligand- to coordinated 2,2′-bipyridine may well take hydroxylation in two such species, of bipy and place in redox reactions of the ruthenium(IV) and n+ of phen (56). However, similar investigations of ruthenium(V) complexes [(NH3)3(bipy)RuOH] n+ binuclear copper(II)-diimine (diimine = bipy or and [(NH3)3(bipy)Ru=O] (n = 2 and 3) (138). phen) (127) and copper(II)-bipy-oxalate (128) The calculations indicate a strong dependence on complexes, and of a cadmium-phen-chloride- the nature of the other coordinated ligands, with acetate complex (129) failed to find any indication the addition of hydroxide to Ru3+-coordinated of OH attachment to coordinated bipy or phen. 2,2′-bipyridine seemingly very much less 2+ At the time Gillard retired (1998), his ideas about favourable for, for example, [Ru(bipy)3] (139). the role of covalent hydration and of pseudobases Similarly, theoretical analysis of catalysis of water were viewed with suspicion by many of the oxidation by [(terpy)(bpz)RuIV=O]2+ (terpy = workers in this field. There was a considerable 2,2′:6′,2′-terpyridine; bpz = 2,2′-bipyrazine) amount of spectroscopic and kinetic evidence revealed a possible pathway through addition of to support his postulates, but much of it was hydroxyl to the coordinated terpy (140). somewhat equivocal or circumstantial, and often An interesting variant on the Gillard covalent applied to the ligand molecules rather than to their hydration mechanism has been suggested based metal complexes. There was a complete lack of a on studies of intermediates isolated from the structural determination for a covalent hydrate, reaction of phen with copper nitrate in weakly pseudobase or Reissert intermediate isolated alkaline solution to give a binuclear product. Here from an inorganic complex reacted with water or it is proposed that the attacking nucleophile may – – hydroxide (130, 131). Despite the evidence from be [Cu(phen)2(OH)] rather than free OH (141). circular dichroism spectra for covalent hydration of The opening years of the 21st century have seen coordinated phenanthroline in solutions containing a resurgence of interest in the species formed by 2+ [Ni(phen)3]I2·3H2O (35), a structure determination [Pt(bipy)2] in alkaline solution. Here, in contrast – by the Gillard group – failed to provide any to the above recent intimations of the formation of evidence of ligand-water interaction in the solid covalent hydrates or pseudobases, the formation of state (20, 21, 132). In fact the water molecules such intermediates or products, or indeed of earlier- + occur, together with the iodide ions, in layers postulated square-pyramidal [Pt(bipy)2(OH)] separating the layers of complex cations. species, now seems very unlikely. In a Gillardesque After Gillard’s retirement it became steadily more move (142) a group from Otago (New Zealand) 2+ apparent that covalent hydration and pseudobase re-investigated the much-studied [Pt(bipy)2] - formation play a role in at least some redox hydroxide-water system and obtained results best systems involving ruthenium-diimine complexes – interpreted in terms of a fairly stable adduct in as adumbrated by Nord as early as 1975 (112). which one bipy ligand is effectively monodentate Thus Ledney and Dutta furnished spectroscopic and the hydroxide acts as a normal ligand in the evidence for intermediate addition of hydroxide square planar array around the metal, (13, with 2+ to coordinated bipy in the [Ru(bipy)3] -catalysed X = OH) (143). Supporting evidence for a structure

30 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040223 Johnson Matthey Technol. Rev., 2021, 65, (1) of this type was subsequently provided by a study route complementary to Gillard’s covalent hydrate 2+ of the reactions of [Pt(bipy)2] with over 20 route, could lower the barrier to hydroxide attack bases, mainly pyridine derivatives (144). Further at the metal centre. Indeed the situation may well support was provided by extensions to the range be even more complicated, with the transition state of incoming ligands studied and of instrumental or intermediate involving interactions between techniques (145), and to ternary platinum(II)- water or hydroxide and both the metal ion and a bipy-nitro complexes (146). It is a matter of ligand nitrogen and the observed kinetics may well some regret that these authors apparently did not be further complicated by hydration changes for examine analogous systems where the flexible the initial state as well as for the transition state 2,2′-bipyridine is replaced by the more sterically or intermediate as the hydroxide concentration demanding 1,10-phenanthroline (119, 147). changes. Such complications are, of course, even 2+ – 2 The second decade of the 21st century has, on the more relevant to the k3[Fe(LL)3 ][OH ] term other hand, seen Gillard’s work and speculations on in the rate law of Equation (vi) – this term is mechanisms of nucleophilic (OH–; CN–) attack at appreciable at hydroxide concentrations above a 2+ [M(diimine)3] (M = Fe; Ru) increasingly ignored level as low as about 0.05 molar, but is important or forgotten. Thus, for example, there is no mention at high hydroxide concentrations. of a Gillard-type mechanism in a recently published study of micellar effects on base hydrolysis of 2.1.2 More Controversies Fe2+ complexes of sulfonated and unsulfonated phenyl-1,2,4-triazine complexes (148, 149), In the mid-1980s, while continuing with his while there was no consideration of covalently provocative work on reactions of coordinated hydrated intermediates or similar species in the N-heterocycles, Gillard weighed into another DFT treatment of 18O kinetic isotope effects on controversial area which had been a matter of III 4+ catalysis by the cis,cis-{[Ru (bipy)2(OH2)}2O] interest and concern for almost two centuries. This cation mentioned above (150). concerned the ingestion and consequent effects To summarise the current situation regarding of aluminium and its compounds. These may Gillard’s covalent hydrates and pseudobases: enter the body in several ways, intentionally or (a) It is possible that they are intermediates in some otherwise. They may be ingested in food or drink, of the ruthenium redox systems mentioned introduced intentionally in medicines or cosmetics, above or incidentally in water used for dialysis, even (b) Their occurrence is most unlikely in platinum- drinking water, or may be inhaled or absorbed by bipy-hydroxide-water systems workers in mining, processing or manufacturing (c) The position in regard to hydroxide (and industries. The questions then arise as to whether cyanide) attack at diimine complexes of iron(II) the aluminium becomes involved in metabolic or ruthenium(II) is still unclear, as outlined in processes and whether it is toxic (152–157). the next paragraph. The possibility that the ingestion of aluminium It is still difficult for some, as it was in 1962 might lead to the onset of various maladies 2+ – (85), to accept that the k2[Fe(diimine)3 ][OH ] had been raised occasionally since early in the term in the long-known rate law of Equation (vi) 19th century – the likely first case of presumed arises from direct attack of hydroxide on aluminium poisoning was reported in 1828. the low-spin d6 metal centre. Margerum and Arguments over whether or not aluminium Morgenthaler suggested transient intermediates compounds were a threat to human health – – – with incoming OH , CN or N3 interacting with and well-being probably started during the Fe and simultaneously with a pyridine ring of the subsequent law suit (See pp. 39–40 of (152)). leaving ligand. This was probably the source of Alum has been added to flour used in baking Nord’s suggested intermediate (See Scheme II of since the early 19th century; from about 1880 to (151)) in the dissociative redox reaction of tris- 1920 there raged the ‘baking powder wars’ during diimine complexes of iron(III) or ruthenium(III) in which the relative merits and demerits (culinary basic solution, where interaction between the H of and toxicity) of added alum versus added tartrate the hydroxide and ligand nitrogen of the leaving (more expensive) were hotly debated. As early as ligand – as outlined in 14 – rather than the O of the 1857 a Dr John Snow reported in The Lancet that hydroxide with ligand C, assists the dissociation. there seemed to be a connection between the Such an intermediate in the base hydrolysis of the development of rickets in young children and the iron(II) or ruthenium(II) complexes, providing a consumption of bread made with alum-containing

Sheffield | Kent | Cardiff

1965 1970 1975 1980 1985 1990 1995 2000

Adducts of coordination compoundsa 15 1 2 2 2 1 2 1 1 1 1 1

Optically-active coordination compoundsb 51 2 5 4 2 3 3 6 4 2 4 1 1 1 1 1 1 1 2 1 2 2 1* Isomers of copper-amino-acid complexesc 7 2 1 2 1 1 Reactions of complexes of cobaltd 11 1 3 2 1 1 1 2 Coordination compounds and microorganismse 12 1 1 1 1 3 1 1 1 2 Metal complexes as indicators for solvent structuref 5 1 3 1 Oxidants containing rhodiumg 10 2 1 1 1 1 1 1 2

Equilibria in complexes of N-heterocyclic moleculesh 51 1 1 1 4 8 2 9 3 2 t t 1t 2 2 4 1 1 Sulphides of the platinum group elementsi 13 1 1 1 1 1 1 2 2 1 1 1 Oxovanadium(IV) amino acidsj 8 1 1 2 1 1 1 1

1965 1970 1975 1980 1985 1990 1995 2000 Scheme II. Summary of Gillard’s main series of publications. The sequence of numbers for each series details the number of publications in that series for each year; the part number for the final part of each series is given in the shaded box at the end of each series block. It is interesting to follow the detailed chronological progress of individual series, some showing steady progress, others illustrating a very sporadic progress. Explanation of symbols: * A brief Erratum to Part 51 appeared the following year (1996); ♦ The untraced Parts 35 to 39 and 41 to 43 (see (6)) should have appeared in the period 1982 to 1984. a Table S1; b Table S2; c Table S3; d Table S4; e Table S5; f Table S7; g Table S6; h Table S9; i Table S8; j Table S11

flour. His article was reproduced in 2003 in the the Cups that Cheer’ (159). Gillard’s interest in this International Journal of Epidemiology, where area grew out of: it was followed by commentaries from A. (a) his work on complexes and their effects on Hardy, M. Dunnigan and N. Paneth – see pages microorganisms, such as his investigations 336–343 of Volume 32 (153–156). More recently into the bactericidal effects of various platinum Chesney has reconsidered the possible role of group metal (pgm) complexes (Section 3.4, aluminium in the development of rickets (157). Part I (86) on research at Canterbury) Questions over the absorption and subsequent (b) a developing interest in the research of Margaret effects (especially in relation to kidney function Farago and her group at Imperial College on and to the onset of dementia) of aluminium in the effects of added inorganic species on the food and drink were causing widespread concern growth of aquatic plants (160–162) and controversy in the 1970s. Gillard made his (c) another developing interest, into food materials, minimal sally into the field, in 1986, through the subsequently reflected in a significant review topic of significant incorporation of aluminium on metal-protein interactions with Stuart Laurie into food from aluminium cookware (cf. (158), (163). one of many papers on this subject during this In this letter Gillard indicated that his interest in period). the possible harmful effects of ingested aluminium Gillard’s reading of this paper on the leaching of arose from four papers on medical aspects, an aluminium from saucepans (158) prompted him, in article on the mobilisation of aluminium from 1986, to write a letter to Nature entitled ‘Beware cookware and three publications dealing with

32 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15864407040223 Johnson Matthey Technol. Rev., 2021, 65, (1) the accumulation of aluminium by tea plants by the American Chemical Society at Lehigh (164–167). The topics, titles and citations for these University in 1970. There and thereafter there were papers are set out in Table S10. heated disputes over its nature, or indeed its very In practice his contribution to the argument, existence, until it became increasingly clear that its simply counselling readers to moderate their intake exceptional properties were due to its content of of tea, was miniscule, and he never returned to this colloidal silica (see, for example (190)). Polywater subject (at least in print). Uncharacteristically, he soon moved from chemical science to pathological did not even respond to the prompt and succinct science (see for example (191)) to philosophy comment criticising this, and other articles on (see, for example (192, 193)). Gillard must have beverages and foods containing only low levels revelled in the arguments. Fortunately for him he of aluminium, on the grounds that pickles and did not rush into print on this subject – despite his baking powders often contain much higher levels interest in the effects of solutes on water structure of aluminium (168). In retrospect Gillard was in the late 1960s (cf. Table S7). wise to exit this area promptly, for although he expressed no further interest in this topic, concern 2.1.3 Oxometallates and Amino and controversy over the possible health hazards Acids of ingesting aluminium have continued to generate a large number of publications right up to the In 1982 Gillard published the first of his papers present day (169–175). on molybdenum(VI) amino acid complexes that Gillard played a minor role in two other long- comprised one of his shortest series; the second running controversies, the so-called Rupp and last followed in 1990. These dealt with affair and the question of the (non)existence of speciation in aqueous molybdate solutions to which 1,10-phenanthroline-N,N′-dioxide. In the case L = aspartate (194) or (R)-cysteine (195) had of the former his role was of elucidation rather been added. The latter investigation also included than participation (176); in the latter case he analogous tungstate systems. In 1988 he began twice detailed and rebutted the first (177), and publishing a somewhat longer series, of eight later (178–181), claims for its synthesis and parts, on oxovanadium(IV) complexes of amino the reasons for failure (182, 183). However, acids (Table S11). Gillard and his Portuguese subsequently a successful synthesis of the N,N′- colleagues seemed to be particularly keen to dioxide was achieved using HOF·CH3CN as oxidant. present their results in the oxovanadium(IV) area Its structure was shown to involve a novel helical at conferences; a number of abstracts appeared in geometry to minimise steric interactions between reports on conference proceedings in the Journal the two oxygen atoms (184). In fact Gillard was of Inorganic Biochemistry between 1991 and 1997 right in dismissing all the then-extant (1989) (Table S12). claims for having prepared 1,10-phenanthroline- There was also a two-paper series on N-salicylidene- N,N′-dioxide – the particularly powerful oxidant amino acidate complexes of oxovanadium(IV)

HOF·CH3CN was needed (in 1999) to overcome – Part 1 dealt with their crystal and molecular the aromatic resonance energy and force the structures and with their spectroscopic properties phenanthroline moiety away from its heavily- (196), while Part 2 detailed the chemistry of the favoured planarity (185, 186). N-salicylidene-glycylglycinato complex (197). This Gillard was also always keen to argue with minimal series from the mid-1990s was followed, colleagues about current controversies. Thus he in 2004, by a paper on cysteine and penicillamine was fascinated by the ‘polywater’ saga (187). There derivatives (198). It had been preceded, as early had been vague intimations of strange behaviour as 1970, by a paper on oxovanadium(IV) and of water in close proximity to silica as early as oxovanadium(V) complexes of N-salicylidene-amino the 1920s, but the controversy over ‘polywater’ acids and their esters (199). Whereas the formation (‘anomalous water’, ‘orthowater’) only started in of salicylaldimine (200, 201) metal complexes is 1966, when the eminent Russian scientist Boris V. long-known (202–206) and often studied (207, Derjaguin presented the work of the obscure Nikolaj 208), the corresponding ketimine ligands and their Fedyakin (188) at a Faraday Society Discussion at complexes are much more reluctant to form (209, Nottingham (189) and subsequently (July, 1967) 210). However, the Gillard group showed that bis- at a conference at Meriden, NH, USA. The status of paeonolato-copper(II) (paeonol, whose formula is polywater probably reached its zenith with the First shown as 15 (210–212) and, in a three-dimensional International Conference on Polywater, sponsored representation with ligating positions indicated, in

16, is 2-hydroxy-4-methoxy-acetophenone) reacts, for he became a prolific enthusiastic researcher on gentle heating with concentrated aqueous whose love of practical chemistry and publishing his ammonia, to give bis-paeoliminato-copper(II), results was obvious to all who knew him. He was a – [Cu{2-O ,4-OMe-C6H3-C(=NH)Me}2]. The product, driven man, and publishing was centrally important which crystallises anhydrous as the trans isomer, is to him. His publications spanned a wide area of of interest as a square-planar copper(II) complex. coordination chemistry that included significant In contrast, the parent bis-paeonolato complex contributions from that of the pgms rhodium and crystallises from aqueous media mainly as a platinum itself, and he helped to bring this subject dihydrate, presumably containing an octahedral as a whole to the forefront of scientific awareness (or tetragonal) copper(II) centre (213, 214). These at a time when new types of compounds were being investigations into oxovanadium(IV) complexes identified (75, 227). Fundamentally Gillard was a of amino acid derivatives were later extended to preparative chemist who characterised what he peptide (215) and to dipeptide (216, 217) ligands. prepared using chemical and a variety of physical While working with his Portuguese colleagues techniques then available to him. on oxovanadium complexes Gillard also extended Gillard’s formative research began at Oxford, his career-long interest in amino acids to forensic supervised by Harry Irving. This was followed by dentistry and to archaeology. The first project a prolific PhD with Geoffrey Wilkinson at Imperial (218) involved both fields, being an investigation College, in whose laboratory he prepared a wide into the determination of aspartic acid in dental range of new coordination complexes especially those collagen to establish age at death. This approach of rhodium and other kinetically-inert metal centres. depends on the build-up over time of D-aspartyl Later he worked independently on an increasing residues in tooth enamel. The next project was number of topics maintaining a fast pace at Sheffield concerned with the racemisation of amino acids in University, the University of Kent at Canterbury and bone, the isolation of the relevant microorganisms finally at Cardiff University. Gillard always worked (cf. the series ‘Coordination Compounds and at a frenetic pace. Perhaps in some instances this Micro-Organisms’; Table S5 in Part I (86)), and led to premature conclusions being rushed into print the detection of the enzymes involved (219). This in some of his many short publications. This state project was quickly followed by an investigation of of affairs could have been modified had time been the mineralisation of fibres in burial environments taken to complete additional work and discuss his (220) and a study of the usefulness of Fourier findings more fully with colleagues. transform infrared (FTIR) microscopy in examining His major scientific legacy is contained in some remnants of dyes in long-buried textile fibres (221). 400 scientific papers and review articles, of which we have been able to mention less than a 2.1.4 Into Retirement quarter in the present article. He organised many of his publications – over a third – into series, as About a dozen papers co-authored by Gillard documented in Tables S1 to S9 and Table S11. appeared after his retirement in 1998. Five of these Scheme II details titles, time ranges and were published in 2000, of which two were concerned numbers of parts, and thus gives some idea of the with nicotine, three with his favourite pgm, rhodium relation of his main areas of interest and activity (201, 222–225). Gillard’s last chemical publication to his progress through his career. Scheme III appeared in 2004 (198). It dealt with the structure details his publishing activity during his life. Both of the cysteine and penicillamine N-salicylidene- Scheme II and Scheme III illustrate how his aminoacidato complexes of oxovanadium(IV) in research career waxed and waned, and how it the solid state and in solution. This publication is related to his various affiliations. noteworthy both as an illustration of his many links Another major aspect of his legacy was, and is, with Portugal (nine Portuguese co-authors from five personified by all the research colleagues who went locations) and the wide range of characterisation round the world and made significant achievements techniques used (226). in their own right. Of more than a hundred such colleagues many became professors or gained 3. Legacy and Conclusions positions of importance in academia or industry. He also influenced inorganic chemists of his generation Although it is unclear why Gillard studied chemistry through frequent participation in conferences and rather than the originally intended mathematics at symposia, both in his presentations and in the Oxford University it was a career-defining decision course of discussions. In 1998 a particularly happy

1936 Born

1954 To RAF 1955 1956 To Oxford 1957 1958 1959 1960 To BBHa 2 1961 To Imperial 1 1962 1963 9 1964 To Sheffield 18 1965 14 1966 To Kent 23 1967 21 1968 17 1969 16 1970 19 Fig. 1. Robert D. Gillard with his long time 1971 14 1972 9 Australian friend Professor Brice Bosnich (left) at 1973 To Cardiff 11 Gillard’s retirement event held in Cardiff in 1998 1974 11 (Courtesy of Professor A. W. Addison) 1975 6 1976 9 1977 21 1978 10 1979 16 no Festschrift was published to commemorate his 1980 7 career in research. 1981 6 1982 6 The chemical highlights of his research career 1983 6 included the synthesis of a totally inorganic 1984 8 optically active platinum polysulfide which is a 1985 8 1986 6 significant example of a chiral compound with no 1987 6 carbon centre. Expanding further the coordination 1988 10 1989 15 chemistry of cobalt(III) complexes pioneered by 1990 5 Alfred Werner (228, 229), especially their optically 1991 5 1992 8 active properties, was central in Gillard’s career. 1993 9 Starting at Imperial College he worked tirelessly 1994 7 1995 8 on the chemistry of new rhodium(III) complexes 1996 9 and on correcting the formulation of long-known 1997 3 1998 Retired 6 ones. 1999 1 One of Gillard’s great personal fascinations was 2000 5 2001 2 to read the older chemical literature, like the 2002 early work of Delépine and Poulenc on rhodium 2003 1 chemistry and old transition metal cyanide 2004 1 chemistry. His extremely well used (former British 2013 Died Library book) copy of the second edition (1948) of “Cyanogen Compounds” by H. E. Williams (230, Scheme III. Gillard’s life and publications, relating 231) was littered with inserted scraps of paper annual numbers of articles published (third noting particularly intriguing observations. Such column, depicted diagrammatically on the right of information inspired him to do experiments and, the Scheme) to year and to the major changes in with modern techniques, extend understanding his life and career. aThe abbreviation BBH denotes in these areas (233, 232). Complementing (and Burt, Boulton and Haywood Ltd. It is interesting to note the early surge in publications in the overshadowing) this interest in the past, Gillard’s mid-1960s and the quite rapid decrease in his interests constantly developed and diversified publishing rate after the moderate rate of the first throughout his career. As late as 1994, only half of the 1990s a little time before he completely retired, he published the results of work in another area new to him which might be called archeological meeting marking his retirement was held in Cardiff, chemistry (outlined at the end of Section 2.1.3)! attended by many of his research colleagues and This illustrates the sense of enjoyment Gillard got former students (Figure 1). It is a great pity that from the entire research process from first idea,

the manuscript in various stages of preparation and for providing many valuable comments, to Nick Davidson and Gillian Powell for information on Gillard’s time in St Edmund Hall, to John Todd for searching the early records of the University of Kent for information relevant to Gillard’s time in Canterbury, to Daniel Flowerday for details of the 1981 platinum group metals conference in Bristol, and to Leslie Sims for providing details of the Cwmbach Male Voice Choir. Special thanks are due to Gillard’s daughter Fiona Hammett, his first wife Mrs Diane Gillard, and his second wife the late Mrs Anne Gillard for family details. Several people Fig. 2. One of the last photographs of Robert D. provided photographs of considerable interest and Gillard, having lunch with old friends in Cardiff in those included in each of the present articles are 2012 (Courtesy Dr J. G. Jones) acknowledged in the Figure captions.

doing experiments, coming to conclusions and 5. Supplementary Material publishing the results. He worked very hard and was successful in several The Supplementary Tables S1–S12 may be found areas outside his chemical career. His musical in the Supplementary Information included with abilities have been highlighted in Part I (86) and he the online version of Part I (86). was something of a linguist speaking Portuguese as The authors have full listings of the publications well as some German, Italian, French, Welsh and in each of the multi-part series, and a preliminary even able to converse in Hungarian and some Dutch. version of a complete bibliography. Any reader It is said that in retirement he became interested wishing to acquire any of these should contact in the Victorian and Edwardian eras, writing a either of the authors. Anyone wishing to know more biography of the Portuguese diplomat the Marquis about the historical background to Gillard’s brief de Soveral, Envoy Extraordinaire to the Court of St letter on the question of the toxicity of aluminium James, and completing a first draft of an account in tea, or consult a timeline for the dinitrate anion, – of the popularity of tattoos among the Victorian [(NO3)2H] , is invited to email Dr Burgess via the aristocracy. His daughters aptly characterised him Editorial Office. as “a Renaissance chemist” (234, 235). Gillard was always very personable and he exuded a fun for 6. References and Notes and an enjoyment of life that was infectious to all who met him (Figure 2). It is not surprising that 1. The inorganic/organic borderline species tris- he had a huge number of colleagues around the carbonatocobalt(III) provides a link to Gillard’s world (his daughters’ obituary lists 16 countries series “Optically-Active Coordination Compounds” with which he had chemical contacts) who thought – Part 28 of which reports the resolution of this complex by the use of (+)[Co(en) ]3+ (2) of him as a good friend. Gillard’s passing was a 3 2. R. D. Gillard, P. R. Mitchell and M. G. Price, shock to his family and everyone who knew him, J. Chem. Soc., Dalton Trans., 1972, (12), 1211– and he is deeply missed by all. 1213 3. P. Cartwright, R. D. Gillard, R. Sillanpaa and 4. Acknowledgements J. Valkonen, Polyhedron, 1987, 6, (9), 1775– 1779 Thanks are due to the many people who provided 4. A. N. Buckley, H. J. Wouterlood, P. S. Cartwright a tremendous amount of information, material and and R. D. Gillard, Inorg. Chim. Acta, 1988, 143, reminiscences about Gillard. These include the (1), 77–80 late Brice Bosnich, Tony Addison, the late Malcolm 5. R. D. Gillard, Chem. Brit., 1984, 20, (11), 1022– Green, Michael Mingos, Brian Heaton, John Jones, 1024 Ed Constable, Jon McCleverty, Bill Griffith, the late 6. It is impossible to give a reliable estimate for the Tony Poë, Nicholas Payne and the late Paul O’Brien. number of publications in this Series as several We are most grateful to Colin Hubbard for reading part numbers appear to be missing. The existence

of two Part 17s and two Part 50s complicates the 1988, 7, (13), 1175–1186 situation, while the fact that P. A. Williams is the 21. R. D. Gillard, S. H. Mitchell and W. T. Robinson, sole author of Part 23 adds a further complication Polyhedron, 1989, 8, (22), 2649–2655 7. The final paper is dated 2001, three years after 22. R. D. Gillard and S. H. Mitchell, Polyhedron, he retired and 28 years after Part 1 was published 1989, 8, (18), 2245–2249 8. In Part 34 caerulomycin, which contains a 23. Covalent hydration involves the addition of bipy moiety, provides an exotic example of an water across a ring carbon-nitrogen bond N-heterocyclic ligand – see (9) – and a link to the in a heterocyclic compound, a pseudobase “Coordination Compounds and Micro-Organisms” is generated by addition of an anion (e.g. series hydroxide) to a ring carbon atom in a (generally 9. S. Dholakia and R. D. Gillard, J. Chem. Soc., quaternised) nitrogen heterocycle – either way Dalton Trans., 1984, (10), 2245–2248 producing a four-coordinate carbon bearing an

10. Fe(bipy)2(CN)2 and Fe(phen)2(CN)2 are so-called activated nucleophile Schilt-Barbieri compounds (11, 12) 24. However Gillard had earlier hinted at the possibility 11. G. A. Barbieri, Atti Accad. Lincei, 1934, 20, 273– of the occurrence of covalent hydration, for 278 example in his 1969 paper with Brian Heaton on + 12. A. A. Schilt, J. Am. Chem. Soc., 1960, 82, (12), the properties of complexes [M(LL)2X2] (M = Rh 3000–3005 or Ir, LL = bipy or phen, X = Cl or Br – see (25) 13. Though ESR was rarely used in the studies 25. R. D. Gillard and B. T. Heaton, J. Chem. Soc. A, reported in this Series, Gillard used this technique 1969, 451–454 extensively in later work on oxovanadium(IV) 26. R. D. Gillard and J. R. Lyons, J. Chem. Soc., complexes (see, for example, Parts 7 and 8 of Chem. Commun., 1973, (16), 585–586 the series “Oxovanadium(IV) – Amino-Acids” – 27. R. D. Gillard, Inorg. Chim. Acta, 1974, 11, L21– Table S11 – and (14). ESR also featured in his L22 work on silver(II) (15) and in a few of his rhodium 28. “Equilibria in Complexes of N-Heterocyclic studies. These latter included investigation Molecules. Part III. An Explanation For of Rh(II) species in zeolite catalysts (16) and Classical Anomalies Among Complexes of of the paramagnetic dioxygen complexes of + 1,10-Phenanthrolines and 2,2′-Bipyridyls”: A rhodium cis- and trans-[Rh(O2)(en)2Cl] , 3+ 3+ lecture delivered at the Bressanone Conference [[Rh(en)2Cl]2(μ-O2)] and [(RhL4Cl)2(μ-O2)] (L = 4-methylpyridine). Here ESR showed that on “Stability and Reactivity of Coordination – Compounds”, in August, 1974, and based on the complexes contain the Rh(III)-O2 moiety lectures at Canterbury (1967), Coleraine (1974), with the unpaired electron localised on the O2 (17). Despite his extensive work on copper, he Cambridge (1974) and Gregynog (1974) (29) seems very rarely to have obtained ESR spectra 29. R. D. Gillard, Coord. Chem. Rev., 1975, 16, of copper complexes. However, his study of (1–2), 67–94 equilibria at high pH in copper(II)/amino-acid 30. The pH-rate profile of solvolysis, racemisation solutions (18) does provide one instance 3+ and photoracemisation of [Cr(bipy)3] and the 14. J. Costa Pessoa, L. F. Vilas Boas and R. D. Gillard, pH dependence of its luminescence behaviour Polyhedron, 1989, 8, (13–14), 1745–1747 suggest the intermediacy of a cation-hydroxide adduct – see (31) and references therein 15. J. C. Evans, R. D. Gillard, R. J. Lancashire and P. H. Morgan, J. Chem. Soc. Dalton Trans., 1980, 31. P. S. Cartwright and R. D. Gillard, Polyhedron, (8), 1277–1281 1989, 8, (11), 1453–1455 16. I. J. Ellison, R. D. Gillard and J. P. Maher, Trans. 32. Diimine complexes of cobalt(II), nickel(II) and Met. Chem., 2000, 25, (6), 626–627 copper(II) feature in Parts 22 (33) and 28 (34); the pH-dependent racemisation of [Ni(phen) ]2+ 17. J. B. Raynor, R. D. Gillard and J. D. Pedrosa de 3 – cf. [Cr(bipy) ]3+ in (30) – is discussed in Part Jesus, J. Chem. Soc., Dalton Trans., 1982, (6), 3 10 (35) 1165–1166 33. R. D. Gillard and P. A. Williams, Trans. Met. 18. R. D. Gillard, R. J. Lancashire and P. O’Brien, Chem., 1979, 4, (1), 18–23 Trans. Met. Chem., 1980, 5, (1), 340–345 19. X-Ray powder patterns were used, alongside 34. J. A. Arce Sagüés, R. D. Gillard and P. A. Williams, colour and hydration data, to characterise various Inorg. Chim. Acta, 1979, 36, L411–L412 tris-(1,10-phenanthroline)nickel(II) salt hydrates 35. R. D. Gillard and P. A. Williams, Trans. Met. (20), and X-ray structure determinations carried Chem., 1977, 2, (1), 14–18 out on the iodide trihydrate of this complex (21) 36. Pteridines undergo nucleophilic addition and on the dimorphs of fac-[Ir(py)3Cl3] (22) reactions, such as covalent hydration, particularly 20. R. D. Gillard and S. H. Mitchell, Polyhedron, easily (37)

37. A. Albert, D. J. Brown and G. Cheeseman, 55, (3), 511–549 J. Chem. Soc., 1952, 1620–1630 60. F. D. Popp, ‘Reissert Compounds’, in “Advances in 38. A. Albert, Adv. Heterocyclic Chem., 1976, 20, Heterocyclic Chemistry”, eds. A. R. Katritzky and 117–143 A. J. Boulton, Academic Press, Cambridge, USA, 39. A. Albert, Angew. Chem., 1967, 79, (21), 913– Vol. 9, 1968, pp. 1–25 922 61. F. D. Popp, ‘Developments in the Chemistry of 40. A. Albert, Angew. Chem., Int. Ed., 1967, 6, (11), Reissert Compounds (1968-1978)’, in “Advances 919–928 in Heterocyclic Chemistry”, eds. A. R. Katritzky 41. A. Albert and W. L. Armarego, Adv. Heterocycl. and A. J. Boulton, Elsevier Inc, Amsterdam, The Chem., 1965, 4, 1–42 Netherlands, Vol. 24, 1979, pp. 187–214 42. D. D. Perrin, Adv. Heterocycl. Chem., 1965, 4, 62. J. Meisenheimer, Justus Liebigs Ann. Chem., 43–73 1902, 323, (2), 205–246 43. Pyridine has long been known to associate 63. C. F. Bernasconi, Accts. Chem. Res., 1978, 11, strongly with water, e.g. in ternary pyridine/ (4), 147–152 water/organic cosolvent media (44), but seems 64. F. Terrier, Chem. Rev., 1982, 82, (2), 77–152 unwilling to form a covalent bond 65. G. A. Artamkina, M. P. Egorov and I. P. Beletskaya, 44. J. R. Johnson, P. J. Kilpatrick, S. D. Christian and Chem. Rev., 1982, 82, (4), 427–459 H. E. Affsprung, J. Phys. Chem., 1968, 72, (9), 66. J. V. Janovsky and L. Erb, Ber. Deutsch. Chem. 3223–3229 Ges., 1886, 19, (2), 2155–2158 45. T. M. Lowry, “Optical Rotatory Power”, Longmans, 67. J. W. Bunting and W. G. Meathrel, Can. J. Chem., Green and Co, London, UK, 1935, pp. 329–333 1974, 52, (6), 975–980 46. N. S. A. Davies and R. D. Gillard, Trans. Met. 68. A. L. Black and L. A. Summers, Tetrahedron, Chem., 2000, 25, (6), 628–629 1968, 24, (21), 6453–6457 47. (48); Gillard was the (co)author of two reviews on the Pfeiffer effect (49, 50) – the former from 69. J. W. Bunting, Adv. Heterocycl. Chem., 1980, 25, a conference held at University of Sussex 1–82 48. R. D. Gillard, K. W. Johns and P. A. Williams, 70. R. D. Gillard, Coord. Chem. Rev., 1983, 50, (3), J. Chem. Soc., Chem. Commun., 1979, (8), 357– 303–309 358 71. R. D. Gillard, Comm. Inorg. Chem., 1986, 5, (4), 49. R. D. Gillard, ‘The Origin of the Pfeiffer Effect’, 175–199 Proceedings of the NATO Advanced Study 72. N. Serpone and M. Z. Hoffman, J. Chem. Educ., Institute, University of Sussex, UK, 10th–22nd 1983, 60, (1), 853–860 September, 1978, “Optical Activity and Chiral 73. M. A. Jamieson, N. Serpone and Discrimination”, Series C – Mathematical and M. Z. Hoffman, Coord. Chem. Rev., 1981, Physical Sciences, ed. S. F. Mason, Vol. 48, 39, (1–2), 121–179 Springer Science and Business Media, Dordrecht, Holland, 1979, pp. 353–367 74. It should be added that references 98 to 101 of Constable’s recent review (see page 298 of (75)) 50. R. D. Gillard and P. A. Williams, Int. Rev. Phys. argue that covalent hydration plays no role in Chem., 1986, 5, (2–3), 301–305 reactions of this type 51. M. S. Henry and M. Z. Hoffman, J. Am. Chem. 75. E. C. Constable, Polyhedron, 2016, 103, (Part Soc., 1977, 99, (15), 5201–5203 B), 295–306 52. M. S. Henry and M. Z. Hoffman, J. Phys. Chem., 76. D. A. House, P. R. Norman and R. W. Hay, Inorg. 1979, 83, (5), 618–625 Chim. Acta, 1980, 45, L117–L119 53. A. Sarkar and S. Chakravorti, J. Luminescence, 1995, 63, (3), 143–148 77. This hint appears in his riposte (70) to Serpone’s criticism (78). It is also possible that steric 54. R. D. Gillard and D. P. J. Hall, J. Chem. Soc., effects are significant, while both here and in the Chem. Commun., 1988, (17), 1163–1164 equilibria shown in Equation (iv) back-donation 55. P. Tomasik and A. Woszczyk, J. Heterocyclic of electrons from the metal may also play a role Chem., 1979, 16, (6), 1283–1286 78. N. Serpone, G. Ponterini, M. A. Jamieson, 56. X.-M. Zhang, M.-L. Tong and X.-M. Chen, Angew. F. Bolletta and M. Maestri, Coord. Chem. Rev., Chem., Int. Ed., 2002, 41, (6), 1029–1031 1983, 50, (3), 209–302 57. A. Reissert, Ber. Deutsch. Chem. Ges., 1905, 38, 79. O. Mønsted and G. Nord, J. Chem. Soc., Dalton (2), 1603–1614 Trans., 1981, (12), 2599 58. A. Reissert, Ber. Deutsch. Chem. Ges., 1905, 38, 80. This observation is a byproduct of the study of 3415–3435 2+ reactions of complexes [M(LL)3] . M = Fe or Ru. 59. W. E. McEwen and R. L. Cobb, Chem. Rev., 1955, LL = bipy or phen, with cyanide mentioned later

in the text – see (81) 1986, 14, (4), 315–319 81. R. D. Gillard, L. A. P. Kane-Maguire and 101. M. T. Beck and E. C. Porszolt, J. Coord. Chem., P. A. Williams, Transition Met. Chem., 1976, 1, 1971, 1, (1), 57–66 (6), 247 102. E. C. Constable, Polyhedron, 1983, 2, (7), 551– 82. A. Sengül and R. D. Gillard, Trans. Met. Chem., 572 1998, 23, (6), 663–666 103. The session at the Autumn Meeting was part 83. A. M. F. Gameiro, R. D. Gillard, N. H. Rees, of a joint Dalton and Perkin (i.e. inorganic and J. Schulte and A. Sengül, Croatica Chem. Acta, organic) symposium on “Reactions of Coordinated 2001, 74, (3), 641–665 Ligands”, which included an oral contribution 84. D. W. Margerum, J. Am. Chem. Soc., 1957, 79, by E. C. Constable and K. R. Seddon entitled (11), 2728–2733 “Reactivity of Coordinated 2,2’-Bipyridine and 1,10-Phenanthroline”. Nord and Gillard both 85. D. W. Margerum and L. P. Morgenthaler, J. Am. made oral presentations at the IMDG meeting Chem. Soc., 1962, 84, (5), 706–709 104. R. D. Gillard and R. J. Wademan, J. Chem. Soc., 86. J. Burgess and M. V. Twigg, Johnson Matthey Chem. Commun., 1981, (10), 448–449 Technol. Rev., 2021, 65, (1), 4–22 105. K. R. Seddon, J. E. Turp, E. C. Constable and 87. (88). Some early spectroscopic evidence for O. Wernberg, J. Chem. Soc., Dalton Trans., 1987, pseudobase formation from M(5NO phen) 2+, 2 3 (2), 293–296 M = Fe(II), Ru(II) and 5NO2phen = 5-nitro- 1,10-phenanthroline, appeared in a non-Series 106. P. B. Hitchcock, K. R. Seddon, J. E. Turp, communication (89); the electron-withdrawing Y. Z. Yousif, J. A. Zora, E. C. Constable and 5-nitro-substituent promotes reaction with O. Wernberg, J. Chem. Soc., Dalton Trans., 1988, nucleophilic hydroxide (7), 1837–1842 88. R. D. Gillard, D. W. Knight and P. A. Williams, 107. R. D. Gillard and C. T. Hughes, J. Chem. Soc., Trans. Met. Chem., 1980, 5, (1), 321–324 Chem. Commun., 1977, (21), 776–777 89. R. D. Gillard, C. T. Hughes and P. A. Williams, 108. R. D. Gillard and R. J. Wademan, J. Chem. Soc., Trans. Met. Chem., 1976, 1, (2), 51–52 Dalton Trans., 1981, (12), 2599–2600 90. (91). It should be noted that NMR spectra of 109. O. Farver, O. Mønsted and G. Nord, J. Am. Chem. Soc., 1979, 101, (20), 6118–6120 Fe(bipy)2(CN)2 (92) and of Ru(bipy)2(CN)2 (93) have been interpreted in terms of the effects of 110. G. Nord, Comm. Inorg. Chem., 1985, 4, (4), shielding by the CN group rather than in terms of 193–211 covalent hydration 111. Though earlier Nord had suggested the 91. R. D. Gillard, C. T. Hughes, L. A. P. Kane-Maguire intermediacy of a pseudobase “... highly reactive 2+ and P. A. Williams, Trans. Met. Chem., 1976, 1, precursor complex … [M(LL)2(R’-OH)] where (3), 114–118 R’-OH is probably the pseudo-base…”, in her 92. B. V. Agarwala, K. V. Ramanathan and discussion of the kinetics of reduction of tris(2,2’- C. L. Khetrapal, J. Coord. Chem., 1985, 14, (2), bipyridine) and tris(1,10-phenanthroline) 133–137 complexes of iron(III) and osmium(III) by hydroxide ion (112) 93. M. Maruyama, H. Matsuzawa and Y. Kaizu, Inorg. Chim. Acta, 1995, 237, (1–2), 159–162 112. G. Nord and O. Wernberg, J. Chem. Soc., Dalton Trans., 1975, (10), 845–849 94. J. A. Arce Sagüés, R. D. Gillard and P. A. Williams, Trans. Met. Chem., 1989, 14, (2), 110–114 113. R. D. Gillard, R. J. Lancashire and P. A. Williams, J. Chem. Soc., Dalton Trans., 1979, (1), 190– 95. D. W. W. Anderson, P. Roberts, M. V. Twigg and 192 M. B. Williams, Inorg. Chim. Acta, 1979, 34, L281–L283 114. (115). Other examples of tris-bipy complexes of iridium(III) containing a monodentate bipy plus 96. R. D. Gillard, R. P. Houghton and J. N. Tucker, water or hydroxide to complete the octahedral J. Chem. Soc., Dalton Trans., 1980, (11), 2102– environment of the metal include (116–118). 2017 Monodentate phen bonded to platinum(II) has

97. R. D. Gillard and R. E. E. Hill, J. Chem. Soc., been demonstrated in [PtCl(PEt3)2(phen)]BF4, Dalton Trans., 1974, (11), 1217–1236 see (119) 98. R. D. Gillard and H.-U. Hummel, Trans. Met. 115. R. J. Watts, J. S. Harrington and J. Van Houten, Chem., 1985, 10, (9), 348–349 J. Am. Chem. Soc., 1977, 99, (7), 2179–2187 99. S.-Zhan, Q.-Meng, X.-You, G. Wang and P.- 116. J. L. Kahl, K. W. Hanck and K. DeArmond, J. Phys. J. Zheng, Polyhedron, 1996, 15, (15), 2655– Chem., 1978, 82, (5), 540–545 2658 117. R. J. Watts and S. F. Bergeron, J. Phys. Chem., 100. R. D. Gillard and H. U. Hummel, J. Coord. Chem., 1979, 83, (3), 424–425

2+ 118. S. F. Bergeron and R. J. Watts, J. Am. Chem. [Ni(phen)3] -water systems cannot be totally Soc., 1979, 101, (12), 3151–3156 ruled out 119. K. R. Dixon, Inorg. Chem., 1977, 16, (10), 133. M. Ledney and P. K. Dutta, J. Am. Chem. Soc., 2618–2624 1995, 117, (29), 7687–7695 120. J. A. A. Sagüés, R. D. Gillard, R. J. Lancashire 134. P. K. Ghosh, B. S. Brunschwig, M. Chou, C. Creutz and P. A. Williams, J. Chem. Soc., Dalton Trans., and N. Sutin, J. Am. Chem. Soc., 1984, 106, 1979, (1), 193–198 (17), 4772–4783 121. (122). Three years later the waters of 135. J. K. Hurst, Coord. Chem. Rev., 2005, 249, crystallisation in the analogous perchlorate salt (3–4), 313–328

[Ir(bipy)3](ClO4)3·2⅓H2O were shown to be 136. H. Yamada, W. F. Siems, T. Koike and J. K. Hurst, close to 5,5’ positions of the ligands; no oxygen J. Am. Chem. Soc., 2004, 126, (31), 9786–9795 atoms were found close to the 3 (3’) positions as 137. J. L. Cape, W. F. Siems and J. K. Hurst, Inorg. proposed in Gillard’s covalent hydration structure Chem., 2009, 48, 8729–8735 (123) 138. These entities were chosen as models for 122. W. A. Wickramasinghe, P. H. Bird and N. Serpone, intermediates in the catalysis of water oxidation J. Chem. Soc., Chem. Commun., 1981, (24), III 4+ by cis,cis-{[Ru (bipy)2(OH2)}2O] 1284–1286 139. A. Ozkanlar, J. L. Cape, J. K. Hurst and A. E. Clark, 123. A. C. Hazell and R. G. Hazell, Acta Cryst., 1984, Inorg. Chem., 2011, 50, (17), 8177–8187 C40, (5), 806–811 140. L.-P. Wang, Q. Wu and T. Van Voorhis, Inorg. 124. There is also a link to his interest in mineral Chem., 2010, 49, (10), 4543–4553 materials, in that he invoked covalent 141. K. B. Szpakolski, K. Latham, C. J. Rix, J. M. White, hydration to explain aspects of the behaviour of B. Moubaraki and K. S. Murray, Chem. Eur. J., 1,10-phenanthroline complexes of iron(II) and 2010, 16, (5), 1691–1696 of copper(II) on clay (hectorite, the smectite Na (Mg,Li) Si O (OH) ) surfaces (125). A 142. The words we have italicised in the title “The 0.3 3 4 10 2 2+ few years later Krenske et al. (126) suggested nature of the [Pt(bipy)2] ion in aqueous alkaline that the dependence of absorption spectra solution: a new look at an old problem” irresistibly and luminescence yields on water content for recall Gillard’s re-examinations of such entities 2+ as Rhodium Blue and Tipper’s Compound [Ru(bipy)3] absorbed on smectite membranes could be rationalised in terms of covalent 143. C. S. McInnes, B. R. Clare, W. R. Redmond, hydration – quoting several Gillard references C. R. Clark and A. G. Blackman, Dalton Trans., (though not the Clays and Clay Minerals (125) 2003, (11), 2215–2218 paper!) in support 144. Y. Kawanishi, T. Funaki, T. Yatabe, Y. Suzuki, 125. R. D. Gillard and P. A. Williams, Clays Clay Miner., S. Miyamoto, Y. Shimoi and S. Abe, Inorg. Chem., 1978, 26, 178–179 2008, 47, (9), 3477–3479 126. D. Krenske, S. Abdo, H. Van Damme, M. Cruz 145. G. Cavigliasso, R. Stranger, W. K. C. Lo, and J. J. Fripiat, J. Phys. Chem., 1980, 84, (19), J. D. Crowley and A. G. Blackman, Polyhedron, 2447–2457 2013, 64, 238–246 127. J.-P. Zhang, Y.-Y. Lin, Y.-Q. Weng and X.- 146. W. K. C. Lo, R. J. Shepherd, R. Stranger, M. Chen, Inorg. Chim. Acta, 2006, 359, (11), A. G. Blackman and G. Cavigliasso, Polyhedron, 3666–3670 2017, 130, 145–153 128. Q. Huang, L.-H. Diao and X.-H. Yin, Z. Kristallogr. 147. However, monodentate phen bound to NCS, 2010, 225, (4), 781–782 platinum(II) is not unknown – as in, for instance, [PtCl(PEt3)2(phen)]BF4 (see (118)) 129. S.-F. Wu, J.-Z. Liu, M.-X. Meng and W.-Q. Luo, Z. Kristallogr. NCS, 2012, 227, (2), 163–164 148. See (149) and five earlier references (from 2012 to 2017) to Bellam et al. therein 130. Though it must be said that Bunting cites Gillard 149. R. Bellam, N. R. Anipindi and D. Jaganyi, J. Mol. in this connection in his 1980 review (131) Liquids, 2018, 258, 57–65 131. J. W. Bunting, Adv. Heterocycl. Chem., 1980, 25, 150. A. M. Angeles-Boza, M. Z. Ertem, R. Sarma, 1–82 C. H. Ibanez, S. Maji, A. Llobet, C. J. Cramer and 132. (21). The authors comment (their italics) J. P. Roth, Chem. Sci., 2014, 5, (3), 1141–1152 “Covalent hydration was proposed for this 151. G. Nord, B. Pedersen and E. Bjergbakke, J. Am. system in solution … such modification of the Chem. Soc., 1983, 105, (7), 1913–1919 ligated 1,10-phenanthroline is not present in this particular solid”. It should be added that, in view 152. E. E. Smith, “Aluminum Compounds in Food”, of the extensive polymorphism – at least seven Paul B. Hoeber, New York, USA, 1928

other forms – known for [Ni(phen)3]I2·nH2O 153. J. Snow, Int. J. Epidemiol., 2003, 32, (3), 336– (20) the occurrence of covalent hydration in 337

154. A. Hardy, Int. J. Epidemiol., 2003, 32, (3), 337– 2018, 14, (1), 35–41 340 176. R. D. Gillard, Chem. Brit., 1985, 21, (6), 535 155. M. Dunnigan, Int. J. Epidemiol., 2003, 32, (3), 177. F. Linsker and R. L. Evans, J. Am. Chem. Soc., 340–341 1946, 68, (3), 403 156. N. Paneth, Int. J. Epidemiol., 2003, 32, (3), 341– 178. Gillard does not cite sources for some of these 343 claims, but implies that they were made in 157. R. W. Chesney, Pediatr. Nephrol., 2012, 27, (1), manuscripts submitted for him to referee – 3–6 he was a commendably active peer reviewer. 158. P. Mattsson, Vår Foda, 1981, 33, (6), 231–236 Several papers reporting preparations of a number of its complexes, including those 159. A. M. Coriat and R. D. Gillard, Nature, 1986, of VO2+, ZrO2+, UO 2+, Sn4+ and Th4+ (see 321, (6070), 570 2 (179, 180) and references therein), appeared 160. An article detailing the effects of various platinum around this time. This group claimed to have metal species on the growth of water hyacinths prepared their 1,10-phenanthroline-N,N'-dioxide provides a useful perspective (161); a Gillard- (phenO2) ligand by hydrogen peroxide/glacial contemporaneous reference is (162) – but Gillard acetic acid oxidation of 1,10-phenanthroline, will have been aware of Margaret Farago’s work a method which works well for the conversion well before 1979 of 2,2'-bipyridine to its N,N'-dioxide (181) but 161. M. E. Farago and P. J. Parsons, Chem. Speciat. is unlikely to be successful for the hoped-for

