Johnson Matthey Technology Review

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JOHNSON MATTHEY TECHNOLOGY REVIEW

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

Johnson Matthey Technology Review is published by Johnson Matthey Plc.

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

Contents Volume 59, Issue 4, October 2015

291 Guest Editorial: Water Technologies at Johnson Matthey By Nick Garner

293 “Heavy Metals in Water: Presence, Removal and Safety” A book review by Edward Rosenberg

298 “Particle-Stabilized Emulsions and Colloids: Formation and Applications” A book review by Cecilia Bernardini

303 Interplay between Silver and Gold Nanoparticles in Production of Hydrogen from Methanol By Hany M. AbdelDayem

313 Carbon Formation in Steam Reforming and Effect of Potassium Promotion By Mikael Carlsson

319 “Electrochemical Power Sources: Batteries, Fuel Cells, and Supercapacitors” A book review by Billy Wu

322 Selective Removal of Mercury from Gold Bearing Streams By James G. Stevens

331 In the Lab: Uranium Capture From High Sulfate and Nitrate Waste Streams with Modiﬁ ed Silica Polyamine Composites Featuring Professor Edward Rosenberg

334 New Smopex® Ion Exchange Materials for the Removal of Selenium from Industrial Efﬂ uent Streams By Carl Mac Namara, Javier Torroba and Adam Deacon

353 Johnson Matthey Highlights http://dx.doi.org/10.1595/205651315X688037 Johnson Matthey Technol. Rev., 2015, 59, (4), 291–292 JOHNSON MATTHEY TECHNOLOGY REVIEW www.technology.matthey.com

Guest Editorial Water Technologies at Johnson Matthey

In this issue the theme is water remediation. Johnson need to clean up effl uents from industrial processes Matthey is working on a number of high technology such as mining, agriculture and manufacturing. purifi cation products for applications in the water Pollutants including metals, non-metals and organic industry. We are focusing our research and development compounds may be present due to either man-made or efforts on creating technology to remove a range of low natural processes. level toxic contaminants, such as mercury, from water. An example of such a pollutant, selenium, is discussed Johnson Matthey is known for its expertise in adsorbent in the article by Mac Namara et al. in the present issue materials, such as Smopex®, with which readers of this of the Johnson Matthey Technology Review (3). In journal may be familiar for their use in the recovery of this article the performance and mechanisms of a new precious metals from both waste and product streams material based on the Smopex® range of ion exchange (1, 2). In 2013 the company acquired further advanced materials is described for Se remediation in effl uents ion exchange technology from Purity Systems Inc, from coal combustion plants and oil refi neries. A common forming the company’s Water Technologies business. co-contaminant is sulfur which poses signifi cant This combination of technology fi ts well with Johnson problems for previous generations of ion exchangers, Matthey’s core competences in advanced materials and although the technique of ion exchange offers attractive catalysts. We place particular emphasis on some key benefi ts over existing technologies (whether chemical challenges facing the mining and chemicals industries, or biological) which all have disadvantages in terms where problem contaminants, increasing legislative of high cost or high volumes of materials required. requirements and focus on environmental and cost Strong-base functionalised materials were identifi ed issues often mean current technologies are being by Johnson Matthey as being the most promising stretched. candidates for selective sorption of selenium ions and the article presents results and fundamental studies on Providing Effective Solutions these materials showing promising results in both fi xed bed and continuous stirred tank reactor trials. The need for clean water is of major signifi cance across Gold mining is another area which suffers from the the world, with growing populations requiring access presence of water soluble pollutants, in this case to improved quality water resources. Environmental species of the heavy metal mercury which is frequently legislation and regulation mean that there is increasing associated with gold in ore deposits. The health and

From mining to molecules – Johnson Matthey’s innovative processes and advanced scavenger technologies, built on and underpinned by continuous research and development, can help recover valuable metals and purify active pharmaceutical ingredients

291 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688037 Johnson Matthey Technol. Rev., 2015, 59, (4) environmental implications of mercury are well-known, naturally occurring substances, bioremediation and even however it is a major challenge to remove the mercury waste products are in use for removing heavy metals from the gold processing circuit; technical difﬁ culties from water – but more technically advanced materials also exist since the most widely used method for are required for heavy metal contamination arising from extracting gold, employing cyanide as lixiviant, also high technology industries in developed countries. For extracts mercury and other metals along with the gold. example, ion exchange is the go-to technology in the It is therefore essential to identify a method that will USA, where it constitutes a multi-billion dollar a year remove only the mercury; any loss of gold during the market. The technique of ion exchange shows great process is deemed unacceptable. Johnson Matthey promise to help remediate wastewater streams around has now developed solid adsorbents which can achieve the world and provide safer, cleaner water for greater selective adsorption of mercury from gold cyanide numbers of people than ever before. bearing process streams and the technique is described in detail in this issue of the journal (4). Testing of the NICK GARNER material in real process feeds is described and a pilot Group Director, Corporate and Strategic plant trial is now underway in Nevada, USA. Development Johnson Matthey Plc, Orchard Road, Royston, A Collaborative Approach Hertfordshire, SG8 5HE, UK Johnson Matthey is always open to new collaborative Email: [email protected] efforts to solve problems for our customers. One such collaboration is with Professor Edward Rosenberg, References University of Montana, USA. He develops advanced silica polyamine composite materials for metal ion 1. S. Phillips and P. Kauppinen, Platinum Metals Rev., separations and recovery from industrial and mining 2010, 54, (1), 69 waste streams. Most recently these materials are being 2. J. Frankham and P. Kauppinen, Platinum Metals Rev., applied for uranium remediation with the University of 2010, 54, (3), 200 the Witwatersrand in South Africa, and a forthcoming 3. C. Mac Namara, J. Torroba and A. Deacon, Johnson article in this journal is expected to present some further Matthey Technol. Rev., 2015, 59, (4), 334 details on this project. 4. J. G. Stevens, Johnson Matthey Technol. Rev., 2015, It is worth noting that many techniques based on 59, (4), 322

“Heavy Metals in Water: Presence, Removal and Safety”

Edited by Sanjay K. Sharma (Jaipur Engineering College and Research Centre (JECRC), India), Royal Society of Chemistry, Cambridge, UK, 2015, 357 pages, ISBN: 978-1-84973-885-9, £175.00, €218.75, US$290.00

Reviewed by Edward Rosenberg production where more technically advanced, but more Department of Chemistry and Biochemistry expensive materials are employed in the industrially University of Montana, Missoula, Montana 59812, USA developed countries. The specifi c metal contamination problems presented are arsenic (Chapter 5), iron and Email: [email protected] manganese (Chapter 6), fl uoride (Chapter 13) and chromium (Chapter 16), with the remaining chapters dealing with techniques and general surveys of heavy Introduction metal contamination. Chapters 7 and 8 stand out as chapters that deal with Chinese government policies “Heavy Metals in Water: Presence, Removal and on toxic metal contamination and should be very useful Safety” is published by the Royal Society of Chemistry for foreign entrepreneurs wanting to establish new and consists of 16 independent chapters. The chapters businesses in metals related industries. can be broadly divided into two groups: those covering The editor of this volume, Sanjay K. Sharma, is the techniques and processes used to deal with heavy currently Professor and Head of the Department metal pollution and those discussing a particular of Chemistry at JECRC University, India. He has pollutant or pollution problem. The chapters are divided edited many volumes closely related to this one and approximately equally between these two topics. The was recently appointed editor for the series ‘Green techniques presented include the use of modern Chemistry for Sustainability’. approaches such as photocatalysis and nanotechnology (Chapters 2, 4 and 9) but by and large the volume Heavy Metals in Aquatic Media emphasises the use of naturally occurring substances, waste products and bioremediation for removing heavy It is beyond the scope of this review to give a detailed metals from water (Chapters 3, 10, 11, 14 and 15). This analysis of each chapter. A brief summary of each is understandable in light of the fact that most of the chapter will be provided with critical comments on the contributors come from developing countries where scientifi c contents, its relevance to the topic and where the emphasis for remediation is on low-cost readily it complements or is redundant with the other chapters. accessible technologies. As a consequence the volume Chapter 1, ‘Contamination of Heavy Metals in does not deal with heavy metal contamination resulting Aquatic Media: Transport, Toxicity and Technologies from high technology industries such as nuclear power, for Remediation’ coauthored by the editor serves as computer manufacturing and related electronics a general introduction to the topic and deals with the

293 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689009 Johnson Matthey Technol. Rev., 2015, 59, (4) sources of heavy metal contamination, associated nanoparticles as adsorption media for heavy metals. health risks and brief summaries of remediation The chapter begins with a repetition of the same topics methodologies, all of which are handled in more summarised in Chapters 1–3, sources and health detail in later chapters. It concentrates on the removal effects of heavy metals, but does include selenium in the of iron and manganese. This is strange as an entire list. The chapter then goes on to discuss the synthesis chapter devoted to this subject is found later in the of magnetic nanoparticles, focusing heavily on iron, volume (Chapter 6). A useful summary of the health and then goes on to explain the different materials risks associated with heavy metals in water is provided and methods for coating the iron nanoparticles. in table form but several important contaminants This is a useful summary of the currently available are omitted. For example, uranium has become an technologies. The kinetic and isotherm models for important contaminant as a result of the development the nanoparticles as applied to adsorption of heavy of nuclear energy and selenium remains a problem metals are also discussed. This is fairly standard for for the oil and coal industries. Both of these metals all adsorptive materials and the authors would have pose signifi cant health risks. There are a few notable been better off giving more details on the synthesis of misstatements in the chapter. For example the authors the iron-glutamic acid nanocages and on regeneration defi ne a heavy metal as having densities in the range of magnetic nanoparticles in general. A minor point of 3.5–7.9 g cm–3 while mercury has a density of is that one of the equations given is not correct, it –3 13.7 g cm and many third row transition metals have should read Fe2O3, not Fe2O4. The correct equation densities of 19–22 g cm–3. Cadmium is defi ned as the (Equation (i)) is given below. most toxic heavy metal although the allowable release Fe O + 2H+  -Fe O + Fe2+ + H O (i) level of mercury is lower than that of cadmium. 3 4 2 3 2 Chapter 9, ‘Use of Nanotechnology against Heavy Photocatalysis and Nanotechnology Metals Present in Water’, provides a brief overview of nano-adsorbents. This is a developing area of research Chapter 2, ‘Photocatalytic Processes for the Removal that has not seen much use in the remediation of Toxic Metal Ions’ describes the photocatalytic industry. The author does a good job of putting this reduction of metals using titanium dioxide (TiO2) as fi eld in perspective including a discussion of the the photocatalyst. The appeal of this method is that the environmental dangers of nanomaterials, a topic that is electron holes created by the incident light can oxidise often sidestepped by other workers in this area. organic contaminants and the electrons released could be used to reduce metal ions in the same waste stream. Removing Heavy Metals From Water In the absence of organic contaminants water needs to be oxidised. The authors do a good job of outlining Chapter 3, ‘Removal of Dissolved Metals by the basic process and the relevant kinetic parameters, Bioremediation’, is perhaps the least useful and although one of the diagrams (taken from another most superfi cial chapter in the book. The usual list of source) is not adequately explained in the text. There is metals is followed by a cursory summary of the same an appropriate discussion of the problems associated remediation techniques outlined in Chapters 1 and 2. with scale-up of this technique followed by a case-by- The actual subject of the chapter is bioremediation, case discussion of the reduction of specifi c metals on which is summarised in three or four pages and consists the bench scale. The tables in this chapter are basically of a laundry list of bacterial strains that absorb heavy redundant with those in Chapter 1 but it is interesting metals with no conceptual or mechanistic insights. to note that the allowable release levels use the World Chapter 10, ‘Modifi ed and New Adsorbents for Health Organization (WHO) values, which are different Removal of Heavy Metals from Wastewater’ presents than those in Chapter 1. a survey of industrial waste byproducts and modifi ed Chapter 4, ‘Functionalized Magnetic Nanoparticles agricultural and biomaterials for heavy metal for Heavy Metals Removal from Aqueous Solutions’ adsorption. Like the other chapters in this volume, addresses the timely and interesting topic of magnetic the author starts out with the usual list of toxic metals,