Bioavail., 1994, 6, (1), 1–12 analogous conversion of phen into phenO2 162. M. E. Farago, W. A. Mullen and J. B. Payne, Inorg. 179. A. K. Srivastava, S. Sharma and R. K. Agarwal, Chim. Acta, 1979, 34, 151–154 Inorg. Chim. Acta, 1982, 61, 235–239 163. R. D. Gillard and S. H. Laurie, ‘Metal-Protein 180. R. K. Agarwal and H. K. Rawat, Thermochim. Interactions’, in “Biochemistry of Food Proteins”, Acta, 1986, 99, 367–371 ed. B. J. F. Hudson, Ch. 5, Springer Science and 181. P. G. Simpson, A. Vinciguerra and J. V. Quagliano, Business Media, Dordrecht, The Netherlands, Inorg. Chem., 1963, 2, (2), 282–286 1992, pp. 155–196 182. R. D. Gillard, Inorg. Chim. Acta, 1981, 53, L173 164. Recent references testifying to continuing interest in the absorption of aluminium by tea 183. R. D. Gillard, Inorg. Chim. Acta, 1989, 156, (2), plants, its concentration in their shoots and 155 leaves, its extraction in tea infusions, and levels 184. S. Rozen and S. Dayan, Angew. Chem., Int. Ed., of aluminium in typical cups of tea include (165– 1999, 38, (23), 3471–3473 167) 185. The situation here recalls the difficulty of doubly- 165. T. Karak and R. M. Bhagat, Food Res. Int., 2010, protonating 1,10-phenanthroline – see (186) 43, (9), 2234–2252 186. C. Swinnerton and M. V. Twigg, Trans. Met. 166. M. Kröppl, M. Zeiner, I. J. Cindrić and G. Stingeder, Chem., 1978, 3, (1), 25–27 Eur. Chem. Bull., 2012, 1, (9), 382–386 187. F. Franks, “Polywater”, The Massachusetts Institute 167. R. F. Milani, M. A. Morgano and S. Cadore, LWT – of Technology Press, Cambridge, USA, 1981 Food Sci. Technol., 2016, 68, 491–498 188. N. Fedyakin, Kollodniy Zhurnal, 1962, 24, 497– 168. G. M. Wild, Nature, 1987, 326, (6112), 434 501 169. A typical example of the state of the controversy 189. B. V. Derjaguin, Disc. Faraday Soc., 1966, 42, towards the end of the 20th century is provided 109–119 by the linked references (170–172) – of which 190. B. V. Derjaguin, Z. M. Zorin, Ya. I. Rabinovich and the last is of particular reference to the topic of N. V. Churaev, J. Colloid Interface Sci., 1974, 46, aluminium in tea. The current situation can be (3), 437–441 traced through, for example (173–175) 191. D. L. Rousseau, Am. Sci., 1992, 80, (1), 54– 170. D. G. Munoz, Arch. Neurol., 1998, 55, (5), 737– 63 739 192. W. Gratzer, “The Undergrowth of Science: 171. W. F. Forbes and G. B. Hill, Arch. Neurol., 1998, Delusion, Self-Deception and Human Frailty”, 55, (5), 740–741 Oxford University Press, New York, USA, 2000, 172. V. Hachinski, Arch. Neurol., 1998, 55, (5), 742 328 pp 173. C. Exley, Environ. Sci.: Processes Impacts, 2013, 193. J. van Brakel, ‘Pure Chemical Substance’, in 15, (10), 1807–1816 “Stuff: The Nature of Chemical Substance”, ed. 174. A. Mirza, A. King, Claire Troakes and C. Exley, K. Ruthenberg and J. van Brakel, Königshausen J. Trace Elem. Med. Biol., 2017, 40, 30–36 & Neumann, Würzburg, Germany, 2008, Ch. 9, 175. A. Seidowsky, E. Dupuis, T. Drueke, S Dard, pp. 145–162 Z. A. Massy and B. Canaud, Nephrol. Ther., 194. A. M. V. S. V. Cavaleiro, J. D. Pedrosa de Jesus,

V. M. S. Gil, R. D. Gillard and P. A. Williams, 210. P. Pfeiffer, E. Buchholz and O. Bauer, J. Prakt. Trans. Met. Chem., 1982, 7, (2), 75–79 Chem., 1931, 129, (1), 163–177 195. A. M. V. S. V. Cavaleiro, J. D. Pedrosa de Jesus, 211. The trivial name paeonol (US peonol, German V. M. S. Gil, R. D. Gillard and P. A. Williams, Inorg. päonol) for 4-methoxy-acetophenone dates from Chim. Acta, 1990, 172, (1), 25–33 the time it was used in perfumery. It has also been 196. I. Cavaco, J. Costa Pessoa, D. Costa, M. T. Duarte, used in traditional medicine – and, more recently, R. D. Gillard and P. Matias, J. Chem. Soc., Dalton shown to have analgesic, anti-inflammatory. and Trans., 1994, (2), 149–157 anti-mutagenic properties. Pfeiffer’s laboratory studied its metal-complexing reactions in the 197. I. Cavaco, J. Costa Pessoa, S. M. Luz, M. T. Duarte, 1920s (209, 212). The name paeonol was derived P. M. Matias, R. T. Henriques and R. D. Gillard, from the Greek – Paiõn (Παιών) was Apollo’s Polyhedron, 1995, 14, (3), 429–439 title as physician of the gods. Presumably the 198. J. Costa Pessoa, M. J. Calhorda, I. Cavaco, perfumery usage of paeonol comes from the P. J. Costa, I. Correia, D. Costa, L. F. Vilas- fragrance of paeony (Greek παιωνία) flowers Boas, V. Félix, R. D. Gillard, R. T. Henriques and 212. P. Pfeiffer, S. Golther and O. Angern, Chem. Ber., R. Wiggins, Dalton Trans., 2004, (18), 2855– 1927, 60, (2), 305–313 2866 213. R. A. Wiggins and R. D. Gillard, 1970, unpublished 199. J. J. R. Frausto da Silva, R. Wootton and R. D. Gillard, J. Chem. Soc. A, 1970, 3369–3372 214. E. R. J. Sillanpää, A. Al-Dhahir and R. D. Gillard, Polyhedron, 1991, 10, (17), 2051–2055 200. Analogous complexes can readily be prepared from other aromatic o-hydroxyaldehydes – see 215. J. Costa Pessoa, S. M. Luz and R. D. Gillard, for example (201) J. Chem. Soc., Dalton Trans., 1997, (4), 569–576 201. J. Costa Pessoa, I. Cavaco, I. Correia, 216. J. Costa Pessoa, T. Gajda, R. D. Gillard, T. Kiss, M. T. Duarte, R. D. Gillard, R. T. Henriques, S. M. Luz, J. J. G. Moura, I. Tomaz, J. P. Telo and F. J. Higes, C. Madeira and I. Tomaz, Inorg. Chim. I. Török. J. Chem. Soc., Dalton Trans., 1998, Acta, 1999, 293, (1), 1–11 (21), 3587–3600 202. Ettling’s preparation of bis-salicylaldimine- 217. J. Costa Pessoa, I. Cavaco, I. Correia, D Costa, copper(II), reported in 1840 (203), provided R. T. Henriques and R. D. Gillard, Inorg. Chim. one of the earliest examples of a reaction of Acta, 2000, 305, (1), 7–13 a coordinated ligand. Ettling wrote at length 218. R. D. Gillard, A. M. Pollard, P. A. Sutton and about salicylate(s) (203) in an oil steam- D. K. Whittaker, Archaeometry, 1990, 32, (1), distilled (in 1835) from meadowsweet (Spiraea 61–70 ulmaria) flowers by the apothecary Pagenstecher 219. A. M. Child, R. D. Gillard and A. M. Pollard, and analysed by Löwig in Zürich (204, 205). J. Arch. Sci., 1993, 20, (2), 159–168 Copper(II) and nickel(II) complexes of amino acid ester derivatives of salicaldehyde were 220. R. D. Gillard, S. M. Hardman, R. G. Thomas and described in (206) D. E. Watkinson, Stud. Conserv., 1994, 39, (2), 132–140 203. C. Ettling, Justus Liebigs Ann. Chem., 1840, 35, (3), 241–276 221. 221. R. D. Gillard, S. M. Hardman, R. G. Thomas and D. E. Watkinson, Stud. Conserv., 1994, 39, 204. K. J. Löwig, Ann. Phys., 1835, 112, (11), 383– (3), 187–192 403 205. M. Vallett, J. Pharm. Sci. Access., 1836, 22, 187– 222. Indeed one paper was concerned with both 200 subjects – (223) dealt with rhodium(III) complexes with enantiomers of nicotine 206. P. Pfeiffer, W. Offerman and H. Werner, J. Prakt. Chem. (Leipzig), 1942, 159, 313–333 223. R. D. Gillard and E. Lekkas, Trans. Met. Chem., 2000, 25, (6), 617–621 207. D. St. C. Black, in “Comprehensive Coordination Chemistry”, eds. G. Wilkinson, R. D. Gillard and 224. This final paper (198) and his 1999 paper J. A. McCleverty, Vol. 1, Pergamon Press, Oxford, on the same complexes (201), reflect both UK, 1987, p. 434 his long-standing interest in complexes of aminoacids and their derivatives and his 208. D. St. C. Black, in “Comprehensive Coordination fruitful Portuguese collaborations. These two Chemistry”, eds. G. Wilkinson, R. D. Gillard and papers also complement Part IV of Gillard’s J. A. McCleverty, Vol. 6, Pergamon Press, Oxford, “Oxovanadium(IV) – Amino-Acids” series UK, 1987, p. 156 (Table S11) on oxovanadium complexes of 209. Gillard thought that the “adduct” of aniline with cysteine and of penicillamine (225) bis-paeonolatocopper(II) made by Pfeiffer and 225. J. Costa Pessoa, L. F. Vilas Boas and R. D. Gillard, his students – see p. 174 of (210) for the copper- Polyhedron, 1990, 9, (17), 2101–2125 paeonol complex and its aniline solvate – was in fact probably a copper(II)-ketimine complex 226. To quote from the abstract and text of this

article: “…characterised by elemental analysis, 232. It is a matter of considerable regret that Gillard spectroscopy (UV-VIS, CD, EPR), TG, DSC and did not write a book on chemistry. A volume magnetic susceptibility measurements (9–295 K) on coordination complexes which combined an ... the use of molecular mechanics and density outline of their historical context with accounts functional calculations … DFT calculations for of their preparation, characterisation and both types of tautomers…” properties, would have been particularly valuable 227. As was recently stated “Bob was an undervalued and appreciated, as would a book expanding his member of the generation who brought review on circular dichroism (cf. (233)) coordination chemistry to the front of scientific 233. R. D. Gillard, ‘The Cotton Effect in Coordination awareness.” – see page 303 of (228) Compounds‘, in “Progress in Inorganic Chemistry”, 228. G. B. Kauffman, “Alfred Werner: Founder of ed. F. A. Cotton, Vol. 7, John Wiley and Sons Inc, Coordination Chemistry”, Springer-Verlag, New York, USA, 1966, pp. 215–276 Heidelberg, Germany, 1966 234. (235). This obituary also mentions a popular 229. G. B. Kauffman, Coord. Chem. Rev., 1972, 9, public lecture entitled “Is God left handed?”, (3–4), 339–363 reflecting his long standing fascination by 230. (231). The first edition was published in 1915 by chirality and an unpublished essay on chemistry J. & A. Churchill (London) under the title “The in Sherlock Holmes books Chemistry of Cyanogen Compounds” 235. I. Gillard and F. Hammett, ‘Robert David Gillard 231. H. E. Williams, “Cyanogen Compounds”, 2nd (1956)’, St Edmund Hall Magazine (Oxford), Edn., E. Arnold and Co, London, UK, 1948 2013, 18, (4), 167–173

The Authors

www.technology.matthey.com

Optimising Metal Content in Platinum Group Metal Ammonia Oxidation Catalysts A technical review of catalyst design methods

Julie Ashcroft selling the first platinum gauze pack to the UK Johnson Matthey, Orchard Road, Royston, Munitions Invention department in October 1916. Hertfordshire, SG8 5HE, UK This pack contained woven wires manufactured from pure platinum and required 1 kg of platinum Email: [email protected] to manufacture 1 tonne of nitric acid (2). In the century that followed, a number of technological advances resulted in lighter and more efficient Platinum-based knitted gauzes are the most catalyst packs and reduced the installed metal efficient catalysts for the production of nitric oxide, content per tonne of nitric acid produced from as a precursor to the manufacture of nitric acid and kilograms to milligrams. caprolactam. Decades of research and optimisation The first advancement in technology was the have resulted in a greater understanding of substitution of pure platinum with a 10% rhodium ammonia oxidation kinetics and associated metal 90% platinum alloy, first patented by DuPont, USA, movement within these catalyst packs, along with in 1929 (3). Rhodium increased the mechanical the development of beneficial binary and ternary strength of the alloy and reduced in situ metal alloys. The design of a pack has evolved from the loss, allowing the catalyst to be operated at higher simple addition or removal of metal to modelling temperatures and benefit from the associated the optimal installed metal content and distribution. increase in selectivity to nitric oxide (4). This review discusses the fundamental kinetics and Despite the reduction in metal loss from the in situ metal loss for ammonia oxidation catalysts optimisation in alloy, the metal lost from the catalyst in nitric acid applications and outlines how they in situ remained high and formed a significant can, in conjunction with prevailing platinum part of the plant operating costs. To reduce the group metal (pgm) market conditions and plant cost of the metal loss, gauzes comprising woven key performance indicators (KPIs), influence the palladium-alloy wires were developed in 1968, optimal catalyst design. using a gold-palladium alloy, with up to 20% gold (5). This catchment pack sacrificially collects 1. Introduction and History platinum whilst losing palladium, reducing the cost of the total metal loss. Further research to The production of nitric acid, a key industrial reduce the cost of the catchment system resulted process, requires the synthesis of nitric oxide in the development of nickel-palladium and as a precursor to the desired end product. tungsten‑palladium alloys (6, 7) with research Nitric acid, with a current annual production showing that a low concentration of the base metal of 65.9 million tonnes, is primarily used in the was required for high collection efficiency (8). As production of ammonium nitrate for fertiliser a result, alloys used in catchment systems today applications, with the remainder (around 20%) typically contain 95% palladium. used in industrial applications including the The 1990s saw a significant change in the design production of explosives (1). and manufacture of pgm catalysts, with Johnson Johnson Matthey has manufactured pgm Matthey introducing new technology where knitted ammonia oxidation catalysts for over 100 years, gauzes superseded woven. As well as introducing

44 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16012842414480 Johnson Matthey Technol. Rev., 2021, 65, (1) more flexibility to catalyst design, the new industrial nitric acid plants operated at atmospheric technology was observed to have a positive impact pressure and were restricted to low production on the catalyst selectivity to nitric oxide (6). This rates. In the late 1920s, nitric acid plants were technology is now the standard for ammonia constructed that could operate under pressure, oxidation catalysts and is offered by all major resulting in more compact plants that could achieve catalyst suppliers. higher rates (10). Gauze manufacturers make use of different Today, new nitric acid plants are typically technologies, i.e. warp and weft knitting, to dual‑pressure plants or ultra-high mono-pressure manufacture the catalyst gauzes. Recent advances plants (10). Dual-pressure plants utilise a low driven by knitting include the manufacture of very operating pressure for the ammonia oxidation high-density gauzes to concentrate metal towards reactor to ensure high selectivity to nitric oxide the top of the pack. There are different methods to (typically 3–6 barg) and increase the pressure in achieve this: Johnson Matthey has achieved this by the downstream absorption column to improve the minimising the open area of a weft-knitted gauze, column efficiency. Ultra-high mono-pressure plants using high density structures that increase the operate at a single pressure, typically 10–13 barg, metal content per gauze by over 90% compared to benefiting from lower capital costs but achieving the basic knit structure. Other manufacturers such lower plant efficiencies. as Umicore, Belgium, limited to greater open areas, have developed three-dimensional knit structures 2. Understanding Ammonia to achieve a high-density gauze (9). Oxidation The mid-2000s saw a fundamental change in ammonia oxidation catalyst design, with ternary 2.1 Reaction Fundamentals rhodium-platinum-palladium alloys, containing high concentrations of palladium, replacing Ammonia reacts with oxygen to produce nitric traditional rhodium-platinum alloys in the lower oxide, nitrogen and nitrous oxide. The selectivity of portion of the pack. Johnson Matthey developed ammonia to each reactant is dependent on process ECO-CATTM gauzes using these principles, and this conditions and catalyst specification. Nitrogen new technology saw a reduction in installed weight is the favoured product at lower temperatures, required to produce a set amount of nitric acid. The whilst nitric oxide formation is favoured at higher length of time the plant could run with a catalyst temperatures and at lower pressures. At low pack (known industrially as the ‘campaign’) was temperatures, NO is formed but is bound strongly increased. Lower density palladium-based alloys to the platinum surface. This allows the NO to react in the lower layers collect volatilised platinum with ammonia to form N2 which will then desorb. from the top layers, with the captured platinum At high temperatures, the NO formed will desorb being used later in the campaign as the reaction from the catalyst surface before this secondary progresses further into the pack. Variations on this reaction occurs, increasing the catalyst selectivity technology are now the standard offering by most to NO (11). catalyst suppliers for medium-pressure nitric acid In industrial production, the reaction is mass plants. transfer limited due to the relatively slow diffusion Historically, catalyst packs were designed using of reactants to the wire surface compared to the rules of thumb and best estimates. An increased almost instantaneous surface kinetics (12). In understanding of ammonia oxidation and primary industrial operation, the catalyst operates between metal losses within pgm catalysts has resulted in 850–930°C and plant operating pressures range the creation of bespoke design tools, which allow from atmospheric pressure to 13 barg. Typical Johnson Matthey to optimise the metal content selectivity to nitric oxide ranges from 90–97% and distribution required within a catalyst pack. (Equations (i)–(iii)): Using these tools, catalyst designs can be tailored to plant operating conditions and KPIs. A tailored 4NH3 + 5O2 → 4NO + 6H2O design can result in higher selectivity to nitric ΔH0 = –907.28 kJ (i) oxide, along with increased campaign lengths and 4NH3 + 4O2 → 2N2O + 6H2O reduced metal content in catalyst packs. ΔH0 = –1104.9 kJ (ii)

Improvements have also been observed in nitric 4NH3 + 3O2 → 2N2 + 6H2O acid plant designs over the last 100 years. The first ΔH0 = –1269.0 kJ (iii)

In addition to these primary reactions, ammonia nitrous oxide at all temperatures as demonstrated can react with nitric oxide to produce nitrogen and through ab initio modelling of the selectivity of nitrous oxide, which reduces the efficiency of the ammonia oxidation to NO, NO2, N2 and N2O on plant (Equations (iv) and (v)): pure platinum and palladium-platinum alloys

(Figure 1). The benefit of lower selectivity to N2O

4NH3 + 6NO → 5N2 + 6H2O (iv) is partially offset by an increase in selectivity to

2NH3 + 8NO → 5N2O + 3H2O (v) N2: palladium-based alloys can help reduce N2O emissions but at the cost of reduced selectivity As a result, an effective catalyst will be designed to to NO (16). The benefit of using palladium-based convert 100% of ammonia in the top few layers of alloys has been demonstrated within the nitric the catalyst pack at the beginning of a campaign; acid industry, with catalyst packs using ECO‑CATTM although the reaction will progress further through technology or other catalyst manufacturers the pack as platinum is lost and the top layers of ternary alloy technologies, shown to reduce N2O the catalyst lose activity. emissions by up to 30% compared to standard The surface chemistry and metallurgy of the rhodium‑platinum catalyst packs (17). catalyst used impacts the selectivity achieved. Several precious metals have been shown to 2.2 In situ Restructuring and Metal catalyse the ammonia oxidation reaction, including Loss platinum, rhodium, palladium (13), silver and iridium (14). Platinum has the greatest selectivity When exposed to the ammonia-air feed gas, to nitric oxide, making it the most suitable catalyst the pgm wires begin to restructure, forming for nitric acid plants. Rhodium-platinum alloys, high-surface-area dendritic multiplanar crystal containing 3–10% rhodium, are the most common growths, known in the industry as ‘cauliflowers’. alloys offered in ammonia oxidation catalysts today, The formation of cauliflowers results in a although often in conjunction with a rhodium- significant increase in the specific surface area platinum-palladium alloy. of the wire, with 10 to 20-fold increases in While rhodium-containing alloys are primarily used surface area observed after industrial operation, to increase the alloy strength during manufacture with the initial period of restructuring taking and operation (15), rhodium-platinum alloys have several days (18–20). After initial restructuring, been observed to have a higher selectivity to nitric cauliflower formation is greatest in the top layer oxide than pure platinum (4), and palladium- of the catalyst (Figure 2), as this is exposed containing alloys have a reduced selectivity to to the greatest concentration of ammonia, with

(a) (b) NO NO 1.0 NO2 1.0 NO2 N2O N2O N2 N2 0.8 0.8 products products 0.6 0.6 /ΣP /ΣP i i

0.4 0.4 Selectivity, P Selectivity, Selectivity, P Selectivity, 0.2 0.2

0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 Temperature, °C Temperature, °C Fig. 1. In-house modelling by Johnson Matthey using ab initio kinetics and density functional theory (DFT) demonstrates the significant reduction in selectivity to 2N O for: (a) a pure platinum alloy; (b) a palladium- doped platinum alloy across a range of temperatures. An increase in selectivity to N2 at higher temperatures is also demonstrated (16)

(a) (b) (c)

Fig. 2. (a) layer 1; (b) layer 3 and (c) layer 5 of a catalyst pack composed of 5% rhodium 95% platinum alloy after 5 days operation at 4 barg showing a high level of restructuring and cauliflower growth in the top layer and negligible restructuring on the bottom layer. Images are at 4000 × magnification (21) layers further down the pack seeing little to no metal is installed to maintain a high selectivity for restructuring until later in the campaign. the desired campaign length. As the campaign The restructuring mechanism is poorly understood, progresses, the catalyst begins to lose a significant with several mechanisms suggested for the amount of platinum in the top gauze layers, production of cauliflower structures at the surface with the wires becoming enriched with rhodium. of platinum alloy wires. The prevailing theory is Post‑campaign analyses of catalysts have observed that the adsorption and subsequent reaction of NHx surface rhodium levels of over 40% in extreme species results in localised areas of high temperature cases that suffered mechanical loss of platinum, which promotes the dissociation of surface PtOx and with further research observing 33% rhodium

RhOx; the presence of gas phase platinum species enrichment in non-damaged gauzes (14). As a during ammonia oxidation has been demonstrated result of the platinum deficiency, full conversion by mass spectrometry (22). The hotspots occur of ammonia is no longer achieved in these top close to surface defects, such as grain boundaries, layers and the reaction zone stretches further into and this is where the restructuring process is the catalyst. Lower gauze layers will be exposed observed to begin (14). As these gaseous species to the ammonia oxidation reaction and begin contact regions of the gauze where the reaction restructuring. is slower, condensation occurs due to reduced The design of the lower half of the catalyst pack is temperatures, which forms new crystallites (23). key to maintaining a high selectivity to nitric oxide These colder regions can be found very close to the in the later stages of the campaign: installing too hotspots, which results in a localised temperature many catalyst gauzes or not correctly distributing gradient that drives the chemical vapour transport the metal within the pack can result in residual of the metal oxides. Some of these metal oxides ammonia reacting with nitric oxide. These reactions will condense on the wire surface in these colder result in both a reduction in selectivity to nitric regions; further ammonia oxidation then occurs at oxide and an increase in unwanted side products, this deposition and this rapidly results in the growth N2 and N2O. These reactions are suppressed in the of cauliflower structures observed in activated and presence of excess oxygen (24), and as a result spent gauzes (18). are more likely to occur in lower layers of the pack. Whilst this vapour-phase mechanism results Conversely, if too few layers are installed then in capture of a proportion of the metal, some is 100% conversion of ammonia may not be achieved lost downstream of the gauze pack. The addition as the campaign progresses, resulting in ammonia of palladium and rhodium will reduce the overall slip downstream of the catalyst. metal loss from the pack, as these alloys see a The use of ternary rhodium-platinum-palladium reduction in platinum volatility compared to pure alloys in the lower portion of the catalyst pack is a platinum (14). Catalyst packs will typically lose key feature of ECO-CATTM packs. The concentration half of the installed pgm content at the end of a of palladium in the alloy can vary from low levels to campaign. constituting the majority of the alloy and allows the The movement and loss of metal within the lower half of the pack to function as part catalyst, catalyst pack is a key constraint during the pack part catchment. During operation, volatilised design phase, and it is critical to ensure sufficient platinum from the top rhodium-platinum layers

47 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16012842414480 Johnson Matthey Technol. Rev., 2021, 65, (1) is captured on the lower layers, increasing the 2.3 Industrial Operation platinum content on the surface of the wire, while palladium is lost from the catalyst. A successful Industrial ammonia oxidation is carried out at high ECO-CATTM catalyst will have a wire surface temperatures (ranging from 850–930°C) and at covered in platinum at the point the ammonia pressures ranging from atmospheric pressure to begins to break through and react on the layer, and 14 barg. Depending on the daily acid production, the selectively react to produce mostly nitric oxide. flux of ammonia (known in industry as the nitrogen The restructuring mechanism within ECO-CATTM loading) can range from 3–100 tonnes of nitrogen packs differs from rhodium-platinum cauliflower per m2 of cross sectional area of the ammonia –2 – 1 formation. Surface growths are more block-like oxidation burner per day (teN m day ; units are (Figure 3), showing a resemblance to catchment based on nitrogen from ammonia, excluding air). restructuring (Figure 4) and the lower layers At industrial operating conditions, the selectivity to will fuse together during operation; a similar nitric oxide can range from 90–97%, with the large mechanism has been observed with catchment range due primarily to the reduction in efficiency gauzes which are always separated with steel caused when operating at higher pressures. In layers to prevent this. As a result of this formation, addition to reduced selectivity, higher pressure the gauze open area is reduced, and the pressure plants also see an increased rate of metal loss from drop over the system increases. the catalyst in situ. Platinum capture is also possible through the Nitric acid plants in industry have a wide range use of palladium-based catchment gauzes and of operating conditions; however, they can broadly glass-wool platinum filters. Glass-wool filters see be defined in four categories. These categories are recovery rates of 10–20% (5), and catchment derived from plotting ammonia oxidation pressure packs can recover from 25% to 95% of platinum against nitrogen loading (Figure 5). lost in the catalyst gauzes depending on the weight and distribution of palladium installed (6). • Atmospheric plants, with nitrogen loadings of –2 –1 95% palladium alloys used in the catchment pack <10 teN m day and pressures of <1 barg, exhibit a different end of life morphology than the were the first type of nitric acid plants to be built rhodium‑platinum and rhodium-platinum‑palladium industrially, and benefit from high selectivity to catalyst gauzes, with the observed restructuring nitric oxide. However, due to the low nitrogen driven by the collection of platinum. However, loading, the acid production rates are low palladium and palladium‑based alloys can catalyse • Low-medium pressure plants, which include the ammonia oxidation reaction, and if exposed most dual-pressure plants. These plants to the NH3-O2 containing gas, will restructure to continue to benefit from relatively high produce cauliflower structures similar to those selectivity to nitric oxide, with efficiencies of observed in rhodium-platinum alloys (14). up to 96.5% observed. Operating at higher

Fig. 3. Scanning electron microscopy (SEM) at Fig. 4. SEM at 250 × magnification showing the 250 × magnification showing the top layer of the top layer of a catchment pack in a high-medium fused structure at the bottom of an ECO-CATTM pressure plant. The observed low open area results pack which had been installed in a high-medium in a high pressure drop as the gas passes through pressure plant this gauze layer

14 Fig. 5. Plotting individual nitric 12 4 acid plant operating pressure against 10 nitrogen loading gives rise to four distinct 8 3 categories of plants. The majority of nitric 6 acid plants will have operating conditions 4 2 that lie within one of the four operating 2 ranges illustrated here 1 Ammonia oxidation pressure, barg Ammonia oxidation

0 20 40 60 80 100 Nitrogen loading, te m–2 day–1

pressure allows for a significant increase in Post-campaign analysis by Johnson Matthey has daily acid production and, due to relatively low observed the open area of used catchment gauzes metal losses, campaign lengths of up to one to be as low as 2%. year can be achieved • High-medium pressure plants have a wide 3. Designing the Optimal Pack range of operating conditions. Some of these plants will benefit from similar design principles The reaction fundamentals, restructuring and loss to low-medium pressure plants, while others of metal and constraints in industrial operation from those for ultra-high-pressure plants must all be considered when designing a catalyst • Ultra-high-pressure plants operate at pack for ammonia oxidation in a nitric acid plant. high pressures (10–13 barg) and high The variables available to a catalyst designer and temperatures (920–930°C). The selectivity manufacturer include optimising the metal content to nitric oxide of around 90% is significantly and metal placement within the pack and selecting lower than other plant types. The intrinsic rate the most appropriate alloys and optimising their of metal loss is substantially higher, limiting placement in the pack. campaigns to around 100 days. As a result of the metal loss, the catalyst designs for these 3.1 Metal Content plants have the highest level of installed pgm weight per tonne of acid. The optimal metal content installed in a catalyst pack is determined by the expected levels of metal In addition to the catalyst selectivity and metal loss during the campaign, which is modelled as loss, the pressure drop over the catalyst and a function of the burner operating pressure and catchment pack can also have an effect on nitrogen loading. For standard packs, the pack will industrial operation. This is most pronounced for be designed such that around 50% of the installed ultra-high-pressure plants: a high system pressure metal content remains after the completion of drop will put strain on the compressor and the the campaign. For optimised packs utilising high plant will not be able to operate at maximum -palladium alloys, the remaining metal content can production rates. The pressure drop over binary be as low as 30% of the installed weight. rhodium-platinum catalysts is relatively low, with A catalyst pack with an insufficient mass of pgm higher pressure drops observed when operating installed can lead to ammonia slip, where the ECO-CATTM catalysts due to the high palladium reaction fails to complete within the catalyst pack. layers fusing together and reducing the system The introduction of ammonia downstream can open area. The catchment system will create the result in the formation of ammonium nitrite and greatest pressure drop as the physical collection ammonium nitrate, which can pose an explosion of platinum can reduce the gauze open area. hazard. Ammonium nitrite can form when NO2,

NO and NH3 are present; significant levels of NO2 primary losses, to determine where the majority form downstream of the gauzes as NO undergoes of the reaction is carried out at different stages oxidation, so avoiding ammonia slip downstream of in the campaign (Figure 6) and ensure sufficient gauze is crucial. Ammonium nitrite is highly unstable metal is installed to prevent ammonia slip later in and can undergo explosive decomposition, which the campaign. can result in the detonation of any ammonium nitrate present (25). 3.2 Alloy Selection Defining the required mass of pgm required for the catalyst pack is critical, however thought must The selection and placement of different alloys also be given to the distribution of metal within within the catalyst pack is critical to achieving high the pack to achieve high selectivity to nitric oxide. performance, with some alloys suited to the top The underlying kinetics of the reaction require part of the pack (the catalytic ‘engine’) and others that the ammonia oxidation is completed within as providing benefits when installed at the bottom of few gauze layers as possible to minimise the side the pack. Having a high platinum content in the reactions between ammonia and nitric oxide. top layer reduces the temperature at which the Modifying the metal concentration within the pack reaction begins to progress, in turn reducing the is achieved by varying the densities of the gauzes. time between start-up and reaching peak efficiency. Johnson Matthey manufactures multiple knit This can be achieved through a combination of alloy structures of varying density which are combined choice, selecting a binary alloy with <5% rhodium with wire diameters ranging from 60 μm to 120 μm, and increasing the metal concentration at the top as well as a range of different binary and ternary of the pack by using high density knit structures. alloys. There are hundreds of configurations Experimental data has demonstrated a reduction possible for each layer within the catalyst pack, in the light-off temperature of around 100°C, and and possible densities range from 300 g m–2 to Johnson Matthey has developed specific products over 1200 g m–2. for plants that struggle with light-off. To ensure the installed metal content is correct, A range of rhodium concentrations are available Johnson Matthey uses theoretical and empirical in binary rhodium-platinum alloys. Lower levels models to estimate the primary loss expected from of rhodium are advised when plants suffer from a catalyst at the required operating pressure and rhodium oxide formation (Figure 7), which is nitrogen loading. Distribution of the metal within significantly less selective to nitric oxide (26), the catalyst is also dependent on the plant category, to minimise or prevent such formation. Plants and can be evaluated using an ammonia oxidation operating at low temperatures (<850°C) are at kinetic model, in conjunction with the expected higher risk of rhodium oxide formation, and the

Fig. 6. Kinetic model 100 output for a gauze design for a low- medium pressure plant 90 showing achieved ammonia conversion 80 throughout the gauze pack at the start of 0 days 70 the campaign and 9 days nine days into the campaign. This shows 60

Ammonia conversion, % Ammonia conversion, sufficient metal has been installed to 50 prevent significant 1 2 3 4 5 6 7 levels of ammonia Gauze layer slip on start-up (zero days) and to carry out the majority of the reaction in the top two gauze layers after nine days online

3.3 Catchment System

In addition to the catalyst gauze pack, a palladium‑alloy catchment (or getter) system will often be installed below the catalyst to minimise the metal loss from the catalyst through platinum capture and palladium loss. Recent changes to the pgm market have reduced the profitability of catchment systems. Palladium prices have continued to rise steadily since 2017, whilst platinum remained relatively flat. For 2020, the platinum market is expected to move into Fig. 7. SEM image of a gauze wire at surplus and the palladium market to fall further 4000 × magnification, showing the formation of into deficit (27), which has further implications characteristic rhodium oxide needles. Optimising rhodium content and placement within the gauze for the optimal catchment pack design. The pack can reduce the likelihood of formation optimal catchment design is a function of the price differential and acceptable pressure drop. risk increases for catalysts with significant levels of 4. Case Study iron contamination. Interest in low-rhodium alloys has increased following the substantial increase in Optimisation of metal content is applicable to all rhodium price, peaking at almost US$12,000 per categories of nitric acid plants and Johnson Matthey troy oz in early 2020 (27). continually reviews existing catalyst designs to Palladium proves a beneficial addition in ensure they are optimal for plant operation and the lower part of the catalyst pack, and the prevailing pgm market conditions. In recent years, introduction of ternary alloys within the pack of significant steps have been taken to optimise metal Johnson Matthey’s ECO-CATTM catalyst results in a content for ultra-high-pressure plants, with design significant reduction in the primary 2N O generated options tailored to the key drivers of each plant, by the catalyst pack (17). The catalyst pack for example maximising daily production rates achieves a higher efficiency towards the end of or maximising plant efficiency from a set level of the expected campaign length due to the capture ammonia feed. and recycling of volatilised platinum, allowing For a significant proportion of nitric acid for campaign lengths to be extended, where producers operating ultra-high-pressure plants, maintenance schedules permit, while maintaining the key driver is to produce the maximum or even reducing the installed metal content. As amount of nitric acid from a gauze pack. In these a result of this technology, campaigns of up to cases, the introduction of palladium alloys is not one year are now commonplace in low-medium recommended due to the increased pressure pressure plants. drop. The catalyst pack is instead optimised by There are a range of ternary alloys used in increasing the platinum weight at the top of the ECO‑CATTM catalysts. Initial developments of pack by using larger wire diameters in the top ECO‑CATTM used only one high-palladium alloy, layers. To maintain a constant pack weight, the but further work has shown that the average total number of layers within the pack is reduced. campaign efficiency can be improved through use of As a result, ammonia oxidation is completed higher a gradient of palladium in the pack. This has been in the pack and the N2 and N2O-producing side demonstrated industrially, with an increase in the reactions are reduced. In addition to selectivity average efficiency of up to 1% reported by nitric benefits, increasing the wire diameter in the acid plants, along with a reduction in N2O emissions. top layers provides sufficient metal for further The optimal level of palladium is influenced by both cauliflower formation in the event of mechanical plant performance and the relative price difference metal loss following a plant trip. between platinum and palladium, balancing the For other ultra-high-pressure producers, typically cost of the installed metal, the net metal loss and operating in locations with higher raw material any financial implications resulting from a change costs, the key driver is to maximise the efficiency in N2O emissions. of the gauze pack over a set campaign length.

As a slight increase in pressure drop is tolerable catchment technologies are not always suitable for these plants, pack optimisation includes the for ultra-high-pressure plants due to the increased use of ECO‑CATTM design principles. Initial design pressure drop. optimisation work for a plant in this category Optimisation of metal content within the combined the use of high-density knit structures catalyst pack is now the expected standard for all in the top portion of the pack with high palladium medium‑pressure plants, with the development alloys in the lower portion of the pack. The of high-density knit structures and ECO‑CATTM changes resulted in improving the conversion technology resulting in improvements to conversion efficiency towards the end of the campaign, and efficiency, reduction in installed metal content and as a result increasing the average campaign the extension of campaign lengths. In recent years, efficiency. Further design changes, including the further progress has been made in optimising catalyst reduction of palladium content whilst increasing packs for ultra‑high‑pressure nitric acid plants the concentration of palladium higher in the pack, operating at >10 bar and with nitrogen loading –2 –1 resulted in a reduction in N2O emissions from the values in excess of 60 teN m day , designing the gauze system. pack to minimise unwanted side reactions. While the high intrinsic metal loss continues to constrain 5. Conclusion the maximum campaign length for these plants, the optimisation of pgm content within the pack has The catalysis of ammonia oxidation using pgm resulted in an improvement in plant efficiency and, gauzes has been used industrially for over where targeted, a reduction in N2O emissions. 100 years. Step changes in performance have been Despite being a mature and conservative achieved throughout the decades, reducing the industry, the operational targets for nitric acid required pgm metal content required in a catalyst plants continue to increase to support the global pack and introducing a range of alloys and knitted demand for fertiliser. Many plants will seek to structures from which to construct a catalyst pack. operate in excess of the nameplate capacity and Research into ammonia oxidation has continued to carry out de-bottlenecking studies to support support the development of catalysts with higher this. Changing operational targets should always selectivity to NO and reduced N2O emissions, be communicated to the catalyst supplier, as a giving rise to the current binary and ternary alloys change in plant loading, temperature or pressure used in catalysts today. is likely to require an updated catalyst design. The The underlying reaction kinetics and metal dynamic pgm market also influences the optimal movement provide a basis for designing the optimal pack design; catchment packs see a reduction in ammonia oxidation catalyst and have been used profitability when the price of palladium exceeds within Johnson Matthey to develop plant category that of platinum. Additionally, at the time of writing specific design rules. Achieving a high selectivity rhodium was seeing extremely high prices which to nitric oxide can be achieved by promoting near form a significant portion of the pack cost, leading 100% ammonia conversion in the top layers, and to increasing interest in low-rhodium alloys. the use of ternary alloys in the lower catalyst The optimisation of a catalyst pack is never layers promotes recycling of platinum to maintain a complete process, and continual monitoring a high efficiency towards the end of the campaign. of catalyst performance and an open dialogue A wide range of knit structures, wire diameters between plant operator and catalyst supplier are and alloys are available to tailor the catalyst to critical in creating and maintaining an optimal pack the plant operating conditions and to distribute the design. metal within the pack in a way that promotes the ECO-CATTM is a trademark of Johnson Matthey highest selectivity to NO throughout the campaign. PLC. The concentration of rhodium and palladium in the top catalyst layers can be optimised to address References plant‑specific issues, including the formation of rhodium oxide and helping the catalyst to light-off 1. R. Gubler, B. Suresh, H. He and Y. Yamaguchi, at a lower temperature. ‘Nitric Acid’, in “Chemical Economics Handbook”, Other factors influence the optimal design, with IHS Markit, London, UK, May, 2020, 116 pp the acceptable pressure drop over the catalyst 2. H. Frankland, C. Brown, H. Goddin, O. Kay and pack being key. Despite having significant benefits, T. Bünnagel, Johnson Matthey Technol. Rev., TM including reduced N2O emissions, ECO-CAT and 2017, 61, (3), 183

3. C. W. Davis, E. I. Du Pont de Nemours & Co, 16. J. Mugo and G. Jones, internal Johnson Matthey ‘Process of Oxidizing Ammonia’, US Patent report, unpublished, 2018 1,706,055; 1929 17. Fraunhofer ISI, ECOFYS BV and Öko-Institut eV, 4. S. L. Handforth and J. N. Tilley, Ind. Eng. Chem., “Methodology for the Free Allocation of Emission 1934, 26, (12), 1287 Allowances in the EU ETS Post 2012: Sector 5. A. E. Heywood, Platinum Metals Rev., 1973, Report for the Chemical Industry”, Study Contract 17, (4), 118 07.0307/2008/515770/ETU/C2, Ecofys Project Number PECSNL082164, European Commission, 6. B. T. Horner, Platinum Metals Rev., 1993, 37, (2), 76 Brussels, Belgium, November, 2009, 101 pp 7. A. Bazhenov and G. Gushin, ‘Catalysts for Ammonia 18. A. N. Salanov, E. A. Suprun, A. N. Serkova, Oxidation’, Nitrogen+Syngas, September– N. M. Chesnokova, E. F. Sutormina, L. A. Isupova October, 2017, 349, 1 and V. N. Parmon, Kinet. Catal., 2020, 61, (3), 8. Y. Ning, Z. Yang and H. Zhao, Platinum Metals 421 Rev., 1996, 40, (2), 80 19. E. Bergene, O. Tronstad and A. Holmen, J. Catal., 9. H. Gölitzer, D. Köenigs, J. Neumann and T. Stoll, 1996, 160, (2), 141 Umicore AG & Co KG, ‘Three-Dimensional Catalyst 20. R. J. Farrauto and H. C. Lee, Ind. Eng. Chem. Res., Gauzes Knitted in Two Layers’, European Patent, 1990, 29, (7), 1125 1,358,010; 2002 21. M. Wilson and H. Goddin, internal Johnson Matthey 10. M. C. E. Groves, ‘Nitric Acid’, in “Kirk-Othmer report, unpublished, 2017 Encyclopedia of Chemical Technology”, John Wiley & Sons Inc, Hoboken, USA, 2020 22. O. Nilsen, A. Kjekshus and H. Fellvåg, Appl. Catal. 11. T. Pignet and L. D. Schmidt, J. Catal., 1975, A: Gen., 2001, 207, (1–2), 43 40, (2), 212 23. M. R. Lyubovsky and V. V. Barelko, J. Catal., 1994, 12. M. Warner and B. S. Haynes, Proc. Combust. Inst., 149, (1), 23 2015, 35, (2), 2215 24. E. V. Kondratenko and J. Pérez-Ramírez, Appl. 13. J. Pérez-Ramírez, E. V. Kondratenko, G. Novell- Catal. A: Gen., 2005, 289, (1), 97 Leruth and J. M. Ricart, J. Catal., 2009, 261, (2), 25. G. M. Lawrence, Proc. Safe. Prog., 1989, 8, (1), 217 33 14. L. Hannevold, O. Nilsen, A. Kjekshus and H. Fjellvåg, Appl. Catal. A: Gen., 2005, 284, (1– 26. F. Sperner and W. Hohmann, Platinum Metals 2), 163 Rev., 1976, 20, (1), 12 15. J. A. Busby, A. G. Knapton and A. E. R. Budd, Proc. 27. A. Cowley, “Pgm Market Report”, Johnson Matthey, Fert. Soc., 1978, 169, 39 London, UK, February, 2020, 40 pp

The Author

Julie Ashcroft joined Johnson Matthey in 2013 after graduating from the University of Cambridge, UK. Having worked as a technical sales engineer within the nitric acid industry for a number of years, she is currently a Senior Process Engineer in the ammonia and nitric acid engineering team.

www.technology.matthey.com

A Re-assessment of the Thermodynamic Properties of Osmium Improved value for the enthalpy of fusion

John W. Arblaster degree of correlation for the platinum group metals Droitwich, Worcestershire, UK (pgms). However the derived entropy of fusion value for osmium was based on values for the other pgms Email: [email protected] available at that time but since then the values for both palladium and platinum have been revised so that the entropy of fusion value for osmium would The thermodynamic properties were reviewed by the also be revised leading to a new estimate of 68.0 ± author in 1995. A new assessment of the enthalpy of 1.7 kJ mol–1 for the enthalpy of fusion. This would then fusion at 68.0 ± 1.7 kJ mol–1 leads to a revision of require the thermodynamic properties of the liquid the thermodynamic properties of the liquid phase and phase to also be updated. A comment is included on although the enthalpy of sublimation at 298.15 K is an independent much lower estimate of the enthalpy retained as 788 ± 4 kJ mol–1 the normal boiling point of fusion. Wherever possible measurements have is revised to 5565 K at one atmosphere pressure. been corrected to the International Temperature Scale (ITS-90) and to the currently accepted atomic Introduction weight of 190.23 ± 0.03 (4).

The thermodynamic properties of osmium were Low Temperature Solid Phase reviewed by the author in 1995 (1) with a further review in 2005 (2) to estimate a most likely value Selected values in the normal and superconducting for the melting point at 3400 ± 50 K to replace the states are based on the specific heat measurements poor quality experimental values which were being of Okaz and Keesom (0.18 K to 4.2 K) (5) including quoted in the literature. More recently Burakovsky et a superconducting transition temperature of al. (3) have estimated a value of 3370 ± 75 K in good 0.638 ± 0.002 K, an electronic specific heat agreement with the above selected value. In the 1995 coefficient (γ) of 2.050 ± 0.003 mJ mol–1 K–2 and review the enthalpy of fusion was unknown but was a limiting Debye temperature (ΘD) of 467 ± 6 K. estimated from a relationship between the entropy Specific heat values up to 5 K in both the normal of fusion and the melting point which showed a high and superconducting states are given in Table I.