294 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689009 Johnson Matthey Technol. Rev., 2015, 59, (4) their sources, health risks and methods of removal. affecting effi ciency. The chapter would have benefi tted The use of industrial and agricultural wastes is a good from more details on these topics rather than devoting addition to this volume as is the discussion of modifi ed half the chapter to information already covered biopolymers. A very interesting magnetic core-shell elsewhere in the book. particle modifi ed with a bio-hydrogel is reported but this fi gure is barely readable and should have been Contamination Problems corrected prior to publication. An otherwise useful chapter is compromised by the statement that batch Overall Chapter 5, ‘Arsenic Contamination: An equilibrium studies can be used for designing industrial Overview’ is an excellent chapter that summarises processes. This is a signifi cant misstatement as this all aspects of the most pervasive water contaminant type of study is only the beginning of the process, worldwide. The natural and anthropogenic sources followed up by kinetic studies, adsorbent regeneration of arsenic contamination are nicely described and studies and evaluation of usable lifetime. the health risks and the different methods of arsenic Chapter 11, ‘Natural Clays/Clay Minerals and removal are explained. In the conclusions section Modifi ed Forms for Heavy Metals Removal’ presents a the reader is left with the impression that none of thorough and comprehensive survey of the use of clays the currently available removal techniques are in and modifi ed clay. The fi rst few pages are devoted to widespread use. Although all the current methods allowable limits and then methods of treatment for have their disadvantages, processes using iron(III) activation of the clays. A comprehensive list of the chloride (FeCl3) precipitation, adsorption onto Fe applications of the various mineral clays to specifi c particles (the ferrihydrite process) and composite heavy metals is provided. The complex structures materials are commercially available and are being and structural modifi cations of the wide range of used effectively. The most recent advances using ion clay minerals available has prompted workers in the exchange technologies are not covered at all and this fi eld to develop new isotherm mathematical models is a glaring omission in an otherwise excellent review. for evaluating adsorption parameters. The chapter The properties of elemental arsenic are listed, however, presents a list of the more recently developed isotherm this has nothing to do with the topic of this chapter and models in addition to the more common Langmuir and should have been deleted. Freundlich models and the clays to which they have Chapter 6, ‘Removal of Iron and Manganese from been applied. Unfortunately, the chapter is already Water – Chemistry and Engineering Considerations’ quite long and there was no critical evaluation of these deals with removal of these metals from ground and models. surface waters. The chapter begins with an excellent Chapter 14, ‘Use of Industrial and Agricultural description of the aqueous redox chemistry of iron and Waste in Removal of Heavy Metals Present in Water’ manganese. These metals rank low on the toxicity describes a wide range of materials for this application index and are mainly a problem for the construction (everything from banana peel to walnut dust), along with industry. Indeed, this chapter is a contribution from a the methods used to modify them and their capacities civil engineering fi rm. The chapter goes on to describe for divalent metals and chromium. The authors discuss the effective oxidation-fi ltration systems used to the methods in detail and some interesting images are remove these metals in the construction industry. There included that describe the surface changes resulting are many other commercially available adsorbents from modifi cation of the surface of the waste product. for removal of these metals but none of these are Chapter 15, ‘Biosorption of Metals – From the Basics discussed in this chapter and so it is a narrowly to High Value Catalysts Production’ targets biosorption conceived contribution to the volume. by living organisms. The fi rst six pages of the chapter, Chapter 13, ‘Fluorides in Different Types of Aquatic devoted to the sources and toxic effects of the metals Systems and their Correlation with Metals and arsenic, cadmium, chromium, copper, nickel, lead and Metalloids’ deals with fl uoride contamination arising zinc, are probably not necessary in light of the other primarily from the use of fl uorine containing industrial chapters. The chapter goes on to briefl y describe chemicals. Sulfur tetrafl uoride (SF4) used for termite the mechanisms of biosorption and the parameters extermination in the USA is missing from the otherwise

295 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689009 Johnson Matthey Technol. Rev., 2015, 59, (4) fairly complete list. The chapter was hard to follow. Taken together the two chapters provide an excellent The salt contents of an apparently random list of water overview of the challenges facing the tanning industry sources are given in a table but are not discussed in today. the text. The authors attempt to correlate the presence of fl uoride with various cations and estimates of Al/F Chinese Economy speciation as a function of pH are discussed in detail. The authors go on to explain the correlation between Chapter 7, ‘Heavy Metal Pollution in Water Resources the presence of fl uoride with arsenic in ground in China – Occurrences and Public Health Implications’ waters but the discussion and the data presented are and Chapter 8, ‘Heavy Metals Distribution in Surface confusing and unconvincing. A useful but, perhaps over Water Samples of Taihu Lake, China’ are unique in interpreted chapter overall. this volume in that they both deal with the distribution Chapter 12, ‘Heavy Metals in Tannery Wastewater of heavy metals in the environment. Chapter 7 deals and Sludge: Environmental Concerns and Future with the sources of heavy metals across the Chinese Challenges’ and Chapter 16, ‘Chromium in Tannery economy while Chapter 8 focuses on the details Wastewater’ both deal with the problem of chromium of metal pollutants in one of China’s largest lakes in the wastewater and sludge associated with tanning (Figure 2). Both chapters contain an enormous leather. The chapters are complementary rather than amount of information that will be useful for planning redundant. Chapter 12 focuses more on the distribution future environmental clean-up in China and for foreign of tanning sites worldwide and the demographics of investors in the Chinese economy looking to get risk from the toxic effects of Cr(VI). The legal discharge involved in balancing Chinese economic growth with limits for Cr in various countries are included with some sustainability. countries providing only Cr(III) limits, and estimates of the most at-risk populations are given. Chapter 16 Summary contains much more detail on the chemistry of the tanning process and methods of treatment (Figure 1) The volume covers the fi eld of heavy metals in water including recovery and reuse of the chromium salts. very well for the most part. In general the chapters

The bacteria reduces Cr(VI) to Cr(III) and then the Cr(III) is retained in the zeolite by ion exchange CrVI CrVI VI III Cr Cr CrIII

Formation of a bioﬁ lm Na+ VI Cr Na+ Catalysts to be applied in oxidation III III reactions of volatile M M Cr Cr organic compounds Heat treatment – calcination Fig. 1. New method of chromium removal from wastewater and catalytic reutilisation in volatile organic compounds oxidation (Reproduced by permission of Royal Society of Chemistry)

US$5 billion dollar a year market and represents the –1 N Pb, mg l go-to technology for heavy metal removal from water. 0–0.01 W E The volume would also have beneﬁ tted from more 0.01–0.02 careful editing. As mentioned several times in this S 0.02–0.05 0.05–0.1 review there is too much repetition of the sources, health effects and allowable release levels of the various heavy metal pollutants. This could have been covered in one introductory chapter (as it is) and omitted from the subsequent chapters. Overall however, this volume is a useful addition to the area of heavy metal pollution and remediation.

“Heavy Metals in Water: 0 5 10 20 Presence, Removal and Kilometers Safety” 1:500,000 Fig. 2. Lead concentration distribution in Taihu Lake, China (Reproduced by permission of Royal Society of Chemistry)

are well written and organised for facile retrieval of data. The references are recent and cite the most important journals in the ﬁ eld. The one glaring omission is the cursory treatment of ion exchange. In the USA the use of ion exchange materials constitutes a

The Reviewer

Edward Rosenberg received his doctorate at Cornell University, USA, and held post-doctoral fellowships at the University of London, UK, and the California Institute of Technology, USA. He is the author of 180 peer-reviewed publications, ﬁ ve boo k chapters, eight patents and one book in the areas of environmental and organometallic chemistry. He has received awards for his research and student mentoring from the University of Montana and has had visiting faculty fellowships in Italy, Israel and South Africa.

“Particle-Stabilized Emulsions and Colloids: Formation and Applications”

Edited by To Ngai (The Chinese University of Hong Kong, China) and Stefan A. F. Bon (University of Warwick, UK), RSC Soft Matter Series, No. 3, Royal Society of Chemistry, Cambridge, UK, 2015, 337 pages, ISBN: 978-1-84973-881-1, £175.00, €218.75, US$290.00

Reviewed by Cecilia Bernardini is devoted to the use of solid particles as a means to Johnson Matthey Technology Centre, stabilise emulsions and more complex colloidal systems. Blounts Court, Sonning Common, Reading, The ambition of the book is to offer a comprehensive RG4 9NH, UK overview of not only the fundamental science behind Pickering emulsions and their stabilisation mechanism, Email: [email protected] but also of the current and future range of useful industrial applications, with the aim of fostering further development of these emerging technologies. “Particle-Stabilized Emulsions and Colloids: Formation The target audience is therefore the colloid science and Applications”, edited by To Ngai and Stefan A. F. community at large, both in academia and in industry, Bon, is the third book of the Royal Society of Chemistry rather than a general, non-specialised audience. Given (RSC) Soft Matter Series, published in 2015. Both the broad scope of the applications illustrated, only a editors have extensive expertise in polymer chemistry selection of the most relevant chapters will be reviewed. and its application to colloid science. Professor Ngai’s research interests focus on interparticle interactions at The Pickering Stabilisation Phenomenon fl uid interfaces and using emulsions as templates for functional materials, whereas Professor Bon’s current The fi rst chapter is written by Stefan Bon and it is a very research area is supracolloidal polymer chemistry, short and basic introduction to the Pickering stabilisation focusing on the design of assembled supracolloidal phenomenon, with a brief historical perspective. The structures and the synthesis of their colloidal and following chapter, authored by Bum Jun Park (Kyung macromolecular building blocks through a combination Hee University, South Korea), Daeyeon Lee (University of polymer chemistry, colloid science, soft matter of Pennsylvania, USA) and Eric M. Furst (University of physics and chemical engineering. Delaware, USA), is a more extensive description of the This series, edited by Hans-Jürgen Butt, Ian W. physical-chemical interactions of particles adsorbed Hamley, Howard A. Stone and Chi Wu, provides a at fl uid-fl uid interfaces: from the wettability of a single review of recent developments in soft matter research. particle, homogeneous or amphiphilic, to more complex The scope of this volume is quite focused: the book topics, such as the interactions between pairs of