Table I Low Temperature Specific Heat Data Up To 5 K Temperature, K Cºsa, mJ mol–1 K–1 Cºnb, mJ mol–1 K–1 Temperature, K Cºp, mJ mol–1 K–1 0.2 0.093 0.410 1.0 2.07 0.3 0.525 0.616 2.0 4.25 0.4 1.19 0.821 3.0 6.67 0.5 1.94 1.03 4.0 9.43 0.6 2.79 1.23 5.0 12.7 0.638 3.14 1.32 – – aSuperconducting state bNon-superconducting state (in magnetic field)

Above 4 K selected specific heat values are Liquid Phase initially based on the measurements by Naumov et al. (6 K to 316 K) (6). However above 280 K Selected values of the enthalpies and entropies these measurements show an abrupt increase of fusion of the Groups 8 to 10 elements with a of 0.5 J mol–1 K–1 and a further abrupt increase close-packed structure are given in Table VII. of 0.3 J mol–1 K–1 above 300 K. Naumov et al. Only the enthalpy of fusion of osmium is unknown. attempted to accommodate these values but the References (10–14) represent the latest reviews on selected specific heat curve showed an unnatural the thermodynamic properties of the pgms by the sharp change in slope above 270 K. Therefore present author. From an evaluation of the entropies the selected values of Naumov et al. above 250 K of fusion of the elements, Chekhovskoi and Kats were rejected and instead specific heat values to (15) proposed that the entropy of fusion (ΔSºM)

298.15 K were obtained by joining smoothly with and the melting point (TM) could be related by the the high temperature enthalpy measurements of equation ΔSºM = A TM + B. In the previous review (1) Ramanauskas et al. (7). In the original review of different values were proposed for the entropies of the low temperature data only the specific heat fusion of palladium (8.80 J mol–1 K–1) and platinum values were given consisting above 50 K of 10 K (10.45 J mol–1 K–1) leading to an estimate of the intervals to 100 K and then 20 K intervals above entropy of fusion for osmium of 20.6 J mol–1 K–1. this temperature as well as the value at 298.15 K. With the revised values it is clear that, although This minimalist approach is now considered to be of the right order, the entropy of fusion of nickel is unsatisfactory and therefore comprehensive low discrepant and has therefore been disregarded. The temperature thermodynamic data are now given at other six values were fitted to the equation with 5 K intervals from 5 K to 50 K and at 10 K intervals A = 6.6954 × 10–3 and B = –2.7630 and a standard above this temperature up to 290 K and then the deviation of the fit of ± 0.193 J mol–1 K–1. However value at 298.15 K as given in Table II. in order that the derived entropy of fusion of osmium has a similar accuracy to those of the input High Temperature Solid Phase values then the accuracy is expanded to a 95% confidence level leading to an entropy of fusion In the high temperature region, after correction for of 20.0014 ± 0.387 J mol–1 K–1 and based on a temperature scale and atomic weight, the enthalpy melting point 3400 ± 50 K to an enthalpy of fusion measurements of Ramanauskas et al. (1155 K to of 68,005 ± 1653 J mol–1. Based on neighbouring 2961 K) (7) were fitted to the following equation elements then a liquid specific heat of 50 J mol–1 K–1 with an overall accuracy of ± 200 J mol–1 (0.4%) was proposed in the original paper (1) and therefore (Equation (i)): the enthalpy of liquid osmium can now be expressed as Equation (ii): –1 HºT – Hº298.15 K (J mol ) = 26.1938 T –1 HºT – Hº298.15 K (J mol ) = 50.0000 T + 816.2 + 1.32318 × 10–4 T2 + 3.85960 × 10–7 T3 (ii) + 3.99978 × 10–11 T4 + 150378/T – 8336.36 (i) Equivalent specific heat and entropy equations This equation was used to represent selected corresponding to the above equation are given in enthalpy values from 298.15 K to 3400 K. Table III, the free energy equation in Table IV and Equivalent specific heat and entropy equations derived thermodynamic values in Table VI. It should corresponding to the above equation are given in now be possible to accurately determine the melting Table III, the free energy equations in Table IV, point and enthalpy of fusion of osmium since the metal transitions values associated with the free energy is available in high purity in a coherent form whilst functions in Table V and derived thermodynamic the enthalpies of fusion of other high melting point values in Table VI. The actual equation given elements such as rhenium (3458 K) and tungsten by Ramanauskas et al. to represent the enthalpy (3687 K) have been successfully determined. measurements over the experimental temperature range agrees with Equation (i) to within 0.2%. Gas Phase The only other enthalpy measurements were obtained by Jaeger and Rosenbohm (693 K to Based on a standard state pressure of 1 bar the 1877 K) (8) and compared to the selected values thermodynamic properties of the monatomic vary from 1.6% low at 693 K to an estimated 1.2% gas were calculated from the 295 energy levels low at 1600 K to 1.4% low at 1877 K. listed by Van Kleef and Klinkenberg (16) and

Table II Low Temperature Thermodynamic Data Above 5 K a b c d d Temperature, Cºp , HºT – Hº0 K , SºT , –GºT – Hº0 K , –(GºT – Hº0 K)/T , K J mol–1 K–1 J mol–1 J mol–1 K–1 J mol–1 J mol–1 K–1 5 0.0127 0.0286 0.0111 0.0266 0.00532 10 0.0417 0.153 0.0272 0.119 0.0119 15 0.116 0.519 0.0559 0.319 0.0213 20 0.290 1.475 0.110 0.719 0.0360 25 0.636 3.704 0.208 1.490 0.0596 30 1.252 8.302 0.374 2.910 0.0970 35 2.104 16.61 0.628 5.376 0.154 40 3.139 29.65 0.975 9.346 0.234 45 4.322 48.25 1.412 15.27 0.339 50 5.604 73.03 1.933 23.60 0.472 60 8.205 142.2 3.186 48.99 0.817 70 10.563 236.2 4.631 87.96 1.257 80 12.661 352.6 6.182 142.0 1.775 90 14.448 488.4 7.780 211.8 2.353 100 15.939 640.6 9.381 297.6 2.976 110 17.182 806.4 10.961 399.3 3.630 120 18.231 983.6 12.502 516.7 4.305 130 19.132 1170 13.997 649.2 4.994 140 19.912 1366 15.445 796.4 5.689 150 20.577 1568 16.842 957.9 6.386 160 21.085 1777 18.187 1133 7.082 170 21.533 1990 19.479 1322 7.774 180 21.975 2207 20.722 1523 8.459 190 22.377 2429 21.921 1736 9.136 200 22.695 2655 23.078 1961 9.804 210 22.928 2883 24.191 2197 10.463 220 23.178 3113 25.263 2445 11.111 230 23.441 3346 26.928 2702 11.749 240 23.715 3582 27.302 2970 12.377 250 23.929 3820 28.275 3248 12.993 260 24.119 4061 29.217 3536 13.599 270 24.290 4303 30.130 3832 14.195 280 24.444 4546 31.017 4138 14.780 290 24.584 4791 31.877 4453 15.355 298.15 24.688 4992 32.560 4715 15.816 a CºP is specific heat b HºT – H0 K is enthalpy c SºT is entropy d –GºT – Hº 0 K and –(GºT – Hº0 K)/T are free energy functions

Gluck et al. (17) using the method outlined and Reif (20) and Carrera et al. (21). Normally by Kolsky et al. (18) together with the the experimental temperature values would 2018 Fundamental Constants (19). Derived therefore be accepted but in the case of such thermodynamic values are given in Table VIII. values above 2000 K the difference from the current scale, ITS‑90, becomes significant. Since Enthalpy of Sublimation the measurements were carried out in 1962 and 1964 then they would ultimately be associated No temperature scales were given with the with the International Practical Temperature Scale measurements of the vapour pressures by Panish (IPTS-1948) and were therefore corrected to the

Table III Thermodynamic Equations Discussion of Alternative Estimates Above 298.15 K of the Enthalpy of Fusion of Osmium Solid: 298.15 K to 3400 K a –1 –1 –4 Based on various assumptions Fokin et al. (22) Cºp , J mol K = 26.1938 + 2.64636 × 10 T + 1.15788 × 10–6 T2 + 1.599912 × 10–10 T3 proposed that the enthalpy of fusion for osmium – 150378/T2 was only in the range 30 kJ mol–1 to 40 kJ mol–1 b –1 HºT – Hº298.15 K , J mol = 26.1938 T or half of the above derived value. One of the main –4 2 –7 3 + 1.32318 × 10 T + 3.85960 × 10 T arguments was that by using the Chekhovskoi- –11 4 + 3.99978 × 10 T + 150378/T – 8336.36 Kats equation the entropy of fusion for rhenium c –1 –1 –1 –1 SºT , J mol K = 26.1938 ln(T) was estimated to be 20.0 J mol K whereas –4 –7 2 + 2.64636 × 10 T + 5.78940 × 10 T the actual value is only 9.85 J mol–1 K–1 (23) + 5.33304 × 10–11 T3 and therefore if the estimate for rhenium was so + 75189/T2 – 117.6597 completely wrong then it would also be possible Liquid: 3400 K to 5600 K that the estimate for the neighbouring element Cº a, J mol–1 K–1 = 50.0000 p osmium at 19.0 J mol–1 K–1 could also be wrong. b –1 HºT – H298.15 K , J mol = 50.0000 T + 816.2 However, Fokin et al. completely misunderstood c –1 –1 SºT , J mol K = 50.0000 ln(T) – 281.5442 how the estimated values were arrived at. It a CºP is specific heat was initially assumed that Group 7 rhenium b HºT – H298.15 K is enthalpy c would behave like Groups 8 to 10 (the pgms) SºT is entropy whereas all that the experimental value proved was that Group 7 elements behaved completely independently of Groups 8 to 10 and therefore Table IV Free Energy Equations Above showed the same deviations as other transition 298.15 K metal groups. For example, the entropies of fusion Solid: 298.15 K to 3400 K of Group 5 elements vanadium, niobium and a –1 –1 –1 –1 –1 GºT – Hº298.15 K , J mol = 143.8535 T – 1.32318 tantalum at 10.46 J mol K , 11.13 J mol K –4 2 –7 3 × 10 T – 1.92980 × 10 T – 1.33326 and 10.25 J mol–1 K–1 (24) showed no trend with × 10–11 T4 + 75189/ T – 26.1938 T ln(T) temperature whilst the entropies of fusion of the Group – 8336.36 6 elements chromium, molybdenum and tungsten Liquid: 3400 K to 5600 K at 13.89 J mol–1 K–1, 13.53 J mol–1 K–1 and 13.66 –1 GºT – Hº298.15 K, J mol = 331.5442 T – 50.0000 T J mol–1 K–1 (24) were virtually identical. Therefore ln(T) + 816.2 it would not be surprising if Group 7 elements aGº – Hº is the free energy function T 298.15 K would also behave completely independently. In fact for the transition metals only the Groups 8 to 10 elements showed a high degree of correlation ITS-90 scale on this basis. Derived enthalpies of with the Chekhovskoi-Kats equation. However sublimation are given in Table IX. The selected in order to prove their point that osmium does enthalpy of sublimation of 788 ± 4 kJ mol–1 is behave differently to the other pgms, Fokin et al. basically an unweighted average but slightly used the equation: σM = Z ΔHM ρSM d where σM is biased towards the measurements of Carrera et the surface tension at the melting point, ΔHM is al. (21). the enthalpy of fusion, ρSM is the density of the solid at the melting point and d is the interatomic Vapour Pressure Equations distance. This equation was applied to a number of elements but there is virtually no correlation for The vapour pressure equations are given in the values of Z with values varying between 1.2 to Table X. For the solid the evaluation was for free 3.3. For osmium Fokin et al. selected an arbitrary energy functions for the solid and the gas at 50 K rounded value of Z = 2 for osmium and values intervals from 1700 K to 3400 K and for the liquid of surface tension and liquid density determined at 50 K intervals from 3400 K to 5600 K and were by Paradis et al. (25) to arrive at an enthalpy of fitted to Equation (iii): fusion of only 32 kJ mol–1 which is considerably less than the value of 39.0 ± 1.4 kJ mol–1 (9) selected ln(p, bar) = A + B ln(T) + C/T + D T + E T2 (iii) for the analogue element ruthenium whereas for A review of the vapour pressure data is given in the other pgms the enthalpy of fusion is always Table XI. greater for the heavier analogue. This much lower

Table V Transition Values Involved with the Free Energy Equations –1 –1 –1 Transition Temperature, K ΔHM, J mol ΔSM, J mol K Fusion 3400 68005.00 20.0014

Table VI High Temperature Thermodynamic Data for the Condensed Phases Hº – Hº b –(Gº – Hº )/ Temperature, K Cº a J mol–1 K–1 T 298.15 K , Sº c J mol–1 K–1 T 298.15 K p , J mol–1 T , Td, J mol–1 K–1 298.15 24.688 0 32.560 32.560 300 24.711 46 32.712 32.560 400 25.555 2564 39.951 33.541 500 26.034 5145 45.709 35.419 600 26.386 7767 50.488 37.543 700 26.694 10,421 54.579 39.692 800 26.994 13,105 58.163 41.781 900 27.301 15,820 61.360 43.782 1000 27.626 18,566 64.253 45.687 1100 27.975 21,346 66.902 47.497 1200 28.351 24,162 69.352 49.217 1300 28.757 27,017 71.637 50.855 1400 29.196 29,914 73.784 52.417 1500 29.669 32,857 75.814 53.909 1600 30.178 35,849 77.745 55.339 1700 30.724 38,894 79.591 56.712 1800 31.308 41,996 81.363 58.033 1900 31.932 45,157 83.073 59.306 2000 32.597 48,383 84.727 60.536 2100 33.303 51,678 86.335 61.726 2200 34.053 55,045 87.902 62.880 2300 34.846 58,490 89.432 64.002 2400 35.684 62,016 90.933 65.093 2500 36.568 65,628 92.407 66.156 2600 37.499 69,331 93.859 67.193 2700 38.478 73,130 95.293 68.208 2800 39.506 77,028 96.710 69.200 2900 40.583 81,032 98.115 70.173 3000 41.712 85,147 99.510 71.128 3100 42.892 89,377 100.897 72.066 3200 44.125 93,727 102.278 72.988 3300 45.412 98,203 103.655 73.897 3400 (solid) 46.751 102,811 105.031 74.792 3400 (liquid) 50.000 170,816 125.032 74.792 3500 50.000 175,816 126.482 76.249 3600 50.000 180,816 127.890 77.664 3700 50.000 185,816 129.260 79.040 3800 50.000 190,816 130.594 80.379 3900 50.000 195,816 131.892 81.683 4000 50.000 200,816 133.158 82.954 4100 50.000 205,816 134.393 84.194 4200 50.000 210,816 135.598 85.403 (Continued)

Table VI Continued Hº – Hº b –(Gº – Hº )/ Temperature, K Cº a J mol–1 K–1 T 298.15 K , Sº c J mol–1 K–1 T 298.15 K p , J mol–1 T , Td, J mol–1 K–1 4300 50.000 215,816 136.774 86.585 4400 50.000 220,816 137.924 87.738 4500 50.000 225,816 139.047 88.866 4600 50.000 230,816 140.146 89.969 4700 50.000 235,816 141.222 91.048 4800 50.000 240,816 142.274 92.104 4900 50.000 245,816 143.305 93.139 5000 50.000 250,816 144.306 94.152 5100 50.000 255,816 145.311 95.146 5200 50.000 260,816 146.276 96.120 5300 50.000 265,816 147.229 97.075 5400 50.000 270,816 148.164 98.012 5500 50.000 275,816 149.081 98.933 5600 50.000 280,816 149.982 99.836 a CºP is specific heat b HºT – H298.15 K is enthalpy c SºT is entropy d –(GºT – Hº295.15 K)/T is the free energy functions

Table VII Enthalpies and Entropies of Fusion for the Groups 8 to 10 Elements Enthalpy of fusion, Entropy of fusion, Element Melting point, K Reference J mol–1 J mol–1 K-1 Cobalt 1768 16056 ± 369 9.08 ± 0.21 (9)

Nickel 1728 17042 ± 376 9.86 ± 0.22 (9)

Ruthenium 2606 39040 ± 1400 14.98 ± 0.54 (10) Rhodium 2236 27295 ± 850 12.21 ± 0.38 (11) Palladium 1828.0 17340 ± 730 9.48 ± 0.40 (12) Iridium 2719 41335 ± 1128 15.20 ± 0.41 (13) Platinum 2041.3 22110 ± 940 10.83 ± 0.46 (14)

Table VIII Thermodynamic Properties of the Gaseous Phase Hº – Hº b –(Gº – Hº )/ Temperature, K Cº a J mol–1 K–1 T 298.15 K , Sº c J mol–1 K–1 T 298.15 K p , J mol–1 T , Td, J mol–1 K–1 298.15 20.788 0 192.579 192.579 300 20.788 38 192.707 192.579 400 20.810 2118 198.689 193.394 500 20.901 4203 203.341 194.936 600 21.102 6302 207.168 196.665 700 21.432 8428 210.444 198.404 800 21.887 10,592 213.334 200.093 900 22.453 12,809 215.944 201.712 1000 23.104 15,086 218.342 203.256 1100 23.812 27,431 220.577 204.731 1200 24.545 29,849 222.680 206.140 1300 25.278 22,340 224.674 207.489 (Continued)

Table VIII Continued Hº – Hº b –(Gº – Hº )/ Temperature, K Cº a J mol–1 K–1 T 298.15 K , Sº c J mol–1 K–1 T 298.15 K p , J mol–1 T , Td, J mol–1 K–1 1400 25.988 24,904 226.574 208.785 1500 26.659 27,537 228.390 210.032 1600 27.283 30,234 230.130 211.234 1700 27.854 32,991 231.802 212.395 1800 28.374 35,803 233.409 213.518 1900 28.844 38,665 234.956 214.606 2000 29.269 41,571 236.446 215.661 2100 29.656 44,517 237.884 216.685 2200 30.009 47,501 239.272 217.681 2300 30.337 50,518 240.613 218.649 2400 30.642 53,567 241.911 219.591 2500 30.931 56,646 243.167 220.509 2600 31.207 59,753 244.386 221.404 2700 31.473 62,887 245.569 222.277 2800 31.732 66,047 246.718 223.130 2900 31.986 69,233 247.836 223.962 3000 32.234 72,444 248.925 224.776 3100 32.480 75,680 249.986 225.573 3200 32.722 78,940 251.021 226.352 3300 32.961 82,224 252.031 227.115 3400 33.197 85,532 253.019 227.862 3500 33.430 88,864 253.984 228.595 3600 33.660 92,218 254.929 229.313 3700 33.885 95,596 255.855 230.018 3800 34.107 98,995 256.761 230.710 3900 34.323 102,417 257.650 231.389 4000 34.535 105,860 258.522 232.057 4100 34.742 109,324 259.377 232.713 4200 34.943 112,808 260.217 233.357 4300 35.138 116,312 261.041 233.992 4400 35.327 119,835 261.851 234.616 4500 35.510 123,377 262.647 235.230 4600 35.687 126,937 263.429 235.834 4700 35.858 130,514 264.199 236.430 4800 36.023 134,108 264.956 237.016 4900 36.182 137,719 265.700 237.594 5000 36.325 141,345 266.432 238.164 5100 36.483 144,986 267.153 238.725 5200 36.625 148,641 267.863 239.278 5300 36.762 152,311 268.562 239.824 5400 36.895 155,993 269.251 240.363 5500 37.022 159,689 269.929 240.894 5600 37.145 163,398 270.597 241.419 a CºP is specific heat b HºT – H298.15 K is enthalpy c SºT is entropy d –1 –(GºT – Hº295.15 K)/T is the free energy functions; Hº298.15 K – Hº0 K = 6197.4 J mol

Table IX Enthalpies of Sublimation at 298.15 K Temperature ΔHº (II)c, ΔHº (III)c, Authors Reference Methoda 298.15 K 298.15 K range, Kb kJ mol–1 kJ mol–1 Panish and Reif (19) L 2376–2718 807 ± 35 784.3 ± 1.3 Carrera et al. (20) L 2159–2595 773 ± 13 790.7 ± 0.7 Selected 788 ± 4 aL: Langmuir free evaporation bTemperature ranges corrected to temperature scale ITS-90 c ΔHº298.15 K (II) and ΔHº298.15 K (III) are the Second Law and Third Law enthalpies of sublimation at 298.15 K

Table X Vapour Pressure Equationsa Temperature Phase A B C D E range, K Solid 1700–3400 26.82612 –1.17464 –95030.60 5.68917 × 10–4 –6.25849 × 10–8 Liquid 3400–5600 45.02206 –3.41958 –93542.51 2.64385 × 10–4 –5.78416 × 10–9 a Equation (iii)

Table XI Vapour Pressure a –1 b –1 Temperature, K Pressure, bar ΔGºT , J mol ΔHºT , J mol Pressure, bar Temperature, K 298.15 2.03 × 10–130 740,290 788,000 10–15 1780 300 1.44 × 10–129 739,994 787,992 10–14 1861 400 2.81 × 10–95 724,059 787,554 10–13 1950 500 1.03 × 10–74 708,242 787,058 10–12 2048 600 5.14 × 10–61 692,527 786,535 10–11 2156 700 3.09 × 10–51 676,901 786,007 10–10 2277 800 6.59 × 10–44 661,350 785,487 10–9 2411 900 3.28 × 10–38 645,863 784,989 10–8 2563 1000 1.18 × 10–33 630,430 784,520 10–7 2736 1100 6.23 × 10–30 615,043 784,085 10–6 2934 1200 7.88 × 10–27 599,693 783,687 10–5 3163 1300 3.31 × 10–24 584,375 783,323 10–4 3435 1400 5.86 × 10–22 569,084 782,990 10–3 3792 1500 5.19 × 10–20 553,816 782,680 10–2 4235 1600 2.62 × 10–18 538,568 782,385 10–1 4804 1700 8.32 × 10–17 523,338 782,097 1 5559.70 1800 1.80 × 10–15 508,126 781,807 NBPc 5564.74 1900 2.81 × 10–14 492,929 781,508 – – 2000 3.33 × 10–13 477,749 781,188 – – 2100 3.12 × 10–12 462,586 780,839 – – 2200 2.38 × 10–11 447,439 780,456 – – 2300 1.52 × 10–10 432,312 780,028 – – 2400 8.32 × 10–10 417,204 779,551 – – 2500 3.97 × 10–9 402,117 779,018 – – 2600 1.68 × 10–8 387,052 778,422 – – 2700 6.36 × 10–8 372,012 777,757 – – 2800 2.19 × 10–7 356,998 777,019 – – 2900 6.92 × 10–7 342,011 776,201 – – 3000 2.02 × 10–6 327,054 775,297 – – 3100 5.51 × 10–6 312,129 774,303 – – 3200 1.41 × 10–5 297,237 773,213 – – (Continued)

Table XI Continued a –1 b –1 Temperature, K Pressure, bar ΔGºT , J mol ΔHºT , J mol Pressure, bar Temperature, K 3300 3.39 × 10–5 282,381 772,021 – – 3400 (solid) 7.75 × 10–5 267,563 770,721 – – 3400 (liquid) 7.75 × 10–5 267,563 702,716 – – 3500 1.58 × 10–4 254,788 701,048 – – 3600 3.08 × 10–4 242,061 699,402 – – 3700 5.78 × 10–4 229,380 697,780 – – 3800 1.05 × 10–3 216,742 696,179 – – 3900 1.84 × 10–3 204,146 694,601 – – 4000 3.15 × 10–3 191,590 693,044 – – 4100 5.23 × 10–3 179,073 691,508 – – 4200 8.48 × 10–3 166,593 689,992 – – 4300 1.34 × 10–2 154,148 688,496 – – 4400 2.08 × 10–2 141,739 687,019 – – 4500 3.15 × 10–2 129,363 685,561 – – 4600 4.69 × 10–2 117,018 684,121 – – 4700 6.84 × 10–2 104,706 682,698 – – 4800 9.87 × 10–2 92,422 681,292 – – 4900 0.140 80,169 679,903 – – 5000 0.195 67,943 678,529 – – 5100 0.269 55,745 677,170 – – 5200 0.365 43,574 675,820 – – 5300 0.490 31,428 674,495 – – 5400 0.651 19,307 673,177 – – 5500 0.854 7,210 671,873 – – 5559.70 1.000 0 671,100 – – 5600 1.110 –4,863 670,582 – – a ΔGºT is the free energy of formation at 1 bar standard state pressure and temperature T b –1 ΔHºT is the enthalpy of sublimation at temperature T enthalpy of sublimation at 0 K: ΔHº0 = 786.795 ± 4.000 kJ mol cNBP is the normal boiling point at one atmosphere pressure (1.01325 bar)

55 Fig. 1. The specific 50

–1 heat values of

K ruthenium and

–1 45 osmium at reduced 40 temperature (T/TM)

Specific heat, J mol 25

20 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Reduced temperature, T/TM Ru Os

value for the enthalpy of fusion would suggest that (T/TM) as indicated in Figure 1 are very similar and the thermal properties of osmium should then be show virtually the same behaviour suggesting that distinct from those of the other pgms but this is not they are genuine analogues of each other whilst the case. For example, the specific heat values of the extrapolated melting point of osmium obtained ruthenium (10) and osmium at reduced temperature by applying the same incremental difference as

62 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15898131243119 Johnson Matthey Technol. Rev., 2021, 65, (1) between iridium and platinum agrees closely with 7. G. Ramanauskas, V. D. Tarasov, V. Ya. Chekhovskoi, the selected value and again suggesting a common N. L. Korenovskii and V. P. Polyakova, Vysokochist. Groups 8 to 10 behaviour. Veshchestva., 1988, (4), 149, in Russian Further, the chemical properties of ruthenium and 8. F. M. Jaeger and E. Rosenbohm, Proc. R. Acad. osmium are virtually identical forming the same Amsterdam, 1931, 34, (1), 85 type of compounds with similar properties. These 9. S. Stølen and F. Grønvold, Thermochim. Acta, 1999, 327, (1–2), 1 are examples where osmium behaves exactly like 10. J. W. Arblaster, Calphad, 1995, 19, (3), 339 the other pgms and on these grounds it is suggested 11. J. W. Arblaster, Calphad, 1995, 19, (3), 357 that the very low value for the enthalpy of fusion as 12. J. W. Arblaster, Johnson Matthey Technol. Rev., suggested by Fokin et al. is inconsistent with this 2018, 62, (1), 48 behaviour and that osmium would obey the same 13. J. W. Arblaster, Calphad, 1995, 19, (3), 365 periodic trend as suggested by the other pgms and 14. J. W. Arblaster, Platinum Metals Rev., 2005, 49, that its entropy of fusion can be determined by the (3), 141 Chekhovskoi-Kats equation. This would suggest 15. V. Ya. Chekhovskoi and S. A. Kats, High Temp.– anomalies in the input values selected by Fokin et High Pressures, 1981, 13, (6), 611 al., especially in the selection of Z = 2 for osmium 16. Th. A. M. Van Kleef and P. F. A. Klinkenberg, since the value for the analogue ruthenium is only Physica, 1961, 27, (1), 83 1.5 whilst the value for the neighbouring element 17. G. G. Gluck, Y. Bordarier, J. Bauche and Th. iridium is only 1.2 where the selection of such A. M. Van Kleef, Physica, 1964, 30, (11), 2068 values would lead to higher enthalpies of fusion 18. H. G. Kolsky, R. M. Gilmer and P. W. Gilles, for osmium. It is suggested that in view of the “The Thermodynamic Properties of 54 Elements lack of any real correlation for Z that the value for Considered as Ideal Monatomic Gases”, LA 2110, osmium may well be independent and could even US Atomic Energy Commission, Washington, USA, be 1.0 leading to an enthalpy of fusion similar to 15th March, 1957, 138 pp that obtained from the Chekhovskoi-Kats equation. 19. E. Tiesinga, P. J. Mohr, D. B. Newell and B. N. Taylor, Therefore until the actual enthalpy of fusion of ‘The CODATA Internationally Recommended 2018 osmium is determined it is assumed that it behaves Values of the Fundamental Physical Constants’, as a normal Groups 8 to 10 element. NIST Standard Reference Database 121, Version 8.0, National Institute of Standards and Technology, Gaithersburg, USA, May, 2019 Conclusions 20. M. B. Panish and L. Reif, J. Chem. Phys., 1962, 37, (1), 128 Estimated entropy and enthalpy values of fusion of 21. N. J. Carrera, R. F. Walker and E. R. Plante, J. Res. osmium have been revised leading to corrections of Nat. Bur. Stand., 1964, 68A, (3), 325 the thermodynamic properties of the liquid phase 22. L. R. Fokin, E. Yu. Kulyamina and V. Yu. Zitserman, and therefore to the vapour pressure curve above High Temp., 2019, 57, (1), 54 the melting point. The revisions are based on the 23. J. W. Arblaster, Calphad, 1996, 20, (3), 343 assumption that osmium behaves as a normal 24. M. G. Frohberg, Thermochim. Acta, 1999, 337, Group 8 to 10 element and contradicts recent (1–2), 7 suggestions that its behaviour could be abnormal. 25. P.-F. Paradis, T. Ishikawa and N. Koike, J. Appl. Phys., 2006, 100, (10), 103523 References

1. J. W. Arblaster, Calphad, 1995, 19, (3), 349 The Author 2. J. W. Arblaster, Platinum Metals Rev., 2005, 49, John W. Arblaster is interested (4), 166 in the history of science and the 3. L. Burakovsky, N. Burakovsky and D. L. Preston, evaluation of the thermodynamic Phys. Rev. B, 2015, 92, (17), 174105 and crystallographic properties 4. ‘Standard Atomic Weights’, Commission on Isotopic of the elements. Now retired, Abundances and Atomic Weights (CIAAW), 2019 he previously worked as a 5. A. M. Okaz and P. H. Keesom, Physica, 1973, 69, metallurgical chemist in a number (1), 97 of commercial laboratories and 6. V. N. Naumov, I. E. Paukov, G. Ramanauskas and V. Ya. Chekhovskoi, Zh. Fiz. Khim., 1988, 62, was involved in the analysis of a (1), 25, translated into English in Russ. J. Phys. wide range of ferrous and non- Chem., 1988, 62, (1), 12 ferrous alloys.

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Comprehensive Review on High Hydrogen Permselectivity of Palladium Based Membranes: Part I Research progress of concentration polarisation phenomena

Hasan Mohd Faizal** through a palladium based membrane is governed Automotive Development Centre, School of by Sieverts’ Law and quantified with Fick’s First Mechanical Engineering, Faculty of Engineering, Law. Since the 20th century, the fabrication of Universiti Teknologi Malaysia, 81310 UTM Johor high-performance palladium based membrane Bahru, Johor, Malaysia; School of Mechanical for enhanced hydrogen recovery performance Engineering, Faculty of Engineering, Universiti has become practical. However, along with the Teknologi Malaysia, 81310 UTM Johor Bahru, improvement in hydrogen recovery performance, Johor, Malaysia concentration polarisation becomes unavoidable because hydrogen permeation flux starts to affect Bemgba B. Nyakuma hydrogen concentration at the membrane surface. School of Chemical and Energy Engineering, Various parametric studies have investigated the Faculty of Engineering, Universiti Teknologi effects of membrane thickness, hydrogen molar Malaysia, 81310 Skudai, Johor Bahru, Malaysia fraction and total upstream and downstream Mohd Rosdzimin Abdul Rahman* pressures on concentration polarisation level. The influence of membrane temperature, permeability, Department of Mechanical Engineering, Faculty type and number of species in the hydrogen of Engineering, Universiti Pertahanan Nasional mixture, diffusivity of the hydrogen mixture, Malaysia, Kem Sg. Besi, 57000, Kuala Lumpur, system configurations and flow patterns are also Malaysia reported and comprehensively reviewed in this Md. Mizanur Rahman, N. B. paper. Part II will complete the presentation. Kamaruzaman, S. Syahrullail School of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 1. Introduction and Background 81310 UTM Johor Bahru, Johor, Malaysia Hydrogen purification by palladium-based Email: *[email protected]; membrane is one of the most well-known **[email protected] techniques for supplying high purity hydrogen to low temperature operated (1) PEFC in various electronic devices such as tablets, laptop computers and small Palladium based membranes are widely used vehicles. The compact methanol steam reformer for supplying ultra-high purity hydrogen to a that consists of a catalytic burner, reformer and polymer electrolyte fuel cell (PEFC) installed on hydrogen purification device in a single package small vehicles and various electronic devices. was developed almost two decades ago (2). Over Compared to pressure swing adsorption (PSA), the years, various integrated systems, operating the use of palladium based membrane is more conditions and geometries have been investigated economical for small size (small capacity) to obtain the optimum operating conditions for applications. The transportation of hydrogen reliable performance (3–9). As a result, the compact

64 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15814149544965 Johnson Matthey Technol. Rev., 2021, 65, (1) reformer has several advantages (i.e. simple and permeability. However, this type of membrane is compact) compared to complex systems with not commercialised due to the expensive processes separated units (10). required to convert the palladium-yttrium alloy into Due to the difficulty and high risk of exposure the functional separation membrane (28). to accidents, the transportation and storage of Hydrogen permeation through palladium based gaseous hydrogen are undesirable. Alternatively, membranes is based on the solution-diffusion the utilisation of alcohols is more practical due mechanism (29). When a membrane with to their existence in a liquid form at ambient sufficient thickness is operated at sufficiently high conditions. Methanol for instance has a relatively temperature, the diffusion of hydrogen atoms high hydrogen:carbon ratio and moderate reaction through the metallic lattice becomes dominant, temperature compared to other alcohols (10, 11). thus permeation flux can be estimated accurately Based on the two step equations of methanol by using Sieverts’ Law (30, 31) that is quantified in steam reforming, the reaction between vaporised Fick’s First Law as follows, Equation (i): methanol and steam produces hydrogen and carbon f = q (√P – √P ) (i) dioxide along with carbon monoxide. However, the H2,1 H2,2 d concentration of carbon monoxide in the electrode of PEFC must be equal to or less than 10 ppm (12). where f is the hydrogen permeation mole flux, q Otherwise, the carbon monoxide could deteriorate is the hydrogen permeance coefficient, d is the the electrode performance by reducing the active membrane thickness, PH2,1 is the hydrogen partial surface area for reaction and lowering the partial pressure at membrane surface of upstream (or pressure of hydrogen (13–15). retentate) side and PH2,2 is the hydrogen partial Hydrogen purification by palladium based pressure at membrane surface of downstream (or membrane is preferable rather than selective permeate) side. The Sieverts’ equation as shown carbon monoxide methanation and selective carbon by Equation (i) states that hydrogen permeation is monoxide oxidation (16), in which very high purity governed by the difference in the square root of the of hydrogen and very high permeation flux can be hydrogen partial pressure between the upstream obtained (17–19). PSA is an alternative technique to and downstream side. produce very high purity hydrogen (20). However, Membrane temperature (22, 32) and membrane the high costs of installing PSA is not economical thickness (33–35) are among the important for small size (or small capacity) applications (21). parameters that determine compliance with In addition to the well-known inhibitive carbon Sieverts’ Law. When the temperature of the monoxide produced from the aforementioned membrane is sufficiently high, the adsorption and methanol steam reforming, carbon dioxide and dissociation of hydrogen atoms at the membrane excessive methanol from the same reaction (22), surface are very fast. Therefore, the diffusion hydrogen sulfide (23, 24) and trace amounts of of hydrogen atoms through the metallic lattice ammonia (20, 25) from coal gasification process, becomes a controlling step for permeation. In this dehydrogenated methanol and ethanol (26) also case, the hydrogen permeation flux is found to be affect adversely the performance of palladium linear with respect to the difference in the square membranes through its inhibitive mechanism. root of hydrogen partial pressures between the Alloying palladium with other metals such as upstream and downstream side. For the case of silver and copper is necessary during membrane the palladium/copper membrane, Ma found that fabrication to prevent embrittlement when the Sieverts’ Law is valid only when the membrane membrane is used under hydrogen atmosphere temperature is set above 573 K (32), whereas and below 573 K and 2 MPa (10). It was found for the palladium/silver membrane, the law can that the maximum amount of permeated hydrogen be applied even if the temperature is lower than is obtained when the silver content is 23% (27). 500 K (22). In addition, the palladium/silver membrane shows The different findings by several groups of better performance compared to the palladium/ researchers had proven that there is no exact copper membrane from the viewpoint of hydrogen value of thickness that can be set as a limit for permeability. Consequently, there is growing compliance with Sieverts’ Law. Ward and Dao (33) interest in the palladium/silver membrane. and Federico et al. (34) found that the membrane In addition to these two types of alloys, the thickness should be higher than 10 µm in order to palladium-yttrium membrane has also been apply the Sieverts’ equation (Equation (i)). Other previously examined due to its superior hydrogen research groups discovered that the Sieverts’ Law

65 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15814149544965 Johnson Matthey Technol. Rev., 2021, 65, (1) is still valid even though the membrane thickness is surface, thus could trigger the phenomenon of below 10 µm (22, 35). These contrary findings are concentration polarisation. supposed to be caused by difficulty in quantifying The concentration polarisation phenomenon causes various uncontrolled factors such as surface the accumulation of the less permeable species processes (36), surface poisoning (37) and grain and the depletion of the more permeable species boundaries (38). in the boundary layer adjacent to the membrane, For the case of pure hydrogen, hydrogen thus generating a concentration gradient in the permeation is found to follow the correlation of boundary layer (42). Therefore, in such situation, Sieverts’ equation regardless of feed flow rate. an additional elementary step is essential for the However, for mixtures, when the feed flow rate of solution-diffusion mechanism of the membrane. hydrogen becomes sufficiently high, the hydrogen This involves the transportation of molecular permeation ratio (fraction of fed hydrogen hydrogen from (to) the bulk gas phase to (from) the that permeates membrane) (39) becomes low. gas layer adjacent to the surface at the upstream Therefore, the hydrogen permeation flux can (downstream) side (33). As a consequence, if the be predicted accurately by Sieverts’ equation inlet hydrogen partial pressure is directly substituted

(Equation (i)). Further, the term PH2,1 in Equation into the Sieverts’ equation (Equation (i)), the (i) can be predicted from the bulk value of hydrogen hydrogen partial pressure at the membrane surface mole fraction at the upstream side, which indicates of upstream side is overestimated, which causes a that hydrogen partial pressures at the membrane significant deviation from the actual permeation flux. surface and the bulk flow are uniform, as illustrated Chen et al. observed that such deviation implies the by Line 1 in Figure 1. level of concentration polarisation for a palladium When a mixture of hydrogen with relatively based membrane (41). Based on the analytical low feed flow rate (or high permeation ratio) is study for multicomponent hydrogen mixtures, used, the direct substitution of inlet hydrogen Caravella et al. confirmed that such deviation is partial pressure (PH2,in) value into the Sieverts’ caused by the effect of multicomponent external equation causes an overestimation of PH2,1 from mass transfer (such as concentration polarisation) the actual permeation flux. This indicates there in addition to non-ideal diffusion through the is a nonuniformity of hydrogen mole fraction in membrane (43). the boundary layer near to membrane surface, Technological advances in membrane materials, as illustrated by Line 2 in Figure 1. In this case, modification and fabrication (42) since the end of several research groups (39–41) mentioned that the 20th century have stimulated the fabrication the hydrogen permeation flux could start to affect of very thin membranes in order to substantially the hydrogen concentration at the membrane improve the permeation performance. Therefore, the phenomenon of concentration polarisation could not be avoided due to the use of membranes Upstream side Downstream side with minimal thickness. However, some Inlet side researchers (44) have recommended installation Membrane of baffles in the membrane reactor to decrease P H2,1 A the polarisation effect. For palladium based Line 1 P membranes, concentration polarisation has been extensively investigated in the past decade (40) Line 2 B although concentration polarisation had been investigated in the 1990s for separation of a P H2,1 gas-vapour mixture (45). Therefore, significant Pressure research interest in the theoretical understanding of the phenomenon has resulted in numerous P x H2,2 methods for estimating hydrogen flux for such conditions. Bulk side Boundary layer Therefore, a comprehensive review of the transport Near to membrane phenomena for palladium based membranes surface is performed, particularly on concentration Fig. 1. Schematic diagram of hydrogen partial polarisation. In addition to the background of the pressure profile scenario related to palladium based membranes already highlighted, current information on various

66 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15814149544965 Johnson Matthey Technol. Rev., 2021, 65, (1) parametric studies and theoretical approaches for and technological advances on the transport predicting hydrogen permeation flux under the phenomena of palladium based membranes. It also influence of concentration polarisation are covered. presents the various prediction methods applicable Therefore, this review presents critical scientific to hydrogen permeation under the influence of knowledge and current research on concentration concentration polarisation that could serve as a polarisation. The coverage of the present review is future reference for researchers and industrial significantly different from the published works of practitioners. Adhikari and Fernando (46), Rei (47), Gallucci et al. (21), Al-Mufachi et al. (28), Li et al. (48), Conde 2. Factors Affecting Concentration et al. (49) and Peters and Caravella (50). Adhikari Polarisation in Palladium Based and Fernando comprehensively reviewed the Membranes classification of hydrogen purification membranes, along with the advantages and disadvantages of In this section, a review of case studies on each type of membrane (46). The study emphasised concentration polarisation is presented. The various the superior quality of palladium based membranes studies and types of membrane used for each study in producing ultra-high purity hydrogen due to very are presented in Table I, whereas the parameters high selectivity (46). Similarly, Rei (47) reviewed considered for studying such phenomena are listed the advances in permeation through palladium in Table II. Based on Table I, it is evident that most based membranes for the case of a hydrogen common membranes are tubular, as illustrated by mixture based on case studies in Taiwan. Several the various configurations in Figures 2(a)–2(c). discoveries on new phenomena of hydrogen The studies by Faizal et al. (55, 60) examined the permeation such as perturbation of hydrogen phenomenon of concentration polarisation for flat permeation due to palladium lattice expansion and sheet type membranes, which are widely used in hydrogen spillover in the membrane reactor have compact reformers for hydrogen production (4–5, been described (47). Gallucci et al. also highlighted 65–66). The common configuration for a flat sheet the problem of concentration polarisation in the type membrane is illustrated by Figure 2(d). It membrane reactor, although this was not the main is interesting to note that various configurations topic of the study (21). The authors mainly focused and fluid flow conditions have been considered on the route of commercialisation and application for studying concentration polarisation. Based of various types of membranes (21). The review of on Table II, it is evident that the most common several palladium alloy membranes was presented varied parameters for concentration polarisation by Al-Mufachi et al. (28). The paper highlighted are operating pressure, hydrogen mixture the advantages and disadvantages of each type composition, feed flow rate and Reynolds number as of membrane in terms of hydrogen permeability, well as membrane temperature. However, several tensile strength and fabrication costs (28). studies have focused on geometry improvement to Subsequently, Li et al. reviewed the thermal and suppress concentration polarisation, as elaborated chemical stability of palladium based membranes in this section. which are considered the two most critical issues In the early 21st century, Hou and Hughes were for the commercialisation of the membranes (48). among the earliest groups who observed the In 2017, a review on palladium based membranes concentration polarisation during an experiment was performed by Conde et al. (49). The paper involving hydrogen permeation with a palladium presented a review of the alloying elements for based membrane (67). The authors confirmed palladium based membranes and their effect on the existence of the concentration polarisation the membrane properties. Finally, a most recent phenomenon during their experiment for a overview by Peters and Caravella (50) has covered membrane with a thickness of 5 µm to 6 µm. the scopes of manufacturing process of palladium However, the effect was not severe due to the membranes, membrane materials, membrane relatively high feed gas velocity, which was a 5% modules and reactor design, as well as applications decrease in hydrogen concentration for the various of palladium based membranes. mixing ratios of the binary mixture of hydrogen Based on these published review articles, it can and nitrogen (67). be said that the subject of transport phenomena, Zhang et al. (51) were one of the pioneer particularly on the concentration polarisation, groups who comprehensively investigated is the novelty for the present review. This paper the concentration polarisation phenomenon presents coverage of recent research features specifically for palladium based membranes. The

Table I Types of Study and Membranes Used for Investigation on Concentration Polarisation Phenomena Type of study for Membrane No. References concentration Thicknessa, Type polarisation µm Experiment and Porous ceramic tube supported palladium 1. Zhang et al. (51) – modelling membrane Palladium/20 wt% silver tubular Experiment and 2. Pizzi et al. (52) membranes deposited on ceramic 2.5 analytical supports Catalano et al. Experiment and Palladium/20 wt% silver tubular 3. 2.5 (40) analytical membrane with ceramic support Caravella et al. Tubular type self-supported palladium 4. Modelling and analytical 1–150 (53) based membrane Coroneo et al. Simulation and Tubular palladium/silver membrane 5. 3 (44) experiment deposited on tube Caravella et al. Self-supported tubular palladium based 6. Modelling and analytical 60 (54) membrane Self-supported tubular type palladium 7. Chen et al. (39) Numerical simulation – based membrane Self-supported tubular type palladium 8. Chen et al. (41) Numerical simulation – based membrane Experiment and Self-supported circular flat sheet type 9. Faizal et al. (55) 25 analytical palladium/23 wt% silver membrane Self-supported tubular palladium 10. Chen et al. (56) Numerical simulation – membrane Palladium and palladium/copper 11. Chen et al. (57) Experiment membrane with porous stainless steel 6.5–7.0 support Self-supported tubular palladium 12. Chen et al. (58) Numerical simulation – membrane Nekhamkina and Self-supported tubular palladium 13. Analytical – Sheintuch (59) membrane Palladium/copper tubular membrane with 14. Zhao et al. (23) Experiment 5 ceramic substrate Experiment and Self-supported circular flat sheet type 15. Faizal et al. (60) 25 analytical palladium/23 wt% silver membrane Nakajima et al. Experiment and Tubular palladium/silver membrane with 16. – (61) numerical ceramic support Analytical and Caravella and Sun simulation (for case Self-supported tubular palladium based 17 – (62) study of water-gas membrane shift reaction) Palladium layer deposited on yttria stabilised zirconia (YSZ) support 11 18. Kian et al. (63) Experiment (tubular) Palladium/gold layer deposited on YSZ 8 deposited on Al2O3 substrate (tubular) Pd Ag based tubular membrane Simulation and 0.85 0.15 19. Helmi et al. (64) supported on Al O porous in fluidised 4.5 experiment 2 3 membrane reactor a Sign – indicates not available effect of various parameters such as pressure, modelling particularly for tubular membranes with temperature, feed gas flow rate and permeability porous ceramic supports. The authors found that was investigated experimentally. Also, the when the feed gas flow rate is increased, the parameters were interpreted through mathematical concentration polarisation is weakened, resulting

Table II Varied Parameters for Studying the Effect on Concentration Polarisation

Reference Parameters Feed flow rate: 0–5.1 × 10–5 m3 s–1. Pressure: 101.3–405.3 kPa (difference in total Zhang et al. (51) pressure). Temperature: 623–773 K Pressure: 20–600 kPa (difference in total pressure). Inlet H concentration: 88 vol% Pizzi et al. (52) 2 and 50 vol% Feed flow rate (m3 s–1): (1.67–5.00) × 10–5 m3 s–1 (at normal condition). Pressure: up Catalano et al. (40) to 600 kPa (difference in total pressure). Inlet 2H concentration: 50 vol% and 88 vol%. Temperature: 673–773 K. Binary (H2:N2, H2:CH4) and ternary mixtures (H2:N2:CH4) Membrane thickness: 1–150 µm. Permeance: 0.1–20 mmol m–2 s–1 Pa–0.5. Reynolds Number: 2100–8000. Upstream total pressure: 200–1000 kPa. Downstream total Caravella et al. (53) pressure: 100–800 kPa. Inlet H2 concentration: 0–1 molar fraction. Temperature: 573–773 K Coroneo et al. (44) 0, 2 and 3 (number of baffles) Total upstream pressure: 400–1000 kPa. CO partial pressure: 0–1000 kPa. Inlet H Caravella et al. (54) 2 concentration: 0–1 molar fraction. Ternary (H2:CO:N2) and binary mixture (H2:CO) Permeance: 10–3–1 mmol m–2 s–1 Pa–0.5. Reynolds number: 20–2000. Pressure: Chen et al. (39) 506.5–3039 kPa. Inlet H2 concentration: 0.20–0.80 molar fraction Feed flow rate: 2.713 × 10–4–4.3408 × 10–3 mol s–1. Reynolds number: 10–50 Chen et al. (41) (retentate side) and 2–2000 (permeate side). Flow pattern: countercurrent and cocurrent modes. Position of the feed flow: in lumen or shell side Feed flow rate: 1.489 × 10–5– 2.976 × 10–4 mol s–1. Upstream total pressure: 200– Faizal et al. (55) 300 kPa Reynolds number: 20–2000 (permeate side) and 20–800 (retentate). Pressure Chen et al. (56) difference: 506.5–3039 kPa. Shell diameter: 25–100 mm Feed flow rate: 1.67–3.33 × 10–6 m3 s–1. Pressure: 50.7–405.2 kPa (H partial Chen et al. (57) 2 pressure difference). Inlet 2H concentration: 50–100 vol% Chen et al. (58) Baffle patterns, positions at shell wall, ratio of baffle length to radius (0–0.75) 0.5 Nekhamkina and Pressure: 31.62–632.46 Pa (initial driving force). Inlet H2 concentration: 0.50–0.88 Sheintuch (59) (molar fraction). Separation parameter, Γ(<29) Feed flow rate: 4.17 × 10–6–4.00 × 10–3 m3 s–1. Inlet H concentration: 0.50–0.90 Zhao et al. (23) 2 (molar fraction). Temperature: 673–773 K. H2S concentration: 7–35 ppm Feed flow rate: 2.78 × 10–5–2.50 × 10–4 mol s–1. Inlet H concentration: 0.70–0.80 Faizal et al. (60) 2 (molar fraction). Various species in H2 mixture (H2:N2, H2:Ar, H2:He and H2:CO2) Feed flow rate: 5.0 × 10–4–1.5 × 10–3 Nm3 s–1 m–2. Internal diameter of reactor Nakajima et al. (61) vessel: 1.66–2.39 × 10–2 m Total upstream pressure: 150–600 kPa. Various ternary and quaternary mixtures as well as a senary mixture → H :Ar, H ,He,H :CH , H :H O, H :CO:He (example of ternary Kian et al. (63) 2 2 2 4 2 2 2 mixture), H2:CO2:CO:He (example of quaternary mixture), H2:CO2,H2O:CH4:CO:He. Gas hourly space velocity (GHSV): 221–882 h–1. Flow rates: 276–1078 ml min–1

Helmi et al. (64) Relative fluidisation velocity: 1.3–3.3. 2H mole fraction: 0.1, 0.25 and 0.45 in higher hydrogen permeation. For instance, for that the observed phenomena are mainly due to the case of membrane temperature of 723 K, the higher removal rate of accumulated nitrogen in when the feed flow rate was 5 ml –1s (equivalent the boundary layer at higher feed flow rates (51). to 5 × 10–6 m3 s–1), the concentration polarisation However, an increase in pressure at the retentate degree for hydrogen (ratio of hydrogen permeation or upstream side (at constant permeated pressure) flux with concentration polarisation to hydrogen increases concentration polarisation, as clearly permeation flux without concentration) was described by the mathematical modelling developed around 0.54. However, when the feed flow rate in the study. Based on their study, at constant was increased to around 14 ml s–1 (equivalent to temperature of 723 K, for the case of pressure of 14 × 10–6 m3 s–1), the concentration polarisation 2 atm (equivalent to 202.6 kPa), the concentration degree for hydrogen became 1, that is no effect of polarisation degree for hydrogen already reached a concentration polarisation on hydrogen permeation value of 1 (no effect of concentration polarisation) flux was found. Furthermore, the authors reported when the feed flow rate was around 14 ml–1 s

degree for hydrogen still not reached value of (a) 1 even though feed flow rate was increased to Upstream/shell –1 –6 3 –1 Downstream/lumen side 32 ml s (equivalent to 32 × 10 m s ). Based side on the model, the observed trend is related Retentate Feed/upstream to the proportional relation between the mass flow flow transfer coefficient of the retentate side and the diffusion coefficient, which is reciprocal to Membrane Permeate/ operating pressure (51). Therefore, the findings of downstream Zhang et al. corroborated the previous findings by flow Morguez and Sanchez (68), which reported that the effect of selectivity is less significant compared to (b) feed gas flow rate, pressure and permeability (68). Permeate/downstream The authors also observed that the temperature flow range used in their experiment did not trigger the Feed/upstream flow concentration polarisation phenomenon. However, Retentate Membrane the authors did not rule out the possibility of flow concentration polarisation when the hydrogen permeation rate is enhanced due to the increase in Upstream/lumen side temperature (51). Downstream/shell side Pizzi et al. performed an experimental study for ultra-thin (~2.5 µm thickness) palladium/silver (c) membranes deposited on ceramic supports. The Permeate/downstream flow authors observed that pronounced concentration Feed/upstream polarisation occurred regardless of the composition flow mixture (52). As a result, the phenomenon was

Retentate evident despite the concentration of nitrogen in flow a binary mixture of H2:N2 being relatively very low (12 vol%) (52). For this case, it was found Tubular membrane that when Sieverts’ driving force is set to 0.8 bar0.5 (equivalent to 253 Pa0.5), the hydrogen permeate (d) flux is only 0.26 mol –1s m–2, that is significantly Feed/upstream flow lower if compared to the permeate flux obtained when the concentration polarisation effect is negligible (permeate flux of 0.68 mol –1s m–2). Retentate The lack of extensive studies on the detailed flow mechanism of concentration polarisation specifically Upstream side for palladium based membranes prompted Membrane Catalano et al. (40) to explore this research area. Downstream side The findings of Catalano et al. demonstrated a similar trend with that of Zhang et al. (51) in terms of the effect of feed flow rate, and Pizzi et al. (52) in terms of the effect of mixture composition on the Permeate/downstream flow concentration polarisation phenomena. Compared to previous research, a fundamental investigation

Fig. 2. (a) Tubular type membrane configuration on the ternary mixture of H2:N2:CH4 (volume ratio (feed flow is issued through shell side); (b) of 50:25:25) was also conducted for the first time. tubular type membrane configuration (feed flow The findings showed that the permeation flux was is issued through lumen side); (c) tubular type marginally less than the flux for the binary mixture membrane with ‘finger-like’ configuration (front of H :N (volume ratio of 50:50) but almost similar view); (d) circular flat sheet type membrane 2 2 configuration (front view) to the H2:CH4 (volume ratio of 50:50) mixture. Therefore, the findings reveal that there was a very slight decrease in permeation flux, which occurred (equivalent to 14 × 10–6 m3 s–1). However, for when nitrogen was replaced with methane (40). the case of higher pressure of 4 atm (equivalent In this case, previous researchers have confirmed to 405.2 kPa), the concentration polarisation that the inhibitive effect caused by methane is very

70 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15814149544965 Johnson Matthey Technol. Rev., 2021, 65, (1) minimal, thus can be neglected (69). Based on the severity of concentration polarisation increases findings of Catalanoet al. (40) and Jung et al. (69), when the temperature and permeance are it is evident that the severity level of concentration increased whereas the total downstream pressure polarisation is independent on number of species and membrane thickness are reduced (53). As (binary or ternary) in the non-inhibitive hydrogen example, when Reynolds number, hydrogen mixture. Catalano et al. also have interpreted retentate molar fraction, temperature, pressure at the level of concentration polarisation for various retentate side and pressure at permeate side were operating conditions using a dimensionless set to 2100, 0.40, 500°C, 1000 kPa and 200 kPa, polarisation number, which is defined as the ratio respectively, the CPC increased significantly from of gas phase to membrane sensitivity factor (40). around 0.13 to around 0.65 (thus concentration The polarisation number of much higher than polarisation effect become stronger) when 1 (S>>1) means concentration polarisation is membrane thickness was reduced from 50 µm dominant, whereas when the number is much to 5 µm. Similar to the assertion by Morgues et less than 1 (S<<1), resistance by the metal al. (68), Caravella et al. (53) also found that the membrane controls the entire permeation process. hydrogen flux itself plays a significant role during The authors discovered that for both cases of concentration polarisation. ternary and binary hydrogen mixtures with feed Caravella et al. extended their analytical study to flow rates of 1 Nl min–1 to 3 Nl min–1 and relatively cover the concentration polarisation phenomena low inlet hydrogen concentration (50 vol% H2), for a hydrogen mixture that contains well-known the concentration polarisation becomes dominant, inhibitive species of carbon monoxide (22, 71–76). that is S>>1. However the value of S is reduced In this study (54), the authors simultaneously when hydrogen concentration or feed flow rate considered the effects of concentration polarisation is increased. As example, for the case of binary and inhibition by carbon monoxide by merging mixture of H2:N2 (50 vol% H2 and 50 vol% N2) with their previously introduced approach (53) and operating temperature and total pressure of 673 K the approach by Barbieri et al. (77). Similar to and 600 kPa, respectively, when the feed flow their previous study (53), the authors introduced rate was increased from 1 Nl min–1 to 3 Nl min–1, a parameter so-called permeation reduction S reduced significantly from 6 to 2.5. coefficient (PRC) that includes both polarisation and Most of the studies on concentration polarisation carbon monoxide inhibitive effects simultaneously. elaborated previously used palladium based Interestingly, it was found that when the membrane with support that influences the polarisation and carbon monoxide inhibition occur hydrogen permeation process (70). Conversely, at the same time, the hydrogen permeation flux Caravella et al. (53) examined the concentration obtained is lower compared to the flux obtained polarisation phenomenon on self-supported when both phenomena occurred separately. This palladium-based membrane in which case the is mainly due to the polarisation of the inhibitor effect of support was eliminated. In addition carbon monoxide toward the membrane surface, to the previous studies, other researchers (40, which increases the carbon monoxide partial 51–52), have analysed the broader range of pressure at the surface (54). The researchers upstream hydrogen molar fraction, total upstream found that for binary mixture of hydrogen and pressure, downstream total pressure, operating carbon monoxide, when operating temperature, membrane temperature, membrane thickness and membrane thickness, Reynolds number, hydrogen permeability to develop polarisation maps as a very upstream molar fraction, total upstream pressure useful guide for membrane reactor designer. The and downstream pressure are set to approximately term concentration polarisation coefficient (CPC) 647 K, 60 µm, 1200, 0.60, 1000 kPa and 200 kPa, was introduced in the maps as a demonstration for respectively, the permeating flux for the cases of the level of concentration polarisation. Here, when polarisation only, inhibition by carbon monoxide the value of CPC is 0, it means no polarisation only, and simultaneous polarisation and inhibition occurs while the value of 1 indicates the occurrence are around 85 mmol m–2 s–1, 71.5 mmol m–2 s–1 of total or maximum polarisation. Additional and 67 mmol m–2 s–1 (54). The inhibition of carbon parameters were considered in this study that were monoxide under the influence of concentration not covered by the other previous researchers, polarisation is expected since the low feed flow namely: operating temperature, permeance, total rate is generally applied, for instance, during downstream pressure and membrane thickness. steam reforming for application in small portable Furthermore, the analysis demonstrated that the electrical devices (8, 66).