298 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689126 Johnson Matthey Technol. Rev., 2015, 59, (4) homogeneous and amphiphilic particles, with a focus Next the topic of AuNPs, functionalised with polymer on effects of geometrical anisotropy and non-spherical brushes to stabilise emulsions, is highlighted. objects. Pair interactions are discussed not only from Interesting NP complexes with core-shell structures are a theoretical viewpoint, but also by illustrating direct made from AuNPs and iron oxide NPs with PS brushes. measurements done with optical laser tweezers and AuNP-stabilised emulsions are used as templates to then related to bulk property measurements; further fabricate hollow hybrid capsules. experiments reviewed include the effect of additives Finally, the stabilisation of emulsions by Janus disks (salt and surfactant) and the evolution of interactions is summarised, with examples that include preparing with time. amphiphilic Janus Laponite disks at the oil-water interface (Figure 1) and using metal-supporting Janus particles as interfacial catalysts. Polymer-brush Modiﬁ ed Particles

Chapter 3 is nearly entirely dedicated to applications Pickering Suspension, Mini-Emulsion and of polymer brush-modifi ed clay layers or gold Emulsion Polymerisation nanoparticles (AuNPs) in Pickering emulsions. The The terminology used in Chapter 4 by Stefan Bon chapter has been written by Hanying Zhao and Jia is quite technical and confusion is likely for those Tian (Nankai University, China). Brushes are generally who are unfamiliar with these topics. The opening sought after for their responsiveness to environmental paragraph explains the peculiarities of each system, conditions. First clay layers with block copolymer followed by historical perspective and the more recent brushes are discussed: examples of preparations with developments in suspension polymerisation, such as poly(dimethylaminoethyl methacrylate) (PDMAEMA) and preparing deliberately armoured composite polymer poly(methyl methacrylate) (PMMA) block copolymers particles. Examples of suspension polymerisations are given. Then clay layers with homopolymer include using titanium dioxide, Laponite particles and brushes are presented; fi nally examples of mixed iron oxide with different polymers and formulating homopolymer brushes and hydrophilic faces with inverse Pickering suspension polymerisation systems. hydrophobic polystyrene (PS) brushes on the edges Pickering mini-emulsion polymerisation was fi rst are introduced. reported in the literature by Landfester et al. in 2001 (2).

Solvent

Solvent Laponite disks B PS particle

Solvent Micelles C

Fig. 1. Schematic outline for the synthesis of Janus Laponite disks: A a Laponite disk with hydrophobic PS brushes on one side and hydrophilic quaternised PDMAEMA on the other side; B a Laponite disk with PS brushes on one side; C a polymeric micelle with Laponite disks (Reprinted with permission from (1). Copyright (2013) American Chemical Society)

More examples of using Laponite disks as stabilisers silica particles. The characteristic size of bijels domains are given. A successful synthesis needs particles that follows from the ratio of the particle diameter and the are able to adhere and cover the emulsion droplet, particle volume fraction used: this allows tuning of thus curvature is crucial and constraints on the size this size from the nanoscale to hundreds of microns. of particles are present: typically the particles need to Other liquids successfully tested are nitromethane and be less than 200 nm in diameter, unless fl exibility and ethanediol. bending around the droplet are possible. Apart from molecular liquids, polymers are also used The fi nal part of the chapter concentrates on to obtain bijels. Notably, there are some key differences Pickering emulsion polymerisation: general references with molecular liquids, thoroughly discussed in the are suggested for further reading, and then a standard chapter. recipe for a free radical emulsion polymerisation is An essential element in the fabrication of bijels is presented. The difference, in the case of Pickering the particles: colloidal silica particles have been used emulsions, is the replacement of surfactants with extensively, but more recently more exotic particles solid, often nanosized particles: clay disks, amphiphilic have been investigated as alternatives, (mainly with polymer Janus particles, and silica NPs. An extensive simulations) such as anisotropic or fi eld-responsive historical overview of the fi eld is provided with several particles, magnetic particles and graphene oxide (GO) examples, followed by a mechanistic insight on the sheets. process, although work on the area is still limited. Characterisation of bijel morphology is then The outlook on technological application of armoured introduced, followed by studies on the link between nanocomposite polymer latexes is promising, although morphology and mechanical properties. Among the still in its infancy: the latter developments in the up-and-coming applications investigated so far, the use synthesis techniques will allow more applied studies of three-dimensional (3D) bijels (Figure 2) to create and a potential application has recently been reported porous materials is mentioned. by Wang et al. (3), where soft armoured latexes added Materials with bicontinuous structures can be to waterborne adhesives induce a marked increase in used in catalysis, sensors and gas storage, allowing tack adhesion energy. simultaneous optimisation of active surface area and mass transport. Other potential applications include Bicontinuous Emulsions Stabilised by delivery (for example a dye and a bleach in haircare Colloidal Particles products or separate chemical reactants that form the desired product if released simultaneously at target, Chapter 6 by Joe W. Tavacoli (Université Paris- through a specifi c trigger) through bijel capsules Sud, France), Job H. J. Thijssen and Paul S. Clegg and development of cross-fl ow microreactors. The (University of Edinburgh, UK) describes how interfaces challenges are the scaling up of fabrication, cost densely coated by particles can behave like an elastic sheet. Therefore unlike surfactant-stabilised systems, they do not necessarily form spherical droplets and their shape is dependent on the process history, allowing formation of liquid bicontinuous architectures (called bicontinuous interfacially jammed emulsion 100 nm gels or bijels), which consist of two tortuously entwined percolated liquid phases that are separated and stabilised by solid particles. To adopt a bicontinuous morphology, two immiscible liquids must be induced through a critical quench, while the liquid-liquid interface is populated by neutrally wetted particles. Bicontinuous domains evolve when the system is quenched into the 100 μm spinodal region of its phase diagram. This process was fi rst described by Stratford et al. (4) by computational Fig. 2. Silver monolith with continuous pores on two widely methods and later experimentally obtained by Herzig separated length scales (Reprinted with permission from (6). et al. (5), using a water-lutidine system and Stoeber Copyright (2011) American Chemical Society)

300 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689126 Johnson Matthey Technol. Rev., 2015, 59, (4) of materials and use of benign materials instead of higher mechanical strength and more control over the hazardous components. Finally, after dealing with release of encapsulated agents. The large variety liquid-liquid bicontinuous systems separated by a of particles used allows for an easy introduction of particle-coated, solid interface, the idea of creating a functionality onto the microcapsule surface: magnetic structure which has two co-continuous solid domains particles can be used to direct the microcapsules into separated by a single liquid domain, called a bigel, is an area of the vessel or onto a specifi c delivery target. considered. Other responsive materials have been used too. The main use of colloidosomes is for encapsulation and Hollow Spheres and Microcapsules Fabrication subsequent controlled release of drugs or substances: through Particle-stabilised Emulsions of more widespread interest for commercial applications are the approaches for encapsulation of highly volatile, Chapter 9, written by Simon Biggs (The University low molecular weight molecules which are poorly of Queensland, Australia) and Olivier Cayre (The soluble, or sensitive to their environment (perfumes), University of Leeds, UK), explains the fabrication however none so far have succeeded in forming an of colloidosomes, hollow core-shell microcapsules impermeable shell. The variable pore size features in which the capsule wall consists of close-packed have also been explored in order to encapsulate larger colloidal particles that have been permanently locked molecular weight materials or even NPs. to each other. Colloidosomes display a range of unique features: the shell thickness can be manipulated by Conclusion choosing different-sized colloids; the porosity of the shell can be similarly adjusted; the system has inherent The book gives a very extensive coverage of particle- fl exibility due to the large range of particles that are stabilised emulsions and it is a useful reference for used; and using particles solely as stabilisers of the industrial or academic researchers who are already emulsion yields enhanced stability. familiar with the colloid science fi eld, but need to deepen The manufacture of colloidosomes include three main their knowledge into this rather specifi c, although vast, categories of methods that can lock the particles at the branch of colloid science. The content is very technical interface. The fi rst method involves using a sol-gel or and the style of presentation of the different topics is a polymerisation reaction, either in the droplet or at rather heterogeneous throughout the chapters: both the interface, leading to the formation of a shell that aspects hinder somewhat the overall readability. entraps the colloid particles permanently. A second Frequent overlapping of closely-related themes, method uses precipitation, by solvent extraction, of developed to a different degree of extent, or across several an existing polymer added to the dispersed phase. chapters, can sometimes confuse the reader. As such, the Another way to solidify the capsule surface is to perform volume is more suitable to a specialised audience rather a physical or chemical modifi cation of the particles, for than to a general one, with little or no previous knowledge instance fusing, by heating the particles above their of the areas covered by the book. glass transition temperature, or by using chemical cross-linkers to bind adjacent particles. Finally it is “Particle-Stabilized possible to adsorb one or more additional layers of Emulsions and polymer or polyelectrolyte onto the particle monolayer Colloids: Formation and with two main advantages: adjacent particles are Applications” bridged on the surface of emulsion droplets and the permeability of the microcapsule shell so obtained is further controllable, and usually decreased, by this route. The main drawback consists of extra washing steps to remove the excess of stabilising particles and polymer or polyelectrolyte. When the capsules have been synthesised, post-processing involves removing the oil phase to obtain a water-in-water microcapsule dispersion: often a gelling agent is added to the aqueous phase to impart

References 4 K. Stratford, R. Adhikari, I. Pagonabarraga, J.-C. Desplat and M. E. Cates, Science, 2005, 309, 1 J. Liu, G. Liu, M. Zhang, P. Sun and H. Zhao, (5744), 2198 Macromolecules, 2013, 46, (15), 5974 2 F. Tiarks, K. Landfester and M. Antonietti, Langmuir, 5 E. M. Herzig, K. A. White, A. B. Schoﬁ eld, W. C. K. Poon 2001, 17, (19), 5775 and P. S. Clegg, Nature Mater., 2007, 6, (12), 966 3 T. Wang, P. J. Colver, S. A. F. Bon and J. L. Keddie, 6 M. N. Lee and A. Mohraz, J. Am. Chem. Soc., 2011, Soft Matter, 2009, 5, (20), 3842 133, (18), 6945

The Reviewer

Cecilia Bernardini studied Chemistry at Universitá degli Studi di Milano, Italy, and obtained a doctorate degree in colloid science in 2012 at Wageningen Universiteit, The Netherlands, under the supervision of Professor Martien Cohen-Stuart and Professor Frans Leermakers. Since July 2012 she works as a Coating Scientist at Johnson Matthey Technology Centre, Sonning Common, UK, on process chemistry research for automotive catalyst applications.