The phenomenon of concentration polarisation various parameters above is generally similar to was featured by the two-dimensional numerical the values described quantitatively by previous method for tubular type (39) and flat sheet experimental and analytical studies (40, 51–54). type (78) membranes. In general, for permeation Chen et al. introduced an important parameter with tubular type membrane under concentration called hydrogen permeation ratio (HPR) to indicate polarisation influence, the direction of hydrogen the level of concentration polarisation (39). The concentration decrease for binary mixture of H2:N2 HPR is defined as the ratio of hydrogen permeation is from the region around the leading edge (inlet rate across the membrane to the hydrogen feed part) to the tailing edge (outlet part) (39). This rate at the inlet. The authors concluded that a phenomenon is featured by Figure 3, in which decrease in value of HPR indicates that the severity dimensionless hydrogen concentration gradient of concentration polarisation is diminished. As an decreases from the leading edge to the tailing edge example, for the case of binary mixture of H2:N2 of the membrane surface regardless of hydrogen (H2 mole fraction of 0.50) with pressure difference volume percentage. However, for the case with and membrane permeance of 30 atm and 10–4 mol vertical flow towards flat sheet type membrane m–2 s–1 Pa–0.5, respectively, HPR decreased from surface, the hydrogen concentration is highest 96 to 3 when Reynolds number was increased at the centre of the membrane, and decreases in from 20 to 2000, thus severity of concentration radial direction, as shown by Figure 4 (78). These polarisation is significantly reduced (39). It is studies investigated the hydrogen concentration interesting to note that when the aforementioned distribution around the membrane surface for four important parameters (operating pressure, various important parameters such as operating hydrogen molar fraction, feed flow rate and pressure, hydrogen molar fraction, feed flow rate membrane permeance) were set in such a way (or Reynolds number) and membrane permeance. to cause the effect of concentration polarisation The qualitative simulated results reveal that to become very significant, the hydrogen the hydrogen concentration decreases at the concentration gradient is very high at the leading membrane surface due to the effect of hydrogen edge of the membrane (in the region near to flux itself during the phenomena. However, the the inlet) and then decays faster. Due to this severity level of concentration polarisation for the phenomenon, there was almost no driving force for

100 1.1 X /X = 1 H2,1 H2,in(ref) 1.0

–1 [–] 10 0.9 in(ref) , mole fraction, mole fraction, 2 2 H

X 0.8 / ,1 concentration gradient concentration 2 2 H

X Inlet –2 0.7 10 radius 0.19 mol s–1 m–2 20 vol% H2 0.38 mol s–1 m–2 40 vol% H2 Normalised H –1 –2 60 vol% H2 0.6 0.57 mol s m 80 vol% H2 0.76 mol s–1 m–2 100 vol% H2 0.95 mol s–1 m–2

Dimensionless H 0.5 10–3 0 0.2 0.4 0.6 0.8 1.0 0 0.05 0.10 0.15 0.20 Normalised radial distance, r/rmax[–] Z, m Fig. 3. Dimensionless hydrogen concentration Fig. 4. Radial profile of hydrogen concentration on gradient from the leading edge (Z = 0.025 m) to the membrane surface as a function of mean mole the tailing edge (Z = 0.175 m) of the membrane flux (feed flow rate divided by effective membrane surface, for the case of Reynolds number of 200, surface area) for the case of inlet hydrogen mole pressure difference of 30 atm and permeance of fraction of 0.75, membrane temperature of 623 K, 10–3 Reprinted from (39) Copyright (2011), with total upstream pressure of 0.25 MPa and total permission from Elsevier downstream pressure of 0.10 MPa (78)

72 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15814149544965 Johnson Matthey Technol. Rev., 2021, 65, (1) permeation in most of the remaining membrane the case of countercurrent mode with the use length, as demonstrated by Figure 5 (refer to of sweep gas in the shell side (outside tubular the case of permeance (K) of 10–3) (39). Figure membrane and inside shell), the improvement 5 also demonstrates that when the permeance in hydrogen flux was improved by 12.3% is increased, the tendency for concentration when the sweep flow rate was increased from polarisation to occur increases. Based on Nagy et 2.713 mol s–1 to 4.3408 mol s–1, thus indicating al. (79), a convex shape can be observed for the the importance of sweep flow rate in improving hydrogen concentration curve in a boundary layer hydrogen flux. In this case, the optimum flow during concentration polarisation phenomena, rate of sweep gas can be estimated from the once the convective flow starts to play a role in arctangent function (41, 85) of feed gas flow the diffusive flow of the layer (80, 81). Based on rate, once the flow rate or Reynolds number of these numerical simulation studies, Chen et al. the feed gas is specified. Here, the flow rate of also asserted that the concentration polarisation sweep gas is considered optimum when the flow phenomenon is not significant when the hydrogen rate can give the maximum hydrogen permeation permeation ratio (H2 permeation rate:H2 feed rate) flux and sufficiently high hydrogen recovery (up is less than 30%. It is important to note that even to 95% H2 recovery) could be maintained (41). if the concentration polarisation can be improved It is interesting to note that the coupling (and more hydrogen flux can be obtained), the between feed gas and sweep gas will give better HPR that indicates hydrogen recovery becomes separation performance when countercurrent smaller, and this becomes a shortcoming for the mode is applied (5, 41). It is also interesting membrane performance (39). to note that whether the feed gas is issued in Subsequently, Chen et al. extended their the lumen side (inside tubular membrane) numerical simulation to the tubular type or the shell side (outside tubular membrane membrane with simultaneous use of feed flow and and inside the shell) during countercurrent sweep flow (41) under concentration polarisation mode, the difference in hydrogen flux for both influence. The advantage of using sweep flow to configurations is negligible. Therefore, this improve hydrogen permeation flux (and to abate implies the independence of hydrogen flux on concentration polarisation) has been confirmed the position of the feed flow. Also, the difference by previous researchers (6, 7, 82–84). When in hydrogen permeation flux between cocurrent the sweep flow rate at the permeated side is mode and countercurrent mode can be reduced increased, the hydrogen partial pressure at the by increasing the feed flow rate. Similar to the membrane surface of the permeated side can feed flow rate, a decrease in the sweep flow rate be reduced, thus hydrogen permeation flux causes the effect of concentration polarisation to increases (and concentration polarisation is become stronger. weakened) due to increase in hydrogen partial Part II (86) continues the discussion and provides pressure difference (7, 83). For instance, in the conclusions.

Y Fig. 5. Distributions of H2 Inlet Y concentration contour A B C D H2,inlet 1.0 for various hydrogen Inlet Axis of 0.9 permeance values (H2:N2 symmetry mixture, inlet H = 50%, 0.8 2 Reynolds number = 200 0.7 and pressure difference 0.6 = 30 atm). Reprinted 0.5 from (39) Copyright

0.4 (2011), with permission from Elsevier 0.3 Tubular membrane Tubular membrane 0.2

0.1 –6 –5 –4 –3 K = 10 K = 10 K = 10 K = 10 0 Min. 0

Acknowledgement 17. G. J. Grashoff, C. E. Pilkington and C. W. Corti, Platinum Metals Rev., 1983, 27, (4), 157 The authors acknowledge the Ministry of Higher 18. Y.-M. Lin and M.-H. Rei, Sep. Purif. Technol., 2001, Education Malaysia and Universiti Teknologi 25, (1–3), 87 Malaysia for giving cooperation and full support in 19. A. Basile, A. Iulianelli, T. Longo, S. Liguori and this research. The authors wish to thank Research M. De Falco, ‘Pd-Based Selective Membrane State- Management Centre (RMC) for the Tier 2 Grant of-the-Art’, in “Membrane Reactors for Hydrogen (Vote No.: 16J28) from Ministry of Higher Education Production Processes”, eds. M. De Falco, L. Marrelli and G. Iaquaniello, Springer-Verlag London Ltd, and Universiti Teknologi Malaysia. London, UK, 2011, pp. 21–55 20. T. A. Peters, J. M. Polfus, M. Stange, P. Veenstra, References A. Nijmeijer and R. Bredesen, Fuel Process. Technol., 2016, 152, 259 1. E. Carcadea, H. Ene, D. B. Ingham, R. Lazar, 21. F. Gallucci, E. Fernandez, P. Corengia and M. van L. Ma, M. Pourkashanian and I. Stefanescu, Int. Sint Annaland, Chem. Eng. Sci., 2013, 92, 40 Commun. Heat Mass Transf., 2005, 32, (10), 1273 22. S. H. Israni and M. P. Harold, Ind. Eng. Chem. Res., 2010, 49, (21), 10242 2. B. Emonts, J. Bøgild Hansen, S. Lœgsgaard Jørgensen, B. Höhlein and R. Peters, J. Power 23. L. Zhao, A. Goldbach, C. Bao and H. Xu, J. Membr. Sources, 1998, 71, (1–2), 288 Sci., 2015, 496, 301 3. E. Kikuchi, Catal. Today, 2000, 56, (1–3), 97 24. C.-H. Chen and Y. H. Ma, J. Membr. Sci., 2010, 362, (1–2), 535 4. B. Arstad, H. Venvik, H. Klette, J. C. Walmsley, W. M. Tucho, R. Holmestad, A. Holmen and 25. J. Zhang, H. Xu and W. Li, J. Membr. Sci., 2006, R. Bredesen, Catal. Today, 2006, 118, (1–2), 63 277, (1-2), 85 5. A. Basile, G. Tereschenko, N. Orekhova, 26. H. Dannetun, L. Wilzén and L.-G. Petersson, Surf. M. M. Ermilova, F. Gallucci and A. Iulianel, Int. Sci., 1996, 357–358, 804 J. Hydrogen Energy, 2006, 31, (12), 1615 27. S. Uemiya, N. Sato, H. Ando, Y. Kude, T. Matsuda 6. A. Basile, S. Tosti, G. Capannelli, G. Vitulli, and E. Kikuchi, J. Membr. Sci., 1991, 56, (3), 303 A. Iulianelli, F. Gallucci and E. Drioli, Catal. Today, 28. N. A. Al-Mufachi, N. V Rees and R. Steinberger- 2006, 118, (1–2), 237 Wilkens, Renew. Sustain. Energy Rev., 2015, 47, 7. A. Iulianelli, T. Longo and A. Basile, Int. J. Hydrogen 540 Energy, 2008, 33, (20), 5583 29. G. L. Holleck, J. Phys. Chem., 1970, 74, (3), 503 8. A. S. Damle, J. Power Sources, 2009, 186, (1), 30. A. Sieverts and G. Zapf, Zeit. Phys. Chem., 1935, 167 174, (1), 359 9. S. H. Israni and M. P. Harold, J. Membr. Sci., 2011, 31. R. W. Baker, “Membrane Technology and 369, (1–2), 375 Applications”, 3rd Edn., John Wiley and Sons Ltd, 10. V. Gepert, M. Kilgus, T. Schiestel, H. Brunner, Chichester, UK, 2012, 575 pp G. Eigenberger and C. Merten, Fuel Cells, 2006, 32. Y. H. Ma, ‘Hydrogen Separation Membranes’, 6, (6), 472 in “Advanced Membrane Technology and 11. S. Fujimoto, H. Ishihara and S. Tsuruno, JSME Int. Applications”, eds. N. N. Li, A. G. Fane, W. S. W. Ho J., 1987, 30, (267), 1437 and T. Matsuura, John Wiley & Sons Inc, Hoboken, USA, 2008, pp. 671–684 12. K. Kusakabe, Membrane, 2005, 30, (1), 2 33. T. L. Ward and T. Dao, J. Membr. Sci., 1999, 13. K. Narusawa, M. Hayashida, D. Kurashima, 153, (2), 211 K. Wakabayashi and Y. Kamiya, JSME Int. J. Ser. B, 2003, 46, (4), 643 34. F. Guazzone, E. E. Engwall and Y. H. Ma, Catal. Today, 2006, 118, (1–2), 24 14. K. Narusawa, M. Hayashida, D. Kurashima, K. Murooka, K. Wakabayashi and Y. Kamiya, Trans. 35. S. Uemiya, T. Matsuda and E. Kikuchi, J. Membr. Japan Soc. Mech. Eng. Ser. B, 2003, 69, (687), Sci., 1991, 56, (3), 315 2553 36. J. P. Collins and J. D. Way, Ind. Eng. Chem. Res., 15. J.-Y. Lee, J. Joo, J. K. Lee, S. Uhm, E. S. Lee, 1993, 32, (12), 3006 J. H. Jang, N.-K. Kim, Y.-C. Lee and J. Lee, Korean 37. A. B. Antoniazzi, A. A. Haasz and P. C. Stangeby, J. Chem. Eng., 2010, 27, (3), 843 J. Nucl. Mater., 1989, 162–164, 1065 16. B. Höhlein, M. Boe, J. Bøgild-Hansen, 38. S. Yan, H. Maeda, K. Kusakabe and S. Morooka, P. Bröckerhoff, G. Colsman, B. Emonts, R. Menzer Ind. Eng. Chem. Res., 1994, 33, (3), 616 and E. Riedel, J. Power Sources, 1996, 61, (1–2), 39. W.-H. Chen, W.-Z. Syu and C.-I. Hung, Int. 143 J. Hydrogen Energy, 2011, 36, (22), 14734

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86. H. M. Faizal, B. B. Nyakuma, M. R. A. Rahman, S. Syahrullail, Johnson Matthey Technol. Rev., Md. Mizanur Rahman, N. B. Kamaruzaman and 2021, 65, (1), 77

The Authors

Mohd Faizal Hasan currently is working at the Faculty of Engineering, Universiti Teknologi Malaysia (UTM). He obtained his PhD degree in Engineering from Keio University, Japan. He is actively doing research on densification, torrefaction and gasification of palm biomass, characteristics of hydrogen permeation through palladium/silver purification membranes for fuel cell applications and methanol steam reforming for hydrogen production. In addition to research activities, he is currently teaching Thermodynamics and Applied Thermodynamics.

Bemgba B. Nyakuma obtained his doctoral degree from UTM and currently works at the School of Chemical and Energy Engineering, UTM Skudai Campus, Johor Bahru, Malaysia. He is actively doing research and producing articles in biomass and coal related pretreatment, conversion and utilisation technologies.

Mohd Rosdzimin Abdul Rahman currently is working at the Faculty of Engineering, Universiti Pertahanan Nasional Malaysia. He obtained his PhD degree in Engineering from Keio University, Japan. He is actively doing research on thermal and reactive fluid dynamics.

Md. Mizanur Rahman obtained his PhD in Mechanical Engineering from Aalto University School of Engineering, Finland; MSc in Sustainable Energy Engineering from Royal Institute of Technology (KTH), Sweden; and BSc in Mechanical Engineering from Khulna University of Engineering and Technology, Bangladesh. His research interests include energy economics, renewable energy technologies, biomass digestion and gasification, multicriteria-based rural electrification, energy policy, modelling and optimisation and sustainable energy systems.

Natrah Kamaruzaman obtained her undergraduate level degree from University of The Ryukyus, Japan. Then she obtained Master and Doctoral Degrees from UTM. Currently, she is specialising in microelectronic cooling and heat transfer. Her research interests are focusing on heat transfer, computational fluid dynamics, fluid flow and microchannel and microneedle areas.

Syahrullail Samion is currently working at the Faculty of Engineering, UTM. He obtained his undergraduate, master and doctoral degrees from Kagoshima University, Japan. His areas of expertise are tribology in metal forming, friction and wear tests (tribotester), biolubricants, palm oil as lubricant and fluid mechanics. He is also teaching mechanics of fluids (undergraduate course) and research methodology (master course).

www.technology.matthey.com

Comprehensive Review on High Hydrogen Permselectivity of Palladium Based Membranes: Part II Hydrogen permeation flux under concentration polarisation influence

Hasan Mohd Faizal** under concentration polarisation influence, which Automotive Development Centre, School of will be a useful guide for academics and industrial Mechanical Engineering, Faculty of Engineering, practitioners. Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia; School of Mechanical 1. Factors Affecting Concentration Engineering, Faculty of Engineering, Universiti Polarisation in Palladium Based Teknologi Malaysia, 81310 UTM Johor Bahru, Membranes Johor, Malaysia Miguel et al. examined the decrease in the Bemgba B. Nyakuma hydrogen concentration along the membrane School of Chemical and Energy Engineering, length of a finger-like configuration (1–3) for the Faculty of Engineering, Universiti Teknologi case of binary hydrogen mixtures with inhibitive Malaysia, 81310 Skudai, Johor Bahru, Malaysia carbon monoxide or carbon dioxide, by replacing Mohd Rosdzimin Abdul Rahman* the terms of feed partial pressure of inhibitive species and the difference in the square root of Department of Mechanical Engineering, Faculty hydrogen partial pressure in the Sieverts’ Langmuir of Engineering, Universiti Pertahanan Nasional equation (4) with the average partial pressure Malaysia, Kem Sg. Besi, 57000, Kuala Lumpur, of inhibitive species and logarithm mean driving Malaysia force (5), respectively. In this case, the authors Md. Mizanur Rahman, N. B. obtained an excellent concordance between the Kamaruzaman, S. Syahrullail predicted results obtained from their rearranged School of Mechanical Engineering, Faculty of Sieverts’ Langmuir equation with the actual

Engineering, Universiti Teknologi Malaysia, hydrogen permeation flux (2), which proved the 81310 UTM Johor Bahru, Johor, Malaysia existence of concentration polarisation during the permeation. Email: *[email protected]; With the apparent advantage of using sweep **[email protected] gas during hydrogen permeation (6), Chen et al. have further investigated the concentration polarisation phenomena under sweep gas and This article completes the presentation of various baffles implementation (7). The flows of feed gas techniques reducing concentration polarisation in and sweep gas were in the form of countercurrent palladium based membranes for supplying ultra- mode. It is interesting to note that higher hydrogen high purity hydrogen to a polymer electrolyte fuel flux can be obtained from the membrane when a cell (PEFC), such as the implementation of baffles smaller diameter of the shell (smaller distance and the use of microchannel configuration. The between the shell and tubular membrane) is used. present paper also reviews and reports the current As an example, when the pressure difference, methods for estimating hydrogen permeation flux temperature, mass flow rate of feed gas and

Reynolds number of flow at the permeate side to 20% when inlet hydrogen partial pressure was were set to 9 atm, 623 K, 267.48 mg s–1 and increased from 0.150 MPa to 0.225 MPa. Therefore, 1000, respectively, the hydrogen flux for the cases the concentration polarisation was strengthened. of large, medium and small shell were 0.88 mol Compared to the previous studies, the changes in m–2 s–1, 0.96 mol m–2 s–1 and 1.03 mol m–2 s–1, the concentration polarisation level concerning the respectively. This is due to the reduction of boundary inlet hydrogen partial pressure and feed flow rate layer thickness, as demonstrated by the numerical are similar qualitatively (10). results (7). Besides, the introduction of baffles to Chen et al. performed experimental studies the shell side causes disturbance to the boundary on a H2:N2 mixture permeation test using high layer and more hydrogen is directed towards the permeance tubular palladium based membranes membrane surface. Therefore, concentration (palladium and palladium/copper membrane with polarisation is weakened and more permeated porous stainless steel) (11). Here, the thickness hydrogen can be obtained. Interestingly, due to the applied was from 6–7 µm. Similar to the previous trade-off between the installation cost and slight study on ultrathin high permeance palladium/ improvement in permeation performance when silver membrane with ceramic support (thickness more baffles are installed, one baffle installation has of 2.5 µm) (12), the authors revealed that been recommended (7). In this case, Coroneo et concentration polarisation was most affected by the al. also have asserted there should be an optimum concentration of the hydrogen feed, notably when number of baffles installed after observing just a the hydrogen concentration was decreased from slight improvement in permeated flow when the 75 vol% to 50 vol%. The severity of concentration number of baffles is increased, from around 38% polarisation becomes higher even though the (two baffles configuration) to just around 46% hydrogen partial pressure difference has been set (three baffles configuration) (8). to the same value. In this case, it can be noticed Further investigation by Chen et al. (9) then that in order to obtain the same hydrogen partial discovered the optimum baffles configuration pressure difference, when the hydrogen partial for minimising concentration polarisation while pressure at the permeated side is set constant, obtaining maximum hydrogen recovery. In this higher total upstream pressure is necessary for case, the authors emphasised the importance of smaller feed hydrogen fraction, and this increases concentrating hydrogen at the membrane surface the levels of concentration polarisation. Within through the flow contraction mechanism. The their selected operating condition, feed flow rate optimum conditions for baffle installations are as and hydrogen partial pressure difference cause follows: (a) installation of single baffle at shell concentration polarisation as well, but with wall; (b) installation at the leading edge of the minor influence compared to the feed hydrogen membrane and (c) use of a sufficiently high ratio concentration factor (11). Zhao et al. performed of baffle length to shell radius (ratio of 0.75) (9). a permeation test for a mixture that was almost

Faizal et al. (10) investigated the effect of similar to coal gasification product (<40% H2 and hydrogen partial pressure and feed flow rate on <40 ppm H2S) to simultaneously determine the the level of concentration polarisation for flat sheet effect of sulfur contamination and concentration palladium/silver membrane, despite widespread polarisation. The authors found that the influence research interest in tubular type membranes. A third of concentration polarisation was dominant for the degree polynomial equation has been introduced mixture with lower hydrogen composition (50% as a tool to predict hydrogen permeation flux for mole fraction) especially at a low feed flow rate such geometry. Based on the predicted profile of due to the minor effect of sulfur poisoning in the hydrogen mole fraction at the membrane surface, specified condition (13). the difference between the predicted average As a continuation of their previous study (10), hydrogen mole fraction at the membrane surface Faizal et al. (14) investigated the concentration and hydrogen mole fraction at the inlet becomes polarisation phenomena for various binary hydrogen larger at higher inlet hydrogen partial pressure. mixtures with different inlet hydrogen mole

For the case of a binary mixture of H2:N2 (inlet fraction (0.70–0.80) and species (nitrogen, argon, hydrogen mole fraction of 0.75), when operating helium and carbon dioxide). It is interesting to note temperature, feed mole flux and hydrogen partial that a mixture of hydrogen and argon was used pressure at downstream (permeate) side were set to due to different chemical characteristics, whereas 623 K, 0.40 mol m–2 s–1 and 0.10 MPa, respectively, the mixture of hydrogen and helium was used due the aforementioned difference increased from 9% to the different binary diffusivity compared to the

78 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16019176538189 Johnson Matthey Technol. Rev., 2021, 65, (1) hydrogen mixture that contained nitrogen (14). significant role when methane is used instead The authors compared the analytical results of carbon monoxide. In the same year, Helmi et calculated by using their previously introduced al. also proved the concentration polarisation theoretical equation that takes into account the phenomenon for the case of a fluidised membrane effect of hydrogen permeation itself (10) with reactor (19). The good agreement between the the actual hydrogen permeation flux. The study developed model that considers concentration demonstrated an excellent concordance between polarisation (so called ‘1D/kd’) with experimental the estimated hydrogen permeation flux and the results elucidates that this phenomenon becomes actual flux regardless of inlet hydrogen mole fraction more significant when lower inlet hydrogen mole and species, thus elucidates the significant effect fraction and higher inlet velocity are used (19). of hydrogen permeation itself on the decrease in Based on the comprehensive review of the hydrogen concentration at the membrane surface research scenarios in the previous paragraphs, during concentration polarisation. Therefore, it is it is evident that concentration polarisation is interesting to note that the severity of concentration an undesirable phenomenon. This is because polarisation is determined by feed flow rate and the ability of the membrane to permeate a very inlet hydrogen mole fraction, but not the different high amount of hydrogen cannot be fully utilised. chemical characteristics and binary diffusivity of However, it is unavoidable due to the advances in the mixtures (14). membrane technology that cause the fabrication of Nakajima et al. reduced the boundary layer very thin membranes. Consequently, the external thickness to abate concentration polarisation mass transfer becomes the permeation controlling by improving physical geometry of a reactor step instead of diffusion in the metallic lattice. vessel containing a tubular palladium/silver Although the development of compact devices with membrane (15). In this case, the reduction of sufficiently high hydrogen recovery is possible, the boundary layer thickness was performed by concentration polarisation level becomes high due narrowing the path between the membrane surface to the significant effect of permeation itself during and the inner surface of the vessel shell (15). For the permeation. instance, by narrowing the path from 23.9 mm to In brief, several techniques have been introduced 16.6 mm, the amount of produced hydrogen can to address such disadvantages and effectively be improved by around 25% even though there is reduce the severity of concentration polarisation. only a 2% increment in methane conversion during For example, baffles may be implemented (7, 8) hydrogen production from natural gas (for feed flow in the membrane reactor and the path between rate of 9 Nml min–1 cm–2) (15). Such improvement membrane surface and inner vessel wall may be is due to the reduction in distance between the narrowed (7, 15) to reduce the boundary layer at inner surface of the vessel and the membrane the membrane surface (15). The implementation surface, which has also been observed through of a spherical particles bed between tubular the previous numerical simulation performed by membranes in a membrane reactor has been Chen et al. (7). confirmed to reduce the effect of concentration As an alternative to the numerical simulation polarisation, due to the increase in the interparticle technique performed by Chen et al. (6, 16), velocity and the increase in the Reynolds number the profiles of average axial concentration and between the particles and membrane surfaces (20). concentration at the membrane surface can also Helmi et al. have used fluidising particles inside a be obtained by determining an analytical solution fluidised membrane reactor to significantly reduce of the governed ordinary differential equation the concentration polarisation effect due to better (ODE) (17). Further discussion on the analytical mixing of gases (21). The permeation system solution is presented in the following section. with microchannel configuration (22, 23) also has In 2018, Kian et al. investigated the concentration been suggested as a technique for concentration polarisation phenomenon for the case of various polarisation abatement due to the ability to ternary mixtures (18). They found the hydrogen decrease boundary layer thickness near to the permeation behaviour for ternary mixtures is membrane surface (22, 24). The combined usage of similar to the case of binary mixtures, in which baffles and perforated pipe to reduce concentration competitive adsorption becomes dominant polarisation effect has been demonstrated by when strong inhibitive carbon monoxide is used Peters et al. (25) due to the creation of turbulence. in the ternary hydrogen mixture (H2:CO2:CO) Recently, an integrated compact system that while concentration polarisation starts to play a consists of combustor, prereformer, reformer and

79 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16019176538189 Johnson Matthey Technol. Rev., 2021, 65, (1) hydrogen separator (palladium membrane) in a polarisation coefficient that becomes as an single module was developed by Wunsch et al. for indicator for concentration polarisation level. The small hydrogen demand applications (26). Since values of CPC were obtained by solving Equation the height of the integrated system was relatively (i) for various operating conditions and the values very small (12.4 mm) with eleven plates arranged were plotted with respect to hydrogen retentate in a stack, the concentration polarisation effect molar fraction. Here, when the value of CPC is 0, it could be minimised, thus improving hydrogen yield means no concentration polarisation occurs while as well as hydrogen productivity (26). the CPC value of 1 indicates maximum level of concentration polarisation that has been defined as Membrane 2. Estimation Methods for Hydrogen ‘total polarisation’ in their study. π is the membrane permeance that can be obtained from Permeation under Concentration permeation test for pure hydrogen. Meanwhile, Polarisation Influence DFBulk is bulk driving force that can be obtained In 2008, Chen et al. introduced a constant from the difference in square root of hydrogen concentration method with the aim to characterise partial pressure between the feed side and the membrane by eliminating the effect of permeate side when the effect of concentration concentration polarisation (27). Based on this polarisation is negligible. For those who want to method, the tubular membrane reactor was filled evaluate the concentration polarisation level by with non-H2 species first and followed by H2 to using these maps, only knowledge of the operating obtain the desired hydrogen partial pressure condition of the membrane reactor is required. without allowing any permeation. Once permeation The CPC can be determined manually from the began, the hydrogen that permeated out of the maps. Finally, hydrogen permeation flux can be membrane was made up or replaced by the fresh predicted by substitution of the CPC value into hydrogen from the pressurised tank to maintain Equation (i) and followed by solving the equation. the hydrogen partial pressure at the retentate side. Despite the simplicity of this prediction method, Therefore, the variation of hydrogen concentration the determined CPC actually does not account at the membrane surface could be eliminated (27). for the remaining length (and remaining area) However, this technique is not practical for of the tubular membrane where no permeation industrial application since the flow stream at the occurs anymore due to very fast decay of driving upstream side of industrial membrane reactor is force at the region around the inlet. This situation usually in plug flow, and not perfectly mixed flow, occurs when concentration polarisation becomes in which concentration polarisation usually could significant (16), thus hydrogen concentration is be triggered. overestimated when the aforementioned CPC Extensive studies on concentration polarisation value is used. This weakness was then solved phenomena have been performed by Caravella et through the introduction of the powerful parameter al. (28) and Catalano et al. (12). Caravella et al. (28) so called effective average CPC (EAC) (30). have created concentration polarisation maps which The determination of EAC is stated as follows, are very useful for hydrogen purification system Equation (ii): design. Here, the maps are a two-dimensional [Permeation rate with polarisation] graph of concentration polarisation coefficient EAC = 1 – versus hydrogen retentate molar fraction that was [Permeation rate without polarisation] derived based on Equation (i): (ii) ∫L Flux (z)dz = 1 – 0 L Flux (z) H flux( elementary steps) = (i) ∫ dz 2 0 1– CPC(z) (1 – CPC) ϖmembrane DF bulk

where H2flux (elementary steps) is the hydrogen where L, z and CPC are the membrane length, permeation flux that is calculated by considering membrane axial abscissa and concentration all the permeation elementary steps (external polarisation coefficient, respectively. Here, the local mass transfer, superficial adsorption, diffusion value of CPC for each position on the membrane through the palladium-based bulk and superficial in z-direction is determined analogously as was desorption) (29), in which in this case, involved introduced previously (28). Meanwhile, the complex procedures. Meanwhile, the concentration hydrogen concentration profile and the respective polarisation coefficient (CPC) is a concentration profile of hydrogen permeation flux can be obtained

80 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16019176538189 Johnson Matthey Technol. Rev., 2021, 65, (1) by simultaneously solving the external mass As a continuity of the previous study (28), transfer and hydrogen permeation equations (30). Caravella et al. (31) considered simultaneously both Here, the calculation of mass transfer coefficient concentration polarisation and inhibition by carbon is as reported by Caravella et al. (29). Finally, monoxide species in their model, by introducing Equation (ii) can be solved to obtain the value of the permeation reduction coefficient. Similar to the EAC. Based on the previous individual elaboration prediction technique introduced previously (28), on the prediction techniques using CPC (28) and the permeation reduction coefficients were plotted EAC (30), it can be said that the use of EAC is more for different operating conditions, or so called desirable, since it has the ability to represent the ‘permeation reduction maps’. The simple relation real behaviour of a hydrogen permeation device. It for the permeation reduction coefficient as shown is interesting to note that once the EAC maps have by Equation (vi) was derived from definitions of CPC been prepared, similar to the previous technique of and inhibitive coefficient (IC), that were obtained using the CPC maps (28), the hydrogen permeation from previous studies by Caravella et al. (28) and flux can be estimated by simply substituting the Barbieri et al. (4), respectively through complex value of the EAC obtained from the maps into calculation steps, Equation (vi): Equation (ii). PRC = 1 – (1 – CPC)(1 – IC) (vi) Similarly, Catalano et al. (12) concluded the existence of non-negligible resistance to hydrogen where PRC is the permeation reduction coefficient, transport in the gaseous phase itself, in addition to CPC is the concentration polarisation coefficient resistance caused by the membrane. For the case and IC is the inhibition coefficient. To predict the of a hydrogen mixture, the authors demonstrated hydrogen permeation flux for certain operating a significant deviation from Sieverts’ Equation conditions, the value of PRC is determined manually (Equation (iii)) when the hydrogen partial pressure from the ‘permeation reduction maps’ and then the of the bulk gas is substituted into the equation. value is substituted into Equation (vii) as follows: To compensate for this situation, semi-empirical Sieverts retentate permeate equations were developed for a tubular type JH2 = (1 – PRC)ϖ (√PH2 – √PH2 ) (vii) membrane (membrane thickness of 2.5 µm) as follows (12), Equations (iv) and (v): where JH2 is the hydrogen permeation flux, PRC is the permeation reduction coefficient,Sieverts ϖ is q f = (√PH ,1 – √PH ,2) (iii) the permeance which is similar to the hydrogen d 2 2 permeance coefficient, obtained from pure- retentate permeate p – p hydrogen test. Meanwhile, PP− ret H2,int H22H NH2,int = kGln (iv) is the bulk driving force for hydrogen permeation, ( p – p ) ret H2,ret that is bulk difference in square root of hydrogen partial pressures between the retentate and N = K p 0.5 – p0.5 (v) H2,int H2( H2,int H2,per) permeate side. As one of the solutions for the difficulty in where NH2,int is the hydrogen flux crossing the obtaining a general relation that consists of membrane interface and is defined as the hydrogen several interdependent parameters as has been flux within the gas-metal interface, kG is the mentioned by Morgues et al. (32), Faizal et al. (10, mass transport coefficient, pret is the pressure at 14, 33) have introduced a theoretical approach the retentate side, pH2,int is the hydrogen partial for hydrogen permeation through a flat sheet pressure at gas-metal interface, pH2,ret is the palladium based membrane after observing a hydrogen partial pressure at retentate side, pH2,per significant deviation between the actual permeation is the hydrogen partial pressure at permeate side flux and the estimated flux by Sieverts’ equation and K is the hydrogen permeance obtained from (Equation (iii)) when inlet hydrogen concentration H2 pure hydrogen experiment. It is interesting to note was used (33). As asserted by previous researchers that once the value of kG is obtained by solving on the significant effect of permeation flux during Equations (iv) and (v) simultaneously and by using concentration polarisation phenomena (6, 12, 16), experimental data of NH2,int, the same value of kG the term of hydrogen partial pressure at membrane can then be used to estimate hydrogen permeation surface of upstream side in the Sieverts’ equation flux for the cases with different hydrogen partial (Equation (iii)) has been modified to consider pressure difference. the effect of hydrogen permeation flux during

81 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16019176538189 Johnson Matthey Technol. Rev., 2021, 65, (1) permeation. The modification of Equation (iii) leads of both phenomena. This is because the previous to the formation of Equation (viii) as follows (14): approach by Barbieri et al. (4) only considers the feed hydrogen partial pressure for prediction. q fH2,in – fp fp = P – P (viii) However, the model introduced by Miguel et al. is d f – f in √ H2,2 (√ in p ) semi-empirical and therefore, experimental data is where fp is the estimated hydrogen permeation necessary in order to determine certain parameters mole flux, q is the hydrogen permeance in the correction factor. The combination of coefficient, d is the membrane thickness, fH2,in is correction factor and rearranged Sieverts’ equation the mole flux of hydrogen from inlet (feed flow forms the rearranged Sieverts’-Langmuir equation rate of hydrogen divided by effective membrane as shown below (2), Equation (ix): surface area), fin is the mole flux of the mixture K p LH from inlet (feed flow rate of mixture divided by SL i ι 2 JH2 = 1 – α ΔPln (ix) ( 1 + Ki pι ) δ effective membrane surface area), Pin is the total pressure at inlet (total upstream pressure) and Kp where the term α i ι is the correction factor pH ,2 is the hydrogen partial pressure at membrane 2 1 + Kpi ι surface of the downstream side. In order to due to adsorption of inhibitive species on the predict fp, the values of operating parameters L membrane surface and the term H2 is the are substituted into Equation (viii) and followed ∆Pln δ by solving the equation for fp. Surprisingly, the rearranged Sieverts’ equation. modified equation as shown by Equation (viii) can Here, J SL is the hydrogen permeation flux, a is H2 estimate accurately hydrogen permeation flux the Sieverts’-Langmuir reduction factor, Ki is the for any noninhibitive binary hydrogen mixture Langmuir’s adsorption constant for species (CO or with different chemical characteristics and binary CO2), pι is the average partial pressure of species diffusivity, along with any hydrogen concentration (CO or CO2) between the feed and retentate and with any mole flux of mixture from the inlet. sides, LH2 is the hydrogen permeance or hydrogen For instance, when the mole flux of mixture from permeation coefficient,δ is the membrane thickness the inlet is increased, the effect of concentration and DPln is the logarithm mean driving force that is polarisation is weakened. Therefore, the estimated determined based on theory of heat exchanger for flux obtained from Equation (viii) approaches the parallel flow case (2, 5). It is important to note flux obtained from Equation (iii) due to the weaker that the values of a and Ki for carbon monoxide effect of fp. The introduced method is supposed and carbon dioxide are dependent on operating to be very useful for reactors with similar type of temperature, thus these values need to be membrane used (34–36). fitted with experimental data first before using To prevent membrane damage due to mechanical Equation (ix) to estimate hydrogen permeation stress, the palladium/silver tubular membrane was flux. created in the form of a ‘finger-like’ configuration, A prediction method for hydrogen permeation thus allowing the free elongation and contraction of capacity (length of membrane for hydrogen the membrane (2). For this kind of configuration, permeation) has been introduced by Xie et al. (38) the way to predict hydrogen permeation flux through computer programming. The investigation is supposed to be similar to that for the tubular was performed analytically for seven important type membrane with common configurations scenarios with a different flow pattern on both (Figures 2(a) and 2(b) in Part I (37)) because sides (upstream and downstream side). In this the hydrogen mixture similarly flows horizontally case, the concept is similar to that performed by along the membrane length for both cases. In the Faizal et al. (10, 14, 33), in which the effect of research performed by Miguel et al. (2), a model hydrogen permeation rate is taken into account to simultaneously consider both concentration when determining hydrogen partial pressure at polarisation and inhibition by carbon monoxide the membrane surface of the upstream side, as or carbon dioxide has been introduced based on shown by the following Equation (x) (example for the logarithm-mean driving force (for considering the scenario with plug flow at upstream side, and concentration polarisation effect) and correction no sweep gas): factor due to inhibitive effect (4). Compared to M1 – Mx the approach by Barbieri et al. (4), the model P (x) = P (x) H M + M – M 1 introduced by Miguel et al. could provide more 1 2 x accurate results for the simultaneous occurrence

per where PH(x) is the hydrogen partial pressure of of retentate side, P is the hydrogen partial Hw2 an infinitesimal permeation capacity (infinitesimal pressure at membrane surface of permeate side, membrane length for permeation), dx in the high P ret is the average hydrogen partial pressure H2 pressure (upstream) side, M1 is the feed flow rate of at retentate side, Γ is the separation parameter, hydrogen at upstream side, M2 is the feed flow rate D is the diffusion coefficient of hydrogen in the of nonpermeable gas at upstream side, P1 is the total gaseous phase, Sh is the Sherwood number, d is ret pressure of upstream side and Mx is the hydrogen the characteristic length, p is the pressure at permeation rate through a membrane for length retentate side, KH2 is the hydrogen permeability from 0 to x. Then, the local hydrogen permeation and ctot is the total molar density. To predict the rate through the infinitesimal permeation capacity profile of hydrogen permeation flux, the algebraic dx can be predicted by substituting Equation (x) functions presented by Nekhamkina et al. (17) into the Sieverts’ equation as follows, Equation (xi): need to be solved.

0.5 M1 – Mx 0.5 dMx = C P1 – P2 dx (xi) (( M1 + M2 – Mx ) ) 3. Conclusions where C is the constant for membrane and P2 is the The background of the scenarios related to palladium total pressure of downstream side. In their study, based membranes has been elaborated. It was Equation (xi) is rearranged and then followed by found that concentration polarisation becomes integration of x with respect to Mx in order to unavoidable in parallel with advances in membrane predict the permeation capacity (membrane length technology. The scenario of parametric studies on the for separation) x as shown by Equation (xii): concentration polarisation phenomenon specifically for palladium based membranes was reviewed M – M –1 Mx 1 x 0.5 0.5 x = ∫ C P1 – CP2 dMx (xii) comprehensively. Based on the present review, it is 0 [ ( M1 + M2 – Mx ) ] evident that an increase in total upstream pressure, Differently to other techniques, the technique membrane temperature and permeance promotes introduced by Xie et al. (38) is used to estimate concentration polarisation. The same trend is also the value of x instead of Mx. achieved when the feed flow rate, inlet hydrogen Recently, algebraic functions that can be used to concentration, total downstream pressure and predict the profiles of hydrogen permeation flux membrane thickness are reduced. When the ratio under the influence of the concentration polarisation of hydrogen permeation rate to hydrogen feed rate phenomenon have been introduced (17). at the inlet becomes sufficiently high, the effect Concentration polarisation is accounted through of hydrogen permeation flux on the hydrogen the use of an effectiveness factor which was derived concentration decrease at the membrane surface in the previous study (39). The effectiveness factor becomes significant, thus concentration polarisation is the ratio of actual permeation flux over the becomes strong. Therefore, it can be said that an calculated flux based on the average concentration, increase in hydrogen recovery percentage leads to and it is a function of separation parameter that a stronger tendency for concentration polarisation represents the ratio of diffusive to permeation flux. to occur, and as a consequence, larger deviation The effectiveness factor and separation parameter from the hydrogen permeation flux estimated by that obeys Sieverts’ Law can be expressed as Sieverts’ equation could be observed. For both Equation (xiii) and Equation (xiv), respectively (2): tubular type and flat sheet type membranes,

ret per when concentration polarisation is triggered, the P – P √ H2w √ H2w hydrogen concentration decreases in the horizontal η = (xiii) ret per direction (from the leading edge to the tailing edge) PH – PH w √< 2 > √ 2 and radial direction, respectively. Furthermore, DSh Γ = (xiv) the existence of inhibitive species such as carbon ret dΚ √p monoxide in the hydrogen mixture somehow causes the membrane performance in terms of where (Equation (xv)): hydrogen permeation flux to become worse due to the simultaneous occurrence of concentration KH Κ = 2 (xv) polarisation and inhibition by carbon monoxide. c tot Meanwhile, the concentration polarisation level P ret Here, η is the effectiveness factor, Hw2 is the does not depend on the number of noninhibitive hydrogen partial pressure at membrane surface species, chemical characteristics of noninhibitive

83 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16019176538189 Johnson Matthey Technol. Rev., 2021, 65, (1) species or binary diffusivity in the hydrogen mixture 4. G. Barbieri, F. Scura, F. Lentini, G. De Luca and E. Drioli, Sep. Purif. Technol., 2008, 61, (2), 217 (binary or ternary mixture). Several techniques have been identified to 5. F. P. Incropera and D. P. DeWitt, “Fundamentals of effectively abate concentration polarisation such Heat and Mass Transfer”, 4th Edn., John Wiley & Sons, Inc., Hoboken, USA, 1996 as coupling of upstream flow with sweep flow in countercurrent mode, installation of baffles in 6. W.-H. Chen, W.-Z. Syu, C.-I. Hung, Y.-L. Lin and C.- the appropriate number, size and position, and C. Yang, Int. J. Hydrogen Energy, 2012, 37, (17), 12666 by narrowing the space for upstream flow, that is reduction of the distance between the shell 7. W.-H. Chen, W.-Z. Syu, C.-I. Hung, Y.-L. Lin and C.- and membrane. Basically, the aforementioned C. Yang, Int. J. Hydrogen Energy, 2013, 38, (2), 1145 techniques were implemented to increase the hydrogen concentration at the membrane surface 8. M. Coroneo, G. Montante and A. Paglianti, Ind. of upstream side, thus concentration polarisation Eng. Chem. Res., 2010, 49, (19), 9300 could be reduced. 9. W.-H. Chen, C.-H. Lin and Y.-L. Lin, J. Membr. Sci., Finally, several estimation methods for hydrogen 2014, 472, 45 permeation flux have been reported for different 10. H. M. Faizal, R. Kizu, Y. Kawamura, T. Yokomori membrane configurations. Several methods are and T. Ueda, J. Therm. Sci. Technol., 2013, 8, (1), empirical, in which experimental data is necessary 120 to obtain certain coefficients while some of the 11. W.-H. Chen, M.-H. Hsia, Y.-H. Chi, Y.-L. Lin and C.- methods can be used by just substituting the C. Yang, Appl. Energy, 2014, 113, 41 operating parameters into the introduced equation. 12. J. Catalano, M. G. Baschetti and G. C. Sarti, J. For future work, it is suggested that the Membr. Sci., 2009, 339, (1–2), 57 methods to estimate hydrogen permeation flux 13. L. Zhao, A. Goldbach, C. Bao and H. Xu, J. Membr. for the application of steam reforming should Sci., 2015, 496, 301 be intensively developed for various operating 14. H. M. Faizal, Y. Kawasaki, T. Yokomori and T. Ueda, conditions. In this case, the detailed chemical Sep. Purif. Technol., 2015, 149, 208 kinetics must be considered to obtain the accurate 15. T. Nakajima, T. Kume, Y. Ikeda, M. Shiraki, H. mixture composition near the membrane surface. Kurokawa, T. Iseki, M. Kajitani, H. Tanaka, H. The competitive adsorption by excessive steam Hikosaka, Y. Takagi and M. Ito, Int. J. Hydrogen and excessive vaporised alcohols (methanol for Energy, 2015, 40, (35), 11451 instance) in addition to carbon monoxide during the 16. W.-H. Chen, W.-Z. Syu and C.-I. Hung, Int. J. occurrence of steam reforming reaction should also Hydrogen Energy, 2011, 36, (22), 14734 be taken into account in the future development of 17. O. Nekhamkina and M. Sheintuch, J. Membr. Sci., estimation methods. 2016, 500, 136 18. K. Kian, C. M. Woodall, J. Wilcox and S. Liguori, Acknowledgement Environments, 2018, 5, (12), 128

The authors acknowledge the Ministry of Higher 19. A. Helmi, R. J. W. Voncken, A. J. Raijmakers, I. Education Malaysia and Universiti Teknologi Roghair, F. Gallucci and M. van Sint Annaland, Chem. Eng. J., 2018, 332, 464 Malaysia for giving cooperation and full support in this research. The authors wish to thank Research 20. A. Caravella, L. Melone, Y. Sun, A. Brunetti, E. Management Centre (RMC) for the Tier 2 Grant Drioli and G. Barbieri, Int. J. Hydrogen Energy, (Vote No.: 16J28) from Ministry of Higher Education 2016, 41, (4), 2660 and Universiti Teknologi Malaysia. 21. A. Helmi, E. Fernandez, J. Melendez, D. A. P. Tanaka, F. Gallucci and M. van Sint Annaland, Molecules, 2016, 21, (3), 376 References 22. T. A. Peters, J. M. Polfus, M. Stange, P. Veenstra, A. 1. F. Gallucci, A. Basile, S. Tosti, A. Iulianelli and E. Nijmeijer and R. Bredesen, Fuel Process. Technol., Drioli, Int. J. Hydrogen Energy, 2007, 32, (9), 2016, 152, 259 1201 23. A. L. Mejdell, M. Jøndahl, T. A. Peters, R. Bredesen 2. C. V. Miguel, A. Mendes, S. Tosti and L. M. Madeira, and H. J. Venvik, J. Membr. Sci., 2009, 327, (1– Int. J. Hydrogen Energy, 2012, 37, (17), 12680 2), 6 3. F. Gallucci, F. Chiaravalloti, S. Tosti, E. Drioli and A. 24. T. Boeltken, M. Belimov, P. Pfeifer, T. A. Peters, R. Basile, Int. J. Hydrogen Energy, 2007, 32, (12), Bredesen and R. Dittmeyer, Chem. Eng. Process.: 1837 Process Intensif., 2013, 67, 136

25. T. A. Peters, P. M. Rørvik, T. O. Sunde, M. 33. H. M. Faizal, M. Kuwabara, R. Kizu, T. Yokomori Stange, F. Roness, T. R. Reinertsen, J. H. Ræder, and T. Ueda, J. Therm. Sci. Technol., 2012, 7, (1), Y. Larring and R. Bredesen, Energy Proc., 2017, 135 114, 37 34. B. Arstad, H. Venvik, H. Klette, J. C. Walmsley, 26. A. Wunsch, P. Kant, M. Mohr, K. Haas-Santo, P. W. M. Tucho, R. Holmestad, A. Holmen and R. Pfeifer and R. Dittmeyer, Membranes, 2018, Bredesen, Catal. Today, 2006, 118, (1–2), 63 8, (4), 107 35. A. Unemoto, A. Kaimai, K. Sato, T. Otake, K. 27. S. C. Chen, C. C. Y. Hung, G. C. Tu and M. H. Rei, Yashiro, J. Mizusaki, T. Kawada, T. Tsuneki, Y. Int. J. Hydrogen Energy, 2008, 33, (7), 1880 Shirasaki and I. Yasuda, Int. J. Hydrogen Energy, 28. A. Caravella, G. Barbieri and E. Drioli, Sep. Purif. 2007, 32, (14), 2881 Technol., 2009, 66, (3), 613 36. A. S. Damle, J. Power Sources, 2009, 186, (1), 29. A. Caravella, G. Barbieri and E. Drioli, Chem. Eng. 167 Sci., 2008, 63, (8), 2149 37. H. M. Faizal, B. B. Nyakuma, M. R. A. Rahman, 30. A. Caravella and Y. Sun, Int. J. Hydrogen Energy, Md. Mizanur Rahman, N. B. Kamaruzaman and S. 2016, 41, (27), 11653 Syahrullail, Johnson Matthey Technol. Rev., 2021, 65, (1), 64 31. A. Caravella, F. Scura, G. Barbieri and E. Drioli, J. Phys. Chem. B, 2010, 114, (38), 12264 38. D. Xie, N. Lu, F. Wang and S. Fan, Int. J. Hydrogen Energy, 2013, 38, (25), 10802 32. A. Mourgues and J. Sanchez, J. Membr. Sci., 2005, 252, (1–2), 133 39. M. Sheintuch, Chem. Eng. J., 2015, 278, 363

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Lattice Dynamical Study of Platinum by use of Van der Waals Three Body Force Shell Model Good agreement between experimental and theoretical results

U. C. Srivastava In the present manuscript the author has used Department of Applied Physics, Amity VTBFSM for theoretical calculation. The pioneering Institute of Applied Sciences, Amity University, work of Kellerman (1) for ionic interactions in the Sector-125, Noida-201313, Uttar Pradesh, India alkali halides has attracted considerable attention theoretically as well as experimentally. Löwdin’s Email: [email protected]; (2, 3) and Lundqvist’s (4–6) theory for ionic solids [email protected] leads to the first important term as many body force which includes the three-body component. The Heitler-London theory and the free-electron The present article considers the lattice approximations will employ the combined effects dynamical study of platinum by use of the Van of VWI and TBI in RSM (7). The effects of VWI der Waals three body force shell model (VTBFSM) and TBI in the framework of ion polarisable RSM due to high stiffness constant11 C and C12. The (IPRSM) are effective up to the second neighbour model uses the frequencies of the optical and with short-range, VWI and TBI interaction. vibrational branches in the direction [100] and The experimental investigation for the phonon phonon density of states (DOS). The study of dispersion curve of platinum has been done with phonon spectra is important in determining coherent inelastic neutron scattering, variation in the mechanical, electrical and thermodynamic Debye temperature and Raman spectra (8–10). properties of elements and their alloys. The The elastic constants and dielectric constants (11), present model incorporates the effect of Van physical and natural properties of platinum have der Waals interactions (VWI) and three-body been elucidated by expedient of theoretical models interactions (TBI) into the rigid shell model (12–16), which has also successfully described (RSM) with face-centred cubic (fcc) structure, their interesting properties. After the failure of operative up to the second neighbours in short the Kellerman rigid-ion model (RIM) then Karo range interactions. The available measured data and Hardy (17) used a deformation dipole model, for platinum agrees well with our results. Woods et al. (7) and Dick and Overhauser (18) used a RSM to report lattice properties of alkali Introduction halides. The other most prominent model was also proposed by some researchers, among them The lattice dynamical study of metallic crystals is Schröder’s (19) breathing shell model, Basu and an interesting field of research. Platinum group Sengupta’s (20) deformable shell model and the metals are highly valuable transition metals which three-body force shell model of Verma and Singh have many useful properties. The electronic (21–23) and Singh et al. (24) for such halides. structure of the platinum metals is of impressive In consideration of the effect of VWI, reported by theoretical and practical importance. Dependable Upadhyaya et al. (25), excellent results have been thermodynamic information is expected to give procured between experiment and theory for ionic the crude material from which lattice dynamics, halides and semiconductors. The betterment of the electronic conveyances and energy states can be present model VTBFSM over others can be realised deduced by genuine understudies of the solid state. from the fact that in the present model relatively

87 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15843541021642 Johnson Matthey Technol. Rev., 2021, 65, (1) fewer numbers of parameters have been able to Here D (q) is the (6 × 6) driving matrix for RSM. interpret numerous and largely divergent physical The dipole-dipole (VWI) energy up to the second properties of materials. This has to motivate the neighbour is expressed as Equation (vi): author to incorporate this model in the present CC study. VWI rS V r (vi) dd V 6r 6

Theory where Sv is the lattice sum and C++ and C–– are the constants ion pairs respectively. By use of the secular The VWI potential originates with the correlation of Equation (v) the expressions for elastic constants can the motions of an electron in various atoms, due to be derived and given as Equations (vii)–(ix): which shifting in electrons of the atom has occurred F 2 1 V with respect to the nucleus and accordingly an atom 2 5.112ZAAA e G m 12 11 22 W G 2 W becomes an electric dipole. The present model thus C11 4 (vii) 4r 1 0 G 2 W consists of the long-range screened Coulomb, VWI, BB11 22 9.3204 HG 2 XW TBI and the short-range lap aversion viable up to the second-neighbour ions in platinum. In metals, F 2 1 V 2 G0.226ZBm 12 AA11 22 W because of angular interaction between electron e (viii) C G 4 W gas and pair of ions, the Cauchy discrepancy arises. 12 4r 4 G 5 W 0 BB 9.3204 2 The significant equation for the crystal potential HG 4 11 22 XW per unit cell can be derived with VTBFSM and is given as Equation (i): F 2 1 V 2 G2.556ZBm 12 AA11 22 W e 4 CRTBIVWI G W (ix) (i) C44 4 4r G 3 W 0 G BB11 22 W where ΦC is long-range Coulomb interaction H 4 X potential. For infinite lattice in a crystal the at equality condition [(dΦ/dr)0=0] we get Coulomb potential energy is given as Equation (ii): Equations (x) and (xi):

22 2 (x) C Ze (ii) BB11 22 20BZ12 .6786 m M r0 C 16 S 22 2 where α and r are the Madelung constant and whereaZZm D1 fZ0 T nd rf00 (xi) m 0 E Z U equilibrium nearest neighbours distance. The methodical expressions in terms of negative The frequency distribution function by use of exponential power laws for the repulsive energy Debye’s model is given by Equation (xii): are given as Equation (iii): Dm hK / (xii) R n raij rij Born Potential , (iii) To determine the phonon DOS for each polarisation R rbij 4exp rij /B -M Potenntial is given by Equation (xiii):

gd Nd/ Nq F ()V where, a (or b) and η (or ρ) are called the strength O B H j X BZ j R and hardness Born parameters and Φ is a short- 22 dq VK /2 4 dK /d (xiii) range lap repulsion potential. The third term ΦTBI 3 3 long-range TBI interaction potential is expressed and NL /2 43 K / as Equation (iv):

22 where N is a normalisation, K is wave vector and TBI Ze F2n V 3 m G frW (iv) L = V. The value of g(ω)dω is the ratio of the r H Z 0 X 0 eigenstates number in the (ω, ω + dω) frequency where f(r)0 is the equilibrium electron wave- interval to the total number of eigenstates jth functions. Since we consider only one ion to be normal mode frequency ωj(q) i.e. phonon wave polarisable and deformable, the basic equations of vector q such that O gd() 1. Singh and Verma’s (21–23) model are modified. The secular determinant equation is given by Numerical Computations Equation (v):  In the present paper the parameters including Dq 2 MI 0 (v) (C11, C12 and C44), polarisabilities (α1, α2), and

lattice constant by (26) have been theoretically Table I Cauchy-Discrepancy and Constant calculated for platinum and given in Table I. By Parameters of Platinum solving Equations (i) and (v) we can obtain the Calculated input Input data for phonon spectra in the first Brillouin zone divided parameters for platinum into evenly spaced miniature cells. The theoretical platinum results were obtained by VTBFSM. We have used Values Values the computed vibration spectra to study the Properties for Properties for specific heat and infrared (IR)/Raman spectra in platinum platinum the present paper. The DOS have been obtained C11 34.67 (26) C11 32.57 by computing the DOS of the frequencies from the C12 25.07 (26) C12 23.97 knowledge of lattice vibrational frequency spectra. C44 7.65 (26) C44 6.35 The values of frequencies are compatible with 2a 3.923 (26) 2a 3.923 theoretical and experimental peaks and Cauchy- a1 0.037 a1 0.0487 discrepancy for lattice dynamics of platinum. a 0.032 a 0.0368 In the Brillouin zone surface the calculated 2 2 phonon dispersion curves for platinum are shown in Figure 1 by using first principles along two The small observed differences are related to high symmetry directions (qqq) Γ-X-Γ. The parallel the shape of the main peaks. The differences vibrational modes show real dispersion with a between the calculated and observed values maximum cleave. The upper branch consists of were found and discussed in Figure 2. There are longitudinal modes, while the lower one is the very important features obtained from the DOS shear‑horizontal mode, along both the Γ-X-Γ vs. frequency curve. The information about the directions. We find that the surface modes for surface and resonance states was found through clean platinum (qqq) undergo a few changes in these differences. Below the Fermi level EF, L-T modes on the clean surface, near the zone resonance-states are expected to be obtained, boundaries and along the X-direction, are replaced mainly because these energies represent the only in the dispersion curves. This is because the continuum and few energy gaps exist at these zone boundaries are moderate and the next two energy values. The experimental reported DOS surface modes are strengthened. The experimental curve (28) may be compared with the present reported results for dispersion relation are model i.e. Figure 2(a). Sharp peaks can be shown in Figure 1(b) (27). On comparing with seen in Figure 2(a) while in Figure 2(b) the the experimental result i.e. Figure 1(a) with distortion in peaks can be seen which justifies Figure 1(b) good agreement can be observed. the superiority of the present theoretical study.

The DOS vs. frequency curve for platinum The specific heat and Debye temperatureD Θ theoretically calculated in the energy range from have been calculated as a function of temperature approximately –7.0 eV to 0.5 eV in bulk is shown T from the lattice frequency spectra as shown in as a solid line in Figure 2. The bulk DOS exhibits Figure 3. Debye temperature ΘD is calculated three main peaks that are accurately produced. from different frequency values. The specific heat

(a) (b) Fig. 1. (a) Phonon ΓΓ∆→ ΧΧ←Σ Λ→ 6.0 dispersion relation 7 [q00] [qq0] [qqq] curve for platinum by L L 6 VTBFSM; (b) phonon dispersion relation to 5 4.0 –1 platinum along the [ε,

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 0.5 1.0 0.75 0.5 ξ 0 q ← Wave vector

(a) (b) Fig. 2. (a) DOS vs. 0.7 frequency curve of

10 platinum with present 0.6 VTBFSM; (b) DOS vs. 9 frequency curve of 0.5 8 platinum. Reprinted

s 7 from (28), with the 0.4 permission of AIP 6 publishing 0.3 5 ), arbitrary units 4 Density of state 0.2 g ( 3

0.1 2

1 0

0 123 456 1234 567 Frequency, THz , 1012 cps

(a) (b) Fig. 3. (a) Debye 270 temperature curve with VTBFSM; (b) 300 Debye temperature curve by Closs

, Fisica. Reprinted by D θ, θ permission of Springer 240 Nature 230

220

200

210 050 100 150 200 250 300 0 50 100 150 Temperature, K Temperature, K

value of platinum has been measured at extended in Table I were used to solve the secular equation temperature (0 K to 300 K). The calculated result is for specified values of wave vectors in the first in reasonable agreement at moderate temperature Brillouin zone, which is split up in an evenly spaced and at very low temperature. The comparison can sample of (1000) wave vectors by Kellerman (1). be seen through experimental results (27). From the symmetry, these 1000 points are reduced to 48 non-equivalent points at which the vibration Discussion frequencies have been obtained by solving the secular determinant. Debye temperature The varying investigated properties are distinctly variations at different temperatures by Macfarlane shown in the present study by successful use of et al. (26) and colossal dielectric constant (CDC) VTBFSM, which has provided the complete lattice curves for platinum crystals have been computed description of platinum. It agrees well with the test by using VTBFSM model. By using the sampling of anisotropy factor A = 2C44/(C11–C12) > unity technique the corresponding values of ӨD have and towards the high frequency end the higher been compared with available experimental peak is found. The determined model parameters data (27–29) and the curve for ӨD vs. absolute

90 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15843541021642 Johnson Matthey Technol. Rev., 2021, 65, (1) temperature (T) was plotted as shown in Figure 1 experimental results at higher temperatures are for platinum. In this temperature range one should seen, though the agreement is almost better take into account the temperature dependence of with VTBFSM. The calculated (ӨD–T) curve for the frequency spectrum, this requires, however, platinum has given excellent agreement with the knowledge of the phonon frequencies at more experimental values (8–10). The DOS curve is than one temperature. The variations of ӨD given in Figure 2 and data points are shown in with specific heats have been used to compute Table III. The two-phonon Raman spectra are phonon frequencies in the first Brillouin zone and sensitised to the high-frequency side while the data points for different points were reported in specific heats are sensitive to its lower side which Table II. The effect of anharmonicity is excluded is stated the reasonableness of VTBFSM for all so slight discrepancies between theoretical and wavelength range.

Table II Assignments for the Observed Peak Positions for Phonon Dispersion Relation in Γ-X-Γ Direction Γ-directions X-directions Γ-directions x-axis y-axis x-axis y-axis x-axis y-axis 0.05283 1.21564 0.4084 5.1927 1.50882 6.4594 2.51486 0.2413 0.09694 1.29258 0.8167 4.3248 1.54589 4.4531 2.58016 0.0742 0.14104 1.29749 1.2251 4.2535 1.54593 4.4548 2.5892 0.3527 0.16361 1.29889 1.9118 4.2383 1.61352 6.3852 2.61588 0.7981 0.1895 1.38452 1.6334 4.5661 1.64652 4.6218 2.633 0.4826 0.19903 1.39083 2.3573 4.4389 1.65054 4.3063 2.67354 1.6334 0.22918 1.39743 1.9861 4.3991 1.67279 4.6961 2.67698 0.7796 0.23438 1.49138 2.7471 6.478 1.70493 6.1439 2.70459 2.0789 0.27413 – 3.174 – 1.74195 4.065 2.73415 1.1694 0.27765 – 2.3944 – 1.75174 5.0302 2.78695 1.5406 0.30512 – 3.5638 – 1.78317 5.8283 2.84847 1.9118 0.33485 – 2.8028 – 1.82898 4.9924 2.91871 2.2645 0.34487 – 3.9907 – 1.8306 5.29 2.98776 5.5313 0.389 – 4.4176 – 1.85263 5.4756 2.99327 2.58 0.39202 – 3.1926 – 1.87869 5.4529 3.04495 5.9397 0.42283 – 3.4153 – 1.87879 5.4385 3.08086 2.8399 0.42875 – 4.8445 – 1.90724 3.5081 3.09402 2.8956 0.47724 – 5.2715 – 1.94376 4.9745 3.11517 6.2738 0.48872 – 3.7865 – 1.97845 3.8125 3.19451 2.9698 0.53007 – 5.6798 – 1.97928 3.6312 – – 0.58951 – 4.1392 – 1.98546 3.174 – – 0.59596 – 6.051 – 2.03477 4.3619 – – 0.67489 – 6.3666 – 2.05494 2.8399 – – 0.68143 – 4.3619 – 2.07071 5.29 – – 0.75796 – 6.478 – 2.0911 2.8566 – – 0.81252 – 4.4733 – 2.09835 2.6172 – – 0.85822 – 6.348 – 2.14018 4.9374 – – 0.86919 – 4.3991 – 2.14743 3.5824 – – 0.90881 – 4.6961 – 2.14883 4.8631 – – 0.95402 – 6.1253 – 2.16345 2.2645 – – 0.97385 – 4.2877 – 2.19499 3.1555 – – 0.9964 – 4.9559 – 2.20077 4.4548 – – 1.03665 – 5.8283 – 2.22853 1.8933 – –

(Continued)

Table II (Continued) Γ-directions X-directions Γ-directions x-axis y-axis x-axis y-axis x-axis y-axis 1.07096 – 5.2715 – 2.24394 4.0093 – – 1.07858 – 4.2506 – 2.25891 1.7262 – – 1.1149 – 5.5313 – 2.28713 3.5824 – – 1.13663 – 5.5154 – 2.32159 3.1555 – – 1.13671 – 5.5128 – 2.32399 1.355 – – 1.18774 – 4.2691 – 2.35607 2.7471 – – 1.21487 – 5.123 – 2.38907 2.2744 – – – – – – 2.39051 2.3016 – – – – – – 2.44978 0.6125 – –

Table III Assignments for the Observed Peak Positions for Combined Density of States y-axis, arbitrary x-axis, THz y-axis, arbitrary units x-axis, THz units 0.0234 3.1403 0.01226 0.23887 1.2142 – 0.50083 – 0.1286 3.2455 0.05927 0.28588 1.2586 – 0.44984 – 0.2224 3.3964 0.10831 0.32272 1.2914 – 0.39884 – 0.3276 3.51 0.15532 0.27586 1.3686 – 0.29481 – 0.4213 3.6352 0.20437 0.229 1.4015 – 0.24381 – 0.562 3.7489 0.27997 0.18418 1.4459 – 0.16746 – 0.6212 3.8852 0.32899 0.12713 1.4788 – 0.14386 – 0.692 4.1503 0.38006 0.13548 1.8082 – 0.00327 – 0.7512 4.4271 0.42908 0.14791 2.0736 – 0.01978 – 0.8219 4.6924 0.48015 0.16033 2.3276 – 0.03832 – 0.8811 4.9576 0.52917 0.17072 2.5589 – 0.06909 – 0.9518 4.9918 0.5782 0.16054 2.7902 – 0.09782 – 1.0111 5.1402 0.62926 0.11778 2.8599 – 0.11827 – 1.104 5.2884 0.65177 0.07299 2.9651 – 0.16528 – 1.1369 5.4367 0.60077 0.03023 3.0818 – 0.2123 – 1.1812 – 0.54978 – – – – –

Conclusion is the availability of experimental (29) and theoretical (30, 31) work on platinum. Therefore, The exploration of model parameters, Debye it may be concluded that the incorporation of temperature and DOS are reported by use of VWI is requisite for the absolute interpretation the present model VTBFSM for platinum. The of the phonon dynamical behaviour of platinum. conformity with experimental data (8–10) of Many researchers have also successfully reported our theoretical peak is very good for platinum. theoretical results for alkali halides (32–42) by A successful explanation of spectra has provided use of the present model. Hence, the present the next best test of any model for their higher model may be understood to provide a powerful range of frequency. Small deviations were and simple approach for a comprehensive study observed at the higher temperature side due of the harmonic as well as anharmonic elastic to harmonic approximation in the Debye curve. properties of the crystals under consideration. Better agreement has been obtained with The only constraint of VTBFSM is the knowledge the available experimental data (16–20) and of certain experimental parameters needed that theoretical results. The motivation of this work can be used as input data.

References 22. R. K. Singh and M. P. Verma, Phys. Status Solidi B, 1969, 36, (1), 335 1. E. W. Kellerman, Philos. Trans. R. Soc. London 23. R. K. Singh and M. P. Verma, Phys. Status Solidi B, Ser. A, 1940, 238, (798), 513 1970, 38, (2), 851 2. P. O. Löwdin, Ark. Mat. Astr. Fys., 1947, 35A, 30 24. R. K. Singh, H. N. Gupta and M. K. Agrawal, Phys. Rev. B, 1978, 17, (2), 894 3. P. O. Löwdin, Philos. Mag. Suppl., 1956, 5, (1) 25. K. S. Upadhyaya, M. Yadav and G. K. Upadhyaya, 4. S. O. Lundqvist, Ark. Fys., 1952, 6, (3), 25 Phys. Status Solidi B, 2002, 229, (3), 1129 5. S. O. Lundqvist, Ark. Fys., 1955, 9, 435 26. R. F. Macfarlane, J. A. Rayne and C. K. Jones, 6. S. O. Lundqvist, Ark. Fys., 1957, 12, 263 Phys. Lett., 1965, 18, (2), 91 7. A. D. B. Woods, W. Cochran and B. N. Brockhouse, 27. H. Closs and M. M. Shukla, Nuovo Cim. B, 1977, Phys. Rev., 1960, 119, (3), 980 42, 213 8. S. Rolandson and G. Raunio, J. Phys. C: Solid 28. A. Konti, J. Chem. Phys., 1971, 55, (8), 3997 State Phys., 1971, 4, (9), 958 29. G. Raunio and S. Rolandson, Phys. Rev. B, 1970, 9. G. Raunio and S. Rolandson, J. Phys. C: Solid 2, (6), 2098 State Phys., 1970, 3, (5), 1013 30. L. P. Sharma, PhD Thesis, Agra University, Agra, Uttar Pradesh, India, 1979 10. G. Raunio and S. Rolandson, Phys. Status Solidi B, 1970, 40, (2), 749 31. R. J. Rollefson and P. P. Peressini, J. Appl. Phys., 1972, 43, (2), 727 11. J. R. Tessman, A. H. Kahn and W. Shockley, Phys. Rev., 1953, 92, (4), 890 32. S. K. Tiwari, L. K. Pandey, L. J. Shukla and K. S. Upadhyaya, Phys. Scr., 2009, 80, (6), 065603 12. H. N. Gupta and R. S. Upadhyaya, Phys. Status Solidi B, 1980, 102, (1), 143 33. U. C. Srivastava and K. S. Upadhyaya, Optoelectron. Adv. Mater., Rapid Commun., 2010, 13. R. K. Singh and H. N. Gupta, Proc. R. Soc. London 4, (9), 1336 Ser. A, 1976, 349, (1658), 289 34. U. C. Srivastava and K. S. Upadhyaya, Phys. Rev. 14. V. Mishra, S. P. Sanyal and R. K. Singh, Philos. Res. Int., 2011, 1, (1), 16 Mag. A, 1987, 55, (5), 583 35. U. C. Srivastava, Optoelectron. Adv. Mater., Rapid 15. R. K. Singh, H. N. Gupta and S. P. Sanyal, Nuov. Commun., 2013, 7, (9–10), 698 Cim. B, 1980, 60, 89 36. J. P. Dubey, R. K. Tiwari, K. S. Upadhyaya and P. K. 16. H. H. Lal and M. P. Verma, J. Phys. C: Solid State Pandey, Turk. J. Phys., 2015, 39, 242 Phys., 1972, 5, (5), 543 37. R. K. Das, B. C. Neog and B. J. Saikia, IOSR J. 17. A. M. Karo and J. R. Hardy, J. Chem. Phys., 1968, Appl. Phys., 2015, 7, (3), 6 48, (7), 3173 38. J. P. Dubey, R. K. Tiwari, K. S. Upadhyaya and P. K. 18. B. G. Dick and A. W. Overhauser, Phys. Rev., Pandey, Turk. J. Phys., 2016, 40, 201 1958, 112, (1), 90 39. U. C. Srivastava, Int. J. Mod Phys. B, 2017, 31, 19. U. Schröder, Solid State Commun., 1966, 4, (7), (4), 1750020 347 40. U. C. Srivastava, J. Sci. Arts, 2017, 2, (39), 309 20. A. N. Basu and S. Sengupta, Phys. Status Solidi B, 41. J. P. Dubey, P. K. Pandey and K. S. Upadhyaya, 1968, 29, (1), 367 AASCIT J. Phys., 2018, 4, (1), 1 21. M. P. Verma and R. K. Singh, Phys. Status Solidi B, 42. U. C. Srivastava and M. P. Srivastava, J. Sci. Arts, 1969, 33, (2) 769 2019, 1, (46), 235

The Author

U. C. Srivastava obtained his MSc (Physics) and PhD (Solid State Physics) degrees from Veer Bahadur Singh Purvanchal University, Jaunpur, Uttar Pradesh, India. He obtained his MTech (Electronics & Telecommunication) degree from Karnataka State Open University, Mysore, India. He currently works as Assistant Professor-III in the Department of Physics, Amity Institute of Applied Sciences, Amity University, India. He has 17 years’ teaching and research experience. His area of research is theoretical lattice dynamical study of ionic crystals. He has published 24 research papers in different national and international journals.

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Electrodeposition of Iridium-Nickel Thin Films on Copper Foam: Effects of Loading and Solution Temperature on Hydrogen Evolution Reaction Performance of Electrocatalyst in Alkaline Water Improved performance and stability of catalyst for hydrogen energy applications

Jianwen Liu, Wangping Wu*, Xiang Tafel slopes of 40–49 mV dec–1 exhibited highly Wang, Yi Zhang efficient electrocatalytic activity for HER. The Electrochemistry and Corrosion Laboratory, electrocatalytic activity of iridium-nickel thin films School of Mechanical Engineering and Rail showed a loading dependence. As the solution Transit, Changzhou University, Changzhou temperature increased from 20°C to 60°C, the 213164, China hydrogen evolution performance of iridium-nickel thin films improved. The apparent activation –1 *Email: [email protected] energy value of Ir88Ni12 film was 7.1 kJ mol . Long-term hydrogen evolution tests exhibited excellent electrocatalytic stability in alkaline Developing novel hydrogen evolution reaction solution. (HER) catalysts with high activity, high stability and low cost is of great importance for the applications of hydrogen energy. In this work, iridium-nickel 1. Introduction thin films were electrodeposited on a copper foam as electrocatalyst for HER, and electrodeposition The continuous increase in energy consumption mechanism of iridium-nickel film was studied. The on the earth could push for alternative routes of morphology and chemical composition of thin films energy generation and storage, in order to make a were determined by scanning electron microscopy transition toward a more sustainable system (1, 2). (SEM) and energy-dispersive spectroscopy (EDS), Hydrogen is increasingly considered to be one of respectively. The electrocatalytic performances the most promising energy carriers (3). Alkaline of the films were estimated by linear sweep water electrolysis derived from renewable energy voltammograms (LSV), electrochemical impedance is a promising technology to produce hydrogen (4). spectroscopy (EIS) and cyclic voltammetry (CV). However, the energy consumption, which is The results show that iridium-nickel thin films directly proportional to cell voltage during the were attached to the substrate of porous structure production of hydrogen by water electrolysis, is and hollow topography. The deposition of nickel currently high (5). In order to resolve the demand was preferable in the electrolyte without the for energy-saving, cathode materials with high addition of additives, and the iridium-nickel thin catalytic activity can be designed to reduce the film was alloyed, resulting in a high deposition overpotential of HER. rate for Ir42Ni58 thin film, and subsequently an Currently, platinum is the most efficient HER increase of iridium content in the thin films of catalyst in alkaline electrolytes due to both low

Ir80Ni20 and Ir88Ni12. Iridium-nickel thin films with overpotential and excellent stability. However, due

94 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15911991747174 Johnson Matthey Technol. Rev., 2021, 65, (1) to the high cost, reducing the use of platinum is of Electrodeposition is a powerful cost-effective and critical importance for developing efficient affordable reliable film process without vacuum (24–28). catalysts for HER. An effective alternative method Electrodeposited nickel-based materials have good to decrease the use of precious metals in catalysts repeatability, simple preparation method, low is by alloying with transition metals such as nickel, requirements on experimental conditions and good cobalt and iron. The properties of precious metals, catalytic properties for HER (29). To achieve the such as their lattice parameters, binding energy electrodeposition process, the operation conditions, and bond length, can be tuned by alloying them such as pH, deposition time, applied current density with non-precious metals to change the adsorption or potential bath temperature and complexing energies and achieve optimal catalytic activity (10). agents should be precisely controlled (25–28). In Nickel provides good catalytic activity, chemical our publications (30–32), the iridium-nickel alloy and mechanical stability and reasonably low cost films were electrodeposited from citrate and oxalic compared to other possible materials (11, 12). acid aqueous solutions. In citrate aqueous solutions, Nickel binary alloys could provide both a reduced nickel-iridium alloys with iridium content as high hydrogen overvoltage at the electrode surface and as 37 at% and Faraday efficiency as high as 44% high quantity of hydrogen absorbed. were obtained, the codeposition of nickel-iridium 2+ Iridium has unique physical and chemical alloy was a normal deposition for IrCl3‑Ni ‑citrate properties, such as high melting point, low oxygen acid system. Recently, iridium-nickel thin film permeability, high chemical stability, superior electrocatalysts were electrodeposited on copper oxidation resistance, high corrosion resistance and foam from hexabromoiridate(III) solution, and the high catalyst activity for both hydrogen and oxygen HER performance of iridium, nickel and iridium- evolution reactions (13–19). Iridium oxides, both nickel thin-film electrocatalysts was studied and anhydrous rutile-type IrO2 and hydrated amorphous published in our laboratory (32). Iridium-nickel forms, have been widely considered as oxygen thin film with large real active area exhibited evolution reaction (OER) catalysts (20). The iridium- highly efficient electrocatalytic activity for HER, nickel alloy system is of great interest for catalytic and achieved a current density of 10 mA cm–2 and corrosive environment applications (20–24). at an overpotential of 60 mV and a Tafel slope of The iridium-nickel alloy is of particular interest 40 mV dec –1, which is superior to pure iridium because of the complete miscibility of iridium and nickel thin films. The copper foam is used and nickel, and the absence of intermetallic to improve catalyst structure to obtain more compounds in the binary phase diagram. Kuttiyiel active sites on a supported electrode. Fabricating et al. (21) synthesised core-shell, hollow-structured porous materials with unique morphology and Ir–Ni–N nanoparticles and then evaluated their HER microstructure to increase the number of active activity. Coupling nickel nitrides with the IrNi cores sites or introducing additional metal or non- enhanced the HER activity of iridium shells to a level metal elements to the catalyst to increase the comparable to that of Pt/C while reducing iridium intrinsic activity of the active site have been loading of the catalyst. Özer et al. (16) explored proven to be effective ways to further enhance the the potentiostatic electrodeposition process of electrocatalytic performance of the catalysts (33). nickel onto polycrystalline iridium and assessed These methods could help catalysts tune the the electrocatalytic properties of the bimetallic electronic structures and optimise the absorption surfaces. An overlayer of nickel on iridium showed energy for HER. The catalysis through increased superior performance in the acidic and alkaline loading may also expose more active sites per conditions. Vázquez-Gómez et al. (22) modified gram (29). However, the research on the effect porous nickel electrodes by electrodeposition of of loadings and solution temperature on hydrogen iridium nuclei from H2IrCl6 solutions at 70°C, evolution performance of electrocatalysts is still with the aim of activating them towards the HER rare and limited. Pierozynski and Mikolajczyk (34) and comparing their performance with those of studied the HER performance of nickel foam and porous nickel electrodes activated by spontaneous palladium-activated nickel foam materials in 0.1 M deposition of noble metals. The current efficiency sodium hydroxide solution over the temperature of iridium deposition was very low (1% or lower), range of 20–60°C. Both Tafel slopes and charge which is not commercially useable. Sawy and transfer coefficient exhibited temperature- Birss (23) studied nanoporous iridium and its oxide dependent behaviour. Devadas and Imae (35) thin films formed by electrodeposition for a wide evaluated the HER efficiency with a small amount variety of electrocatalytic and sensing applications. of platinum nanoparticles immobilised on graphene

95 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15911991747174 Johnson Matthey Technol. Rev., 2021, 65, (1) oxides, carbon nanohorns and carbon nanotubes solution pH was examined by a PHS-3C pH meter by dendrimer-mediated chemical reaction, and and was adjusted to the desired value by adding found that the high HER efficiency from low loading 5.0 M NaOH while being stirred. The volume of the of platinum contribute to the development of low electrolyte in the cell was approximately 15 ml. The cost hydrogen energy or battery systems. solution temperature was controlled by a HH-501 In this study, iridium-nickel thin films were thermostatic bath (Jintan Baita Xinbao Instrument electrodeposited on copper foams with different Factory, China). The mass change of the specimens deposition times. The present work aims to study before and after deposition was recorded by an the effects of loadings and solution temperature analytical balance (FA2004B, resolution 0.1 mg). on the HER performance of iridium-nickel thin Table I shows the composition of the plating bath films in alkaline solutions, simultaneously to and the deposition parameters, three specimens investigate electrodeposition mechanism and were prepared in the same electrolyte, the first the microstructure and chemical composition of specimen #1 was deposited with 1 min, then iridium-nickel thin films. the second specimen #2 for 3 min and the last specimen #3 for 5 min. 2. Experimental The CV of iridium in the bath of 26 mM Na3[Ir(III)Br6], nickel in the bath of 26 mM

2.1 Chemicals and Electrodeposition NiSO4·6H2O and iridium-nickel in the bath of

26 mM Na3[Ir(III)Br6] and 26 mM NiSO4·6H2O were The bath chemistry consisted of trisodium performed, using an electrochemical workstation hexabromoiridate (Na3[Ir(III)Br6]) and nickel (CHI 660E). The electrocatalysts on copper sheets sulfate hexahydrate (NiSO4·6H2O). All chemical with diameter of 1 cm served as the working reagents were analytical grade. The bath solution electrodes. The platinum foil with diameter of 1 cm was purged with pure nitrogen for 10 min before served as the counter-electrode and a reference turning on the current. A nitrogen blanket was Ag/AgCl 3 M KCl electrode was used. The solutions passed over the solution during electrodeposition. for CV tests of iridium-nickel, nickel and iridium In all experiments, a magnetic stir bar was used films were electrolytes at room temperature with a to stir the solution. Copper foam was supplied by scanning rate of 10 mV s–1. Taili Foam Metal Corporation, China (>99.99% copper, porosity ≥98%). The copper foam samples 2.2 Characterisations (10 mm × 10 mm × 0.3 mm) were first immersed in acetone for 40 min and ultrasonically cleaned The microstructure and morphology of the surface for 10 min. The samples were then immersed in a of the films were observed by SEM (SUPRA 55, mixture solution of nitric acid (HNO3)·H2O (volume Sapphire and ZEISS Sigma, ZEISS Microscopy, ratio = 1:1) at room temperature for 5–10 s in a Germany). The chemical composition of the film fume hood. Finally, the samples were washed with was determined by X-ray EDS using x-act detector pure water and then dried in air. A conventional (Oxford Instruments plc, UK). three-electrode cell was used for electrodeposition The chemical composition and elemental states of iridium-nickel thin films on a copper foam sheet of the top surface of the iridium-nickel thin as the working electrode, platinum foil as the film on copper foam before HER performance counter-electrode and a reference Ag/AgCl 3 M KCl testing were performed by X-ray photoelectron electrode. The galvanostatical electrodeposition and spectroscopy (XPS), (ESCALABTM 250Xi, Thermo electrochemical processes were carried out in the Fisher Scientific, USA) with an ultrahigh vacuum cell, using a CHI 660E electrochemical workstation of 3.3 × 10−8 Pa base pressure. The top surface (CH Instruments Inc, China) coupled to a computer of the iridium-nickel thin film on copper foam was with specific data acquisition software installed. The irradiated with an aluminium Kα monochromatic

Table I The Composition of the Plating Bath and Process Parameters 3+ 2+ o –2 No. [Ir(III)Br6] , mM Ni , mM pH Temperature, C j, mA cm Time, min ΔW, mg #1 13.5 40.5 4 80 20 1 2.3 #2 13.5 40.5 4 80 20 5 3.5 #3 13.5 40.5 4 80 20 10 4.2 Note: j = current density, ΔW = weight gain.

source of 1486.6 eV, and the outcome electrons region iC is due to the charging of the double-layer were analysed by a spherical capacitor analyser capacitor. Thus, the current iC is proportional to the using the slit aperture of 0.8 mm. The carbon scanning rate vscan (Equation (ii)): signal for C 1s at 285 eV was taken as an energy iC=×ν (ii) reference for the measured peaks. In order to C scandl identify the elements on the film surface, a low- CV measurements were conducted at different resolution survey spectrum was firstly taken over scanning rates ranging from 10 mV s−1 to −1 a wide energy range of 0–1350 eV, with a pass 160 mV s . The capacitor charging current iC at −1 energy of 150.0 eV at increments of 1.0 eV step . 0.05 V is plotted against the scanning rate nscan,

High-resolution spectra were acquired with a which shows good linear dependence. Then, Cdl is pass energy of 30.0 eV at small increments of obtained from the slope of the linear fitting. The 0.05 eV step−1, to allow precise determination relative ECSA of the iridium-nickel catalysts with of the position of the peaks and their shapes. different loadings are compared by theirC dl values. Curve-fitting was done with Gaussian–Lorentzian Potential values were corrected with the ohmic functions, using the XPSPEAK 4.1 software. Two (iR) drop by the electrolyte resistance obtained fitting parameters, the position of the peak and from EIS measurements taken in a potentiation its full width at half-maximum (FWHM), were fixed mode in the frequency range from 100,000 Hz to within less than about ± 0.2 eV. 0.01 Hz. The solution temperature was controlled in the range of 20–60°C. 2.3 Electrochemical Performance Measurements 3. Results and Discussion

The LSV was tested in an alkaline solution at a 3.1 Cyclic Voltammograms scanning rate of 5 mV s−1 using a three-electrode system. Nitrogen was bubbled through the 1.0 M In order to better understand the electrodeposition potassium hydroxide solution for 1 min before mechanism, the CV measured in electrolytes all electrochemical measurements were done. containing nickel, iridium and both iridium and The iridium-nickel electrocatalysts deposited on nickel salts are shown in Figure 1. The reduction the copper foam electrode served as the working peaks of iridium, nickel and iridium-nickel electrode. The graphite rod and Hg/Hg2Cl2 were were at −0.56 V, −0.37 V and −0.54 V due to used as the counter and reference electrodes, the reduction of Ni(II) to Ni0 and Ir(III) to Ir0, respectively. The catalytic activities of iridium- respectively. It can be found that the reduction nickel thin films with different loadings and solution potentials of iridium-nickel and iridium are very temperatures were compared by LSV testing. close and their peak currents are much higher Furthermore, the potentials were transferred to a reversible hydrogen electrode (RHE) potential. The formula for calibrating the saturated calomel 0.002 Ir Oxidation peak 0 3+ electrode (SCE) to RHE is as follows (Equation (i)): Ir-Ni IrIr3+→Ir4++ e– Ni 0.001 Ev()sE.. RHE = ()vs SCE + ESCE ()0.241V (i) H→H+ + e– + 0.0592pH A 0 Ir The CV of an iridium-nickel thin film was performed

in alkaline solution using an electrochemical Current, –0.001 2+ – workstation (CHI 660E). The iridium-nickel thin film Ni Ni + e →Ni H+ + e–→H0 on the copper foam electrode served as the working –0.002 Ir3+ + e–→Ir0 electrode. In this work, the electrochemically Ir-Ni Reduction peak active surface area (ECSA) of iridium-nickel –0.003 catalysts were compared on a relative scale using –1.0 –0.8–0.6 –0.4 –0.20 Potential, V vs. Ag/AgCl the capacitance of the electrochemical double layer Fig. 1. Cyclic voltammograms of iridium-nickel (Cdl) on the electrode-electrolyte interface. This 3– electrode in the bath of 26 mM [Ir(III)Br6] and comparison is validated as ECSA is proportional 26 mM Ni2+, nickel electrode in the bath of 26 mM 2+ to Cdl. Cdl is obtained by the following procedure Ni and iridium electrode in the bath of 26 mM 3– reported by McCrory et al. (36), which assumes [Ir(III)Br6] that the measured current at the non-Faradaic

97 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15911991747174 Johnson Matthey Technol. Rev., 2021, 65, (1) than that of nickel, which may lead to much more Table II Atomic Composition (at%) iridium content in the iridium-nickel deposits. of Iridium-Nickel Thin Films Without the addition of complexing agent in the Determined by EDS electrolyte, the reduction potential of Ni2+ ions is Content, at% 3– Specimen higher than that of [Ir(III)Br6] ions. Therefore, Nickel Iridium the deposition of nickel was preferable during 1 min, 2.3 mg cm–2 58 42 the electrodeposition process. The oxidation peak 3 min, 3.5 mg cm–2 20 80 for iridium is present at –0.24 V vs. Ag/AgCl. The 5 min, 4.2 mg cm–2 12 88 oxidation peak was mainly attributed to iridium oxidation, Ir0 → Ir(III). However, the oxidation peak for iridium-nickel is present at around to CV curves, nickel is preferentially deposited –0.24~–0.15 V vs. Ag/AgCl because the iridium- because the deposition potential of nickel is higher nickel codeposited film is suggested to be alloyed. than that of iridium. Therefore, the deposition There is no oxidation peak for nickel at negative rate for the first specimen, about 2.3 mg min–1 is potential. For iridium-nickel film, the current peak larger than that of the others. Subsequently, the at –0.54 V was due to the hydrogen adsorption, deposition rate kept stable, and the nickel content while the current peak at around –0.24~–0.15 in the films decreased. The slope of the second V was due to the reoxidation of the codeposited plot is around 0.35, indicating a deposition rate of deposits. For iridium film, the current peak about 0.35 mg min–1. The atomic composition of at –0.56 V was observed due to the hydrogen the film by EDS is listed in Table II. The iridium adsorption, which is higher than the current peak content in the deposits increased remarkably from at –0.23 V due to hydrogen desorption. These 1 min to 5 min. This result is in agreement with the findings indicate that iridium and iridium-nickel above discussion. deposits have a significant facility for hydrogen Figure 3 shows the surface morphology of incorporation into the plated deposits (37). The iridium-nickel electrocatalysts and copper foam. deposition processes of iridium and iridium- Figure 3(a) shows a large number of dendritic nickel films are accompanied by a large amount structures for copper foam with diameters of hydrogen evolution. 30–60 nm. In Figures 3(b) and 3(c), many pores and hollowed topography can be observed. 3.2 Structural Characterisation The grain boundary of the copper foam is clearly visible because of the thin layer. With the increase Figure 2 shows the plots of the mass gain of the of deposition time, the thickness of the film also specimens deposited in one electrolyte with different increases. It can be clearly seen that the iridium- deposition times. There is a nonlinear dependence nickel thin film is attached to the surface of the of the mass gain against deposition time. According substrate (see Figure 3(d)). The cross structure and many pores of copper foam can increase the active area of the thin film, which is advantageous 5 for hydrogen evolution performance. Figure 4 shows the EDS spectrum of the iridium-nickel 4 electrocatalysts with different loadings. The The second line chemical composition of iridium and nickel in the thin films is shown in Table II. EDS elemental 3 mapping to probe nickel and iridium presence

for Ir80Ni20 and the distribution on the substrate 2 surface is shown in Figure 5. A large amount of

Mass gain, mg The first line copper is evenly distributed on the surface. It can 1 be observed that the amount of iridium is larger

than that of nickel in the image. Ir80Ni20 thin film was almost completely covered on the copper foam 0 surface. The phase and crystallographic structure of the films on copper foam were determined by 012345 Deposition time, min X-ray diffraction (XRD), however the signals of the films were not detected due to small loading, Fig. 2. Plots of the mass gain of the specimens the information of copper foam was only present.

Fig. 3. SEM images (a) (b) of copper foam: (a) iridium-nickel films; (b) 2.3 mg cm–2; (c) 3.5 mg cm–2; (d) 4.2 mg cm–2

×100 100 µm ×3,000 5 µm

×3,0005 µm ×3,0005 µm

The copper foam was composed of polycrystalline demonstrating the changes in the alloyed phase, in structure. turn enhancing the catalytic performance (39–41). The chemical composition and elemental states In the O-1s spectrum (see Figure 7(b)), the main of Ir80Ni20 thin films were deeply analysed by XPS peak at 530.5 eV is attributed to O 1s of the oxides, technique. The atomic composition of Ir80Ni20 and the peaks at 531.3 eV and 531.9 eV are usually thin films on copper foam is listed in Table III. ascribed to surface species, such as hydroxyls or Figure 6 shows the XPS depth profile for the top absorbed water of the film. An additional broader surface of iridium-nickel thin films on copper foam. feature peak is present at a higher binding energy The elements copper, iridium, carbon, oxygen and of ~533.2 eV. bromine were determined on the top surface of as-deposited film. Unfortunately, the signal nickel 3.3 Electrocatalytic Behaviour element was not detected, probably due to the low quantities in the film. The large amounts of carbon The electrocatalytic activities for HER of iridium- and oxygen contents were attributed to ordinary nickel thin films with different loadings were adsorption from the environment (see Table III), investigated in 1.0 M KOH solutions. Figure 8 significant amounts of oxides formed on the surface presents the iR-corrected LSV curves and Tafel of the electrode during electrodeposition. The slopes of the bare copper foam and iridium-nickel bromine signal was mainly from the electrolytes. samples. As anticipated, the bare copper foam

The coverage of the Ir80Ni20 thin film on copper displays a relatively low catalytic activity which foam was not so perfect that the signal copper was requires an overpotential of 502.5 mV to drive determined. Figure 7 shows the high-resolution a current density of 10 mA cm–2. In contrast,

XPS spectra of as-deposited Ir80Ni20 thin film on the iridium-nickel catalyst exhibits excellent copper foam. The binding energies of iridium were catalytic activity, demonstrating a negligible onset located at 63.6 eV and 60.6 eV for Ir0 4f5/2 and potential (8.3–18.3 mV) at 1 mA cm–2 for hydrogen 4f7/2, respectively. It was indicated that the film evolution in the electrolyte (see Table IV), which was composed of the metallic state of iridium, are much lower than the onset potential of copper although the nickel signal was not detected foam. Here, the onset potential should always be (Figure 7(a)). The binding energies of Ir0 4f5/2 defined on the basis of a specific current density, and 4f7/2 have weak shifts in iridium-nickel thin where the Tafel constant can be considered as films in contrast with pure iridium (63.8 eV and the onset potential of HER (42, 43). From an 60.8 eV) (38). This is ascribed to the incorporation electrochemical point of view, the Tafel constant of nickel into the electronic structures of iridium, becomes complementary to the Tafel slope.