Interplay between Silver and Gold Nanoparticles in Production of Hydrogen from Methanol

Developing a highly stable bimetallic catalyst for fuel cell applications

By Hany M. AbdelDayem therefore reducing the risk of coke formation and Chemistry Department, Faculty of Science, Ain Shams catalyst fouling. University, 11566 Abbassia, Cairo, Egypt Bimetallic catalysts based on noble metals (gold, silver and platinum) and copper are known to be more Email: [email protected]; active for hydrogen production from methanol than [email protected] monometallic ones (3–18). Ag has the lowest price among noble metals, which makes it ideal for use as an industrial oxidation catalyst. The electrochemical and Hydrogen production from methanol oxidation over steam reforming activity of bimetallic catalysts based silver-gold/zinc oxide (AgAu/ZnO) catalysts was on Ag has been studied extensively (18–22). However, investigated. Bimetallic catalysts produced higher there are few studies in the literature dealing with direct hydrogen yield and lower carbon monoxide and water POM to hydrogen on Ag-based catalysts (9, 23–25). yields than Ag/ZnO catalyst without deactivation during Recently, Ag/ZnO catalyst was found to be active for 72 h on stream at 250ºC. In addition, the presence of POM to hydrogen; however, the catalyst produced Au in the bimetallic catalyst facilitated the preferential high CO yield (~6%) and was rapidly deactivated oxidation of CO to CO2. Structural analysis of bimetallic (25). On the other hand, bimetallic combinations such catalysts indicated that the strong interaction between as Au-Ag signiﬁ cantly improved the activity and the

Ag and Au particles in the nano-range (4.2 nm–7.2 nm) stability of Ag catalyst in CO oxidation to CO2 at low efﬁ ciently enhanced the reducibility of non-selective temperature (26–32). Furthermore, much attention has silver oxide (Ag2O) species. Furthermore dispersion of been focused on using Ag-Au bimetallic catalysts for metal particles in bimetallic AgAu/ZnO catalysts did not other important reactions such as oxidation of alcohols, signiﬁ cantly change after reaction; however, dispersion dechlorination of organochlorides, hydrocarbon- of Ag species in Ag/ZnO catalyst was remarkably selective catalytic reduction of nitrogen oxides (NOx), decreased. decomposition of organic pollutants, hydrogenation of esters and ethylene oxidation (33–40). The activity in a 1. Introduction desired application is determined by the oxidation state of the reactive species, interaction between Ag and Au Direct partial oxidation of methanol (POM) to hydrogen particles, the particle size, shape and location on the reduces the complexity of hydrogen-fuelled proton support controlled by the preparation process and the exchange membrane (PEM) fuel cells. Methanol nature of the support (41, 42). can be easily oxidised into hydrogen at relatively low The main objective of this work was to develop a temperatures (<250ºC) (1, 2). In addition, it contains highly stable bimetallic catalyst based on noble metals no carbon-carbon bond and it has a high H:C ratio (Ag and Au) with high performance in the oxidation

of methanol to hydrogen with low CO formation. The to 8.5 by drop wise addition of 0.5 M Na2CO3 to promote effect of adding Au on the physicochemical properties metal hydroxide precipitation on zinc peroxide (ZnO2). of Ag/ZnO was also studied. The obtained samples were washed, dried, stored and calcined as described above.

2. Experimental 2.1.4 Preparation of Silver and Gold Mechanical 2.1 Catalysts Preparation Mixture 2.1.1 Preparation of Zinc Oxide Support The mechanical mixture (Ag0.5Au0.5Zn)mix catalyst Nano-sized ZnO support was synthesised by direct was prepared by dispersing equal amounts of both precipitation (25). Analytical grade zinc nitrate 2.5 wt% Ag/ZnO and 2.5 wt% Au/ZnO powders

(Zn(NO3)2) and ammonium carbonate ((NH4)2CO3) (prepared by DP) in 200 ml n-pentane to give a total of (Sigma-Aldrich, 99.5%) were fi rst dissolved in deionised 5 wt% metals as theoretical loading. The suspension water to form solutions of 1.5 mol l–1 and 2.25 mol l–1, was stirred vigorously for 20 min and then ultrasonically respectively. The Zn(NO3)2 solution was slowly poured for 5 min. The n-pentane was evaporated at 40ºC and into the (NH4)2CO3 solution with vigorous stirring the obtained solid was dried at 100ºC overnight without and then the precipitate derived from the reaction further calcination. was collected by fi ltration and rinsed three times with 2.2 Catalysts Characterisation high-purity water and ethanol. The product was dried at 80ºC to form the ZnO precursor. Finally, the precursor The Ag and Au content in these catalysts was was calcined at 550ºC for 2 h in a muffl e furnace to determined by atomic absorption spectroscopy (AAS) obtain nanoscale ZnO particles. The average crystal on a Perkin Elmer model 3100. XRD measurements size of ZnO was ca. 35.2 nm, calculated from X-ray were performed on a Philips X’Pert multipurpose diffraction (XRD) by the Debye-Scherrer formula (25). X-ray diffractometer (MPD) using Cu Kα1,2 radiation (λ = 1.5405 Å) for 2θ angles varying from 10º to 2.1.2 Preparation of Monometallic (Silver or 80º. Hydrogen temperature-programmed reduction Gold) Catalysts (H2-TPR) was performed using a ChemBET 300 The preparation of either Au/ZnO2 or Ag/ZnO2 catalyst Quantachrome. 100 mg sample of the freshly calcined with 5.0 wt% as theoretical loading was performed by catalyst was subjected to a heat treatment (20ºC min–1 deposition-precipitation (DP) with sodium carbonate up to 1000ºC) in a gas fl ow (85 ml min–1) composed of

(Na2CO3) at pH 8.5. Gold(III) chloride trihydrate a mixture of 5 vol% hydrogen and 95 vol% nitrogen. (HAuCl4·3H2O) and silver nitrate (AgNO3), both from Prior to the TPR experiments, the samples were heated Sigma-Aldrich, were used as Au and Ag precursors. for 3 h under an inert atmosphere (nitrogen) at 200ºC.

ZnO support was suspended in an aqueous solution The surface areas (SBET) of the various samples were of metal precursor, then the pH was controlled by the determined from the adsorption of nitrogen gas at liquid addition of 0.5 M Na2CO3. After DP, all samples were nitrogen temperature (–195.8ºC) using a NOVA3200e centrifuged, washed with water four times, centrifuged (Quantachrome Instruments, USA). Before the again and dried under vacuum for 2 h at 80ºC. After measurements, all samples were perfectly degassed at drying, the samples were stored at room temperature in 150ºC and 10–4 Torr overnight. Transmission electron desiccators under vacuum, away from light, in order to micrographs were obtained using a JEOL 1200 EX II prevent any alteration (26, 43). Catalysts were calcined transmission electron microscope (TEM) operated at 300ºC for 3 h. with an acceleration voltage of 50 kV. Nitrous oxide (N O) pulse chemisorption was applied to determine 2.1.3 Preparation of Bimetallic Catalysts 2 the Ag degree of dispersion using ChemBET 3000

Preparation of bimetallic Ag1–yAuy (y = 0.1, 0.25 and and the TPR-Win V. 1.50 software; further details can 0.5, where y is the mass fraction of Au with respect be found elsewhere (25). Energy dispersive X-ray to sum of weights of Au and Ag) catalysts supported analyses (EDX) were recorded using a Quanta FEG on ZnO with 5.0 wt% as theoretical loading were also 250 microscope, equipped with EDX spectrometer performed by DP with Na2CO3. The oxide support was (TexSEM Laboratories (TSL) EDAX, AMETEK, Inc, suspended in an aqueous solution of HAuCl4·3H2O and USA). Ultraviolet-visible (UV-Vis) reﬂ ection spectra AgNO3. The initial pH was ~3, which was then adjusted were recorded on a JASCO V-570 spectrophotometer.

2.3 Catalytic Test was examined followed by the change of the rate of hydrogen production at a higher temperature i.e. 350ºC Catalytic tests were performed at atmospheric and at iso-conversion 5% by changing WHSV from pressure in a tubular quartz reactor with 6 mm internal 14.7 × 104 ml h–1 g–1 to 18.9 ×104 ml h–1 g–1. diameter. The reaction was carried out at 250ºC and Furthermore, the external and internal mass transfer in a differential mode at conversion ca. 5% by varying limitations of the catalytic system were tested. the space velocity through changing the catalyst Catalytic tests were carried out using different particle weight. The feed and product gas compositions were sizes of AgZn and Ag0.5Au0.5Zn catalysts in the range determined by online gas chromatography (GC), using 125 μm–1000 μm at the constant WHSV = 8.8 × 104 ml a Bruker 450 GC equipped with three channels. The fi rst h–1 g–1 and 13.2 × 104 ml h–1 g–1, respectively. The is for hydrogen analysis using a thermal conductivity results showed that catalyst of particle sizes 212 μm to detector (TCD). The gas separation was performed by 710 μm exhibited constant conversion without pressure HayeSep Q and 5 Å molecular sieves. Channel two is drop at the constant WHSV. Thus the catalysts were for analysing non-fl ammable gases (O2, N2, CO and ground and sieved to 355 μm−500 μm. In addition, the CO2) using TCD and separation was accomplished by calculation of effectiveness factor (η) for AgZn sample HayeSep Q and MolSieve 13X columns connected in with an average particle diameter of 356 μm showed series. The third channel is for analysing oxygenates that the values are close to 1.0; at iso-conversion 5%, (methanol, formic acid and formaldehyde) and T = 350ºC and WHSV = 8.8 × 104 ml h–1 g–1, indicating separation was accomplished by HayeSep Q and no internal diffusion limitation in the catalytic system. Varian SelectTM columns. The catalyst was diluted The activity of the catalyst was expressed in terms of with silicon dioxide (SiO2) to 10 wt% to prevent experimental rate (Rateexpt = mmole of products per hot-spot formation in the bed. The catalyst activation gram catalyst). The following formula (Equation (i)) was performed in situ by exposing the catalyst to 100 ml was used to calculate the theoretical rate (Ratetheort) of –1 min of 10% H2/N2 and increasing the temperature to hydrogen production over AgAuZn catalysts: 250ºC at 10ºC min–1. This temperature was maintained Rate = Rate of AgZn (1–y) + Rate of for 1 h. Subsequently, the furnace temperature was theort expt expt AuZn (y) (i) lowered to ~100ºC. The partial oxidation experiments were performed under a total fl ow rate of 220 –1 ml min with an O2/methanol molar ratio of 0.5, 3. Results and Discussion balanced with nitrogen and the weight hourly space velocity (WHSV) was from 8.8 × 104 ml h–1 g–1 to In the case of bimetallic samples, the actual Ag and 13.2 × 104 ml h–1 g–1 (WHSV = fl ow rate of feed gas Au fractions are also very close to the nominal values (ml h–1) per weight of catalyst (g)). The catalysts’ stability (Table I). Table I shows the measured values of the

Table I Characterisation of the Catalysts

Metal XRD Nominal Au, Actual S , m2 Catalyst Ag, %a BET dispersionc, crystal fraction %a fraction g–1b % sizef ZnO – – – – 38.8 – AgZn – 4.6 0 – 36.3 42.0 (28.1)e 5.8 AuZn – 0 4.5 – 36.9 46.9 (32.0) 4.2

Ag0.9Au0.1Zn 0.1 4.3 0.4 0.09 36.4 41.5 (37.3) 6.1

Ag0.75Au0.25Zn 0.25 3.5 1.1 0.24 35.7 39.8 (38.2) 6.4

Ag0.5Au0.5Zn 0.5 2.3 2.2 0.49 37.0 38.3 (37.1) 7.2 a Weight percentage from atomic absorption spectroscopy b From N2 adsorption c Ag and/or Au e Value in parenthesis is metal dispersion after reaction f Calculated from XRD peak at 2θ ~38.1º

Ag0.5Au0.5Zn

AgZn Intensity, arbitrary units Intensity, AuZn

37 39 41 43 45 47 2, º Ag0.5Au0.5Zn

AgZn Intensity, arbitrary units Intensity,

AuZn

10 20 30 40 50 60 70 80 2, º

Fig. 1. Wide-angle XRD patterns of the AgZn, AuZn and Ag0.5Au0.5Zn catalysts. Figure inset XRD patterns of the same catalysts in the range 37º to 47º. Peaks marked by the symbols “□”, “+” and “●” indicate the peaks assigned to ZnO, Ag2O and metallic Au, respectively

BET surface areas of catalysts. Interestingly, the BET monometallic AgZn catalyst (Figure 2(a)). Furthermore, surface area of the ZnO support was not signiﬁ cantly as shown from the TEM micrograph of (Ag0.5Au0.5Zn)mix, altered after loading of Ag or Au or both. The results the contact between Ag and Au particles was lower of N2O chemisorptions shows that the degree of than that between these particles in Ag0.5Au0.5Zn dispersion of Ag in Au-containing catalysts (i.e. (Figure 2(c)). This suggests that Ag-Au ensembles bimetallic samples) was less than in AgZn by ca. 4% may be formed due to the interaction between Ag and (shown in Table I). This may suggest an interaction Au. On the other hand, the elemental ratio of Au:Ag by between Ag and Au particles in the bimetallic catalysts. EDX for Ag0.5Au0.5Zn catalyst was 0.86 as shown in the The lower dispersion of Ag and Au particles on ZnO in EDX pattern (Figure 3(a)), i.e. the Ag-Au composite bimetallic catalysts was clearly veriﬁ ed by an increase composition was approaching 1:1. However, the of crystallite size of metals measured by XRD (Table I). Au-Ag ratio for the same catalyst prepared by mechanical Figure 1 shows XRD patterns of AgZn, AuZn and

Ag0.5Au0.5Zn catalysts. All of the diffraction peaks of ZnO could be indexed to the hexagonal phase 5 nm 10 nm reported in the Joint Committee on Powder Diffraction Ag (b) (a) Standards (JCPDS) fi le (36-1451). For AgZn catalyst, Au the peaks characteristic of the cubic Ag2O phase were detected at 2θ = 38.08º and 44.29º (JCPDS fi le, 65-3289 and 42-0874). In the case of AuZn catalyst the Ag peaks at 2θ = 38.17º and 44.38º are characteristic of the single pure metallic Au phase (JCPDS 04-0784). (c) However, in the case of Ag0.5Au0.5Zn catalysts, Au-Ag (d) Ag alloy could not be distinguished from Ag2O and metallic Au Au based on the XRD patterns because the diffraction Au lines characteristic of Au and Ag are overlapped. Ag Based on both XRD and TEM results, both Ag particles and Au particles supported on ZnO in monometallic 2.5 nm 2.5 nm and bimetallic catalysts were in the nano-range Fig. 2. TEM micrographs of the catalysts: (a) AgZn; (b) 4.2 nm–7.2 nm (see Table I and Figure 2). The average and (c) Ag0.5Au0.5Zn prepared by DP method at different size of the Ag particles in the bimetallic Ag0.5Au0.5Zn magnifi cations from ×100,000 to ×350,000 and (d) catalyst (Figure 2(b)) was larger than that in the (Ag0.5Au0.5Zn)mix prepared by mechanical mixing

(a) (b)

Zn L 70.2 25% O K 62.4 7% Ag L 54.6 1 m 46.8 62% Zn K 39.0 6% Au L (c) 31.2 23.4 O K

Count, arbitrary units Zn K 15.6 Ag L2 Au L Ag L Au L 7.8 Ag M Au L3 Au M Ag L Zn K Au L 0 1.3 2.6 3.9 5.2 6.5 7.8 9.1 10.4 11.7 13.0 1 m Energy, keV

Fig. 3. Ag0.5Au0.5Zn: (a) EDX spectrum; (b) EDX mapping for Ag element: the region of mapping corresponds to (a), acquisition time 665.4 s; (c) EDX mapping for Au element; acquisition time 655.4 s

mixing was found to be 1.4, as estimated from the EDX Figure 5 shows the H2-TPR patterns of AgZn, AuZn pattern (Figure 4(a)). It was evident that the outer and Ag1–yAuy/ZnO (y = 0.25 and 0.5) catalysts. The layers of the bimetallic mixture in this case is enriched TPR pattern of AgZn catalyst showed only a major in Au. From EDX mapping of the Ag0.5Au0.5Zn catalyst peak at 192.5ºC and AuZn catalyst showed one main (Figures 3(b) and 3(c)), the contrast between Ag and weak TPR peak at 132.4ºC. These peaks indicated the

Au was fairly clear in the homogeneous distribution presence of Ag2O and gold oxide in the AgZn and AuZn occupying the same location on the ZnO top surface catalysts (45, 46). The fact that the XRD analysis of i.e. in good contact (44). However the EDX mapping of AuZn catalyst did not reveal the presence of any Au the mechanically mixed composition (Figures 4(b) and oxide species (Figure 1), this may suggest that gold 4(c)) shows remarkably aggregated Ag nanoparticles oxide crystallites are highly dispersed on the surface on the top surface of ZnO indicating improper contact and/or Au crystallite sizes are smaller than 5 nm (47). between the composites. Although AgZn and AuZn have approximately the same

(a) (b) 50 Zn L 45 16% O K 40 5% Ag L 35 72% Zn K 1 m 30 7% Au L 25 (c) 20 Zn K Ag L 2 15 O K  Au L

Count, arbitrary units Ag L Zn K 10 Ag M Au L3 Au M Ag L Au L Au L 5 0 1.3 2.6 3.9 5.2 6.5 7.8 9.1 10.4 11.7 13.0 Energy, keV 1 m

Fig. 4. (Ag0.5Au0.5Zn)mix: (a) EDX spectrum; (b) EDX mapping for Ag element: the region of mapping corresponds to (a), acquisition time 665.4 s; (c) EDX mapping for Au element; acquisition time 655.4 s

700

600

Ag0.5Au0.5Zn 500

400 Ag0.75Au0.25Zn 300

200 AgZn

100 AuZn

Thermal conductivity signal, arbitrary units 0 0 50 100 150 200 250 300 350 400 Temperature, ºC

Fig. 5. Temperature-programmed reduction proﬁ les of monometallic AgZn and AuZn catalysts and bimetallic Ag1–yAuyZn (y = 0.25 and 0.5) catalysts

amount (wt%) of Ag and Au (Table I), yet the hydrogen The UV/Vis diffuse-reﬂ ectance spectra of the consumption for AuZn was markedly lower than that monometallic Ag/ZnO and Au/ZnO catalysts as well as for AgZn sample (Figure 5). This indicates that the three bimetallic catalysts (Ag0.75Au0.25Zn, Ag0.5Au0.5Zn majority of Au nanoparticles exist in a metallic state. and (Ag0.5Au0.5Zn)mix) are compared in Figure 6. As On the other hand, the TPR features of Ag1–yAuy/ZnO evident from Figure 6 curve A for Ag/Zn and curve E catalysts did not signiﬁ cantly change compared with for Au/Zn, the obtained spectra of the monometallic AgZn. However, adding Au to AgZn catalyst promoted catalysts reduced at 300ºC show a broad absorption

Ag2O reduction, namely the TPR peak characteristics of band due to the surface plasmon resonance (SPR) Ag2O reduction was shifted toward a lower temperature. of Ag and Au nanoparticles at ca. 480 nm and 546 As the Au content increased from 1.25 wt% to 2.5 wt% nm, respectively (26, 48). In addition, one plasmon the main peak shifted from 179.8ºC to 163.2ºC. band was observed for each bimetallic system and

50 E

D 40 C B 30 A

20 ection, arbitrary units ﬂ Re 10 ection, arbitrary units ﬂ

Re ZnO

0 400 800 1200 1600 2000 0 Wavelength, nm 360 460 560 660 760 860 960 1060 1160 Wavelength, nm

Fig. 6. UV-visible spectra of diffuse reﬂ ectance of the monometallic and bimetallic catalysts: A Ag/Zn; B Ag0.75Au0.25Zn; C Ag0.5Au0.5Zn; D (Ag0.5Au0.5Zn)mix; E Au/ZnO

308 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689117 Johnson Matthey Technol. Rev., 2015, 59, (4) the plasmon maximum was red-shifted from 480 nm in Figure 7. It is clear that adding Au increased the to 540 nm with increasing Au content as shown in selectivity of AgZn catalyst towards hydrogen and

Figure 6 curves B and C, suggesting the formation CO2. The optimal performance in methanol oxidation of Au-Ag alloy (49). However the plasmon band to hydrogen was achieved by Ag0.5Au0.5Zn catalyst. characteristic of (Ag0.5Au0.5Zn)mix (Figure 6 curve D) However, as shown in Figure 7, selectivity toward CO was wider than that of Ag0.5Au0.5Zn prepared by DP. and H2O decreased with increasing Au content and It seems that this peak can decompose to two surface reached a minimum in the case of Ag0.5Au0.5Zn catalyst. plasmon peaks corresponding to the monometallic Furthermore, a complementary investigation to conﬁ rm counterparts. Furthermore, Figure 6 inset shows a typical synergism between Ag and Au, including theoretical band at 400 nm which is characteristic of ZnO (50). calculations, showed that the experimentally measured

Hydrogen, CO, CO2 and H2O production rates at 250ºC rates of hydrogen formation over AgAuZn catalysts and at ca. 5% methanol conversion over monometallic are higher than the calculated ones (Figure 8). On the and bimetallic AgxAu1–xZn catalysts are presented other hand, AgZn catalyst showed a decrease in the

250 4 H CO CO HO 2 2 2 Byproducts formation, mmol g –1 3.5 200 catalyst 3

2.5 150 2

100 1.5

1 catalyst 50 –1

Hydrogen production, mmol g 0.5

0 AgZn Ag0.9Au0.1 Ag0.75Au0.25 Ag0.5Au0.5 AuZn

Fig. 7. Hydrogen and byproducts (CO2, H2O and CO) production rates over monometallic catalysts and bimetallic Ag1–xAuxZn catalysts at 250ºC and iso-conversion 5%

195 Rate (theoretical)

–1 190 Rate (experimental) 185 catalyst 180

175

170

165

160

155

Hydrogen production, mmol g 150

145 Ag0.9Au0.1Zn Ag0.75Au0.25Zn Ag0.5Au0.5Zn (Ag0.5Au0.5 Zn)mix

Fig. 8. Comparison between experimental and theoretical rates of hydrogen production over AgAuZn catalysts at 250ºC and iso-conversion 5%

309 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689117 Johnson Matthey Technol. Rev., 2015, 59, (4) activity during 20 h time-on-stream (TOS) (Figure 9). Adjusting the valency of Ag species could lead to a In contrast, a stable activity was observed after ca. 7 h variation in both hydrogen and CO selectivity. Ag on

TOS for Ag0.5Au0.5Zn catalyst up to 72 h (Figure 9 inset). non-doped AgZn catalyst exists as Ag2O (as shown A signiﬁ cant contribution of CO from methanol by XRD), namely it is in the Ag+ state. Ag+ is regarded decomposition and/or reverse water gas shift as an inactive state for POM to hydrogen (24). On the (Equations (ii) and (iii)) was observed over Ag/ZnO other hand, it was reported that Ag+ is active in the catalyst; however, a decrease in CO formation over conversion of methanol to CO. H2-TPR results showed bimetallic AgAuZn catalysts was observed. This can be that adding Au induced a change in the reduction explained by the presence of Au particles, possibly by proﬁ le of Ag+ species and may result in the more active consuming oxygen with preferential oxidation of CO to species Agn+ (n <1.0) on the AgAuZn catalysts surface.