(a) (a) 1.2

1.0

0.7 KCnt

0.5

0.2

lr Ni lr 0 1 2 3 4 5 6 7 8 9 10 11 Energy, eV

(b) (b) 2.3

1.8

1.4 KCnt 0.9

lr 0.5

Ni lr 0 1 2 3 4 5 6 7 8 9 10 11 Energy, eV (c) (c) 954

763

572 KCnt 381 lr

190 lr Ni 0 1 2 3 4 5 6 7 8 9 10 11 Energy, eV Fig. 5. EDS elemental mapping images of Ir80Ni20 Fig. 4. EDS spectra of the iridium-nickel films: (a) thin film in Fig. 3(c): (a) copper-blue; (b) iridium- 2.3 mg cm–2; (b) 3.5 mg cm–2; (c) 4.2 mg cm–2 red; (c) nickel-green

In order to obtain a current density of has been performed. The Tafel equation is as –2 10 mA cm , Ir42Ni58, Ir80Ni20 and Ir88Ni12 thin follows (44) (Equation (iii)): films require overpotential of 76.4 mV, 60 mV η = bjlog/()j (iii) and 95.4 mV, respectively (see Table IV). These 0 values are already twice the overpotential of where j is the current density, j0 is the exchange 34 mV at the current density of 10 mA cm–2 for current density (i.e. a constant at η = 0 V) and the state-of-the-art Pt/C catalyst tested in the b is the Tafel slope. The equation indicates that same electrolyte. To shed light on insights about excellent catalysts should have both low Tafel the reaction kinetics, a detailed Tafel analysis slopes and high exchange current densities. The

Table III Atomic Composition (at%) of Ir Ni Thin Film on Copper (a) 80 20 9000 Foam Determined by XPS Sum of fitted peaks Ir-4f 8000 Elements Chemical composition, at% Cu2p 24.87 7000 t 6000 Ir4f 2.91 0 Ir 4f7/2 Ir0 C1s 34.37 5000 4f5/2

O1s 37.41 4000

Br3d 0.44 3000 Intensity, arbitrary uni 2000

1000 600,000 0 Cu-2 p 70 68 66 64 62 60 58 56 54 500,000 Binding energy, eV t

400,000 (b)

Ni-2 p 35,000 300,000 O-1 s O-1s 30,000 200,000 d t Intensity, arbitrary uni C-1s f Br-3 100,000 25,000 [OH–] -O-H Ir-4 O2– 0 20,000 H2O

–200 0 200 400 600 800 1000 1200 1400 Binding energy, eV 15,000 Intensity, arbitrary uni

Fig. 6. XPS depth profile for the top surface of 10,000 Ir80Ni20 thin film on copper foam

5000 542 540 538 536 534 532 530 528 526 524 522 potential-dependency of current density j is related Binding energy, eV to the interfacial electrocatalytic reaction n, as the Fig. 7. High-resolution XPS spectra of Ir Ni thin following (Equation (iv)): 80 20 film: (a) Ir-4f; (b) O-1s jn= Fν (iv)

–1 where n is the number of electrons, F is the Faraday’s Ir80Ni20 and Ir88Ni12 thin films were 49 mV dec , constant (96,500 mol C−1). Because the current 40 mV dec–1 and 43 mV dec–1 respectively, indicating density j is potential-dependent, n is also potential- that the Ir80Ni20 electrocatalyst has much higher dependent and consisted of three elementary steps intrinsic activity than other catalysts for HER while as the following Equations (v)–(vii): still being less active than the Pt/C catalyst (28mV Initial discharge or Volmer step: dec–1) (45). The Tafel slopes indicate the HER process of iridium-nickel film follows a Volmer-Heyrovsky He+−+→H (v) ads mechanism, where electrochemical desorption of Atom + ion or Heyrovsky step: hydrogen is regarded as the rate-limiting step, i.e.

the HER rate is determined by both H2O discharge HH+ +→H (vi) ads2 and desorption of H from the catalyst surface

Atom + atom or Tafel step: (46–48). The exchange current density (j0) of iridium-nickel thin films and copper foam were 2HH→ (vii) ads 2 calculated by the Tafel extrapolation method (see The above elementary steps lead to two mechanisms: Table IV), which reflects the catalytic activity Volmer-Heyrovsky and Volmer-Tafel. Three rate of the electrode material under the reaction determining steps, Volmer, Heyrovsky and Tafel thermodynamic equilibrium conditions. The j0 value are possible for the above two mechanisms. The of the Ir80Ni20 thin film was 87.6% of the Pt/C linear portions of the Tafel plots were fitted to the catalyst (0.75 mg cm–2, 15 wt% Pt) (45). Therefore,

Tafel equation, the Tafel slope values for Ir42Ni58, an iridium-nickel thin film has highly efficient

(a) Ir42Ni58, Ir80Ni20 and Ir88Ni12 were 59.82 mF cm–2, 132.91 mF cm–2 and 70.03 mF cm–2, 0 respectively. It indicates that the high hydrogen

evolution performance of Ir80Ni20 catalyst was –10 Ir42Ni58 mainly due to the high exposure of effective active

Ir80Ni20

–2 sites. On the other hand, the scanning range of –20 Ir Ni 88 12 –0.15~0.85V for Ir80Ni20 catalyst is larger than Cu foam the range of –0.25~0.65 V for other catalysts. In j, mA cm –30 our previous publication (32), the initial CV curve

of Ir80Ni20 catalyst was measured at a scanning –40 rate of 10 mV s–1. It was found that iridium oxides are electrochemically formed at high –50 positive potential on the surface of Ir80Ni20 thin –0.5 –0.4 –0.3 –0.2 –0.1 0 Potential, V vs. RHE film during a positive scanning direction, however the formation of iridium oxides cannot easily (b) be reduced to the metal state (32). The formed –0.5 iridium oxides could result in a significant decrease of HER activity. Therefore, when the scanning –0.4 range of Ir42Ni58 and Ir88Ni12 thin films was shifted 189 mV dec–1

V from –0.25 V to 0.65 V, the obtained Cdl values –0.3 Ir42Ni58 should be valid. Therefore, it is inferred that the

Ir80Ni20 hydrogen evolution performance of Ir88Ni12 film –0.2 Ir Ni 88 12 is better than that of Ir42Ni58 film, which might Overpotential, Cu foam be attributed to the increase in iridium content

–0.1 –1 of the film or electrode surface defects, resulting 43 mV dec –1 49 mV dec in increasing the number of effective active sites. 40 mV dec–1 0 The large active surface area could be related 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 to the porous structure and hollow architecture –2 Log(j/mA cm ) with crossed branch structure, which results in a Fig. 8. (a) Linear sweep voltammograms obtained significantly enhanced catalytic activity. The rough in 1 M KOH solution at room temperature and texture and the porous structure of copper foam –1 potential scan rate of 5 mV s ; (b) Tafel plots facilitate fast mass transport for the enhanced

reaction kinetics (82). However, Ir88Ni12 thin film electrocatalytic activity for HER. In addition, a with a good hydrogen evolution performance has summary of the hydrogen evolution performance a larger electrochemical surface area than Ir42Ni58 of electrocatalysts in alkaline solutions is listed in film. The electrocatalytic activity of iridium-nickel detail in Table V (49–81). It can be found that the catalysts show a loading dependence. Tafel slope of the iridium-nickel thin film is low, Electrochemical impedance measurements indicating iridium-nickel thin film has excellent were performed to further investigate the electrocatalytic performance. reaction kinetics of the HER process under The ECSA of the catalyst is proportional to the experimental conditions. Nyquist plots of the electrochemical double-layer capacitance copper foam, Ir80Ni22, iridium and nickel thin

(Cdl). As shown in Figure 9, the Cdl values for films as a function of overpotential are shown

Table IV Comparison of the HER Catalytic Performance of Different Catalysts in 1.0 M KOH at 298 K Onset potential, mV Overpotential, η, mV Tafel slope, Exchange current Samples (at 1 mA cm–2) (at 10 mA cm–2) mV dec–1 density, mA cm–2

Ir42Ni58 8.3 78 49 0.69

Ir80Ni20 11.4 60 40 0.657

Ir88Ni12 18.3 97 43 0.418 Copper foam 336 500 189 0.022 Pt/C (45) 0 40 29.5 0.75

102 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15911991747174 Johnson Matthey Technol. Rev., 2021, 65, (1) Refs. (49) (50, 51) (51–53) (54, 55) (56, 57) (58) (59) (60) (61) (62) (63) (64) (65) (66) (67) (68) (69) (70) (71) (72) (73) (74) (75) (76) (77) (78) (79) –1

b, mVdec 105 175 105 165 95.2 89 121 147 121 – 264.4 130 81 57 283 67 80.9 157 101 102 151 103 113 147 390 121 84 (250) (50) (200) (300) (300) (300) (250) (100) (200) (100) (100) (250) (135) (250) (250) (300) (250) (100) (250) (120) (150) η, mV 323 185 185 187 201.9 170 – 215 – 70 141 141 140 171 – 104 90 – 330 326 – – 430 340 460 1530 119 −3 −3 –2 −2 −3 −3 −3 −4 −3 −6 −2 −3 −3 −1 −3 , mAcm 0 j 3.6 × 10 – 1.86 × 10 – 1.54 × 10 3.10 × 10 3.3 × 10 5.43 6.939 × 10 – 5.385 0.6 1.62 0.24 5.25 3.778 4.6 80.71 × 10 13.2 76.5 × 10 5.85 × 10 49.9 × 10 47 × 10 3.98 × 10 2.66 × 10 2.5 × 10 4 Temperature, °C 30 80 80 80 25 70 25 25 25 90 25 25 25 70 25 80 80 25 25 25 70 30 25 30 85 25 70 Alkaline 32% NaOH 6 M KOH 6 M KOH 6 M KOH 1 M NaOH 1 M NaOH 1 M NaOH 5 M KOH 1 M KOH 3.5% NaCl 30% KOH 30% NaOH 1 M KOH 1 M NaOH 1 M NaOH 28% KOH 28% NaOH 1 M NaOH 1 M NaOH 1 M NaOH 1 M NaOH 1 M NaOH 1 M KOH 32% NaOH 8 M KOH 6 M KOH 25% KOH Methods Electroless plating Electrodeposition ditto ditto ditto ditto Arc melting Electrodeposition ditto ditto ditto ditto ditto ditto Thermal arc spraying Electrodeposition ditto ditto ditto Arc melting Electrodeposition ditto solidification Rapid Electroless plating Heat treatment Melt-spinning Plasma spraying Substrates Mild steel Mild steel Mild steel Mild steel Copper Copper – Carbon steel Copper Copper Nickel Graphite Copper Copper Steel Carbon steel Carbon steel Carbon steel Copper – Carbon cloth Mild steel – Mild steel – – nickel Perforated sheet 33 Ni 2 P 5 67 5 2 C 27 45 10 18 12 8 46 W 10 B P Mo P 90 82 83 71 92 Table V Summary of HER Electrocatalysts in Alkaline Solution on Exchange Current Densities, Overpotential and Tafel Slope Overpotential Densities, on Exchange Current in Alkaline Solution HER Electrocatalysts Table V Summary of Coatings Ni-P Ni-Mo Ni-Mo Ni-Mo-Fe Ni Ni Nickel NiMo Ni-Sn Ni-Fe-C Ni-S NiMn NiCoZn Ni NiTi NiFeZn Ni-S Ni–CeO Ni–LaNi Co CoNiFe Co–Mo Fe Ni-P Platinum Nano-Zr Raney Nickel

in Figure 10. The preparation of the iridium and nickel electrocatalysts was addressed (32). According to alternating current (AC) circuit theory, impedance spectra obtained for a given

Refs. (80) (80) (80) (80) (80) (81) (81) (81) (32) (32) (32) This work This work electrochemical system can be correlated to one or more equivalent circuit (83). Thus, different –1 equivalent circuits were suggested to model the present data and the relevant model with the minimum number of electrical elements. The

b, mVdec 35 48 60 96 72 138 166 42 40 112 69 49 43 model of Ir80Ni22 film consists of the solution

resistance (Rs), the low frequency time constant

(10) characterising the double-layer capacitance (Cdl)

(10) (10) (10) (10) (100) (100) (10) and charge transfer resistance (R ). The potential

(10) (100) (10) (10) (10) ct

η, mV 110 175 180 260 40 298 274 48 60 170 130.1 78 97 dependencies of the obtained data are shown in Table VI. Due to surface heterogeneity of solid

–2 electrodes resulting from surface roughness and formation of porous layers, a constant phase element (CPE) is commonly used to replace the

, mAcm capacitance (C) in a real electrochemical process, 0 j – – – – – 0.32 2.23 7.2 0.657 0.347 0.398 0.69 0.418 which mainly depends on a non-ideal capacitance

behaviour (84, 85). Rct values of iridium film and copper foam are 72.34 Ω cm2 and 76.64 Ω cm2, respectively. While the charge transfer resistance

of nickel and Ir80Ni22 films are large, about 2642 Ω cm2 and 1312 Ω cm2, indicating that

the Rct values of iridium film and copper foam are lower than those of nickel and iridium-nickel

Temperature, °C 25 25 25 25 25 25 25 25 25 25 25 25 25 films. According to the XPS data, there isno

iridium oxide on the catalyst surface for Ir80Ni22 thin films. It can be inferred that the iridium thin film was composed of metallic state. On the other hand, copper foam was immersed in nitric acid

Alkaline 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M NaOH 1 M NaOH 1 M NaOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH solution to activate it before the experiment. The absence of oxides in copper foam may result in a charge transfer resistance that is less than other

electrocatalysts as a result. For nickel and Ir80Ni22 thin films, the top surface might be composed of some nickel oxides. The surface of nickel-rich nickel-iridium thin films consisted of lots of nickel oxides, the amount of nickel oxides was much more than that of metal nickel (unpublished Methods Electrodeposition ditto ditto – Electrodeposition Spontaneous deposition ditto ditto Electrodeposition ditto ditto ditto ditto data). Therefore, there is a contradiction here. The electrocatalytic performance of the thin film involves various factors, such as surface chemical substances, the number of catalytic

) active sites per unit area, and the electronic effect of the thin film metal. At the cathode, the

Substrates foam Nickel foam Nickel foam Nickel – foam Nickel Nickel Nickel Nickel Copper foam Copper foam Copper foam Copper foam Copper foam process of hydrogen reduction for hydrogen gas requires energy to remove electrons from the

Continued metal electrode and connect electrons to protons ( to produce hydrogen. Therefore, the process of =Exchange current density; η= Overpotential, the value in the bracket is the target current density; b=Tafel slope b=Tafel is the target current density; in the bracket the value η= Overpotential, current density; =Exchange 20 58 12

0 transferring electrons from the electrode to the Ni Ni Ni hydrogen ions in the liquid phase has a certain 80 42 88 Table V

Coatings CoFe Iron Cobalt Nickel foam Platinum Porous nickel Porous NiIr Porous NiRu Ir Nickel Iridium Ir Ir Note: j resistance, which is a charge transfer resistance.

(a) (b) 0.04 0.08

10 mV s–1 10 mV s–1 0.03 0.06 20 mV s–1 20 mV s–1 60 mV s–1 60 mV s–1 0.02 80 mV s–1 0.04 80 mV s–1 120 mV s–1 120 mV s–1 –2 0.01 –2 0.02

0 0 j, A cm j, A cm

–0.01 –0.02

–0.04 –0.02

–0.06 –0.03

–0.4 –0.3 –0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 –0.2 0 0.2 0.40.6 0.8 Potential, V vs. RHE Potential, V vs. RHE

18 0.03 Ir42Ni58 10 mV s–1 –1 20 mV s 16 Ir80Ni20 0.02 60 mV s–1 –1 80 mV s –2 14 Ir88Ni12 –2 –1 132.91 mF cm 0.01 120 mV s

–2 12

0 10

j, A cm –2 0.05, mA cm 8 –0.01 70.03 mF cm j∆ 6 –0.02 4 59.82 mF cm–2 –0.03 2

–0.04 –0.3 –0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 20 40 60 80 100 120 Potential, V vs. RHE Scan rate, mV s–1

Fig. 9. CV curves with different scan rates for electrodes in the alkaline solution: (a) Ir42Ni58; (b) Ir80Ni20; (c) Ir88Ni12; (d) differences in current density at 0.05 Vvs . RHE (Δi= ia–ic) plotted against scan rate

Iridium electrode with a low resistance could 52 mV dec–1, 43 mV dec–1 and 40 mV dec–1, accelerate the electron transfer during the respectively. The comparison of the HER catalytic electrocatalytic reaction. performance at different temperatures is shown Figure 11 shows the polarisation curves and Tafel in Table VII. curves of the Ir88Ni12 film at different temperatures. To investigate the essence of the improvement of As shown in Figure 11(a), the electrode has the HER activity of the iridium-nickel electrocatalyst, best hydrogen evolution performance at 60°C, the apparent activation energies (Ea) of the film with only an overpotential of 186 mV to obtain a were determined via the following Arrhenius current density of 30 mA cm–2. At the temperature equations (86) (Equations (viii)–(x)): of 30°C, an overpotential of 212 mV is required.  Ea  The performance of hydrogen evolution improves j0 =⋅Fkc exp  −  (viii)  RT  from 30°C to 60°C. Interestingly, the hydrogen evolution performance of the catalyst has Ea log j0 =− (ix) decreased from 20°C to 30°C. The effect of 2.303RT temperature is not obvious, and the curves are djlog E 0 =− a (x) very close. This result can also be derived from  1  2.303R d   the Tafel slopes (see Figure 11(b)). From 20°C  T  to 60°C, the Tafel slope is 46 mV dec–1, 56 mV dec–1,

(a) (a) 0 600

550 Ir 20°C 500 Ir80Ni20 30°C Ni 450 40°C Cu Foam –5 400 50°C –2

2 100 60°C 350 Ir 90 Ir Ni 80 20 Ni 80 300 Cu Foam 70 j, mA cm

250 2 60 –Z", Ω cm –10 200 50 40 –Z", Ω cm 150 30 100 20 10 50 0 20 40 60 80 100 2 –15 Z', Ω cm –0.14 –0.12 –0.10 –0.08 –0.06 –0.04 –0.02 0 0 200 400 600 800 1000 Potential, V vs.SCE Z', Ω cm2 (b) (b) –0.040

CPEdl 20°C –1 –0.035 52 mV dec 30°C Rs 40°C 50°C V –0.030 Rct 60°C

–0.025 40 mV dec–1 Fig. 10. (a) Nyquist plots of Ir80Ni20 film, iridium film and copper foam for the HER in 1M KOH; (b) 43 mV dec–1 46 mV dec–1 Overpotential, –0.020 the corresponding equivalent electric circuit models for all samples –0.015

56 mV dec–1 –0.010 −2 where j0 is exchange current density (A cm ), F is 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 the Faraday’s constant, k is Kohlrausch coefficient Log(j/mA cm–2) (dimensionless), c is the concentration of reactant Fig. 11. (a) Linear sweep voltammograms obtained (constant), E is the apparent activation energy a in 1.0 M KOH solution at different temperatures and −1 (J mol ), T is the temperature (K), and R is potential scanning rate of 5 mV s–1; (b) Tafel plots

Table VI Electrical Equivalent Circuit Parameters 2 –2 2 Samples Rs, Ω cm Cdl, F cm Rct, Ω cm Iridium 1.218 0.098 72.34

Ir80Ni20 1.485 0.05076 1312 Nickel 13.6 0.003567 2642 Copper foam 1.89 0.005816 76.64

Table VII Comparison of the HER Catalytic Performance in 1.0 M KOH at Different Temperatures Temperature, Onset potential, Overpotential, η, mV Tafel slope, Exchange current °C mV (at 30 mA cm–2) mV dec–1 density, mA cm–2 20 16 192 46 0.54 30 16 212 56 0.62 40 16 204 52 0.53 50 16 193 43 0.44 60 16 186 40 0.40

–1 –1 the gas constant (8.314 J mol K ). The linear –1 relationship between logj0 and 1/T for Ir88Ni12 thin film is displayed in Figure 12. According to Equations (viii)–(x), the apparent activation –2 energy of Ir88Ni12 thin film electrocatalyst is −1 –1 calculated as 7.1 kJ mol , by the slopes of lines. 0 j Ea = 7.1 kJ mol Compared with the nickel cathode with about –3 log, 40 kJ mol–1 in standard electrolyte (87), this result indicates that the Ir88Ni12 thin films can Equation y = a + b*x Plot B –4 Weight No weighting remarkably reduce Ea for HER and accordingly Intercept –4.48426 ± 0.47883 Slope 369.28976 ± 149.28486 Residual sum of squares 0.00755 result in higher electrocatalytic activity. Hence, Pearson's r 0.81916 R-square (COD) 0.67103 the codeposition process of nickel and iridium Adj. R-square 0.56137 –5 species on the copper foam provides a large 0.0030 00.0031 0.0032 0.0033 0.0034 0.0035 number of active centres for hydrogen adsorption, 1/T, K–1 with the synergetic effect giving electronic structure suitable for HER. Fig. 12. Dependence on temperature (Arrhenius plots) of the exchange current density for the AC impedance characterisation of HER on the Ir88Ni12 film Ir88Ni12 electrode in 1.0 M KOH with different temperatures is shown in Figure 13. Nyquist plots of Ir88Ni12 film as a function of overpotential are shown in Figure 13(a). Impedance spectra (a) obtained for a given electrochemical system 18 20°C 30°C can be correlated to an equivalent circuit 16 40°C (Figure 13(b)). The temperature dependence of 14 50°C 60°C Rct and Cdl parameters for the HER of Ir88Ni12 film 12 examined at the temperature range of 20–60°C 2 10 is present in Table VIII. The Ir88Ni12 electrode 8 exhibited single, ‘depressed’ semicircles (a single- –Z", Ω cm step charge-transfer reaction) at all reaction 6 temperatures, in the explored frequency range, it 4 is noted that a high-frequency semicircle electrode 2 porosity response, which is typically observed in alkaline media, was practically indiscernible (34). 0 10 20 30 40 50 The recorded Rs parameter decreased from Z', Ω cm2 1.572 Ω cm2 at 30°C to 1.038 Ω cm2 at 60°C. (b) Simultaneously, the Rct parameter significantly CPE reduced from 48.34 Ω cm2 to 14.75 Ω cm2 for dl the same temperature range (see Table VIII). Rs

The lower Faraday resistance of the Ir88Ni12 R electrode surface accelerates the electron ct transfer during the electrocatalytic reaction. Fig. 13. (a) Nyquist plots of Ir88Ni12 film for the The Cdl parameter was significantly reduced HER in 1M KOH at different temperatures; (b) the –2 –2 from 0.02515 F cm to 0.02885 F cm . The corresponding equivalent electric circuit models effect most likely results from partial blocking

Table VIII Electrochemical Parameters for the HER on Ir88Ni12 Thin Film Electrode in Contact with 1.0 M KOH, Studied over the Temperature Range of 20-60°C Parameters 20°C 30°C 40°C 50°C 60°C

2 Rs, Ω cm 1.572 1.366 1.126 1.041 1.038 –2 Cdl, F cm 0.02515 0.03417 0.02987 0.03334 0.02885 2 Rct, Ω cm 48.34 44.74 24.33 20.53 14.75

107 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15911991747174 Johnson Matthey Technol. Rev., 2021, 65, (1) of electrochemically active electrode surface by electrocatalytic activity of the electrode at low fresh hydrogen bubbles (34). overpotential (see Figure 14(b)). This slight Apart from the catalytic activity, stability is increase in the catalytic activity was possibly another important requirement of catalysts for owing to the reduction of surface oxides during the HER system. In this case, the long-term the initial period of hydrogen evolution, while stability of Ir80Ni20 film electrocatalyst is assessed the observed increase in overpotential as shown in 1.0 M KOH at constant current densities of 10 in Figure 14(a) could be a result of increased mA cm–2 for 10 h (see Figure 14(a)). A slight mass exchange resistance due to the continuous increase in the overpotential has been observed gas bubbling (88). in the V-t curve. The result of the long-term hydrogen evolution tests exhibited excellent electrocatalystic stability in alkaline solution. 4. Conclusions Polarisation curves recorded after 400 cycles Iridium-nickel thin films were successfully prepared on copper foam by electrodeposition under different (a) deposition times. The morphology, chemical 0.6 composition and electrocatalytic performance of iridium-nickel thin films were studied. At the same 0.4 time, the electrodeposition mechanism of iridium- nickel thin film in the electrolyte without the V 0.2 addition of complexing agent was studied. It was found that the deposition of nickel was preferable –2 0 10 mA cm during electrodeposition of iridium-nickel thin film

Overpotential, and the iridium-nickel thin film was alloyed, which

–0.2 could result in high deposition rate for Ir42Ni58 thin film, and subsequently the increase of iridium

–0.4 content in the thin films of 80 Ir Ni20 and Ir88Ni12. Iridium-nickel thin films have highly efficient 0 246810 electrocatalytic activity for HER. The Tafel slopes Time, h of iridium-nickel thin films were 40–49 mV –1 dec , (b) indicating that the HER process of iridium-nickel films followed a Volmer-Heyrovsky mechanism. The 0 Intial Ir80Ni20 thin film possesses the good electrocatalytic After 400 cycles performance due to the incorporation of nickel into

–10 the electronic structures of iridium, enhancing the

–2 catalytic performance. The surface of Ir80Ni20 thin film was composed of metallic state of iridium and –20

j, mA cm some oxides. The electrocatalytic activity of iridium- nickel thin film shows a loading dependence. As the temperature raised from 30°C to 60°C, the hydrogen –30 evolution performance of the iridium-nickel thin film became better. The apparent activation energy –1 –40 value of the Ir88Ni12 film was 7.1 kJ mol . Long- –0.20 –0.15 –0.10 –0.05 0 term hydrogen evolution tests exhibited excellent Overpotential, V vs. RHE stability in the alkaline solution. Fig. 14. (a) Chronoamperometric durability test for Ir80Ni20 film at a constant current density of 10 mA –2 cm for 10 h; (b) polarisation curves of Ir88Ni12 Acknowledgements film initially and after 400 cyclesvs . RHE at a scan rate of 5 mV s–1 in 1.0 M KOH solution This research was partially supported by Changzhou Sci & Tech Program (Grant No. CJ20190041) and the Natural Science Foundation of Jiangsu Province testing for Ir88Ni12 film indicate that there is a (Grant Number: BK20150260). little decay while the overpotential exceeds Wangping Wu designed the study and supervised 0.068 V, instead, a slight improvement of the MSc student, Jianwen Liu, who performed most

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The Authors

Jianwen Liu received his BS and MS degrees from Taizhou Institute of Science and Technology, Nanjing University of Science and Technology (NJUST), China, in 2017, and Changzhou University, China, in 2020, respectively. His research focusses on iridium- based metal electrocatalysts by electrodeposition for hydrogen evolution reactions and electrodeposition of rhodium and electrochemical corrosion. He has published nine papers in peer-reviewed international journals and one patent.

Wangping Wu received his PhD in Materials Processing Engineering from Nanjing University of Aeronautics and Astronautics, China, in 2013. He joined the Tel Aviv University, Israel, as a Postdoctoral Fellow from October 2013, and then jointed the Hochschule Mittweida University of Applied Sciences, Germany, and Technische Universität Chemnitz, Germany, from September 2019 as a visiting scholar by the support of China Scholarship Council (CSC). He currently works as a Senior Lecturer at the School of Mechanical Engineering and Rail Transit, Changzhou University, China. His research focuses on the synthesis, characterisation and performance of films and coatings of the noble metals and their alloys, nanopowders dispersed in polymer and electrochemical additive manufacturing. He has published over 60 papers in peer-reviewed international journals.

Xiang Wang received his BS degree from Changzhou University in 2018. He is currently pursuing his MS degree at Changzhou University. His research focuses on the electrodeposition of the noble metals and their alloys and electrochemical corrosion of metal coatings. Up to now, he has published four papers and one patent.

Professor Yi Zhang is Dean of the Mechanical Engineering and Rail Transit department of Changzhou University. He received his PhD in Mechanical Manufacturing and Automation from University of Science and Technology of China in 2000. His research focuses on modern design methods for electromechanical systems, design of electromechanical drives and control systems and design of industrial automation and monitoring systems. He has published more than 100 academic papers and more than 30 patents, one monograph and three textbooks.

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Different Deformation Behaviour Between Zirconia and Yttria Particles in Dispersion Strengthened Platinum-20% Rhodium Alloys Alloy behaviour during hot forging and cold rolling

Ziyang Wang, Xi Wang, Futao Liu, the latter were compressed along normal direction

Faping Hu, Hao Chen, Guobin Wei and form two cracks on both sides of Y2O3 particles College of Materials Science and Engineering, along the rolling direction. The differences in Chongqing University, No.174 Shazhengjie, hardness and interface bonding properties of these Shapingba, Chongqing, 400044, China two types of particles are supposed to be the main causes of different deformation behaviour during Weiting Liu hot forging and cold rolling. Chongqing Polycomp International Corporation, Jianqiao Industrial Park B, Dadukou District, 1. Introduction Chongqing, 400082, China Platinum and its alloys are very important Weidong Xie* in industries such as the chemical and glass College of Materials Science and Engineering, industries. Some of their main applications are Chongqing University, No.174 Shazhengjie, as follows: a functional structural material in the Shapingba, Chongqing, 400044, China aerospace industry; a catalyst in the nitric acid preparation industry; nozzles for preparing glass *Email: [email protected] fibres; preparing crucibles and utensils that require special properties for use in chemical laboratories. Platinum-rhodium alloys with low rhodium Platinum-20% rhodium strengthened by oxides content can also be used as brazing filler metals of zirconium and yttrium were prepared by in metal welding. Although expensive, their high solidification of platinum-rhodium-(zirconium)- temperature stability and chemical inertia cannot yttrium powder which had been internally oxidised. be replaced by other materials (1, 2). After forging, rolling and annealing, 1 mm plates However, the strength and creep resistance of were obtained. Then the plates were mechanically pure platinum would significantly reduce at high ground to 50–70 μm from rolling-normal direction, temperature because of sharp grain growth. followed by argon ion milling until a hole appeared Therefore, it is crucial to find a way to improve on the centre of the foil to obtain samples which the high temperature creep resistance of platinum were characterised by transmission electron alloys (3). Many methods of strengthening platinum microscopy (TEM), combined with thermodynamic materials are known. Among these different analysis. The existence of spherical ZrO2 and Y2O3 methods, the best methods are solid solution particles was verified with platinum and rhodium strengthening and dispersion strengthening. The present as pure metals at the same time. It was addition of rhodium has been found to have the found that the deformation behaviour of ZrO2 and best solid solution strengthening effect in high

Y2O3 particles was quite different during processing, temperature use of platinum. With increasing where the former basically maintain their spherical rhodium content in platinum-rhodium alloys, the shape and were bonded tightly to matrix, while normal temperature strength, high temperature

112 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15959517568861 Johnson Matthey Technol. Rev., 2021, 65, (1) endurance strength, creep rupture life and creep particle spacing (14, 15). When making samples, activation energy of the alloy increases greatly, we tried to add the yttrium content to 0.1%, and so platinum-rhodium alloys are widely used found that the ingot will crack during rolling, so (4, 5). But the high-temperature mechanical we speculate that different deformation behaviours properties of platinum-rhodium alloys fail to will have a huge impact on alloy strengthening. satisfy the requirement of industrial production In this study, two kinds of platinum alloys with worsening service conditions. Therefore, strengthened by Y2O3 and ZrO2, respectively, were dispersion-strengthened platinum-rhodium alloys prepared and characterised by TEM. A number of were subsequently developed in Johnson Matthey, differences in deformation behaviour of the two UK (6, 7), Engelhard Corporation, USA (8) and particles during processing were found. These Heraeus, Germany (9) from the 1980s to 1990s. findings are expected to lead to new insights into They were prepared by adding zirconium, yttrium developing dispersion strengthened high strength or scandium oxide particles into the platinum- platinum alloys. rhodium alloy under certain process conditions. At present, dispersion-strengthened platinum alloys 2. Experiment such as zirconia grain stabilised (ZGS) platinum and oxide dispersion strengthened (ODS) platinum Nominal composition of the investigated alloys

(10, 11), in which Y2O3 and ZrO2 are two commonly is listed in Table I. The preparation processes used reinforcement particles, are used in industrial have been described in previous research (13) applications. According to the experimental results and can be briefly summarised as follows: Pt-20 of Hu et al. (12), as the zirconium concentration wt% Rh‑0.015 wt% Y without and with 0.1 wt% increases, the platinum-rhodium alloy’s yield stress Zr were smelted in a vacuum induction furnace at (0.2% offset) increases significantly, while the 1800–2000ºC under argon atmosphere, then cast elongation decreases slightly. The work hardening into a water-cooled copper mould to obtain ingots. rate of particle-reinforced samples increases The ingots were hot forged to 10 mm plates which with the increase of the volume concentration of were sheared into pieces, followed by mechanical dispersed particles, which is a typical behaviour milling to fine powder. The size distribution of the of particle dispersion enhancement. In addition, powder was measured using an LS-POP(6) laser based on a reformulation of the Orowan stress particle size analyser (OMEC, China). 97% of the for particle strengthening and by superposing this particles were distributed between 5 μm and 60 stress to the matrix stress, a calculated flow stress μm with an average particle size of 25 μm. The is in good accord with the experimental value. powder was sintered and exposed in air to internally Though the strength of these particle strengthened oxidise at 1150ºC for 4 h. The 30 mm sheets were platinum alloys can be significantly improved, the obtained by hot forging at 1400ºC, then rolled to further enhancement of the strength is difficult due 1 mm sheets at room temperature followed by to the lack of mechanism research and empirical annealing at 1150ºC for 30 min. To prepare the TEM development in the past (12, 13). samples, the sheets from rolling-normal direction Zirconium and yttrium have good plasticity and were mechanically ground to 50–70 μm, followed corrosion resistance. Diffusion strengthening by argon ion milling at 5 kV until a hole appeared improves the metal strength through adding on the centre of the foil. TEM was used to examine a second phase or multiple phases and the the microstructure at 200 kV (JEM-2100 electron essence is the interaction of the second phase microscope (JEOL Ltd, Japan)) and at 300 kV (JEM- and the dislocation (13). Nevertheless, the two 3000F field emission electron microscope (JEOL reinforcement particles, i.e. Y2O3 and ZrO2, are Ltd)). JEM-2100 was used to detect topography usually thought be to the same as strengthening of the particles and energy-dispersive X-ray particles based on Orowan’s equation where the spectroscopy (EDS) was conducted to determine strength increment from the non-deformable the components; JEM‑3000F was used for high- dispersed particles is positively correlated with the resolution transmission electron microscopy

Table I Nominal Compositions of the Investigated Platinum Alloys Alloys Zirconium, wt% Yttrium, wt% Others, wt% Rhodium, wt% Platinum, wt% 1 − 0.015 ≤0.01 20 Bal. 2 0.10 0.015 ≤0.01 20 Bal.

(HR-TEM) observation to determine the structure According to the Gibbs free energy theorem, a of the particles. In this paper, Pt-20Rh-0.015Y is reaction can only occur when the Gibbs free energy referred to as Alloy 1 while Pt-20Rh-0.015Y-0.1Zr is negative and the more negative the Gibbs free is referred to as Alloy 2. energy of a reaction, the more easily the reaction occurs. When the Gibbs free energy is greater 3. Results and Analysis than zero, the reaction cannot happen. Figure 1 is the oxidation tendency of platinum, rhodium, 3.1 Thermomechanical Analysis zirconium and yttrium, which indicates: • ΔG of all the reactions increases as temperature During preparation processes, especially at high increases temperature under oxidising atmosphere, the • Within the temperature range we calculated, metals in the alloys may be oxidised and the ΔG of Y2O3·ZrO2, Y2O3 and ZrO2 are much possible oxides are PtO2, Rh2O3, ZrO2 and Y2O3. smaller than those of Rh2O3, Rh2O and RhO,

Their reaction equations are as follows (Equations which means the formation of Y2O3·ZrO2, Y2O3

(i)–(vi): and ZrO2 are much easier during processing

• ΔG of RhO, Rh2O and Rh2O3 are very close to 3 ΔG = 0 and even greater than 0 at the internal 2Rh(s) + O2(g) → Rh2O3(s) (i) 2 oxidation temperature (1423 K), thus the formation of RhO, Rh2O and Rh2O3 are difficult and can be ignored. 1 2Rh(s) + O2(g) → Rh2O(s) (ii) For platinum, when the temperature rises from 2 room temperature, platinum will react with oxygen and form a PtO film on the metal surface. The 1 2 Rh(s) + O2(g) → RhO(s) (iii) thickness of PtO would grow with the rise of 2 2 temperature. When the temperature reaches about 500ºC, this process would stop. If the temperature 3 2Y(s) + O2(g) → Y2O3(s) (iv) continues to rise, the PtO2 film will be gradually 2 vaporised. Meanwhile, the higher the temperature, the faster the gasification rate. Samples in this 5 2Y(s) + Zr(s) + O2(g) → Y2O3·ZrO2(s) (v) experiment were about 1200ºC, the PtO2 film 2 formed by oxidation had been basically vaporised.

Zr(s) + O2(g) → ZrO2(s) (vi) Therefore, PtO2 is not present in the samples.

Fig. 1. The 0 oxidation tendency of platinum, rhodium, zirconium and yttrium –1

–2000 ΔG, kJ mol

Rh2O3 ΔG = –377458 + 260.87 T Rh2O ΔG = –96672.12 + 158.163 T RhO ΔG = –92319.44 + 186.904 T

Y2O3 ΔG = –1915964 + 290.89 T Y2O3·ZrO2 ΔG = –1915964 + 290.89 T ZrO2 ΔG = –1092145 + 185.22 T –4000 0 1000 2000 3000 Temperature, K

Overall, according to the thermomechanical Table II EDS Analysis Results of the analysis, Y2O3, ZrO2 and Y2O3·ZrO2 are easily Particle in Figure 2(b) formed in the platinum-rhodium-(zirconium)- Element Mass, % Atom, % yttrium alloys. Platinum and rhodium are present Oxygen 7.5 33.7 as almost pure metals. Iron 2.9 3.7 Yttrium 68.3 54.8 3.2 Microstructure and Energy- Platinum 21.3 7.8 Dispersive X-ray Spectroscopy Analysis of Platinum-Rhodium- The particles are so small that it is difficult to (Zirconium)-Yttrium characterise them using normal selected area Figure 2 is the bright field TEM images of Alloy diffraction techniques. Thus, HR-TEM was used to 1 showing the morphology of particles. It shows investigate the structure of the particles. Binary that some of the particles still maintained spherical monolithic ZrO2 is known to exhibit polymorphic shape and each particle was accompanied by two transformations between monoclinic (mP12:P121/c1, cracks on two sides along the rolling direction, as ZrO2-b type), tetragonal (tP6:P42/nmc, ZrO2‑type) shown by particle B. One enlarged image of this and cubic (cF12:Fm3m, CaF2-type) (16). The kind of particle is shown in Figure 2(b). Some monoclinic phase is stable below 1478 K, while particles had been compressed along the normal the tetragonal phase is stable between 1478 K and direction with two cracks along the rolling direction, 2650 K. The cubic phase is stable from 1796 K to such as particle A. A small number of fine particles 2993 K, and exhibits some range of homogeneity. were severely compressed so that the cracks have The fast Fourier transform (FFT) diagrams of ZrO2 closed (particles C). Table II shows the EDS from different zone axes is shown in Figure 4, analysis results of the particle in Figure 2(b), which and when it is compared with the common ZrO2 indicates that the particle is mainly composed of structure (17, 18), we find that two crystal yttrium and oxygen atoms. A small amount of iron structures of ZrO2 were monoclinic and tetragonal. impurity may have come from the TEM equipment. The FFT diagrams of Y2O3 from different zone axes Figure 3 is the bright field TEM images of is shown in Figure 4. When it is compared with

Alloy 2 showing the morphology of the particles. the common Y2O3 structure (19), we find that the

Figure 3(a) shows that the particles (as indicated structure of Y2O3 is body-centred cubic. by red circles) are spherical and uniformly The volume fractions of particles (ZrO2 and Y2O3) distributed in matrix with a diameter range from are difficult to measure by X-ray diffraction (XRD) 20 nm to 70 nm. Note that the particles show a due to low content of the particles or by TEM due good bonding with matrix after hot forging and to the large atomic mass of platinum and rhodium. cold rolling during processing. An enlarged image It is therefore difficult to measure the thickness of of a particle is shown in Figure 3(b) and its EDS the TEM foil. Thus, we approximately calculated analysis results have been listed in Table III, the volume fraction of ZrO2 and Y2O3 as 0.42% and illustrating that the particle is composed of 0.0587%, respectively, based on the weight fraction zirconium and oxygen atoms. of zirconium and yttrium. Note also that a small

(a) RD (b) Fig. 2. Bright-field TEM images in Alloy 1. (a) The Crack overall morphology and distribution of A ND B particles; (b) an C enlarged particle

100 nm 20 nm

(a) (b) Fig. 3. Bright-field TEM images in Alloy 2. (a) The overall morphology and distribution of particles; (b) an enlarged particle

RD ND

50 nm 20 nm

(a) (b) (c) 202

020

50 nm 10 nm

(d) (e) (f) 111 020

(e)

100 nm 4 nm

Fig. 4. TEM and HR-TEM images of ZrO2 particles. (a) Bright-field image; (b) HR-TEM image; (c) FFT diagram of a monoclinic particle with the electron beam approximately parallel to [111] direction; (d) bright-field image; (e) HR-TEM image of the area indicated in (d); (f) FFT diagram of a tetragonal particle with the electron beam approximately parallel to [101]

amount of zirconium and yttrium may be present as Table III EDS Analysis Results of the solute atoms in the matrix ascribed to the extremely Particle in Figure 3(b) low solubility of oxygen atoms in the platinum- Element Mass, % Atom, % rhodium alloy. An oxidation rate of 75% was proved Oxygen 16.6 53.2 to be reasonable, based on the experimental and Zirconium 83.4 46.8 calculated yield stress of the alloys (20, 21).

(a) (b) (c)

211

(b) 121

100 nm 10 nm

Fig. 5. TEM and HR-TEM images of Y2O3 particles. (a) Bright-field image; (b) HR-TEM image of the area indicated in (a); (c) FFT diagram of a body-centred cubic particle with the electron beam approximately parallel to [111] direction

3.3 Analysis of Deformation peeling occurs and voids are generated by finite Behaviour Between Zirconia and element calculation. Belcheko et al. (26) proposed Yttria Particles that the flow of the substrate above and below the undeformed inclusions in the rolling direction Based on our TEM observations, these two particles results in the formation of conical voids. Zhang have totally different deformation behaviours. et al. (27) argued that although there are subtle

Almost all Y2O3 particles have compression differences in the mechanism of void formation deformation along the normal direction with two proposed by different researchers, it is generally cracks on the two sides of particles along the believed that the formation of voids is due to rolling direction, while ZrO2 particles basically the discontinuity of the interface between the maintain their spherical shape and are bonded particles and the matrix. Therefore, the interfacial tightly with matrix. The nanoparticle deformation strength between particles and the matrix is the behaviour in particle strengthened metals has main cause of forming of the adjacent interface been widely researched (22–26). By studying the voids. It is suggested that the interfacial strength deformation behaviour of dispersion-strengthened between Y2O3 particles and the platinum matrix particles in steel, Gove et al. (22, 23) claimed that is not sufficient to withstand the tensile stresses the formation of voids around the particles and the during processing. A crack will form between the matrix is due to the fact that the steel matrix cannot particles and the matrix. As the strain continuously flow around the particles while maintaining contact increases, the matrix work hardens, and when the with them. The strength of the inclusion-substrate hardness of platinum reaches a critical value, the interface is insufficient to withstand the longitudinal Y2O3 particles will be deformed. In addition, the tensile stress caused by the deformation of the agglomeration of the reinforcing phase particles surrounding steel, so the interface is separated and causes an increase in the local volume fraction, voids are generated. As the cavity expands in the which increases the internal stress and causes the rolling direction, the vertical compressive stress is destruction of the particles. The coarsening of the no longer balanced, and the combination of vertical particles reduces the stress required for particle and longitudinal stress causes the steel to partially damage, and the rate of damage of the particles move into the cavity, creating a tapered cavity. increases with size. As for ZrO2 particles, no voids Waudby et al. (24) claimed that the combined are observed because the interfacial strength force of the stress of the steel matrix flowing with between ZrO2 particles and the platinum matrix is the tangential action of the surface of the particle able to withstand the rolling tensile stresses during caused and widened the crack and created a void. processing. ZrO2 is also hard enough to stand Luo et al. (25) proposed that if the resolved normal the stress form matrix so it will keep its spherical stress at the interface reaches a critical value, shape.

4. Conclusions 6. G. L. Selman, J. G. Day and A. A. Bourne, Platinum Metals Rev., 1974, 18, (2), 46

In platinum-rhodium-(zirconium)-yttrium alloys, 7. G. L. Selman and A. A. Bourne, Platinum Metals Rev., 1976, 20, (3), 86 yttrium and zirconium are easily oxidised into Y2O3, 8. Y. Dai, Q. Ma, W. Liu, F. Hu, Y. Zhang and Z. Yang, ZrO2 and Y2O3·ZrO2 and form corresponding oxides while platinum and rhodium are basically present Mater. Sci. Technol., 2018, 34, (6), 654 as pure metals. ZrO2 and Y2O3 particles have been 9. B. Fischer, D. Freund, A. Behrends, D. Lupton and observed in platinum-rhodium-zirconium-yttrium J. Merker, Precious Met., 1998, 22, 333 and platinum-rhodium-yttrium, respectively, and 10. J. Stokes, Platinum Metals Rev., 1987, 31, (2) 54 verified by EDS analysis and HR-TEM observations. 11. M. S. Rowe and A. E. Heywood, Mater. Design, The deformation behaviour of these two oxides 1984, 5, (1), 30 is quite different during processing, though they 12. F. Hu, T. Yu, W. Liu, Y. Yang, G. Wei, X. Luo, H. have the same deformation history. The ZrO2 Tang, N. Hansen, X. Huang and W. Xie, Mater. Sci. particles maintained their spherical shape without Eng. A, 2019, 765, 138305 any visible deformation, while the Y2O3 particles 13. G. L Selman and A. S. Darling, Johnson Matthey were compressed along the normal direction with Plc, ‘Dispersion Strengthening of Platinum two cracks forming on two sides of the particles. Group Metals and Alloys’, US Patent 3709667A;

Insufficient hardness of Y2O3 and relatively 1973 lower interface strength between Y2O3 particles 14. S. K. Karak, T. Chudoba, Z. Witczak, W. Lojkowski and matrix were supposed to be responsible for and I. Manna, Mater. Sci. Eng. A, 2011, 528, deformation of Y2O3 during processing. (25–26), 7475 15. K. Maruyama, H. Yamasaki and T. Hamada, Mater. Acknowledgements Sci. Eng. A, 2009, 510–511, 312 16. N. Sekido, A. Hoshino, M. Fukuzaki, Y. Yamabe- This work was supported by Chongqing Science and Mitarai and T. Maruko, Mater. Sci. Eng. A, 2011, Technology Support Project (No. cstc2017zdcy- 528, (29–30), 8451 zdyfX0070, cstc2018jszx-cyzdX0138) and 17. Yu. E. Gorbunova, V. V. Ilyukhin, V. G. Kuznetsov, Fundamental Research Funds for the Central A. V. Lavrov and S. A. Linde, Doklady Akademii Universities (No. 2019CDCGCL316) and National Nauk SSSR, 1977, 234, (3), 628 Undergraduates Training Program for Innovation 18. G. Katz, J. Am. Ceram. Soc., 1971, 54, (10), (No. 201810611054). 531 19. F. Frey, H. Boysen and T. Vogt, Acta Cryst., 1990, References B46, 724 20. K. Huang, K. Marthinsen, Q. Zhao and R. E. Logé, 1. H. Chen, W. Xie, W. Liu, Z. Wang, Z. Yang, H. Prog. Mater. Sci., 2018, 92, 284 Tang, Y. Dai, G. Wei and W. Xie, Vacuum, 2019, 21. H. Yamazaki, Japanese Patent 163069; 2005 160, 445 22. K. B. Gove and J. A. Charles, Met. Technol., 1974, 2. J. V. Pearce, F. Edler, C. J. Elliott, A. Greenen, 1, (1), 425 P. M. Harris, C. G. Izquierdo, Y.-G. Kim, M. J. Martin, I. M. Smith, D. Tucker and R. I. Veltcheva, 23. T. J. Baker, K. B. Gave and J. A. Charles, Met. Metrologia, 2018, 55, (4), 558 Technol., 1976, 3, (1), 183 3. B. X. Hu, Y. Ning, L. Chen, Q. Shi and C. Jia, 24. P. E. Waudby, W. J. M. Salter and F. B. Pickering, J. Platinum Metals Rev., 2012, 56, (1), 40 Iron Steel Inst., 1973, 211, (7), 486 4. Z. Xie, F. Lu and J. Wang, Precious Met., 1996, 25. C. Luo, Comput. Mater. Sci., 2001, 21, (3), 360 20, 10 26. C. Luo and U. Ståhlberg, J. Mater. Proc. Technol., 5. K. Teichmann, C. H. Liebscher, R. Völkl, S. Vorberg 2001, 114, (1), 87 and U. Glatzel, Platinum Metals Rev., 2011, 55, 27. Z. Zhang and W. Pantleon, Acta Mater., 2018, (4), 217 149, 235

The Authors

Ziyang Wang received his Guobin Wei received his PhD Bachelor’s degree in 2020 from in Materials Science and Chongqing University, China. He Engineering in 2015 from entered Chongqing University, Chongqing University, China. majored in Material Forming and He is vice professor of the Control Engineering in 2016. His School of Materials Science interests include light alloys and and Engineering. His interests composite materials. include magnesium-lithium alloys and the simulation of material forming processes. Xi Wang received his Bachelor’s degree in 2020 from Chongqing University, China, and he Weiting Liu is a senior engineer continued to study at Chongqing and Vice President of Chongqing University for a Master’s degree International Composite in the automobile college. Materials Co Ltd, China, Forging and new energy vehicles supporting and promoting are his research field. research and manufacture of platinum-rhodium alloy bushings and glass fibre. His Futao Liu received his Bachelor’s professional affiliation includes degree in 2020 from Chongqing Deputy Executive Director University, China. He entered in of the Functional Materials Chongqing University, majored Association and he obtained a in Material Forming and Bachelor’s degree in 1993 from Control Engineering in 2016. the Department of Mechanical His dissertation is about the Engineering of Chongqing heat treatment of magnesium- University, China. lithium alloys.

Weidong Xie received his Faping Hu is a PhD candidate PhD in Materials Science and from 2017 in Materials Science Engineering in 2008 from and Engineering at Chongqing Chongqing University, China. University. He visited the He is Vice Dean of the Institute Technical University of Denmark of Scientific Research and as a guest PhD for two years. His Development of Chongqing research is the microstructural University and a council characterisation of magnesium member of the Chinese Society alloys during plastic deformation. for Composite Materials. His interests include light Hao Chen is a PhD candidate alloys, composite materials, from 2020 in Materials Science nanomaterials and foundry. and Engineering at Chongqing University. He studied platinum- rhodium alloys and glass fibre reinforced composites. His current research is on freeze casting.