CO2 (Equation (iv)) (26–32). This suggestion runs in These Ag species may speed up the rate of hydrogen good harmony with the observed increase of the rate formation and decrease the rate of CO formation. of CO2 formation over AgAuZn catalysts especially Similarly, Yang et al. have reported that the reduction Ag0.5Au0.5Zn catalyst. Sasirekha et al. (28) discussed of CuO was enhanced by the presence of Au in the effect of promoting Ag catalyst with Au for the Cu/ZnO (6). The enhanced reducibility of CuO has been preferential oxidation of CO in a hydrogen-rich stream. explained in terms of the tendency of Au to decrease It could be proposed that the formation of bimetallic the strength of the Cu–O bond located in the vicinity of alloy in Au-Ag/cerium(IV) oxide (CeO2) catalyst with Cu. Therefore, it can be suggested that the Ag–O bond Au/Ag ratio of 5:5, which showed a lower reduction was weakened by the presence of Au which seems to temperature, is the reason for its excellent performance be due to a certain degree of interaction between Au toward CO to CO2 reaction. Herein, the probable and Ag oxides in these catalysts. interaction between Au and Ag (XRD, TPR, EDX In order to test the idea that the reduction of Ag2O is mapping, UV/Vis reﬂ ectance and N2O chemisorptions) enhanced by the presence of Au, the (Ag0.5Au0.5Zn)mix may be responsible for improving CO oxidation to CO2. catalyst was also synthesised by mechanical mixing to decrease contact between Ag and Au particles. CH3OH → CO + 2H2 (ii) Interestingly, the (Ag0.5Au0.5Zn)mix catalyst showed a lower hydrogen production rate than that of Ag0.5Au0.5Zn CO2 + H2 → CO + H2O (iii) catalyst prepared by DP (Figure 8). In particular, as

CO + O2 → CO2 (iv) shown from the TEM micrograph and EDX mapping of

400 AgZn Ag0.75Au0.25Zn Ag0.5Au0.5Zn 350 –1

300 catalyst

250

200 –1 300

150 catalyst 250 200 100 150 100 50 50 Hydrogen production, mmol g 0 0 20 40 60 80 Hydrogen production, mmol g Time on stream, h 0 0 2 4 6 8 10 12 14 16 18 20 Time on stream, h

Fig. 9. Time course of the methanol conversion over AgZn catalyst and bimetallic Ag1–yAuyZn (y = 0.25 and 0.5) catalysts at 350ºC and iso-conversion 5%

the (Ag0.5Au0.5Zn)mix, the contact between Ag and Au who generously made resources available to undertake particles was lower than that between these particles this study. in Ag0.5Au0.5Zn (Figure 2(c)). These fi ndings may support the interpretation that the synergetic effects References between Ag and Au were due to the strong interaction 1. L. Mo, X. Zheng and C.-T. Yeh, ChemPhysChem, between Ag and Au nanoparticles (as show by UV-Vis). 2005, 6, (8), 1470 Furthermore, one cannot exclude that the synergetic 2. C.-T. Yeh, Y.-J. Chen and H.-S. Hung, ‘A Process of effects between interacting Ag and Au in Ag0.5Au0.5Zn Producing Hydrogen under Low Temperature’, Taiwan catalyst (prepared by DP) for production of hydrogen Patent 226,308; 2005 may also be due to the hydrogen spillover effect (24). 3. T.-C. Ou, F.-W. Chang and L. S. Roselin, J. Mol. Catal. Hydrogen adsorbed on Ag sites may be spilt over to A: Chem., 2008, 293, (1–2), 8 neighbouring Au particles in high contact (51), which 4. F.-W. Chang, T.-C. Ou, L. S. Roselin, W.-S. Chen, are known to have high affi nity for adsorbing hydrogen S.-C. Lai and H.-M. Wu, J. Mol. Catal. A: Chem., 2009, (52). The spillover hydrogen on Au particles may be 313, (1–2), 55 desorbed as hydrogen through ZnO rather than being 5. V. Dal Santo, A. Gallo, A. Naldoni, M. Guidotti and R. oxidised to water. Psaro, Catal. Today, 2012, 197, (1), 190 Even though adding Au to AgZn catalyst decreased 6. H.-C. Yang, F.-W. Chang and L. S. Roselin, J. Mol. metal dispersion however, dispersion of metal particles Catal. A: Chem., 2007, 276, (1–2), 184 in bimetallic AgAu/ZnO catalysts did not signifi cantly 7. Y.-C. Lin, K. L. Hohn and S. M. Stagg-Williams, Appl. change after reaction compared with AgZn catalyst Catal. A: Gen., 2007, 327, (2), 164 (Table I). This can explain the observed higher stability 8. Y.-J. Huang, K. L. Ng and H.-Y. Huang, Int. J. Hydrogen of bimetallic Ag0.5Au0.5Zn catalyst with 72 h on stream Energy, 2011, 36, (23), 15203 ( Figure 9 inset) during POM reaction at 350ºC. 9. E. Qayyum, V. A. Castillo, K. Warrington, M. A. Barakat and J. N. Kuhn, Catal. Commun., 2012, 28, 128 4. Conclusions 10. H. Zhu, Z. Guo, X. Zhang, K. Han, Y. Guo, F. Wang, Z. Wang and Y. Wei, Int. J. Hydrogen Energy, 2012, 37, Bimetallic AgAuZn catalyst samples produced a lower (1), 873 amount of CO than AgZn catalyst which encourages the 11. C. Pojanavaraphan, A. Luengnaruemitchai and E. use of these catalysts in a hydrogen fuel cell to avoid Gulari, Appl. Catal. A: Gen., 2013, 456, 135 any deactivation. The bimetallic catalysts containing 12. C. Pojanavaraphan, A. Luengnaruemitchai and E. 2.5 wt% Ag and 2.5 wt% Au exhibited the highest Gulari, Int. J. Hydrogen. Energy, 2013, 38, (3), 1348 hydrogen production rate and had the lowest CO production rate. It was suggested that interaction 13. S. Pongstabodee, S. Monyanon and A. Luengnaruemitchai, J. Ind. Eng. Chem., 2012, 18, (4), between Ag and Au particles in the AgAuZn catalyst, 1272 detected from TPR and UV-Vis results, was responsible 14. F.-W. Chang, L. S. Roselin and T.-C. Ou, Appl. Catal. for enhancing the reducibility of Ag2O species in this A: Gen., 2008, 334, (1–2), 147 catalyst. From TEM and EDX mapping investigations it was concluded that the contact between Ag and 15. M.-L. Xu, X.-K. Yang, Y.-J. Zhang, S.-B. Xia, P. Dong and G.-T. Yang, Rare Metals, 2015, 34, (1), 12 Au particles in AgAuZn catalyst prepared by DP is greater than that in a comparable catalyst prepared 16. R. Feng, M. Li and J. Liu, Rare Metals, 2012, 31, (5), by mechanical mixing. The latter catalyst played an 451 important role in the hydrogen spillover effect. The 17. I. T. Schwartz, A. P. Jonke, M. Josowicz and J. Janata, reaction pathway for POM over AgAuZn involved a Catal. Lett., 2013, 143, (7), 636 preferential oxidation of CO to CO2 over Au sites. 18. M. Rashid, T.-S. Jun, Y. Jung and Y. S. Kim, Sens. Actuators B: Chem., 2015, 208, 7 Acknowledgement 19. J. Cao, M. Guo, J. Wu, J. Xu, W. Wang and Z. Chen, The author would like to thank the supporters of the J. Power Sources, 2015, 277, (1), 155 King Faisal University, Saudi Arabia, and the School of 20. Y. Tong, J. Pu, H. Wang, S. Wang, C. Liu and Z. Wang, Catalysis at the Egyptian Petroleum Research Institute J. Electroanal. Chem., 2014, 728, 66

21. Y. Luo, Y. Xiao, G. Cai, Y. Zheng and K. Wei, Fuel, 37. P. M. More, D. L. Nguyen, P. Granger, C. Dujardin, 2012, 93, 533 M. K. Dongare and S. B. Umbarkar, Appl. Catal. B: 22. S. H. Israni and M. P. Harold, J. Membrane Sci., 2011, Environ., 2015, 174–175, 145 369, (1–2), 375 38. D. X. Huy, H.-J. Lee, Y. B. Lee and W. S. Choi, J. Colloid Interface Sci., 2014, 425, 178 23. L. Mo, X. Zheng and C.-T. Yeh, Chem. Commun., 2004, (12), 1426 39. J. Zheng, H. Lin, Y.-n. Wang, X. Zheng, X. Duan and Y. Yuan, J. Catal., 2013, 297, 110 24. L. Mo, A. H. Wan, X. Zheng and C.-T. Yeh, Catal. Today, 2009, 148, (1–2), 124 40. S. Rojluechai, S. Chavadej, J. W. Schwank and V. Meeyoo, Catal. Commun., 2007, 8, (1), 57 25. H. M. AbdelDayem, S. S. Al-Shihry and S. A. Hassan, Ind. Eng. Chem. Res., 2014, 53, (51), 19884 41. G. C. Bond, C. Louis and D. T. Thompson, “Catalysis by Gold”, Catalytic Science Series, Vol. 6, Imperial 26. A. Sandoval, A. Aguilar, C. Louis, A. Traverse and R. College Press, London, UK, 2006 Zanella, J. Catal., 2011, 281, (1), 40 42. T. Mallat and A. Baiker, Ann. Rev. Chem. Biomol. Eng., 27. Z. Qu, G. Ke, Y. Wang, M. Liu, T. Jiang and J. Gao, 2012, 3, 11 Appl. Surf. Sci., 2013, 277, 293 43. F. Zane, V. Trevisan, F. Pinna, M. Signoretto and F. 28. N. Sasirekha, P. Sangeetha and Y.-W. Chen, J. Phys. Menegazzo, Appl. Catal. B: Environ., 2009, 89, (1–2), Chem. C, 2014, 118, (28), 15226 303 29. G. Nagy, T. Benkó, L. Borkó, T. Csay, A. Horváth, K. 44. J. Zhang, B. Lai, Z. Chen, S. Chu, G. Chu and R. Frey and A. Beck, React. Kinet. Mech. Catal., 2015, Peng, Nanoscale Res. Lett., 2014, 9, 237 115, (1), 45 45. Y. Hu, W. Lü, D. Liu, J. Liu, L. Shi and Q. Sun, J. Nat. 30. J. H. Byeon and J.-W. Kim, ACS Appl. Mater. Gas Chem., 2009, 18, (4), 445 Interfaces, 2014, 6, (5), 3105 46. F. Yinga, S. Wang, C.-T. Au and S.-Y. Lai, Gold Bull., 31. Y. Iizuka, R. Inoue, T. Miura, N. Morita, N. Toshima, T. 2010, 43, (4), 241 Honma and H. Oji, Appl. Catal. A: Gen., 2014, 483, 63 47. C.-K. Chang, Y.-J. Chen and C.-t. Yeh, Appl. Catal. A: 32. T. Déronzier, F. Morﬁ n, M. Lomello and J.-L. Rousset, Gen., 1998, 174, (1–2), 13 J. Catal., 2014, 311, 221 48. A. N. Pestryakov, N. E. Bogdanchikova and A. Knop- 33. W. Li, A. Wang, X. Liu and T. Zhang, Appl. Catal. A: Gericke, Catal. Today, 2004, 91–92, 49 Gen., 2012, 433–434, 146 49. S. Link and M. A. El-Sayed, J. Phys. Chem. B, 1999, 34. X. Huang, X. Wang, M. Tan, X. Zou, W. Ding and X. 103, (40), 8410 Lu, Appl. Catal. A: Gen., 2013, 467, 407 50. T. Xu, L. Zhang, H. Cheng and Y. Zhu, Appl. Catal. B: 35. X. Huang, X. Wang, X. Wang, X. Wang, M. Tan, W. Environ., 2011, 101, (3–4), 382 Ding and X. Lu, J. Catal., 2013, 301, 217 51. L.-T. Weng and B. Delmon, Appl. Catal. A: Gen., 1992, 36. L. V. Romashov, L. L. Khemchyan, E. G. Gordeev, 81, (2), 141 I. O. Koshevoy, S. P. Tunik and V. P. Ananikov, 52. C. Kartusch and J. A. van Bokhoven, Gold Bull., 2009, Organometallics, 2014, 33, (21), 6003 42, (4), 343

The Author

Hany AbdelDayem holds a BSc degree in Chemistry and an MSc in catalysis from Ain Shams University, Egypt, as well as a PhD in Chemistry for research on chemical kinetics and catalysis from the Université Catholique de Louvain, Belgium. He subsequently held a Fulbright postdoctoral award at the University of Pittsburgh, USA, and a postdoctoral fellowship from the Agence Universitaire de la Francophonie (AUF) at the Université Catholique de Louvain. In 2001 he was appointed Assistant Professor in the Chemistry Department at Ain Shams University. He joined King Faisal University, Saudi Arabia, as Assistant Professor from 2005 up to 2014. In 2015 he is the appointing Associate Professor in physical chemistry at Ain Shams University. His main focus is the development of heterogeneous catalytic processes, including petrochemical and green energy. He is particularly interested in catalyst synthesis, ex- and in situ characterisation of catalysts and reaction kinetics.

Carbon Formation in Steam Reforming and Effect of Potassium Promotion

Potassium dopants prevent carbon formation and aid catalyst recovery

By Mikael Carlsson Due to the temperatures at which steam reformers Johnson Matthey Plc, operate, carbon is constantly being formed from PO Box 1, Belasis Avenue, Billingham TS23 1LB, UK the hydrocarbon feedstock, with the primary route being through cracking reactions. However, there are Email: [email protected] also carbon removal (or gasifi cation) reactions that simultaneously occur which remove the carbon laid down, meaning there is no net accumulation of carbon in a well-run plant. With a given catalyst loading in Introduction the reformer, the rate of gasifi cation is fi xed by the catalyst type and the process conditions. However, When choosing a reformer catalyst, there are a number the rate of carbon laydown is a function of a number of important things to consider. Steam reforming of conditions such as the catalyst activity, degree of of methane is an endothermic reversible reaction, sulfur poisoning and heat input to the tubes. The rate of whilst steam reforming of higher hydrocarbons is not laydown is therefore more likely to vary compared to the reversible. The activity of the catalyst installed is critical rate of gasifi cation. The selected catalyst should have in determining the reaction rate within the reformer. appropriate activity or alkali promoters to ensure that the However, the steam reforming reaction is diffusion carbon removal rate is faster than the carbon formation limited, so the geometric surface area of the installed rate, which would result in no net carbon laydown. catalyst is directly related to the catalyst activity. This Finally, the catalyst should allow for the lowest possible article will show the mechanisms by which carbon can pressure drop, as this will enable the highest possible form on a catalyst and how a potassium dopant can plant throughput before compressor limits are reached. prevent this and aid catalyst recovery following carbon However the catalyst breakage characteristics are also formation (1). important as all pelleted steam reforming catalysts will Because the reaction is endothermic, the transfer of break due to the forces exerted on them when reformer heat from the burners to the catalyst is just as important tubes expand in operation and then contract during as the activity. Whilst within the reformer itself the plant shutdowns, which will lead to an increase in primary heat transfer mechanism is radiation, within pressure drop. the tube it is convection and conduction. The hottest point inside the tube is the internal tube wall. The size Carbon Formation and shape of the catalyst will impact on the tube-side laminar fi lm layer and therefore on the overall heat The three main reactions for carbon formation are transfer coeffi cient as represented in Figure 1. hydrocarbon cracking (Equations (i) and (ii)), carbon

Tube wall heat Catalyst reaction transfer tube stress carbon formation Flame heat release Gas radial and radiation temperature gradient

Radiation from wall

Tube Gas Radiation from gas emissivity Pressure drop Film heat Shadowing of tubes and transfer circumferential temperature Internal analysis emissivity

Radiation from cofﬁ ns

Fig. 1. Heat transfer balance inside a steam reformer box from ﬂ ames to reactant stream

monoxide disproportionation (the Boudouard reaction) There are three catalyst parameters that can be (Equation (iii)) and carbon monoxide reduction altered to prevent carbon formation. These are the (Equation (iv)). activity, the inherent heat transfer coeffi cient and the catalyst alkali promoter content. CH ⇌ C + 2H ΔH = +75 KJ mol–1 (i) 4 2 298 Increasing the catalytic activity can be achieved by using a higher surface area catalyst due to the diffusion CnHm  nC + (m/2)H2 (ii) limited nature of the reaction mentioned previously. This –1 has a threefold effect; fi rstly, as there is more reforming 2CO ⇌ C + CO2 ΔH298 = –172 KJ mol (iii) reaction near the inlet of the tube, there is a lower –1 process gas temperature due to the increased heat of CO + H2 ⇌ C + H2O ΔH298 = –131 KJ mol (iv) reaction required. Secondly, the hydrocarbon content Cracking or decomposition of hydrocarbons is of the process gas is reduced. And fi nally as more favoured at temperatures above approximately 620ºC hydrogen is produced carbon formation is suppressed. (1148ºF) depending on the hydrocarbon species. The By improving the heat transfer characteristics of the reaction with methane is reversible but with the heavier reforming catalyst, the rate of heat transfer within the hydrocarbons they are not. tube can be increased. Intuitively this would appear to Both the carbon monoxide reduction and increase the process gas temperature thereby making disproportionation reactions are more prevalent at the carbon forming potential worse. However since lower temperatures but at those temperatures the carbon is most likely formed on the inside tube wall concentrations of carbon monoxide would normally which is the hottest part of the process, increasing the be low, depending on recycle rates, so the cracking heat transfer characteristics of the catalyst reduces reactions are normally the most important to consider. this temperature by transferring heat to the bulk of the However, any combination of these reactions can lead catalyst. The additional heat transferred will in turn to detrimental effects on catalyst activity and, if left increase the reaction rate, which will also reduce the untreated, eventually lead to permanent damage and hydrocarbon content of the process gas making carbon carbon build-up. formation less likely. The process gas temperature is

314 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688992 Johnson Matthey Technol. Rev., 2015, 59, (4) also reduced. Overall, this has a similar effect to that of steam and these will aid in any removal of carbon seen by installing a highly active catalyst. that is formed on the surface. As highlighted earlier, Another way of preventing the formation of carbon is depending on the conditions, there are locations within to include a promoter in the catalyst to help increase the reformer where carbon will form on hot surfaces, the rate of carbon gasifi cation; one such promoter is for example, the inner tube wall. This is especially likely if potassium. heavier species slip further down the tube where the wall is hotter. That carbon will have to be removed at a faster Potassium Promotion rate than it is formed in order to prevent any build-up. The history of potassium promoted catalysts goes It is well known that carbon formation on a surface, back to 1975 when a trial was carried out on the whether the support or catalyst, is affected by the No 1 Low Pressure Ammonia Plant in Billingham, UK, acidity of that surface. Positively charged acidic sites (2). During the trial it was shown that the promoted on a surface will increase the rate of carbon formation, catalyst, where the potassium was incorporated in which is partly due to acidic sites catalysing the cracking the support, was successful in the suppression of hot reaction. Alpha alumina, which is a common catalytic bands that had been seen for the previous charge of support, contains acidic sites and adding Group 2 unpromoted catalyst. These hot bands associated with metals such as magnesium or calcium neutralises carbon formation appeared after only a few months these making the surface less acidic. of operation and it was thought at the time that they For a supported nickel catalyst the steam ratio at were due to a plant uprate. Alkali metals were known which a catalyst would run without forming carbon can to inhibit the steam reforming reaction, but during be decreased by approximately 16% compared to an the plant trial no such inhibition was seen due to the undoped alumina through the addition of dopants such way in which the potassium was incorporated into as calcium or magnesium. A way to further increase the support. The effect was confi rmed by laboratory the surface basicity is to add a potassium-containing experimental testing. After nine months of operation compound such as potash as a dopant, which will lead the reformer was inspected and the tubes containing to an increased prevention of carbon formation. For potassium promoted catalyst were running cooler alkalised calcium aluminate catalyst the steam ratio with a more uniform temperature than adjacent tubes, can be reduced by approximately 65% without forming which contained unpromoted catalyst. The material carbon compared to an undoped alumina. The reason was discharged and when examined only a very limited for this is due to both the acceleration of the carbon potassium loss was detected. gasifi cation reaction and the suppression of carbon The Johnson Matthey KATALCOJM catalyst range formation reactions. available today has been designed with different In addition to increasing the surface basicity, the amounts of promoter for various operations. As can potassium will form hydroxide species in the presence be seen in Table I the range spans from unpromoted

Table I Range of Johnson Matthey KATALCOJM Catalysts with Different Potassium Promotion

K2O, wt% Series Feedstock/carbon protection requirement

0 KATALCOJM 23-4 or 57 series Light feed/low C protection

1.5–2.5 KATALCOJM 25-4 series

4–5 KATALCOJM 47 series

6–7 KATALCOJM 46-3

Heavy feed/high C protection

KATALCOJM 23 and 57 series which are used for light shows areas which are rich in aluminium (Figure 2(a)) feedstocks such as methane in combination with a low and potassium (Figure 2(b)). What can be seen is that heat fl ux, up to KATALCOJM 46-3 which contains much where there is a high abundance of potassium there is higher levels of potassium for operations with heavy also high aluminium content. This clearly indicates that naphtha. The two catalysts series with intermediate there are areas of potassium-aluminates which act as levels of potassium promotion are for operations potassium reservoirs for the catalyst. where the feed composition is heavier than methane Froment et al. examined different potassium loadings but lighter than heavy naphtha, for example, liquid on a nickel catalyst and found that in conditions where petroleum gas (LPG). In reality this summary is slightly methane cracking was taking place the presence of oversimplifi ed as both the steam-to-carbon ratio and potassium seemed to have three effects on the carbon the overall heat fl ux also affect the amount of carbon formation (3): (a) it reduced the fi nal level of carbon protection required. formed; (b) it reduced the rate of carbon formation; and Potassium is incorporated into the catalyst in ceramic (c) it apparently delayed the onset of carbon formation, phase reservoirs with a precise stability to regulate which is speculated to be the result of decreasing the the rate of release onto the surface. This leads to the nucleation rate on the catalyst surface. Furthermore, right level of potassium and hydroxide species on the the gasifi cation rate of fi lamentous carbon that had surface to ensure gasifi cation of carbon from all nickel been deposited is also affected by the presence sites throughout the catalyst’s lifetime. of potassium as shown in Figure 3. The rate of The potassium-containing phases present in gasifi cation by steam as a function of the potassium Johnson Matthey catalysts depends on the series but content exhibits a maximum of around 1.6–2.0 wt% typically they are either a potassium-aluminosilicate potassium oxide (K2O) for this catalyst system. or potassium-aluminate which is incorporated in the The presence of a potassium dopant will promote support. The use of a range of phases allows for the the adsorption of water which will in turn increase release of potassium at an appropriate rate under a the carbon gasifi cation (4). The potassium will also range of process conditions and maintains high activity affect the gasifi cation kinetics and increase the carbon in terms of carbon removal. This also ensures that monoxide production rate and, as steam adsorbs any adverse effect on the steam reforming activity is dissociatively, there could be an increase in oxygen on minimised. the surface as a result of the increased number of sites Figure 2 shows an electron probe microanalysis for water adsorption on an alkalised catalyst, which (EPMA) of a potassium-promoted catalyst which clearly leads to an increase of the rate of gasifi cation.