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On Deformation Behaviour of Polycrystalline Iridium at Room Temperature How structure rules by mechanical properties

Peter Panfilov* 1. Introduction Institute of Natural Sciences and Mathematics, Ural Federal University, Kuybysheva str., 48, The platinum group metal iridium is perhaps Ekaterinburg, 620026, Russia the most puzzling metal on Earth due to its property of being cleavable and a plastic solid Irina Milenina simultaneously (1). This refractory face-centred

Urals Innovative Technologies (UralInTech), 18 cubic (fcc) metal (Tmelt = 2446°C) serves as the Kosmonavtov avenue, Yekaterinburg, 620017, structural material for applications under extremely Russia hard conditions (2, 3) such as containers for fuel sources in radioisotope generators for deep Dmitry Zaytsev space missions (4), or crucibles for growing oxide Institute of Natural Sciences and Mathematics, crystals for power lasers (5). Industrial technology Ural Federal University, Kuybysheva str., for refining and processing iridium, based on 48, Ekaterinburg, 620026, Russia; Institute traditional chemical refining methods (2, 6, 7), of High Temperature Electrochemistry, 20 has been developed over the past 60 years Akademicheskaya st., Yekaterinburg, 620137, (8–10). Based on these achievements, it has Russia been shown that polycrystalline iridium exhibits limited plasticity due to intergranular fracture at Alexander Yermakov room temperature, but its plasticity increases Urals Innovative Technologies (UralInTech), 18 considerably under elevated temperatures (9–12). Kosmonavtov avenue, Yekaterinburg, 620017, The segregation of non-metallic impurities on the Russia grain boundaries was considered the cause of poor workability of polycrystalline iridium (12). *Email: [email protected] This type of deformation behaviour agrees with empirical knowledge on deformation and fracture of metals (13, 14). On the other hand, single Deformation and fracture behaviour of cold drawing crystalline iridium behaved unusually: it cleaved iridium wire under tension at room temperature is under tension after considerable elongation examined. High purity polycrystalline iridium was (13, 15), but never failed under compression (16–18). manufactured using pyrometallurgical technology. At room temperature the fracture mode of iridium During the initial stage of cold rolling, iridium wire single crystals was attested as BTF (1, 15), while has its usual grain structure and exhibits brittle brittle intergranular fracture (BIF) was the fracture deformation behaviour: poor plasticity and brittle mode for polycrystalline iridium (11, 19). Analysis transgranular fracture (BTF). However, the wire of the causes of cleavage in iridium has shown that begins demonstrating high plasticity including it satisfies some empirical cleavage criteria (18–20) necking in spite of the brittle fracture mode when due to features in the elastic moduli in comparison the lamellar structure has been formed in iridium with other fcc metals (18, 21). This fact leads to during cold drawing. the conclusion that the inclination to cleavage is an

120 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15815921428640 Johnson Matthey Technol. Rev., 2021, 65, (1) intrinsic property of iridium, whereas impurities only was not carried out in this work. The mechanical reinforce it (18–20, 22, 23). However, the analysis treatment of the pyrometallurgical iridium included: of interatomic bonding in iridium has shown that (a) forging the ingot into sheet at 1500–2000°C in BIF may also be considered as the intrinsic fracture air; and (b) rolling the sheet at ~800°C in air. The mode of polycrystalline iridium (24). resulting metal could be processed like platinum. The pyrometallurgical scheme for the refining of The cold drawing iridium wire, whose diameter iridium, including: (a) oxidation induction melting; varied from 2.7 mm to 0.5 mm, was prepared from (b) electron beam melting; and (c) growing massive this plastic metal. No long-term recrystallisation single crystalline workpieces by electron beam, annealing of this wire was carried out because this became an alternative technology to manufacture procedure leads to embrittlement and failure of ‘plastic’ iridium (25–27). Pyrometallurgical iridium iridium wire due to BIF. Tensile testing was carried demonstrated considerable plasticity prior to out with the help of an Autograph AG-X 50N tensile/ failure under tension in both the single crystalline compression tester (Shimadzu Corporation, Japan) state (28) and the polycrystalline state (29), while (traverse rate of 1 mm min–1) at room temperature. its elastic properties were the same as findings The lengths of working parts of iridium wire samples obtained earlier (30). It was confirmed that the were 100 mm. The structure of the samples before intrinsic fracture mode of this ‘plastic’ iridium is and after testing was examined by conventional BTF, while BIF is induced by harmful non-metallic X-ray diffraction (XRD) technique on the D8 impurities such as carbon and oxygen (31, 32). Advance diffractometer (Bruker Corporation, USA) Indeed, the portion of BTF on the fracture surface with copper kα irradiation. Back surfaces of each of plastic iridium is considerably higher than sample before and after testing were documented BIF (33, 34). Deformation mechanisms and, on a light metallographic microscope. The fracture hence, behaviour of pyrometallurgical iridium surfaces of samples were studied on the scanning were the same as a normal fcc metal excepting electron microscope JSM-6390 (JEOL Ltd, Japan). the special fracture mode (35–38). Recent studies of deformation and fracture behaviour of iridium 3. Results have shown that new participants achieved the technological level that allows ‘plastic’ iridium to The first set of iridium samples consisted of 10 pieces be manufactured (39, 40), including iridium single taken from a commercial parcel of cold drawing crystals (41). Also, the old problem concerning the thin iridium wire produced by UralInTech (Russia) intrinsic fracture mode of polycrystalline iridium or having a diameter of 0.5 mm. The microstructure the competition between BTF and BIF in iridium of this wire had a strongly deformed lamellar remains (42). Therefore, in the present paper, the morphology, where the grains of the polycrystalline deformation and fracture behaviour of iridium wires matrix practically disappeared (Figure 1). The under tension at room temperature are considered main feature of this lamellar structure is the narrow in light of the discussion on this problem. highly elongated grains collected in a bunch like a rope. As a result, deformation tracks, such as slip 2. Materials and Methods bands or twin lamellae, could not be revealed on the surfaces of the samples after deformation. An Pyrometallurgical iridium was used in this work. XRD spectrum taken from the cold drawing iridium It was high purity metal, free of non-metallic wire prior to testing is shown in Figure 2. There contaminants such as carbon and oxygen. The are two high narrow peaks ((200) and (220)) in refining procedure and, hence, impurities content the middle angles of the spectrum taken from the were the same as the metal used in earlier work sample. No visible changes in the spectrum were by our group: non-metallic elements <0.1 ppm; tungsten, molybdenum, niobium, iron, zirconium, copper, gadolinium, yttrium, gallium, nickel, palladium, zinc, magnesium, calcium –0.1–1 ppm; platinum, rhodium ~10 ppm (26, 27, 29, 38). Experience has shown that pyrometallurgical refining could be limited by the first and second 0.5 mm procedures without loss of quality of the metal. Fig. 1. Microstructure of the cold drawing iridium Therefore, the operation of the growth of the wire (diameter 0.5 mm) massive single-crystalline iridium workpieces

(220) Fig. 2. XRD taken 4 from the cold drawing iridium wire (diameter (200) 0.5 mm)

3 3

2 Intensity, × 10 Intensity,

1 (110) (311)

40 60 80 100 120 2θ, º

2.0 Fig. 3. Stress-strain A curves under tension at room temperature: A cold drawing iridium 1.6 wire, diameter 0.5 mm (elongation 29%, necking 23%); B annealed copper wire (elongation 1.2 58%, necking 55%); C C cold drawing iridium wire, diameter 2.7 mm (elongation 3.6%, no

Stress, GPa 0.8 necking); D cold drawing iridium wire, diameter 2 mm after recrystallisation D annealing (elongation 0.4 7%, no necking)

0 12 24 36 Strain, %

revealed after tensile testing of the sample. It for 20 min in a low vacuum and, as a result, their may be concluded that a stable drawing texture Vickers microhardness dropped up to 5 GPa. This is formed in the iridium wire in comparison with operation is also used in the technological process an annealed polycrystalline sheet, which does not for the manufacture of plastic iridium wire. depend on further tensile deformation. The stress-strain curves of the cold drawing The second set of iridium samples consisted iridium wires are shown in Figure 3 and some of 10 cold drawing wires with a diameter of of their mechanical characteristics are collected 2.7 mm taken from the workpiece that was used in Table I. The back surfaces of the deformed to manufacture the thin plastic iridium wire. The samples are shown in Figure 4, while their Vickers microhardness of these samples in the fracture surfaces are given in Figure 5. It is undeformed state was about 7 GPa. The third set clearly visible that the deformation behaviour of of iridium samples contained 10 cold drawing wires the samples from the first set Figure( 3, curve A) with a diameter of 2 mm. In contrast with the second is similar to the behaviour of annealed copper wire set, these samples were annealed at 1000–1200°C (Figure 3, curve B). The long stage of plastic

Table I Mechanical Properties of the Cold Drawing Iridium Wires and Annealed Copper Wires Under Tension at Room Temperature Ultimate tensile stress, Yield stress, σ0,2, MPa Elongation, ε, % Thinning in neck, δ, % σВ, MPa Cold drawing iridium wire (0.5 mm in diameter) ~1000 1850 30 20 Cold drawing iridium wire (2.7 mm in diameter) ~900 1000 3.6 - Cold drawing iridium wire (2 mm in diameter) after annealing ~200 480 7 - Annealed copper wire 20 210 43 55

(a) for cold drawing iridium wire with a diameter of 0.3 mm in the temperature range 20–800°C in (29). 5 mm The deformation behaviour of thick cold drawing (b) iridium wire (Figure 3, curve C) can be attested as brittle: its stress-strain curve has an almost 5 mm rectilinear profile, the yield stress is similar to the (c) ultimate tensile strength, while the deformation prior to failure is small in comparison with the 5 mm previous case. No necking was observed on the (d) back surfaces of the deformed cold drawing thick iridium wires (Figure 4(c)). The fracture mode of the samples agrees with their brittle behaviour, 5 mm it is BTF (Figure 5(b)). The short-term vacuum Fig. 4. Back surfaces after tensile testing at annealing of the thick cold drawing iridium wire at room temperature: (a) cold drawing iridium wire, a temperature close to the point of recrystallisation diameter 0.5 mm (elongation 29%, necking 23%); (b) annealed copper wire, diameter 0.75 of iridium leads to a change in mechanical behaviour mm (elongation 58%, necking 55%); (c) cold from brittle to ductile. Indeed, the behaviour of the drawing iridium wire, diameter 2 mm (elongation stress-strain curve becomes similar to annealed 3.6%, necking 0%); (d) cold drawing iridium wire, copper (Figure 3, curve D) when after a short stage diameter 2.7 mm after recrystallisation (elongation of strengthening follows the plastic flow stage, 7%, necking 0%) while the yield stress and the tensile strength drop considerably (Table I). However, its deformation prior to failure is very small (Table I) for a plastic flow takes place after the short stage of material material and the neck is absent in the deformed strengthening (Figure 3, curves A and B, samples (Figure 4(d)). The fracture mode does respectively). Indeed, the total elongation of both not change from brittle to ductile: it is attested as materials may be estimated as considerable for a a mixture of BTF and BIF (Figure 5(c)). polycrystalline wire sample (30% for iridium and 43% for copper). In addition, there is a clearly 4. Discussion visible advanced necking region on the back surfaces of the deformed samples (thinning of 20% It was shown that the deformation behaviour of for iridium and 55% for copper) (Figures 4(a) cold drawing iridium wire under tension at room and 4(b) and Table I). However, in contrast with temperature depends on its structural state. Wire copper, iridium exhibits much higher yield stress with grains of 50–100 µm behaves as a brittle and ultimate tensile strength (Table I). In spite material and exhibits BTF as the fracture mode. of the features that are inherent to the ductile Vacuum annealing at 1000–1200°C causes a drop deformation behaviour, the fracture mode of the of yield stress of the iridium wire, but does not lead iridium samples from the first set is attested as to significant increase of plasticity, while its fracture BTF in the strongly deformed lamellar structure mode continues to be brittle. On the other hand, (Figure 5(a)). The same findings were obtained thin cold drawing iridium wire having a lamellar

(a)

100 μm 10 μm

(b)

500 μm 100 μm

(c)

500 μm 100 μm

Fig. 5. Fracture surface of the cold drawing iridium wire under tension at room temperature: (a) diameter 0.5 mm (elongation 29%, necking 23%); (b) diameter 2.0 mm (elongation 3.6%, no necking); (c) diameter 2.7 mm after recrystallisation (elongation 7%, no necking)

structure demonstrates considerable elongation iridium (31, 32). Indeed such non-metallic prior to failure and clearly visible necking, despite elements as carbon and oxygen contained in a BTF as its fracture mode. This finding gives the low vacuum (10–2 MPa) induce grain boundaries basis for the conclusion that such lamellar structure brittleness, but the kinetics of the process depends is the correct morphology for plastic polycrystalline on the working temperature and its duration (31). iridium. It is important to note that this morphology For example, under annealing of 20 min at is formed in the iridium workpiece under the cold 1200°C, the portion of BIF on the fracture surface drawing process, while a few short terms annealing is considerably less than the portion of BTF, while at 1000–1200°C are included in the procedure after 24 h annealing BIF covers the whole fracture after some rolling passes (29). surface. It means that the regime of annealing of Earlier, it was shown that BIF is the impurities the cold drawing iridium wire used in this work is induced fracture mode of high purity polycrystalline optimal because the hardness and the yield stress

124 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15815921428640 Johnson Matthey Technol. Rev., 2021, 65, (1) of the iridium workpiece are decreased, but the Refractory Metals”, Ch. 9, Springer Science and cohesion strength of grain boundaries does not Business Media, Dordrecht, The Netherlands, drop. 2014, pp. 609–650 The low plasticity of the thick cold drawing iridium 4. E. A. Franco-Ferreira, G. M. Goodwin, T. G. George wire, which was strongly hardened during preliminary and G. H. Rinehart, Platinum Metals Rev., 1997, 41, (4), 154 processing, may be explained by the supposition that its resource of plasticity is finally exhausted under 5. J. R. Handley, Platinum Metals Rev., 1986, 30, (1), 12 tension as it takes place in iridium single crystals under the same experimental conditions (26, 37). 6. J. D. Ragaini, ‘Iridium Refining’, in “Iridium”, eds. E. K. Ohriner, R. D. Lanam, P. Panfilov and As a result, the cleavage crack can appear on any H. Harada, 129th Annual Meeting and Exhibition, dangerous macroscopic surface defect and, hence, 12th–16th March, 2000, Nashville, Tennessee, such wire is prone to separation by the brittle route USA, Metals and Materials Society (TMS), without necking. Following this logic, the thin cold Warrendale, Pennsylvania, USA, pp. 333–337 drawing iridium wire should behave the same way; 7. E. K. Ohriner, Platinum Metals Rev., 2008, 52, (3), however, it exhibits ductile mechanical behaviour, 186 except its fracture mode. One cause of this puzzling 8. F. D. Richardson, Platinum Metals Rev., 1958, effect may be the special configuration of the defect 2, (3), 83 structure of iridium, whose feature on the microscopic 9. R. W. Douglass and R. I. Jaffee, ‘Elevated- level is a lamellar morphology. Indeed, high purity Temperature Properties of Rhodium, Iridium and polycrystalline iridium is able to undergo severe Ruthenium’, ASTM Proc., 1962, 62, pp. 627–638 deformation under high-pressure torsion at room 10. G. Reinacher, Metall., 1964, 18, 731 temperature when the nanocrystalline structure is 11. B. L. Mordike and C. A. Brookes, Platinum Metals forming in the material (38). It is puzzling, but in this Rev., 1960, 4, (3), 94 case, the surface defects play the role of the initiation 12. C. A. Brookes, J. H. Greenwood and J. L. Routbort, of cleavage in the neck region only (29). Indeed, J. Inst. Metals, 1970, 98, 27 iridium meets some empirical cleavage criteria (18– 13. R. W. K. Honeycombe, “The Plastic Deformation of 20). However, this effect should be considered as Metals”, Edward Arnold, London, UK, 1968 an artefact because in contrast with other cleavable 14. A. S. Argon, “Strengthening Mechanisms in Crystal solids iridium is a plastic material in both the single Plasticity”, Oxford University Press, Oxford, UK, crystalline and polycrystalline states and its inclination 2008, 403 pp to cleavage depends on the structural state. 15. R. W. Douglass, A. Krier and R. I. Jaffee, “High- Temperature Properties and Alloying Behavior of 5. Conclusion the Refractory Platinum-Group Metals”, Report NP-10939, Batelle Memorial Institute, Columbus, The lamellar structure that forms in iridium wire USA, 31st August, 1961 during the cold drawing process provides the 16. H. Hieber, B. L. Mordike and P. Haasen, Platinum excellent mechanical properties of polycrystalline Metals Rev., 1964, 8, (3), 102 iridium under tension: it behaves like a ductile fcc- 17. P. Haasen, H. Hieber and B. L. Mordike, metal excepting the brittle fracture mode. It was Z. Metallkde, 1965, 56, (12), 832 shown that the inclination of iridium to cleavage 18. C. N. Reid and J. L. Routbort, Metall. Trans., 1972, depends on its structural state. 3, 2257 19. S. S. Hecker, D. L. Rohr and D. F. Stein, Metall. 6. Acknowledgement Trans. A, 1978, 9, (4), 481 20. C. Gandhi and M. F. Ashby, Scripta Metall., 1979, The Russian Science Foundation supports this 13, (5), 371 research project (#18-19-00217). 21. R. E. MacFarlane, J. A. Rayne and C. K. Jones, Phys. Lett., 1966, 20, (3), 234 References 22. Yu. N. Gornostyrev, M. I. Katsnelson, N. I. Medvedeva, O. N. Mryasov, A. J. Freeman and 1. C. A. Brookes, J. H. Greenwood and J. L. Routbort, A. V. Trefilov,Phys. Rev. B., 2000, 62, (12), 7802 J. Appl. Phys., 1968, 39, (5), 2391 23. M. J. Cawkwell, D. Nguyen-Manh, C. Woodward, 2. L. B. Hunt, Platinum Metals Rev., 1987, 31, (1), 32 D. G. Pettifor and V. Vitek, Science, 2000, 3. I. L. Shabalin, ‘Iridium’, in “Ultra-High Temperature 309, (5737), 1059 Materials I: Carbon (Graphene/Graphite) and 24. S. P. Chen, Phil. Mag. A, 1992, 66, (1), 1

25. A. V. Yermakov, V. M. Koltygin and E. V Fatyushina, 34. E. P. George, C. G. McKamey, E. K. Ohriner and Platinum Metals Rev., 1992, 36, (3), 146 E. H. Lee, Mater. Sci. Eng.: A, 2001, 319–321, 26. P. Panfilov, A. Yermakov, V. Dmitriev and 466 N. Timofeev, Platinum Metals Rev., 1991, 35. P. Panfilov, A. Yermakov and G. Baturin, J. Mater. 35, (4), 196 Sci. Lett., 1990, 9, (10), 1162 27. N. I. Timofeev, A. V. Yermakov, V. A. Dmitriev and 36. P. Panfilov, V. Novgorodov and G. Baturin, J. Mater. P. E. Panfilov, “Metallurgy and Mechanical Behavior Sci. Lett., 1992, 11, (4), 229 of Iridium”, Urals Branch of Russian Academy of 37. P. Panfilov,J. Mater. Sci., 2007, 42, (19), 8230 Science, Ekaterinburg, Russia, 1996 38. P. Panfilov, A. Yermakov, O. V. Antonova and 28. A. Yermakov, P. Panfilov and R. Adamesku, V. P. Pilyugin, Platinum Metals Rev., 2009, 53, (3), J. Mater. Sci. Lett., 1990, 9, (6), 696 138 29. P. Panfilov, V. Novgorodov and A. Yermakov, 39. P. Wang, J. Yu, X. Zhou and J. Chen, Rare Metal J. Mater. Sci. Lett., 1994, 13, (2), 137 Mater. Eng., 2015, 44, (10), 2363 30. R. A. Adamesku, V. A. Barkhatov and 40. M. Scapin, L. Peroni, C. Torregrosa, A. Perillo- A. V. Yermakov, Vysokochistye Veschestva, Marcone, M. Calviani, L. Gomez-Pereira, F. Leaux 1990, (3), 219 (in Russian) and M. Meyer, Int. J. Impact Eng., 2017, 106, 191 31. P. Panfilov and A. Yermakov, J. Mater. Sci., 2004, 41. B. Wang, S. Li, Y. Wang, H. Zhong, R. Hua, Y. Liu 39, (14), 4543 and X. Luo, Vacuum, 2018, 154, 141 32. P. Panfilov,J. Mater. Sci., 2005, 40, (22), 5983 42. J. Yang, H. Wang, R. Hu, F. Zhang, S. Li, Y. Liu 33. C. T. Liu, H. Inouye and A. C. Schaffhauser,Metall. and X. Luo, Rare Metal Mater. Eng., 2019, 48, (5), Trans. A, 1981, 12, (6), 993 1380

The Authors

Peter Panfilov has a PhD (1993) Dmitry Zaytsev is an Associate and a ScD (2006) in Materials Professor at Ural Federal Science, focusing on deformation University and a Leading and fracture of iridium, from Researcher at the Institute of High- the Ural State University, Temperature Electrochemistry Yekaterinburg, Russia. Russian Academy of Sciences Currently, he is a professor at at Yekaterinburg. He received a the Ural Federal University. He PhD from Ural State University at is developing models for stress Ekaterinburg (2011) and a ScD accommodation mechanisms in from the National University of refractory metals, hard biological Science and Technology (MISIS) tissues and rock materials. at Moscow, Russia (2016).

Irina Milenina holds an MS degree Alexander Yermakov is the in Metallurgical Engineering founder and director of Russian from the Ural Federal University, national pgms manufacturer Yekaterinburg, Russia. UralInTech. He received a PhD in Since joining UralInTech at Metallurgy of Non-Ferrous metals Yekaterinburg in 2010, she has in 1989. The field of his research gained experience working on a activity is metallurgy, processing range of technologies in the field, and applications of pgms. Dr concentrating on metallurgy and Yermakov, in co-authorship with processing of pgms. Currently, Professor Panfilov, is the author Irina works as the Chief of the of many technical publications Research and Development on platinum group metals, Laboratory in UralInTech. including two books on iridium.

www.technology.matthey.com

Platinum Group Metals Recovery Using Secondary Raw Materials (PLATIRUS): Project Overview with a Focus on Processing Spent Autocatalyst Novel pgm recycling technologies ready for demonstration at next scale

Giovanna Nicol Jeroen Spooren, Thomas Abo Atia, Centro Ricerche Fiat Scpa (CRF), Strada Torino, Bart Michielsen, Xochitl Dominguez- 50–10043 Orbassano (TO), Italy Benetton Flemish Institute for Technological Research Emma Goosey (VITO), Boeretang 200, 2400 Mol, Belgium Env-Aqua Solutions Ltd (Env Aqua), 304 Myton Road, Warwick, CV34 6PU, UK Olga Lanaridi Vienna University of Technology (VUT), Deniz Şanlı Yıldız Getreidemarkt 9/163, 1060, Vienna, Austria Ford Otomotiv San AŞ (Ford), R&D Center Akpinar Mah, Hasan Basri Caddesi No:2, 34885 *Email: [email protected] Sancaktepe, İstanbul, Turkey

Elaine Loving PLATInum group metals Recovery Using Secondary Johnson Matthey, Blount’s Court Road, Sonning raw materials (PLATIRUS), a European Union (EU) Common, Reading, RG4 9NH, UK Horizon 2020 project, aims to address the platinum group metal (pgm) supply security within Europe Viet Tu Nguyen, Sofía Riaño by developing novel and greener pgm recycling Katholieke Universiteit Leuven (KU Leuven), processes for autocatalysts, mining and electronic Department of Chemistry, Celestijnenlaan 200F, wastes. The initial focus was on laboratory- PO box 2404, 3001 Leuven (Heverlee), Belgium scale research into ionometallurgical leaching, microwave assisted leaching, solvometallurgical Iakovos Yakoumis leaching, liquid separation, solid phase separation, MONOLITHOS Catalysts & Recycling Ltd electrodeposition, electrochemical process: gas- (MONOLITHOS), 83, Vrilissou Str, 11476 diffusion electrocrystallisation and selective Polygono, Athens, Greece chlorination. These technologies were evaluated against key performance indicators (KPIs) Ana Maria Martinez including recovery, environmental impact and SINTEF AS, Strindveien 4, 7034 Trondheim, process compatibility; with the highest scoring Norway technologies combining to give the selected PLATIRUS flowsheet comprising microwave Amal Siriwardana*, Ainhoa assisted leaching, non-conventional liquid-liquid Unzurrunzaga extraction and gas-diffusion electrocrystallisation. TECNALIA, Parque Tecnológico de San Operating in cascade, the PLATIRUS flowsheet Sebastián Mikeletegi Pasealekua, 2 E-20009 processed ~1.3 kg of spent milled autocatalyst and Donostia-San Sebastián–Gipuzkoa, Spain produced 1.2 g palladium, 0.8 g platinum and 0.1 g

127 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16057842276133 Johnson Matthey Technol. Rev., 2021, 65, (1) rhodium in nitrate form with a 92–99% purity. The are high temperature processes requiring large overall recoveries from feedstock to product were amounts of energy. calculated as 46 ± 10%, 32 ± 8% and 27 ± 3% During the past two decades, alternative for palladium, platinum and rhodium respectively. technologies to the pyrometallurgical processes, The recycled pgm has been manufactured into more specifically smelting, have been evaluated for autocatalysts for validation by end users. This autocatalysts recycling with the focus on reducing paper aims to be a project overview, an in‑depth the environmental impact of pgm recycling (8, 9). technical analysis into each technology is not The hydrometallurgical dissolution of the spent included. It summarises the most promising autocatalyst using aqua regia, cyanide or strong technologies explored, the technology evaluation, acids (HCl, HNO3 or H2SO4) usually in presence operation of the selected technologies in cascade, of an oxidising agent is the most commonly used the planned recycled pgm end user validation and dissolution process (8–12). Among these, cyanide the next steps required to ready the technologies leaching was widely implemented for its high for implementation and to further validate their dissolution efficiency (13, 14). However, due to potential. the severe toxicity and energy consumption, it has been replaced with safer methods. The achieved recovery, efficiency and purity of the proposed 1. Introduction alternative hydrometallurgical methods are still insufficient to compete with the results obtained The pgms comprise six chemically similar elements: from industrially employed pyrometallurgical iridium, osmium, palladium, platinum, rhodium methods. and ruthenium. The primary use of pgms is for Of the techniques reported to separate and their catalytic properties with applications in the purify pgms from the dissolution or leach liquor automotive, chemical manufacture and petroleum irrespective of the selected upstream processes (8, refining industries. The pgms have been denoted by 9, 15–17), the most employed by global refineries the European Commission as critical raw materials are solvent extraction (18, 19) and multi-stage with their significant economic importance and precipitation techniques (20). Although solvent potential supply risk (1). The EU supply stability of extraction is one of the preferred methods because primary source pgms is uncertain given the market of its high efficiency and selectivity, effort continues dominance by a small number of non-EU countries, to find greener and safer organic extractants and the ongoing political, economic and social factors diluents without compromising the efficiency in these regions and the EU’s reliance on imports. already achieved (21–25). Opportunities for Between 2017–2019, recycling only provided process modifications and improvement sit not only 25– 33% of the global demand for palladium, with solvent extraction but with other industrially platinum and rhodium. Recycling can further employed processes all the way through the pgm mitigate the supply risk and ensure the future pgm recycling flowsheet. demand is met both in the EU and globally (2), To the present there are few examples of while also dramatically reducing the environmental complete flowsheets, from feed to product, impact of pgms when compared with primary for the recycling of pgms that do not involve a sources: high energy use, large amounts of waste pyrometallurgical pre-concentration step followed from mines and significant CO2 emissions. by a smelting process (26–29). PLATIRUS is an There are several pgm refining facilities across EU Horizon 2020 project that brings together Europe but almost all rely on pyrometallurgical companies from the pgm supply chain alongside processes, including smelting, as the precursor research organisations to foster the development to hydrometallurgical chemical separation and and upscaling of novel and greener pgm recycling purification processes. Using these existing technologies. This project brings a complete smelting-based recycling routes provided by feed to product flowsheet for the separation and primary producers or refining companies such purification of pgms without the use of smelting as Anglo American Platinum, Impala Platinum, whilst using novel as well as modified traditional Umicore, Johnson Matthey, Heraeus and BASF is processes. conventionally how autocatalysts are recycled in the This paper provides an overview of the project EU (3–7). Though the pyrometallurgical processes summarising the most promising technologies are effective at upgrading the pgm content and explored in the research and innovation phase, hence reducing the levels of impurities, they the technology selection, operation of the selected

128 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16057842276133 Johnson Matthey Technol. Rev., 2021, 65, (1) technologies, planned recycled pgm end user results are presented based on processing the validation and the next steps for the PLATIRUS same waste feedstock, namely milled spent flowsheet. To the best of our knowledge, the autocatalyst feed containing palladium, platinum selected PLATIRUS technologies provide a novel and rhodium (Table I). This feed comprises over flowsheet for the separation and purification of 100 different de-canned and milled autocatalysts pgms which could open new insights for future pilot from the open market. The grain size was <2 mm. scale plants. The feed contained 1588 ± 18 ppm palladium, 912 ± 5 ppm platinum and 327 ± 9 ppm rhodium 2. Overview of Technologies as characterised using X-ray fluorescence spectral analysis. Other feeds have been investigated Explored (Figure 1) but are considered outside of this The project is split into three phases (Figure 1): paper’s scope. (a) research and innovation (R&I) into leaching, Collaboration between the project partners separation and recovery technologies; (b) selection on analytical methods was key to ensure of the best technologies for validation supported by consistency of the project results between them; economic and environmental assessment; and (c) a standard sample with a known and certified upscaling of the selected PLATIRUS technologies concentration of each pgm was used by each and operation in cascade in an industrially relevant partner to evaluate different analytical methods environment. to ensure accuracy and repeatable results. All During the R&I phase, partners investigated results presented, unless otherwise stated, are their respective technologies (Figure 1). Key experimental results.

Waste electrical and electronic End-of-life autocatalyst Slag/tailings/sludge equipment

Pre process WP2

Ionometallurgical leaching Deep Microwave assisted using task specific ionic eutectic Solvometallurgy Leaching leaching liquids solvents

Super- Non- Hybrid sorption critical conventional Non-conventional liquid-liquid WP3-5 CO solid-liquid Separation materials 2 extraction extraction extraction

Gas-diffusion Electrochemical and electro- Electrodeposition Recovery pyrometallurgical methods crystallisation

Industrial validation WP6 + selected technology partners

Life cycle assessment, environmental and economical assessment and selection of PLATIRUS’s process WP7

Exploitation, dissemination, communication WP8

Clustering with other relevant projects WP9

Selection and laboratory Validation in industrial Research and innovation validation environment

Year 1 Year 2 Year 3 Year 4

Fig. 1. PLATIRUS project overview. As of April 2020 Env-Aqua Solutions Ltd is not involved in the project activities

2.1 Ionometallurgical Leaching a very low energy consumption compared to and Reduction Using Deep Eutectic existing pyrometallurgical processes (15). Further Solvents separation steps are needed to separate the three pgms from each other. The DES must be kept close TECNALIA developed a two-stage leaching and to the leaching operation temperature to avoid reduction process using deep eutectic solvents handling issues arising from DES solidification. (DES), comprising choline chloride and oxalic acid (Figure 2). The solvent is first used to leach the 2.2 Microwave Assisted Leaching pgms from the solid feedstock at a temperature <90ºC, followed by a reduction of the pgms to their An advanced leaching process using microwave metallic form, by heating the solution to >100ºC. (MW) technology has been developed by VITO using Centrifugation was employed at this laboratory a laboratory MW digester (flexiWAVE, Milestone scale based on equipment availability, separately Srl, Italy). The feedstock is contacted with 6 M both for the removal of the depleted autocatalysts HCl and H2O2, as an oxidising agent, before being substrate and the pgm product. Recovered DESs heated using MW radiation (2.45 GHz) to the can be recycled for the next leaching batch. reaction temperature of 150ºC. MW heating allows The solvents used are readily prepared from short leaching times, consisting of 15 min heating renewable, non-toxic and naturally occurring with 10 min dwell time at the reaction temperature chemicals when compared to traditional (34). The resultant leachate, containing the pgms, hydrometallurgical processes that employ strong is filtered, to remove the undissolved catalyst acids (15, 31). Another advantage is that it is a substrate, leaving an aqueous solution containing one-pot leaching and reduction process where no the pgms in a chloride matrix. additional reducing agent is required. An increase The MW heating promotes fast, homogeneous in the pgm purity is achieved from the selective volumetric heating enhancing leaching reduction process with a highly pure (90–95%) reproducibility and efficiency, with a positive pgm solid generated, a significant improvement impact on the energy efficiency. Optimisation of from the low content feedstock. Therefore, it is a the leaching process has resulted in a significant simple process for the concentration of pgms, with reduction in the HCl acid concentration (i.e. 6 M HCl) reduced costs and wastes generated, compared required compared to previous results reported in with the cementation process (31–33) and with literature (i.e. 12 M HCl) (34). The necessity of

DES recycling

Concentrated pgm mixture

Spent autocatalysts

Recovery Recovery

87 ± 4% Pd 90 ± 1% Pd 87 ± 5% Pt 45 ± 2% Pt Selectivity ≈99% 42 ± 4% Rh 12 ± 8% Rh

Mass purity Choline 90-95% pgms chloride/oxalic acid Leaching Reduction

Fig. 2. Diagram of TECNALIA’s leaching process using DESs (selectivity is recovery of pgm divided by recovery of impurities)

the H2O2 addition is dependent on the feedstock the first, an innovative method for the selective (Table I, data provided in presence of 10 v/v% leaching of palladium from the feedstock

H2O2 (31%)). In some circumstances its addition was developed, avoiding the co-extraction of has been shown to have a significant impact on the platinum and rhodium whose recovery can be pgm leachability. Through modelling and analyses achieved from the palladium depleted feedstock of the reactor head-space gas, it was shown that by varying the process conditions. Dilute and

H2O2 addition increased the formation of Cl2 and concentrated solutions of FeCl3 in acetonitrile are

H2 gas, which must be considered in the process used for the selective dissolution of palladium safety assessment. and the dissolution of platinum and rhodium, respectively. This solvometallurgical approach 2.3 Solvometallurgical Leaching provides a preconcentration of the pgms after which further purification is needed to separate Solvometallurgy is an alternative branch of platinum and rhodium. In the second approach, metallurgy that uses non-aqueous solutions highly concentrated solutions of AlCl3·6H2O and

(35). KU Leuven investigated the oxidative Al(NO3)3·9H2O were used to dissolve palladium dissolution of pgms from the feedstock using selectively from spent autocatalysts; 95% two different solvometallurgical approaches. In palladium was leached in only 15 min at 80ºC (36).

Table I Summary of Technologies Investigated in the PLATIRUS Project and Experimental Results Owner Technology Recoveries, % Output Form 79 ± 4 Pd Ionometallurgical leaching and One solid stream containing TECNALIA 39 ± 5 Pt reduction palladium, platinum, rhodium <10 Rh MW assisted sulfation roasting 96 ± 1 Pd One aqueous chloride + sulfate VITO followed by microwave assisted 85 ± 5 Pt matrix leachate containing leaching (30) >96 Rh palladium, platinum, rhodium 91.8 ± 0.1 Pd One aqueous chloride matrix VITO MW assisted leaching 96 ± 4 Pt leachate containing palladium, 89.9 ± 0.2 Rh platinum, rhodium Leaching 90 ± 4 Pd Two organic streams KU Leuven Solvometallurgical leaching 76 ± 2 Pt • palladium 45 ± 2 Rh • platinum/rhodium 100 ± 3 Pd Ionometallurgical leaching using One ionic liquid stream containing VUT 98 ± 4 Pt ILs platinum, palladium, rhodium 43 ± 4 Rh 100 ± 0.1 Pd Non-conventional liquid-liquid One ionic liquid stream containing VUT 100 ± 0.3 Pt extraction palladium, platinum, rhodium 99 ± 2 Rh 96 ± 3 Pd Non-conventional solid-liquid One ionic liquid stream containing VUT 86 ± 2 Pt extraction palladium, platinum (no rhodium)

Three aqueous streams 81 ± 3 Pd Non-conventional liquid-liquid • palladium KU Leuven 62 ± 2 Pt

Separation extraction • platinum 54 ± 5 Rh • rhodium 97 ± 6 Pd Two aqueous streams Hybrid sorption material VITO 0 Pt • palladium 0 Rh • other metals Close to 100 ± 10 for One solid stream containing SINTEF Electrodeposition Pd, Pt and Rh palladium, platinum, rhodium 35 ± 10 Pd Selective chlorination One solid stream containing SINTEF 40 ± 10 Pt palladium, platinum, rhodium 25 ± 10 Rh Recovery Gas-diffusion 70 ± 2 to close to 100 One solid stream of nanoparticles VITO electrocrystallisation ± 2 for all pgms or colloidal dispersions pellets

2.4 Ionometallurgical Leaching violent exothermic reaction between the feedstock Using Ionic Liquids and oxidising agent is managed safely.

Ionic liquids (IL) are perceived as ‘green solvents’, 2.5 Non-Conventional Liquid-Liquid mainly due to their low volatility, which signifies a and Solid-Liquid Extraction reduction in the degree of negative environmental impact and health hazards, low toxicity and non- Due to the increased interest in ILs, a new concept flammability (37). In metal extraction applications, surfaced: IL immobilisation on solid support they have provided dramatically higher extraction materials, which is the deposition of a fine IL layer efficiencies than commonly used solvents. They on a solid surface. Use of supported IL phases can act either as solvents in the presence of an (SILPs) is an ideal strategy in order to make use extracting agent or as selective extractants, since via of the benefits of IL and simultaneously avoid the modifications in their anion or cation their selectivity inherent complications of IL-based separations towards a certain metal can be tuned (38, 39). such as mass-transport limitations and excessive VUT explored the properties of ILs for both usage of the IL (40). leaching and separation processes (for separation VUT exploited the properties of hydrophobic see Section 2.5). Leaching of the pgms from the ILs in both liquid and solid extraction processes; feedstock was performed using hydrophilic and this was employed in conjunction with the VUT low cost choline-based ILs. The choline-based IL ionometallurgical leaching process (Section 2.4). leaching process was selected as the optimum due In the liquid-liquid extraction process, a solution to its high extraction efficiency and selectivity. The comprising 50 wt% phosphonium-based IL in process operates at mild conditions in the presence n-heptane was used to extract palladium, platinum of an oxidising agent: <100ºC for 4 h in a sealed and rhodium from the loaded pgm hydrophilic vessel with continuous stirring (Figure 3). leachate (generated from ionometallurgical This process leaches the pgms, but further leaching) to the hydrophobic IL phase. Quantitative separation steps are needed to separate the three extraction is obtained after continuous stirring for pgms from each other. Depending on the feedstock, 2 h at room temperature. The pgm-free hydrophilic the loaded IL can be re-used for subsequent IL phase can be recycled to perform the next batch leaching batches until the IL capacity is reached. of IL leaching. No significant performance loss Of note is the mild process conditions used. of the hydrophilic ILs was observed between the The major drawback is the high IL viscosity, different cycles; only five cycles have been tested making implementation on an industrial scale to date. This is particularly attractive to minimise challenging. Nevertheless, dilution of the IL with environmental impact and operating costs. water is feasible without compromising the pgm The alternative approach, solid extraction, relied extraction efficiencies. Furthermore, larger scale on SILPs using hydrophobic phosphonium-based system design and operation must ensure the ILs (Figure 4). The pgms are adsorbed onto the

Fig. 4. VUT’s pure SILP (left - white) and SILPs Fig. 3. Up-scale of VUT’s leaching process (100 g loaded with pgms (middle and right - orange) catalyst)

132 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16057842276133 Johnson Matthey Technol. Rev., 2021, 65, (1) solid enabling their one-step separation from the an aqueous mixture of pgms into their individual main impurities of aluminium, iron and cerium due elements (Figure 5) (41). Generally, the split-anion to their low retention on the solid material. extraction relates to the solvent extraction process The stripping is a two-step process: (a) an where different anions are present in the aqueous acidified thiourea solution strips the impurities and organic phases and the distribution of the IL retained on the SILP; (b) a more concentrated anions strongly favours the IL phase. The pgms acidified thiourea solution strips palladium and are extracted from the chloride leaching solution platinum (rhodium is retained on the solid). using the iodide form of the quaternary ammonium The use of SILPs allows fast and simple separation IL Aliquat® 336, [A336][I], dissolved in p-cymene. of the pgms from other metals with reduced The iodide anions, which have a strong affinity for chemical reagent consumption compared to liquid- the organic phase, coordinate with pgms to form based separations. There is the possibility to re-use stable iodo-complexes that can be extracted to the solid material for further separations without any the ionic liquid phase (42, 43). The split-anion loss in its retention and separation performance, extraction allows not only efficient extraction of to date one recyclability experiment has been pgms without changing from a traditional chloride conducted. An advantage of SILPs compared to most feed solution, but also the selective recovery of the commercially available resins is no pre-equilibration extracted metal complexes from the loaded organic is required; this could have significant impact on phases. The organic is scrubbed and stripped of its cycle times and effluent generation if re-equilibrium pgms, as detailed in Figure 5. is necessary, as part of the load-elution cycle. A The ionic liquid-based split-anion extraction drawback of the process is the retention of rhodium process is simple, selective and effective for the alongside chromium on the SILP and the removal sustainable separation of pgms, using only one requires a suitable stripping agent not yet identified. ionic liquid [A336][I] as the extractant, which can be regenerated for consecutive extraction- 2.6 Non-Conventional Liquid-Liquid stripping cycles. The high viscosity of [A336][I] is a drawback, which has shown, during its pilot scale Extraction application in mixer-settlers, to slow the mass A split-anion solvent extraction process has been transfer. Some measures have been identified to developed by KU Leuven for the separation of reduce the IL viscosity, such as the use of water-

Leaching solution Fig. 5. KU Leuven’s Pt(IV), Pd(II) and Rh(III) in 6.0 M HCl proposed flowsheet for the extraction and separation of pgms Water-saturated from spent automotive [A336]I or Split-anion 3– [RhCl6] catalysts [A336][I]/p-cymene extraction

Loaded organic (Pt/Pd/impurites)

NaCl Scrubbing Impurites

Loaded organic (Pt/Pd)

Pd(II) NH (aq)/NaCl [Pd(NH ) ]2+ 3 stripping 3 4

Loaded organic (Pt)

Pt(IV) CS(NH ) /HCl 4+ 2 2 stripping [Pt(CS(NH2)2)2]

Stripped organic

dil.HCl Scrubbing Impurites

Regenerated ionic liquid

133 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16057842276133 Johnson Matthey Technol. Rev., 2021, 65, (1) saturated ionic liquid or use of green diluents (i.e. The functionalised microspheres show good p-cymene), but the measures can only partially selectivity for palladium over other pgms and resolve the problem. To the best of our knowledge impurities giving a method to remove palladium this is the first time that a process based onIL from acidic solutions. The adsorbed palladium can for the separation of pgms is tested in continuous be easily recovered by stripping with concentrated mode using real pregnant leach solutions as feed. HCl acid resulting in a concentrated acidic solution Other separations using IL have been developed of palladium. The sorbents have a palladium and show good performance but have been capacity of 0.33 mmol g–1 from acidic solutions only tested with synthetic solutions and not in (pH 2). When stripped with 3 M HCl, a seven-fold continuous mode (44, 45). increase in the palladium concentration between the feed (100 mg l–1 palladium) and stripping solution 2.7 Hybrid Sorption Material was observed. No significant performance loss of the sorbents was observed between the different Sorbents are an established method to selectively cycles; only seven cycles have been tested to date. recover palladium and other noble metals for acidic The developed material is a hybrid (ceramic- aqueous streams such as leachates (16, 46–48). organic) adsorbent selective to palladium over the Many materials have been developed by different rest of the pgms with good hydrolytic stability that groups and their performance tested in powder can be regenerated and reused. If a degradation of form. However, to be applied in a continuous way, the organic scavenging groups and hence, decreased the shape has to be optimised to avoid clogging and performance is observed over a higher number of pressure build-up in a column or the material has cycles, the metal oxide support can be recovered and to be modified for easy recovery afterwards with refunctionalised. The biggest drawback is the limited for example a magnetic core (49, 50). Therefore, sorbent capacity, but more developments to improve VITO has developed solid sorbents by first selecting this are ongoing by increasing the specific surface of or forming a suitable solid backbone before the supports and optimising the grafting conditions. grafting active organic scavenging groups onto the support using a green aqueous synthesis method. 2.8 Electrodeposition Two types of regenerable three-dimensional (3D) structured metal oxide supports (Figure 6) have SINTEF investigated extracting pgms from the been investigated: monodisperse microspheres feedstock using pyrometallurgy, employing copper and 3D printed monoliths. These types of as the pgm collector, followed by a molten salt supports have the advantage of reducing mass electrolysis process (Figure 7). diffusion limitations, optimising packing density The pyrometallurgical process was investigated at and decreasing pressure drop while allowing fast two laboratory scales (10 g and 5 kg). The pgm adsorption and desorption cycling times. recovery rates, in the alloy phase, were close to

(a) (b) (c)

100 µm

Fig. 6. (a) Image of VITO’s 3D-printed titania monolith with a diameter of 17 mm; (b) scanning electron microscopy (SEM) image of the titania microsphere support; (c) a column loaded with the developed microsphere sorbents in operation adsorbing palladium from an acidic solution

1 Pyrometallurgical pretreatment step 2 Direct electrolysis step Development of the anode material – + Metal M

Metallic phase

Containing pgms Refined M

← n+ Spent catalyst material M → M +ne– Metallic phase (M) containing pgm

Fig. 7. Schematic representation of the process carried out by SINTEF

100%, and the copper-collector recovery rates were In general, the kinetics of the electrode charge- in the range 82–100%, when using the optimised transfer reaction in molten salts are considered parameters: 10 wt% copper-collector, 10–15 wt% faster due to the high operational temperature (53). calcium oxide, 1600–1650ºC and 1–1.5 h holding The challenges of operating the SINTEF time. This pyrometallurgy step pre-concentrated pyrometallurgical step are the same as those found the pgms by ~10 times from feedstock to the in industry. The separation of pgm microparticles generated metallic phase. from the molten slag phase is impacted by the In the electrolysis step, the copper-pgm alloy is slag viscosity and metal-to-slag interfacial tension used as anode in an electrorefining cell with the and in turn affect the pgm extraction efficiencies. eutectic LiCl-KCl as electrolyte at 450ºC. The The energy consumption was ca. 5.5 kWh kg–1 experiments demonstrated the selective extraction copper recovered in the pyrometallurgical step and of the metal phase (copper), which was recovered ca. 7 kWh kg–1 pgm recovered in the electrolysis at the cathode with a current efficiency of ca. step (equal to 0.3 kWh kg–1 copper refined) 70%. Under these conditions, the pgms (and other under optimised experimental conditions. Energy impurities) remain in the anode residue giving a consumption can be challenging to extrapolate from solid with >99.9% purity. Further separation steps laboratory to industrial scale. Due to the energy are needed to separate the three pgms from each intensiveness of pyrometallurgical processes, other. energy consumption and associated cost must be One significant advantage of pyrometallurgical evaluated at larger scale. over hydrometallurgical processes is the lower reagent use in relation to the feedstock pgm 2.9 Selective Chlorination content (kilograms of reagent per kilogram of pgm) (51). As such, it provides attractive conditions for Molten chloride mixtures can be used as a reaction preconcentrating pgms from very dilute wastes, media in the chlorination of oxide mixtures, ores such as the PLATIRUS feedstocks; the optimised or industrial byproducts. The dissolution reaction conditions of the SINTEF process showcase that. generates chloride compounds at much lower High recovery rates of the copper collector at the temperatures (ca. 450ºC) than those needed in electrolysis cathode have been achieved and it can solid-gas chlorination reactions (ca. 1000ºC). be recycled for the next pyrometallurgical step. This is due to significant solvation effects of the The electrolysis process allows the extraction dissolved metal cation with the chloride ions of the of copper from the pgm-containing copper anode molten chloride media. in a molten salt electrolyte with better selectivity SINTEF investigated the selective recovery of and kinetics as well as lower energy consumption the pgms using molten salts and chlorine gas than in state-of-art copper-refining processes using as an oxidiser and chlorination agent, followed aqueous solutions (52). Cu(I) species are stable in by an electrolysis process (Figure 8). LiCl-KCl the molten salt electrolyte, thus the voltage (and eutectic mixture was chosen as the best candidate. energy) needed in the electrorefining process The feedstock was fed into the reactor at 450ºC is lower than in an analogous aqueous solution without any pretreatment or up-concentration process where Cu(II) are the solely stable species. steps, resulting in a single step pgm extraction

Cl2(g)

– + Anode reaction:

Selective – – 2Cl → Cl2(g) + 2e chlorination Metal fractions In a molten salt media

Molten chloride with pgm-containing waste pgm-containing waste

Fig. 8. Diagram representing the selective chlorination process carried out by SINTEF

e– Fig. 9. Schematic representation of the O 2 VITO’s GDEx process operating with O2. This represents Triple phase boundary the presumptive mechanism, which – 3 e – – e– 1 may be revised Mered HO2 ,OH as we gain further 2 H+ O2 understanding of the process Me 4 ox

Diffusion layer GDE

process. Silicon, magnesium and aluminium were as, at the much higher temperature, chlorination of not dissolved thus remain as a solid sludge at the all other elements contained in the material occurs, bottom of the reactor. The analysis of the residue not only pgms. shows that, after 3 h, the total chlorination and Though faster than gas-solid reactions, the therefore dissolution of the pgms is close to 50%. kinetics are still slow when using chlorine gas and The dissolved pgms are recovered in an electrolysis the use of other gaseous chlorination agents, such process as a metallic-pgm alloy at the cathode as HCl, should be tested. Further optimisation of resulting in chlorine evolution at the anode. Only the process is required to achieve competitive pgm 80% of the dissolved pgms can be accounted for recovery rates, including recovery of the volatile in the molten chloride as pgm-chlorocomplexes; pgm-chlorocomplexes. it is believed unaccounted pgm mass corresponds to formed volatile pgm-chlorocomplexes that 2.10 Gas-Diffusion could be recovered from the off-gas system by Electrocrystallisation condensation; estimates calculated from a mass balance suggested a feed volatilisation of ca. 10% GDEx is defined as the reactive precipitation palladium, 10% platinum and 20% rhodium. between metal precursors in solution and This chlorination process presents clear intermediates from the reduction of gases at a advantages in terms of rate, conversion and gas-diffusion electrode. When the gas is air or selectivity when compared with traditional gas- O2, the O2 reduction reaction leads to hydroxyl solid reaction systems. In the latter, the rate ions and hydrogen peroxide being formed in the and conversion are limited by the contact of the pgm solution, which react, forming oxides or gaseous chlorinating agent and the material to be hydroxides (Figure 9). Alternatively, the process chlorinated, and the reaction occurs non-selectively can run with other gases. The GDEx process

136 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16057842276133 Johnson Matthey Technol. Rev., 2021, 65, (1) is enabled by VITOCoRE® multi-layered gas- electrode porosity. The recovery of the precipitated diffusion electrodes (54). materials is impacted by the unoptimised GDEx uses the cleanest possible reagent, the downstream separation and drying processes. electron, and it is highly versatile, as it can be Some reagents, such as sulfur-based compounds, used to recover many different metals. The process are known to interfere with the process under uses an inexpensive reactor. As it operates in a defined processing conditions, coprecipitating with flow-cell configuration, it is easily up-scalable the pgms. by stacking multiple individual cells, without a reduction in performance. The process is highly 3. Selection Criteria for Industrial reproducible, involves mild operation conditions Validation (room temperature and atmospheric pressure). The process has a low energy consumption, for A key aim of the PLATIRUS project was to develop example ~2-6 kWh kg–1 of materials recovered, technology that could be applied at scale to when compared with electrowinning platinum from bridge the supply gap of pgms in Europe. Novel chloride media reported at 21 kWh kg–1 platinum and sustainable technologies are unlikely to (55). Finally, it is efficient, i.e. 50% current efficiency be commercialised if the economics are not for the formation of the reactive intermediates that competitive. The KPIs were chosen as selection fully react with the metal precursors to achieve the criteria to align with these project objectives, targeted recovery. namely, to develop novel sustainable pgm recovery Notably, the recovery of dilute metals and processes. Dimension reduction, employing simultaneous synthesis of nanostructures with traditional processes of multicriteria decision GDEx is fast, with rates approaching ~3–15 kg per analysis (MCDA), was used to reduce the number day, using a single, inexpensive, electrochemical of KPIs to a practical range (Figure 10). reactor under flow regime. The KPIs identified for PLATIRUS were quantified The best results were achieved with dilute metal and added to an MCDA model. The results were concentrations, and significant optimisation is then scored and normalised to present all KPIs required for solutions with metal concentrations on the same scale and range. At this point, the above 10 g l–1. Especially with high metal KPIs were all considered to be of equal importance concentrations, it is expected that, after a period and contribute the same impact to the overall of operation, the electrode would become clogged technology rating. Whilst reaching this point and require an acid treatment to regenerate the is enough to deliver a review score, the model

Recovery Operation Energy Environmental yields

Operational Total energy Physico- Initial cost consumption chemical recovery

Solid:liquid Process Temperature Haloplatinoids ratio efficiency

Recyclability No. of steps Process risks

Selectivity of leaching Rate

Fig. 10. Key performance indicators used by Env Aqua to enable the selection of the best (combination of) pgm recovery technologies

Table II Evaluation Results of Most Relevant PLATIRUS Process Flowsheetsa MW assisted Solvometallurgical Ionometallurgical Ionometallurgical Leaching leaching leaching (VUT) leaching using DES leaching using DES (VITO) (TEC) (TEC) Liquid-liquid Liquid-liquid Liquid-liquid - extraction extraction extraction Baseline Separation (KU (KU Leuven) (KU Leuven) Leuven) GDEx GDEx GDEx Electrodeposition Recovery (VITO) (VITO) (VITO) (SINTEF) Sustainability 1 2 8 14 9 Efficiency 2 3 8 14 7 Performance 2 3 10 12 4 Cost 1 3 9 14 5 Environmental 1 4 9 14 8 Weighted 1 2 8 14 7 position aUnweighted individual KPI position scores and total weighted position (low score is preferred with 1 scored to the best technology and 14 scored to the worse technology)

Milled PLATIRUS Autocatalysts Autocatalysts Performance autocatalysts flowsheet production testing results

Fig. 11. Key tasks following PLATIRUS flowsheet selection

and the results are not tailored to the specific manufacture and testing of recycled autocatalysts PLATIRUS objectives. To prioritise the most (Figure 11). important factors, weighting of the KPIs was The ~1.3 kg feedstock was a mixture of diesel conducted through analytical hierarchical process. oxidation catalyst (DOC) and three-way catalyst In addition, grouping of criteria was conducted to (TWC) from CRF and FORD milled, blended and help distinguish components relative to the overall characterised by MONOLITHOS (56). The resulting objective using criteria weights. powder contained 2066 ± 24 ppm palladium, One critical aspect which is not referenced in the 2574 ± 15 ppm platinum and 179 ± 5 ppm KPIs is the compatibility of each of the individual rhodium. processes to work in cascade and hence give a In addition to the three selected technologies, complete recycling flowsheet from feed to product. conventional chemical transformations were A total of 13 different combinations of the PLATIRUS employed to convert the PLATIRUS outputs into technologies and the baseline were evaluated. the correct form for autocatalyst production. Table II shows the evaluation results for the most Demonstration was carried out using technology relevant process combinations; the following three and equipment appropriate, available and compatible technologies were selected for industrial compatible to process the ~1.3 kg autocatalyst validation: MW assisted leaching (VITO, Section 2.2), (~15 l pgm solution). Over a six-month period, non-conventional liquid-liquid extraction (KU Leuven, the PLATIRUS team successfully operated the Section 2.6) and GDEx (VITO, Section 2.10). PLATIRUS flowsheet in cascade and processed the feedstock producing 1.2 g palladium, 0.8 g 4. Autocatalyst Material Processing platinum and 0.1 g rhodium in nitrate form with a purity of 92-99% (as a reference market palladium The three selected technologies were demonstrated nitrate solution is sold with a purity of 99.98%) at the VITO and KU Leuven sites to produce recycled (57) (Figure 12). The overall recoveries were palladium, platinum and rhodium to enable the calculated as 46 ± 10% for palladium, 32 ± 8%

Washings: 3 l 3 l Pt strip Pt: 0.8 g Pt: 0.1 g Conventional 67% Overall Pt: 62% Nitrate Purity Pt: Pd: 0.1 g chemical recovery* Pt: conversion 99% Rh: <0.1 g transformations 32%

3 l Pd strip Pd: 1.2 g 100.0% Overall Milled MW Non-conventional Pd: 81% Pd: 67% Nitrate ˜ Purity Pd: liquid-liquid GDEx-Pd recovery* Pd: autocatalyst leaching conversion 92% extraction 46% Total 1.35 kg Pt: 2.8 g Recovery: Leachate: 12 l 15 l Rh Pd: 3.0 g Pt: 95% raffinate Rh: <0.1 g Pd: 88% Conventional Overall Rh: 0.2 g Rh: 54% Nitrate 84% Purity Rh: Rh: 69% chemical recovery* Rh: conversion 99% transformations 27%

Fig. 12. Overview of the mass balance for autocatalyst processing (*Recoveries include losses for analysis)

Fig. 13. MW system at the VITO laboratory used for leaching process

for platinum and 27 ± 3% for rhodium, including of the 15 reactors. Each of the 15 reactors was losses for analysis and equipment start-up loaded with 5 g of the feedstock and 50 ml 6 M which are disproportionately high at this scale of HCl solution. The MW-assisted reaction took operation. In practice, considering optimisation place at 150ºC for 10 min, with a heating time of auxiliary processes and operating in a manner to the set temperature of 15 min. Subsequently, representative of continued industrial operation, the reactor was cooled, opened and the leachate more representative recoveries are estimated, by was vacuum filtered. It is noteworthy that this process modelling, to be between 60–86% from leaching process did not require addition of H2O2 feed to sponge for the three pgms and further as an oxidation agent and thus the formation of optimisation beyond this is possible. hazardous head space gas mixtures (containing

H2 and Cl2 gas) was avoided. During vacuum 4.1 Leaching–Microwave Assisted filtration, the leach residues were washed with 6 M HCl at room temperature. Both leachate and Leaching washing liquids were collected. A laboratory scale MW system (flexiWAVE) Overall, ~1.3 kg of material was leached by equipped with a spinning carousel holding 15 performing 18 MW-leaching runs. Three batches of pressure-sealed Teflon-lined reactors was used to material with different grain sizes and total masses process ~1.3 kg autocatalyst (Figure 13). were processed, hence three leachates (L1, L2, L3) The leaching temperature is measured by a and washing waters (W1, W2, W3) samples were thermowell contained optic fibre, placed in one obtained (Table III).

4.2 Separation: Non-Conventional selective stripping of palladium was achieved by Liquid-Liquid Extraction equilibrating the scrubbed organic phase with aqueous ammonia in NaCl solution in four-stage The leachate containing L1, L2 and L3 (Table III), mixer-settlers at O/A = 3/1. Followed by the as prepared by VITO, was processed using recovery of platinum from the palladium-free loaded the solvent extraction flowsheet developed at organic phase was performed using acidic thiourea

KU Leuven (Figure 5). Due to the low pgm solution [CS(NH2)2/HCl] in four countercurrent concentration of the leaching residue washings, stages (O/A = 2/1). After being washed with these solutions were not processed. A continuous diluted HCl, the IL requires regeneration by contact solvent extraction demonstration using multi-stage with KI to replace the chloride for an iodide anion mixer-settlers was undertaken to process the reforming [A336] [I]. leachate (Figure 14). The work at KU Leuven demonstrated that First, Pd(II) and Pt(IV) were quantitatively continuous solvent extraction is feasible for not only extracted in two countercurrent stages with [A336] the preparation of [A336][I] but also the selective [I] in p-cymene at O/A = 1/3, leaving Rh(III) in recovery of individual pgms, with recoveries of the raffinate. The impurities (mainly aluminium, 81 ± 3% Pd(II) , 62 ± 2% Pt(IV) and 54 ± 5% barium, cerium, iron and tin) in the loaded organic Rh(III), from the feedstock leachate. The mass phase were removed with NaCl solution. Then loss was mainly as a result of the samples taken

Table III Average Properties of the Recovered Leachates and Washing Waters During Leaching Oxidation-reduction pgm recovery, % Solution pH potential, mV vs. Ag/AgCl (2sf) Palladium Platinum Rhodium L1a –0.9 940 86.4 ± 0.1 91.1 ± 0.1 66.7 ± 0.2 L2a –0.8 930 82.1 ± 0.2 89.7 ± 3.1 59.7 ± 2.8 L3a –1.0 940 82.3 ± 2.1 90.5 ± 1.9 67.6 ± 6.0 W1a –0.7 850 4.752 ± 0.001 4.88 ± 0.02 3.8 ± 0.3 W2a –0.8 830 4.72 ± 0.03 5.11 ± 0.02 3.6 ± 0.3 W3a –0.9 840 3.22 ± 0.05 3.6 ± 0.1 2.7 ± 0.3 Average total recoverya (with washings) 87.9 ± 2.7 95.0 ± 1.2 68.0 ± 4.1 Average total recovery (without washings) 83.7 ± 2.0 90.5 ± 0.6 64.7 ± 3.6 a Errors for individual leachates and washing waters are based on duplicate inductively coupled plasma (ICP) analyses. Errors of the average total recoveries are based on relative standard deviation of the individual values

Fig. 14. Extraction of pgms from leachate of spent autocatalyst in two-stage mixer-settlers at KU Leuven. –1 –1 75% v/v [A336][I] in p-cymene (4 ml min ); Feed (6 M HCl + H2O2) (12 ml min ); two stages; O/A = 1/3; retention time 15 min; 298 K

140 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16057842276133 Johnson Matthey Technol. Rev., 2021, 65, (1) for analysis and the mixer-settlers start-up and 5. Autocatalyst Generation and finish steps. These losses are not representative Testing of industrial scale operation in which mixer settlers would operate continuously. To comply with emission regulations, within the The rhodium raffinate and platinum strip were autocatalyst the unburnt hydrocarbons are oxidised further processed using conventional chemical to CO2 and H2O, whilst the toxic gases NOx and transformations to convert the pgms to the nitrate CO are converted to N2 and CO2; these reactions form. The palladium strip was sent for further require a catalyst containing pgms coated onto a processing by the GDEx process. porous monolith (58). In the final validation step of the PLATIRUS 4.3 Recovery: Gas-Diffusion project, the industrial end-users aim to test the recovered pgms for their application as Electrocrystallisation emission control autocatalysts. To achieve this, GDEx attained a recovery of ~70 ± 1% of palladium autocatalysts were generated from three sets of from the KU Leuven strip sample (Figure 15). pgm origins: benchmark systems using market Palladium was recovered with a purity of 91–93%, grade pgm compounds; recycled material from the with platinum, rhodium and aluminium as the cascade activity; synthetic solutions which mimic major impurities, and additional minor impurities the PLATIRUS outputs in chemical form and purity of barium, iron, magnesium, cobalt and copper. profiles. This final stage of the project is underway Only two batches were processed in comparison at the time of writing and the outcomes will be to the eighteen leaching batches performed thanks disseminated at the culmination of the project. to the up concentration in the solvent extraction The autocatalysts were prepared according process. The selectivity of the recovery can be to the patented wet impregnation method of optimised by further investigation into the effect MONOLITHOS (59) (Figure 16). Firstly, the of the different GDEx operational variables, such diameter, length and weight of the cordierites as the influent concentration, hydraulic retention (monolithic carrots or honeycombs) were time, applied potential or current. measured and the amount of washcoat calculated. The palladium sponge generated from the Before the impregnation, an acetone wash step GDEx technology was further processed using of the cordierite was performed before drying conventional chemical transformations to convert the cordierite at 105ºC and cooling to room it to palladium nitrate. temperature. A slurry containing the catalytic

Fig. 15. Electrochemical reactor, pump and recirculation vessels at VITO

Catalyst preparation

Catalytic powder 30 cm

4 cm 7.6 cm

4 cm

Fig. 16. Catalyst preparation steps. Catalyst nanoparticles are prepared via hydrothermal process. Ceramic matrices are used for the catalyst to be impregnated on their surface

Table IV Summary of Autocatalysts Produced in the PLATIRUS Project Type of feed material User Number Typea pgm loading, g ft–3 Recycled CRF 2 TWC: test scale 60 Recycled CRF 2 DOC: test scale 110 Synthetic CRF 2 DOC: test scale 110 Synthetic FORD 1 DOC: full scale 30 Benchmark CRF 2 TWC: test scale 60 Benchmark CRF 2 DOC: test scale 110

Benchmark FORD 1 DOC: full scale 30

aTest scale catalysts have a diameter of 1.5” (3.8 cm) compared to the full-scale catalysts at 12.0” (30.5 cm)

powder and the binder (Al2O3, 10% of the catalytic FORD and CRF will age and test their allocated powder), dissolved in deionised water (volume autocatalysts (Table IV) to evaluate their five times the solid mass) was prepared and the performance as both fresh and aged catalysts for pH adjusted to 7.0 ± 0.1 (at room temperature). steady state and transient operation. The evaluation The cordierites were impregnated with the slurry will be dependent on the organisation and type of and then dried at 105ºC while being rotated. Once autocatalyst but conventionally includes conversion dried, the cordierites were calcined at 350ºC for efficiency of the pollutant gasses vs. temperature:

1 h. The procedure was repeated until the desired (a) NO and NO2 oxidation; (b) hydrocarbon and CO weight increase was achieved and after the final oxidation; (c) exotherm generation. impregnation the cordierites were calcined at 500ºC for 1 h. Finally, the catalysts were placed 6. Next Steps for Selected under an air stream and their final weight was Technologies measured. The loading was calculated according to the weight increase compared with the original Within the remaining time of the PLATIRUS project weight of the cordierite used. to support the flowsheet TRL increase, the following Different types of autocatalysts were produced steps will be realised: using different pgm loading as well as types of cordierites (900 cells per square inch (cpsi) • Optimisation of the selected technologies for the hexagonal, 400 cpsi and 300 cpsi) (Table IV). alternative project feeds: waste electronic and

electrical equipment, metallic foil autocatalysts After the completion of the project, the following and tailings from copper and nickel mining aspects should be among those addressed for • Leaching: VITO will be upscaling to, and testing the PLATIRUS flowsheet and its technologies to the leaching process in, an automated and fully progress to higher TRLs and present an offering monitored litre-scale MW leaching system (10 ready for detailed evaluation by industry: times scale increase) (Figure 17). This will

allow VITO to utilise advanced process control, • Optimisation of H2O2 delivery in the leaching further optimise operational reaction conditions process to minimise decomposition before and understand the potential formation of toxic metal oxidation and explosive gases in the reactor headspace • Minimisation of impurity leaching to reduce • Separation: The loading capacity and the impact on the flowsheet operation and final regeneration of the ionic liquids will undergo product purity detailed investigation and waste minimisation • Identification of a platinum stripping agent will be studied for the solvent extraction process that has • Recovery: Using other gases to oxygen has lower toxicity and is compatible with an shown positive results for pgm recovery and electrochemical recovery process further investigation is required to confirm the • Confirmation of the recyclability of the liquid- exact mechanism to enable optimisation. The liquid extraction organic across its operational GDEx reactor will be further upscaled and the lifetime configuration optimised such that the volume • Study of GDEx electrode maintenance and processed per day could increase by up to degradation to understand the operational life 3000 times with results validated on synthetic span and optimise the cleaning and regeneration solutions process. Modelling from an electrochemical • Advanced environmental and economic engineering perspective is required for larger assessment of the validated technologies in scale-up the industrial environment will be conducted • Optimisation of particle size formation of through a life cycle assessment (LCA) and life the GDEx nanoparticles considering the cycle cost (LCC) with the aim to support further downstream filtration process and end product technology exploitation and scale-up during form required and after the PLATIRUS project. • Colocation and operation of the technologies in closed loop at similar scale to enable the flowsheet to increase to higher TRLs and identify destinations of recycle streams and spent reagents • Optimisation of the auxiliary processes such as filtration, to ensure that the feed to product process is optimised as these auxiliary processes can have a significant impact on the overall pgm recovery and waste minimisation.

7. Conclusions

The PLATIRUS project has successfully achieved its objective to research, evaluate and upscale novel pgm recycling technologies. The PLATIRUS partners have applied a variety of techniques to recover pgms from secondary source material. Following assessment of the technologies against the project KPIs, the three selected technologies were able Fig. 17. VITO’s litre-scale pressure-sealed to operate in cascade to process a batch of milled MW leaching system with anticorrosion hybrid spent autocatalysts. The technology readiness configuration (synthWAVE/ultraWAVE, Milestone levels (TRLs) of the selected processes have Srl, Italy) in nitrogen-flushed confinement increased throughout the duration of the PLATIRUS

Table V Technology Readiness Levels (TRL) of the Selected PLATIRUS Technologies as Described by the EU (60) Post PLATIRUS Post R&I PLATIRUS Technology autocatalyst project start phase project end processing MW assisted leaching 3 4 4 5 Liquid-liquid extraction 3 4 5 5 GDEx 3 4 5 5 PLATIRUS flowsheet 1 2/3 3 4 *TRL1: Basic principles observed; TRL2: Technology concept formulated; TRL3: Experimental proof of concept; TRL4: Technology validated in laboratory; TRL5: Technology validated in industrially relevant environment

project, with the technologies at the start of the Metallurgy of Nickel, Cobalt and Platinum Group project not having been tested in cascade with real Metals”, Elsevier Ltd, Oxford, UK, 610 pp life feeds (Table V). 4. ‘Pgm refining’, Johnson Matthey, London, UK: The recycled pgms have been used to produce https://matthey.com/en/products-and-services/ autocatalyst that will be validated by end users to precious-metal-products/pgm-refining (Accessed on 19th November 2020) close the recycling loop. The recycled pgm product 5. ‘Precious Metal Recycling’, Heraeus Precious purity (92–99%) achieved via the PLATIRUS Metals, Hanau, Germany: https://www.heraeus. flowsheet is of note. If the technologies were com/en/hpm/hpm_services/recycling/recycling_ operated at a more industrially relevant scale overview/recycling.html (Accessed on 19th and the auxiliary processes were optimised, more November 2020) representative pgm recoveries are estimated 6. N. Ritschel, J. Taylor, T. England, B. Peters, by process modelling at ~60–86% for the F. Stoffner, C. Röhlich, S. Voss and H. Winkler, pgms to sponge. The technologies are ready for Heraeus Deutschland GmbH and Co, ‘Process for demonstration at the next scale and evaluation for the Production of a PGM-Enriched Alloy’, US Patent industrially relevant feedstocks to further validate 10,202,669; 2019 their potential. 7. ‘Recycling of PGMs (Platinum Group Metals)’, BASF, Ludwigshafen, Germany: https://www. basf.com/gb/en/who-we-are/sustainability/we- Acknowledgements drive-sustainable-solutions/sustainable-solution- PLATIRUS project has received steering/examples/recycling-of-pgms.html funding from the European (Accessed on 19th November 2020) Union’s Horizon 2020 Research 8. S. K. Padamata, A. S. Yasinskiy, P. V. Polyakov, and Innovation programme under E. A. Pavlov and D. Yu. Varyukhin, Metall. Mater. Grant Agreement n° 730224. Trans. B., 2020, 51, (5), 2413 9. H. B. Trinh, J. Lee, Y. Suh and J. Lee, Waste Manag., 2020, 114, 148 References 10. S. R. Izatt, N. E. Izatt, D. M. Mansur, T. Hughes, R. L. Bruening and J. B. Dale, ‘Sustainable 1. British Geological Survey, Bureau de Recherches Recovery of Precious and Minor Metals from Low- Géologiques et Minières, Deloitte Sustainability, Grade Resources’, 34th International Precious Directorate-General for Internal Market, Metals Institute Annual Conference, Tucson, Industry, Entrepreneurship and SMEs (European USA, 12th–15th June, 2010, The International Commission) and TNO, “Study on the Review Precious Metals Institute, Pensacola, USA, pp. of the List of Critical Raw Materials: Critical 573–592 Raw Materials Factsheets”, European Union, 11. D. Jimenez De Aberasturi, R. Pinedo, I. Ruiz De Luxembourg, June, 2017, 517 pp Larramendi, J. I. Ruiz De Larramendi and T. Rojo, 2. A. Cowley, “Pgm Market Report”, Johnson Matthey, Miner Eng., 2011, 24, (6), 505 London, UK, February, 2020, 40 pp 12. S. Harjanto, Y. Cao, A. Shibayama, I. Naitoh, 3. F. K. Crundwell, M. S. Moats, V. Ramachandran, T. Nanami, K. Kasahara, Y. Okumura, K. Liu and T. G. Robinson and W. G. Davenport, “Extractive T. Fujita, Mater. Trans., 2006, 47, (1), 129

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The Authors

This paper presents results from the collective work performed by the PLATIRUS project consortium. Only the people who edited this paper are cited as authors. The PLATIRUS consortium includes:

• Boliden: Justin Salminen, Visa Saari, Tuomas Lehtola, Toni Nikkanen, Petri Latostenmaa • CRF: Giovanna Nicol, Mattia Giuliano, Mauro Sgroi, Flavio Parussa • Env Aqua: Emma Goosey, Martin Goosey, Rod Kellner • FORD: Deniz Şanlı Yıldız, Barkın Özener, Emrah Kınav, Feyza Gökaliler, Tamer Acet, Naim Dönmez

• Johnson Matthey: Elaine Loving, Julia Vallejo Navarret, Barbara Breeze, Michael Browne, Michael Budd • KU Leuven: Viet Tu Nguyen, Sofía Riaño, Koen Binnemans • MONOLITHOS: Iakovos Yakoumis, Ekaterini Polyzou, Anthi-Maria Sofianou, Anastasia-Maria Moschovi, Marianna Panou, Ioannis Stamatopoulos • PNO: Nader Akil, Proletina Sabotinova • SINTEF: Ana Maria Martinez, Karen Osen, Anne Støre, Kai Tang • TECNALIA: Amal Siriwardana, Ainhoa Unzurrunzaga, Jokin Hidalgo, Carmen del Río, José Luis Aldana, Laura Sánchez • VUT: Olga Lanaridi, Katharina Bica-Schröder • VITO: Jeroen Spooren, Thomas Abo Atia, Wendy Wouters, Bart Michielsen, Kenny Wyns, Evelyn Moens, Nick Gys, Steven Mullens, Mieke Quaghebeur, Xochitl Dominguez-Benetton, Guillermo Pozo Zamora, Omar Martínez Mora

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Johnson Matthey Highlights A selection of recent publications by Johnson Matthey R&D staff and collaborators

Brazing Filler Metals Improved Ambient Stability of Thermally Annealed M. Way, J. Willingham and R. Goodall, Int. Mater. Zinc Nitride Thin Films Rev., 2020, 65, (5), 257 A. Trapalis, I. Farrer, K. Kennedy, A. Kean, J. Sharman and J. Heffernan,AIP Adv., 2020, 10, Brazing is a joining technique that is more (3), 035018 than 5000 years old. It is a versatile technique that is still fundamental to some of the biggest The stability of zinc nitride with a three-order engineering challenges faced today. The ability magnitude increase in degradation time from a to join dissimilar material combinations (for few days in unannealed films to several years example, metal‑ceramic joints) is one of the after annealing is reported in this article. It unique advantages it has over other joining shows that post-growth thermal annealing methods. With widespread interest in brazing significantly improves the stability. Using with filler metal development for many future samples annealed under a flow of nitrogen at technologies, this review discusses practice 200–400ºC, a degradation study was carried out. It showed the stability of the films was strongly and theory of brazing and advanced filler metal dependent on the annealing temperature. A development, progress and opportunities. mechanism for the improvement is proposed and Long- and Short-Range Magnetism in the the result has substantial potential for using zinc Frustrated Double Perovskite Ba2MnWO6 nitride in devices where the application requires H. Mutch, O. Mustonen, H. C. Walker, P. J. Baker, operational stability. G. B. G. Stenning, F. C. Coomer and E. J. Cussen, Structures of Mixed Manganese Ruthenium Phys. Rev. Mater., 2020, 4, (1), 014408 Oxides (Mn1-xRux)O2 Crystallised Under Acidic Using neutron powder diffraction, DC Hydrothermal Conditions magnetometry, muon spin relaxation and INS, L. K. McLeod, G. H. Spikes, R. J. Kashtiban, M. the magnetic and structural properties of the fcc Walker, A. V. Chadwick, J. D. B. Sharman and R. I. double perovskite Ba2MnWO6 were investigated. Walton, Dalton Trans., 2020, 49, (8), 2661 At a Néel temperature of 8(1) K with a This work reports on the synthesis of the solid frustration index, f ≈ 8, Ba2MnWO6 undergoes solution β-(Mn,Ru)O2. Previous reports of mixed type II long‑range antiferromagnetic ordering. manganese–ruthenium oxides do not provide The magnetic coupling constants J1 and J2, were compelling evidence for the formation of a found to equal −0.080 meV and −0.076 meV, genuine mixed oxide. A new synthesis method respectively. They were identified by using INS. was used successfully to form a solid solution. This indicated that both of the magnetic coupling The complete range of techniques, probing constants are antiferromagnetic with similar long- and short-range atomic order and surface magnitudes, in contrast to other known 3d composition and structure provided evidence for metal double perovskites of the form Ba2MWO6. the formation of mixed oxides. The tetragonal Similarly to that observed in the archetypical fcc lattice parameters do not change isotropcially, lattice antiferromagnet manganese oxide, above consistent with the different local environments the Néel temperature, INS techniques and muon of the two cations even though powder XRD spin-relaxation measurements identify a short- indicates an expansion of the unit cell volume on range correlated magnetic state. replacement of manganese by ruthenium in the

148 © 2021 Johnson Matthey https://doi.org/10.1595/205651321X16068971134658 Johnson Matthey Technol. Rev., 2021, 65, (1) rutile structure. The local structure as determined oxygen concentrations, and with a decreasing by EXAFS supports this conclusion. share of platinum, surface segregation of cobalt atoms was already thermodynamically viable in Synthesis and Polymorphism of Mixed Aluminum- the platinum-cobalt systems. Gold was introduced Gallium Oxides as a dopant, which resulted in structural changes D. S. Cook, J. E. Hooper, D. M. Dawson, J. M. Fisher, favouring the segregation of cobalt. Calculations D. Thompsett, S. E. Ashbrook and R. I. Walton, demonstrated that cobalt leakage would be Inorg. Chem., 2020, 59, (6), 3805 significantly supressed through the creation of a An investigation into the synthesis of a new solid cobalt-gold alloy core, owing to modification of the electronic properties. The theoretical framework solution of the oxyhydroxide Ga5–xAlxO7(OH) was conducted using solvothermal reaction used in this study provides a new route for the between gallium acetylacetonate and aluminum design of oxygen reduction catalysts. isopropoxide in 1,4-butanediol at 240ºC. A limited Elucidating the Mechanism of the CO2 Methanation compositional range was produced showing Reaction Over Ni–Fe Hydrotalcite-Derived Catalysts a linear contraction in unit cell volume with an via Surface-Sensitive in situ XPS and NEXAFS increase in aluminium content. This study and G. Giorgianni, C. Mebrahtu, M. E. Schuster, A. I. results provide reference data for understanding Large, G. Held, P. Ferrer, F. Venturini, D. Grinter, R. structure-property relationships in the gallium Palkovits, S. Perathoner, G. Centi, S. Abate and R. oxide polymorphs for applications including Arrigo, Phys. Chem. Chem. Phys., 2020, 22, (34), electronics and photocatalysis as well as the long- 18788 standing study of aluminium oxides as catalyst supports in heterogeneous catalysis. NEXAFS and in situ XPS techniques were implemented to study the chemical nature of Temperature Reversible Synergistic Formation of hydrotalcite-derived nickel and iron-promoted Cerium Oxyhydride and Au Hydride: A Combined hydrotalcite-derived nickel catalysts under XAS and XPDF Study reaction conditions. During the reaction, A. H. Clark, N. Acerbi, P. A. Chater, S. Hayama, hydroxylation of the nickel surface was shown to P. Collier, T. I. Hyde and G. Sankar, Phys. Chem. follow the formation of water. Higher selectivity Chem. Phys., 2020, 22, (34), 18882 towards carbon monoxide was detected when This article provides evidence for the formation of nickel surface hydroxylation was increased. In a gold hydride species at elevated temperature. contrast, a dominant metallic nickel surface was Using high energy resolved fluorescence detection shown to have higher selectivity towards methane. XANES and X-ray total scattering, in situ studies When selectivity to methane was high, electronic on the physical and chemical properties of gold structure analysis revealed the existence of mostly in inverse ceria alumina supported catalysts have Fe(III) species at the surface and a combination been conducted between 295 K and 623 K. It is of Fe(II) and Fe(III) species under the surface. proposed, through modelling of total scattering Fe(II) was observed to have a beneficial effect data to extract the thermal properties of gold on the maintenance of nickel in a metallic state. using Grüneisen theory of volumetric thermal Extending iron oxidation has a negative impact expansion, that the gold hydride formation on methane selectivity, through a more extended occurs synergistally with the formation of a nickel surface hydroxylation. cerium oxyhydride. In a reducing atmosphere, Use of Open Source Monitoring Hardware to the temperature reversible nature of the reaction Improve the Production of MOFs: Using STA-16(Ni) demonstrates the activation of hydrogen without as a Case Study consumption of oxygen from the ceria lattice. F. Massingberd-Mundy, S. Poulston, S. Bennett, H. Adsorbate-Induced Segregation of Cobalt from PtCo H.-M. Yeung and T. Johnson, Sci. Rep., 2020, 10, Nanoparticles: Modeling Au Doping and Core AuCo 17355 Alloying for the Improvement of Fuel Cell Cathode Catalysts Low-budget (< US$100) in situ reaction monitoring was used to demonstrate the ability B. Farkaš, C. B. Perry, G. Jones and N. H. de Leeuw, to improve the MOF synthesis route of STA-16(Ni) J. Phys. Chem. C, 2020, 124, (33), 18321 (Figure 1). In a step-up from previous research, The critical factors affecting segregation in this study demonstrated the production of the platinum-cobalt-gold ternary and platinum-cobalt MOF at atmospheric pressure. After just one binary nanoparticles in the presence of oxidising experiment, an improvement in reaction time species were identified. This was achieved using was predicted, with a reduction of 93%. This time first-principles-based theoretical methods. At low reduction will be of great benefit in both academia

length increased, the 1-olefin:n‑paraffin ratio in the hydrocarbon liquid and wax products was found to decrease significantly.

Establishing Reactivity Descriptors for Platinum Group Metal (PGM)-Free Fe–N–C Catalysts for PEM

Fuel Cells

M. Primbs, Y. Sun, A. Roy, D. Malko, A. Mehmood, M.-T. Sougrati, P.-Y. Blanchard, G. Granozzi, T. Kosmala, G. Daniel, P. Atanassov, J. Sharman, C. Durante, A. Kucernak, D. Jones, F. Jaouen and P. Strasser, Energy Environ. Sci., 2020, 13, (8), 2480 Kinetic ORR activities, TOF and site-density (SD) values were analysed for four of the most active benchmark pgm-free iron/nitrogen doped carbon electrocatalysts using an ex situ gaseous carbon monoxide cryo chemisorption and an in situ electrochemical nitrate reduction. Rational catalyst developments were enabled through the utilisation of ‘reactivity maps’ as new analytical Fig. 1. Representation of the STA-16(Ni) tools to deconvolute ORR reactivities. Substantial framework. Black, light blue, red and purple SD values paired with low TOF were observed for spheres represent carbon, nitrogen, oxygen and microporous catalysts, with the opposite noted phosphorous atoms respectively. Ni–O polyhedra for mesoporous catalysts. It is hoped that this are represented in light grey and green spheres research will help to improve pgm-free fuel cell (r = 2 nm) are used to illustrate the pore voids. cathode catalysts by acting as a reference for Protons omitted for clarity. Reprinted under future knowledge-based research. Creative Commons Attribution 4.0 International (CC BY 4.0) Palladium Dispersion Effects on Wet Methane Oxidation Kinetics and industry, particularly with the minimal cost, P. Velin, C.-R. Florén, M. Skoglundh, A. Raj, low experimental overhead and few resources D. Thompsett, G. Smedler and P.-A. Carlsson, Catal. required. Sci. Technol., 2020, 10, (16), 5460 Palladium-alumina catalysts with systematically Quantitative Carbon Distribution Analysis of varied palladium oxide dispersions were Hydrocarbons, Alcohols and Carboxylic Acids in a prepared by incipient wetness impregnation. Fischer-Tropsch Product from a Co/TiO2 Catalyst During Gas Phase Pilot Plant Operation Controlled calcination was used to achieve a realistic contact between the alumina support R. Partington, J. Clarkson, J. Paterson, K. Sullivan and active palladium oxide nanoparticles. As the and J. Wilson, J. Anal. Sci. Technol., 2020, 11, 42 palladium oxide particle size increased in dry Quantitative distribution analysis of oxygenated conditions, the apparent activation energy for and FT hydrocarbon products was observed for a methane oxidation also increased. It decreased cobalt/titania catalyst used in a fixed bed gas phase in wet conditions. Active sites at the rim of the pilot plant utilising CANSTM (Johnson Matthey, palladium oxide particles in contact with the UK) catalyst carrier devices. This was achieved alumina support were more sensitive to wet through a combination of methods, including conditions than palladium oxide sites farther GC-MS, GCxGC, GC and HPLC. The combination away from the rim, which can be attributed to of techniques ensured a detailed exploration more severe blocking by hydroxyl groups formed of the FT product composition. The average by water dissociation. In order to balance concentration of 1-alcohol, aldehyde, 1-olefin, palladium utilisation and water tolerance in cis- and trans-2‑olefins were also quantified using palladium–alumina catalysts for high methane 13C NMR and 1H NMR analyses. As carbon chain TOF, palladium oxide particles should be ≥2 nm.

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BIORECOVER: New Bio-based Technologies for Recapture of Critical Raw Materials Development of an innovative, sustainable and safe process from primary and secondary sources

Annette Alcasabas will be assessed by end-users. Modelling and Johnson Matthey, 260 Cambridge Science Park, integration of the modular stages and economic and Milton Road, Cambridge, CB4 0WE, UK environmental assessment will be done to develop the most effective and sustainable process. This Felicity Massingberd-Mundy, short feature describes the aims and approach, Barbara Breeze project technologies and intended outputs of the Johnson Matthey, Blounts Court, Sonning BIORECOVER project. Common, Reading, RG4 9NH, UK Introduction Maite Ruiz Pérez, Cristina Martínez García Nowadays, the EU depends on imports to supply Fundación Centro Tecnológico de Investigación CRMs: materials established by the European Multisectorial (CETIM), Parque Empresarial Commission as raw materials of great importance de Alvedro, calle H, 20, 15180 Culleredo, A for industry and the EU economy, with high risk Coruña, Spain associated with their supply and availability in the market. Email: [email protected] Examples of CRM are the REE (scandium, yttrium, lanthanum, cerium and neodymium), magnesium and pgms (platinum, palladium, BIORECOVER brings together diverse expertise with ruthenium, rhodium and iridium). The largest use the goal of developing a new sustainable and safe of the 8350 tonnes of REE is for catalysts (42%) process, essentially based on biotechnology, for followed by glass additives and over 90% of these selective extraction of critical raw materials (CRMs), are imported from China (1–3). In the same way, rare earth elements (REE), magnesium and platinum 85% of the 130,000 tonnes of magnesium annually group metals (pgms). The four-year European Union consumed are imported from China (3, 4); the (EU) H2020 project involves 14 international partners main uses being as magnesium casting alloys for from mining, microbiology, chemistry, engineering, transportation applications and aluminium alloys metallurgy, sustainable process development, for packaging, transportation and construction (3). as well as CRM end-users. Starting from relevant The EU supply of platinum is dominated by South unexploited secondary and primary sources of Africa with around 70% of supply from the country CRMs, BIORECOVER will develop and integrate three alone (3, 5). The pgms are primarily used in the stages for CRM extraction: (a) removal of major production of catalysts for automotive and chemical impurities present in raw materials; (b) mobilisation industries and in electronic applications. With this of CRMs through use of microorganisms; and (c) strong reliance on CRM from outside the EU the development of specific technologies for recovering development of innovative extraction processes metals with high selectivity and purity that meet is essential to extensively exploit raw material the quality requirements for reuse. Downstream sources, primary and secondary, sourced within processes will be developed and recovered metals the EU.

In this context, the BIORECOVER project aims producing samples for end-user testing assuring to reduce the gap between the European supply the product quality requirements for their reuse in and demand for CRM (REE, magnesium and pgm) different applications Figure( 1). by providing innovative, flexible and versatile To increase the efficiency and the sustainability alternative processes based on modular and mainly of the BIORECOVER processes, valorisation of the bio-based technologies. The scientific advances in byproducts and wastes generated will be studied the field of biotechnology (bacteria, microalgae, towards a zero liquid discharge process. These fungi, proteins) will allow the exploitation of CRM BIORECOVER aims will be supported by applying inaccessible by conventional extraction methods. tools such as interactive life cycle analysis and life cycle costing. Additionally, modelling of the overall The BIORECOVER Project process will be performed to develop a decision- making framework to maximise the performance. Project Aims and Approach To obtain a competitive, secure, sustainable and publicly acceptable process, the project will be The BIORECOVER project aims to produce a suite of supported by socio-economic and health and safety versatile and flexible recovery processes applicable analyses. in several conditions (pH, mineral complex, raw To achieve these goals, the BIORECOVER materials), which obtain high recovery yields project has brought together an interdisciplinary (≥90%), selectivity (>95%) and purity (≥99%) consortium involving partners across the whole and delivers both environmental sustainability and value chain from suppliers, scientific experts cost-efficiency in safe conditions. The BIORECOVER to the CRM end users, as well as two small-to- strategy is based on research, integration and medium enterprises specialised in dissemination, optimisation of the following stages at laboratory communication, exploitation and social issues scale: pretreatment to remove major impurities in (Table I). the raw materials, mobilisation of the target CRM into a bioleachate and recovery of metals with high Supply of Raw Materials selectivity and purity. Selection and integration of the best technologies will be carried out (one MYTILINEOS SA, Metallurgy Business Unit (formerly route for each type of raw material), and the known as Aluminium of Greece) is providing a REE- selected processes will be optimised and validated, containing bauxite residue (BR) for the project. BR is

Characterisation and CRMs biorecovery process, simulation and control Downstream conditioning of raw transformation materials of CRMs

Raw materials Y Mg Pt Selection and optimisation of microorganisms, Pd consortia and metabolites BR Microcapsules Microalgae Y La Ce Pd Testing recovered Sc Mg CRM and chemical Pt transformation to Mg low-grade ores final application

Characterisation Conditioning Pretreatment Integration, Treatment Post- of the primary of the primary to remove treatments optimisation and secondary and secondary uninteresting for CRMs and mobilisation for selective sources sources material recovery of validation CRMs Fabrication of products and testing against commercial equivalents Oxygen Bacteria Proteins sensors Fungi pgm low-grade sources Brake pads Bioprocess simulation, automation and control pgm content byproducts Catalysts

Fig. 1. The flow-scheme of BIORECOVER

Table I BIORECOVER Consortium Description Partner Country Type of entitya Role

Mytilineos Anonimi Etairia-Omilos Raw materials characterisation and Greece LE Epicheiriseon (MYTILINEOS) supply

Raw materials characterisation and Magnesitas de Navarra (MAGNA) Spain LE supply; end-users University of Witwatersrand Raw materials characterisation and South Africa RTO (UWITS) supply; recovery process research Raw materials characterisation and Johnson Matthey UK LE supply; recovery process research; end-users University of Copenhagen (UCPH) Denmark RTO Recovery process research University of Coimbra (UC) Portugal RTO Recovery process research Linnaeus University (LNU) Sweden RTO Recovery process research Recovery process research; Fundación Centro Tecnológico dissemination, communication, de Investigación Multisectorial Spain RTO exploitation & environmental and (CETIM). Coordinator social issues University of Cape Town’s Centre for Bioprocess Engineering South Africa RTO Recovery process research Research (CeBER) Técnicas Reunidas (TR) Spain LE Recovery process research Algaenergy Spain SME Recovery process research Francisco Albero (FAE) Spain LE End-users Dissemination, communication, Vertech France SME exploitation & environmental and social issues Dissemination, communication, LGI France SME exploitation & environmental and social issues a Research and technical organisation (RTO), small and medium-sized enterprise (SME), large enterprise (LE) the insoluble material generated during the extraction the steel industry or calcined at around 1300°C of alumina from bauxite ore using the Bayer process. to produce reactive magnesium oxide, used in When bauxite ore is treated with caustic soda, the agriculture, livestock farming and other industrial aluminium hydroxides and oxides contained within and chemical technologies (8). The waste streams are solubilised, leaving behind other bauxite oxides of low-grade mineral and calcination byproducts (mainly iron oxides, silica and titania) to form (MgW) still contain some magnesium, which is insoluble BR. The BR is washed then filter pressed currently not recovered. The low-grade mineral is to reduce storage volume and recover the alkaline deposited in mining dumps, while the calcination solution (6). MYTILINEOS currently recycles ~10% byproducts are used in different industrial and of this residue as an additive for cement production, environmental applications due to their small however the rest is stockpiled as a waste product. BR particle size and basifying characteristics. contains low amounts of some high value REE, such Johnson Matthey (UK) is supplying low-grade as scandium and yttrium, which are not currently residues from its secondary pgm refineries. The recovered (7). BIORECOVER aims to recover these feed intake is a mix of end-of-life pgm products REE and, in doing so, reduce the build-up of and and process residues, from spent catalysts to extract more value from the BR waste. electronics and jewellery scrap. There are four main Magnesium-containing feeds are being supplied by stages in the refinery process. In the smelting step, Magnesitas de Navarra (MAGNA, Spain). Magnesite pgm‑containing bullions are produced, alongside (magnesium carbonate) is mined, crushed, ground a non-metallic stream which goes to second uses and enriched. Then the carbonate is either sintered such as aggregates. The pgm-containing bullions at around 1800°C to produce magnesite sinter for then undergo chemical leaching processes to

153 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15935988177157 Johnson Matthey Technol. Rev., 2021, 65, (1) produce pgm solutions. In chemical separations, competition, increase biomass, give protection the pgm solutions are converted into separate pgm from environmental stress and provide more salts or high purity fine metal powder. These are control at a lower cost (12). then transformed into application-ready products, The first two biological steps in CRM recovery are such as catalytic converters, which are fed back into pretreatment and CRM mobilisation. In both steps, the refinery at end-of-life (9). During the refining BIORECOVER will isolate and cultivate microbial stages, lost material containing low levels of pgm populations present in the feed. These would is recovered and fed back into the smelting stage. be expected to be naturally metal resistant and This recovery is a highly energy intensive process possess other valuable traits. BIORECOVER will use relative to the low levels of pgm present and so metagenomics and metatranscriptomics to identify recovering the precious metal through a bioprocess and characterise these samples, as well as isolates would be more sustainable and lower cost. from existing culture collections, that have the The final raw material is pgm-containing low grade ability to remove impurities (for pretreatment) and material and concentrates which the University of leach CRM from samples (for CRM mobilisation). the Witwatersrand (UWITS, South Africa) is sourcing For pretreatment, University of Copenhagen (UCPH, from Anglo American Platinum in South Africa. Denmark) and University of Coimbra (UC, Portugal) will screen microbial communities, present in either The BIORECOVER Technologies the unconditioned raw material or the UC culture collection, for the ability to remove impurities (iron, Biometallurgy is a proven green and low-cost aluminium, calcium and titanium for BR; silicon, iron technology for the exploitation of metals through the and calcium carbonate for MgW). For pgm‑containing application of different biocatalysts (microorganisms material, UC will use selected mesophilic and and metabolites) (10). These biocatalysts interact thermophilic microbes and UWITS will isolate iron and with metals by selectively concentrating or mobilising sulfur-oxidising bacteria to test removal of copper, them. One crucial limitation of biometallurgy is the nickel, cobalt, zinc and iron from pgm‑containing long retention times that are currently required. To Johnson Matthey refinery residues and Anglo address this, BIORECOVER will identify and develop American Platinum mining waste, respectively. new biocatalysts with improved performance. This For CRM mobilisation, Linnaeus University (LNU, approach, using microorganisms indigenous to Sweden) will screen both indigenous microbial mining and CRM storage sites, will assure fewer communities as well as fungal cultures known to ecological distortions and less time consumption excrete organic acids for the ability to mobilise for adaptation. In terms of leaching efficiency, REE in BR samples. LNU will also screen organisms native strains usually achieve higher cell density in MgW for the ability to leach magnesium and and greater metal extraction rates than exogenous in Johnson Matthey low-grade residues for the microbes. In addition, use of the following state-of- ability to leach pgm. Fundación Centro Tecnológico the-art technologies will facilitate the development de Investigación Multisectorial (CETIM, Spain) of microbial consortia more competent for will optimise CRM mobilisation conditions for biometallurgy (11). microorganisms selected for BR and MgW. UWITS will sample soil around pgm deposits to isolate • metagenomics (sequencing of genomes from cyanide-producing bacteria that can mobilise pgm. environmental samples) For both pretreatment and CRM mobilisation, the • metatranscriptomics (identification of partners will perform further screening, testing expressed transcripts) relevant conditions such as co-cultivation for • proteomics (identification and quantification synergistic effects, and testing with different CRM of proteins) concentrations to optimise activity. • metabolomics (measurement of cellular In the third step, CRM recovery, BIORECOVER will metabolites) develop five different sustainable technologies to • interactomics (understanding cellular bind CRM. Técnicas Reunidas (TR, Spain) will test and interactions). develop selective reusable polymeric microcapsules with ‘almost zero’ extractant consumption to recover Another innovative approach to improve the a wide range of REE and platinum. ALGAENERGY, biomining processes is the immobilisation of Spain, will cultivate and screen different microalgae microorganisms onto supports that will enable species and develop microalgal-based biosorbents the continuous supply of nutrients without to recover mainly yttrium, magnesium, platinum

154 © 2021 Johnson Matthey https://doi.org/10.1595/205651320X15935988177157 Johnson Matthey Technol. Rev., 2021, 65, (1) and palladium. UC will screen planktonic cells that MAGNA will characterise the recovered magnesium produce siderophores and immobilised systems to nanoparticles and compare this with magnesium develop an adsorption process for yttrium and pgm. obtained through conventional technologies to CETIM will screen fungi with known magnesium- assess their suitability for commercial applications binding activity to develop a biotransformation such as agricultural fertilisers. process to make magnesium nanoparticles. Johnson The dissemination of the project’s achievements Matthey will design and synthesise different proteins to a wide range of stakeholders (policy makers, and peptides to develop protein products that can industry groups, potential markets and the adsorb magnesium and platinum. academic community) will be key for the successful For all three steps, data from all groups will be exploitation of the project. Diffusion actions collected by University of Cape Town’s Centre for will include conferences, seminars, workshops, Bioprocess Engineering Research (CeBER, South scientific publications as well as engagement with Africa), who will develop kinetic models, that other relevant projects on CRM recovery. Finally, will be valuable in process optimisation. This will a strategic plan to exploit the results generated facilitate process improvement towards achieving during and after the end of the project will be a target recovery rate (>90%), selectivity (>95%) accomplished in terms of business models and and purity (>99%). intellectual property rights strategy. In the second half of the project the best technology for each step will be selected, integrated into a complete process and scaled- Acknowledgements up to 5 l bioreactors and columns. Along with the This project has received process modelling this will successfully optimise funding from the EU’s Horizon and validate the BIORECOVER strategy for the 2020 research and innovation different CRM feeds. A technical, environmental programme under grant and economic assessment of the selected process agreement No. 821096. The for each feedstock will be conducted to improve the authors would like to thank and acknowledge performance and to facilitate further replicability the consortium partners for their input into the and scaling up of the BIORECOVER technology. preparation of this manuscript.

Project Outputs References

The CRM recovered through the BIORECOVER 1. ‘Light Rare Earths’, Critical Raw Materials (CRM) processes will be tested for end use by industrial Alliance, Brussels, Belgium: https://www. project partners. crmalliance.eu/lrees (Accessed on 12th November 2020) Francisco Albero SAU (FAE, Spain) will test the application of the recovered yttrium and platinum for 2. ‘Heavy Rare Earths’, Critical Raw Materials (CRM) Alliance, Brussels, Belgium: https:// brake pads and oxygen sensors, respectively. FAE www.crmalliance.eu/hrees (Accessed on 12th fabricates brake pads by tape casting then sintering November 2020) advanced ceramic slurries. Yttrium is added to the slurries to reduce the sintering temperature. FAE 3. Deloitte Sustainability, British Geological Survey, also produces oxygen sensors made of zirconia for Bureau de Recherches Géologiques et Minières exhaust gas monitoring. Conductive parts of the and Netherlands Organisation for Applied Scientific Research, “Study on the Review of the oxygen sensors are fabricated by screen-printing List of Critical Raw Materials: Critical Raw Materials platinum inks on ceramic substrates. Factsheets”, European Union, Brussels, Belgium, The use of the recovered platinum, palladium and 2017, 517 pp iridium for commercial catalysts will be tested by Johnson Matthey. Catalysts will be prepared and 4. ‘Magnesium’, Critical Raw Materials (CRM) Alliance, their catalytic activity will be tested on a range Brussels, Belgium: https://www.crmalliance.eu/ of reactions, including carbon monoxide and magnesium (Accessed on 12th November 2020) hydrocarbon oxidation, selective hydrogenation, 5. ‘PGMs’, Critical Raw Materials (CRM) Alliance, selective nitro reductions and carbon-carbon bond Brussels, Belgium: https://www.crmalliance.eu/ forming reactions. pgms (Accessed on 12th November 2020)

6. A. Tabereaux, Light Metal Age, 2019, 77, (1), 9. ‘Pgm Refining’, Johnson Matthey, London, UK: 54 https://matthey.com/en/products-and-services/ 7. E. Balomenos, ‘Bauxite Residue Handling Practice precious-metal-products/pgm-refining (Accessed and Valorisation Research in Aluminium of Greece’, on 12th November 2020) 2nd International Bauxite Residue Valorisation 10. S. Ilyas and M. Kim and J. Lee, J. Chem. Technol. and Best Practices Conference, Athens, Greece, Biotechnol., 2018, 93, (2), 320 7th–10th May, 2018 11. L. Valenzuela, A. Chi, S. Beard, A. Orell, N. Guiliani, 8. ‘How We Work’, Magnesitas de Navarra, Navara, J. Shabanowitz, D. F. Hunt and C. A. Jerez, Spain: https://www.magnesitasnavarras.es/en/ Biotechnol. Adv., 2006, 24, (2), 197 magnesite-products/how-we-work/ (Accessed on 12. R. Branco, T. Sousa, A. P. Piedade and P. V. Morais, 12th November 2020) Chemosphere, 2016, 146, 330

The Authors

Annette Alcasabas is a Lead Scientist in the Biotechnology Department of Johnson Matthey. She has a background in microbial genetics and worked in industrial synthetic biology prior to joining Johnson Matthey in 2016. At Johnson Matthey, she is responsible for the molecular biology workflow used in commercial protein production and protein engineering. Annette is also part of a team that is exploring new opportunities and applications for biotechnology.

Felicity Massingberd-Mundy is a research scientist in the Recycling and Separations Technology department at Johnson Matthey, Sonning Common, UK. She graduated with an MChem from the University of Oxford, UK, in 2019, having conducted her final year research project in the New Applications group at Johnson Matthey. Her current research focuses on CRM recycling, specifically pgm and battery materials recycling.

Barbara Breeze is a Senior Principal Scientist in the Recycling and Separations Technology department at Johnson Matthey. She has experience of new process research and development for the recovery of critical metals from the end-of-life products, with a particular focus on battery materials recycling and pgms recovery.

Maite Ruiz is a Senior Researcher in the ECO BIO technologies department at CETIM in A Coruña. She has experience of recovery processes of CRM, valuable metals and bioactives from different matrices (such as waste electric and electronic equipment (WEEE) and food byproducts), with a special focus on bio-based technologies.

Cristina Martínez García is the Head of the research and development (R&D) ECO BIO technologies department at CETIM. Cristina Martínez holds a PhD and Master’s degree in Environmental Science and Technology as well as a degree in Chemistry from the University of A Coruña, Spain. She has been involved in the development of several national and international R&D projects focused on the development of new technologies in the area of circular economy: resources recovery from organic and inorganic wastes such as wastewater, sludge, minerals and byproducts of animal and vegetable origin to obtain high value commercial compounds.

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