(a) Level (b) Level 6000 1000 5250 875 4500 750 3750 625 3000 500 2250 375 1500 250 750 125 0 0 Average 3966 Average 129

200 μm 200 μm

Fig. 2. EMPA images showing: (a) aluminium; (b) potassium distribution in a catalyst support highlighting areas of K-Al reservoirs

0.16 –1

h PH2O = 3.0 bar –1 0.14 T = 550ºC cat –1 0.12

0.1

0.08

0.06 Hot bands cation by steam, mol C g ﬁ 0.04

0.02 Rate of gasi 0 0 1 2 3 4 Fig. 4. Hot bands shown on tubes after a carbon forming Alkali content, wt% incident

PCO = 0.1 bar PCO = 0.3 bar PCO = 0.3 bar PCO = 0.5 bar PH2 = 0.3 bar PH2 = 0.3 bar PH2 = 0.5 bar PH2 = 0.5 bar

Fig. 3. Carbon gasiﬁ cation rate as a function of potassium loading. (Reprinted with permission from (2). Copyright (2002) American Chemical Society)

Carbon Formation Case Study

Hot bands – clearly reduced Figures 4–6 demonstrate the use of a potassium promoted catalyst to aid recovery after a carbon incident in a reformer on a European ammonia plant. Figure 4 shows hot bands on the reformer tubes that appeared following a carbon incident due to LPG condensate trapped in a line being inadvertently fed into the reformer. This carbon led to an increase in Fig. 5. Tube appearance two months after the carbon incident, illustrating some improvement pressure drop from 3.6 to 5.0 bar (52.2 to 72.5 psi) across the reformer. Although carbon had been formed, the presence of potassium promoted catalyst limited the severity of this incident and a full shutdown was averted. As the plant needed to keep running the operators decided that it would be run at a higher steam-to-carbon ratio in an attempt to promote carbon gasiﬁ cation. Over the following months the pressure drop was decreased to 4.7 bar (68.1 psi) and the extent of hot bands on the tubes decreased, which can be seen in Figure 5. The No hot bands visible measurement of the tube wall temperature revealed a decrease of up to 30ºC (54ºF). This highlights the effect of carbon removal that is promoted by the potassium containing catalyst. After two months of running at an increased steam-to- carbon ratio the plant tripped, providing an opportunity Fig. 6. Tube appearance after plant shutdown and steaming to steam the catalyst prior to restart. When the plant showing conditions returning to normal

317 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688992 Johnson Matthey Technol. Rev., 2015, 59, (4) was restarted no hot bands were observed (as can be release and mobility of the potassium are required seen in Figure 6) and operation was back to normal to keep tube walls free from carbon and also assist with pressure drop at 3.8 bar (52.2 psi). This case in recovery from plant upset conditions resulting in study illustrates both how the KATALCOJM catalyst can carbon formation. slowly recover during normal operation and also the dramatic return to normal operating conditions after References steaming. 1 “Catalyst Handbook”, 2nd Edn., ed. M. V. Twigg, Manson Publishing Ltd, London, UK, 1996 Conclusion 2 L. W. Lord, ICI Internal Report RD/CC430 , 1976 3 J.-W. Snoeck, G. F. Froment and M. Fowles, Ind. Eng. There are a number of mechanisms by which Chem. Res., 2002, 41, (15), 3548 carbon formation can occur on a nickel-based steam 4 R. A. Hadden, J. C. Howe and K. C. Waugh, reforming catalyst, with the cracking of hydrocarbons ‘Hydrocarbon Steam Reforming Catalysts - Alkali most prevalent. Carbon deposition happens when the Induced Resistance to Carbon Formation’, Catalyst formation rate is greater than the removal rate which Deactivation 1991, Illinois, USA, 24th–26th is a function of surface chemistry and the addition of June, 1991, “Proceedings of the 5th International promoters to reduce carbon formation. It is important Symposium”, eds. C. H. Bartholomew and J. B. Butt, that the potassium dopant is added to the catalyst Studies in Surface Science and Catalysis, Vol. 68, in optimised phases with appropriate hydrothermal Elsevier Science Publishing Co, New York, USA, 1991, stability to give a controlled release rate. The pp. 177–184

The Author

Mikael Carlsson joined Synetix/ICI in 2002 after graduating from Napier University in Edinburgh, UK, with a MSc degree in Materials Technology. He also has a BSc in Chemical Engineering from Chalmers University of Technology, Sweden. Mikael has for the last nine years been developing catalysts for the steam reforming area and currently works as Reforming Technical Development Manager.

“Electrochemical Power Sources: Batteries, Fuel Cells, and Supercapacitors”

By Vladimir S. Bagotsky, Alexander M. Skundin and Yury M. Volfkovich (A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Science, Russia), John Wiley & Sons Inc, New Jersey, USA, 2015, 372 pages, ISBN: 978-1-118-46023-6, £66.95, €81.99, US$99.95

Reviewed by Billy Wu The book covers three general topics: batteries, Dyson School of Design Engineering, Imperial College fuel cells and supercapacitors, with each section London, South Kensington Campus, London SW7 discussing fundamental operating principles, material 2AZ, UK considerations and technology prospects.

Email: [email protected] Application of Electrochemical Devices

Electrochemical devices are being employed in “Electrochemical Power Sources: Batteries, Fuel Cells, applications ranging from consumer electronics to and Supercapacitors” is a comprehensive textbook electric vehicles due to their relatively high effi ciencies covering materials, applications and prospects of and environmental friendliness. However, major the aforementioned devices. The high level overview challenges include increasing the energy, power provided makes this book an excellent resource for density and effi ciencies of these devices, especially readers new to electrochemical devices as it avoids when scaling up from a novel material or chemistry. An going into excessive detail of each material, whilst appreciation of the historic works, common challenges providing an overall perspective and outlook. The book and potential future position of a technology is therefore was edited by Alexander Skundin and Yury Volfkovich extremely useful. in honour of the late Vladimir Bagotsky, who is widely recognised for his scientifi c activities in electrochemistry Batteries and for his textbooks on this subject. Skundin and Volfkovich themselves are the chief scientists at the The fi rst part of the book covers thermodynamic A.N. Frumkin Institute of Physical Chemistry and aspects in electrochemical devices with a focus on Electrochemistry of the Russian Academy of Sciences aqueous redox couples which have been historically and are two of the leading experts in batteries and considered for battery applications. The general supercapacitors in Russia with over 200 peer reviewed operating principle of galvanic cells along with articles between them. different electrode chemistries including zinc-based,

319 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688992 Johnson Matthey Technol. Rev., 2015, 59, (4) nickel-based and lead acid batteries are examined. cell (SOFC) and alkaline fuel cell (AFC). With PEMFC, The section follows with an explanation of the fi gures of which are the most relevant for automotive uses, a merit relevant to batteries, and design considerations historical overview is given covering the development when translating the material chemistry into cell of the technology but it also touches on engineering level systems, taking into account the accompanying challenges such as thermal management, cold start and separator and electrolyte. Applications of the technology, water management. The progress and complications current limitations and technology prospects are then of each fuel cell technology then follows with a brief summarised. overview of phosphoric acid (PAFC), direct carbon fuel The next section then leads onto non-aqueous systems cell (DCFC), bacterial fuel cell (BFC) and redox fl ow which are currently the most prevalent in real world battery (RFB). RFB in particular are receiving much applications. Here a general introduction into different attention in the large scale energy storage industry types of electrolytes is provided covering aprotic due to their potential cost savings and increased lifetime non-aqueous solutions, molten salts and solid compared to batteries with the common all-vanadium and electrolytes. Discussion of intercalation based iron-chromium RFB discussed. lithium-ion electrodes then follows with their review The section concludes with discussions about fuel concluding that existing materials will not be able cell applications and the historically installed fuel cell to meet consumer demands for energy density and units for stationary appliances are summarised. Only charging rates. Therefore, they believe a shift to PEMFC has received signifi cant industria l attention alternative materials, i.e. silicon, tin or aluminium will for automotive uses and the authors suggest the initial be required, however cycle life then becomes the gains are in heavy goods vehicles. Their technology limiting factor with microstructural damage leading to outlook underlines their interpretation of the forthcoming poor lifetimes. issues which are developing new catalysts for oxygen Their assessment of forthcoming battery technologies, reduction, catalysts for the complete oxidation of from the perspective of increasing energy density, then ethanol and selective catalysts which do not promote concentrates on conversion based chemistries and the undesired side reactions. thus a move away from the traditional intercalation based mechanisms. Lithium-sulfur, lithium-air and Supercapacitors sodium-ion batteries are discussed with challenges identifi ed as again being lifetime issues associated with The third section focuses on electrochemical double the microstructural changes occurring in the electrodes layer capacitors or supercapacitors, looking initially at and the voltage hysteresis observed upon charge and carbon based systems which have the most industrial discharge limiting device effi ciency. relevance. Here they present a detailed analysis of The fi nal chapters of this section then briefl y cover electrode optimisation for different kinds of carbon such solid state batteries and batteries with molten salt as activated carbon, carbide derivatives, aerogels, electrolytes which researchers have investigated in nanotubes and graphene, with various electrolyte order to enable high device lifetimes, however other types in terms of pore size distribution and the effect of diffi culties such as electrolyte conductivities currently functional groups. The discussion then follows onto a limit their mainstream adoption. range of electrolyte types: aqueous, non-aqueous and ionic liquids. Fuel Cells Observations suggest that eventually further growth in specifi c surface area of electrodes through reducing The second part deals with fuel cells, which has pore sizes will not result in improved supercapacitor relevance in both transport and stationary power energy density due to the infl uence of steric effects. applications. Again, this starts with thermodynamic Thus, there is an increased interest in pseudocapacitor aspects of the device operation and basic defi nitions electrode materials which includes metal oxides such as of components and concepts found in fuel cells. The iridium(IV) oxide (IrO2), manganese(IV) oxide (MnO2) subsequent sections then go into further explanation and ruthenium(IV) oxide (RuO2), conducting polymers of the different types: proton exchange membrane and monomer redox systems. This text gives a fuel cell (PEMFC), direct methanol fuel cell (DMFC), comprehensive overview of these electrode materials, molten carbonate fuel cell (MCFC), solid oxide fuel challenges and prospects, of which a current trend is

320 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688992 Johnson Matthey Technol. Rev., 2015, 59, (4) observed to be the creation of composite electrodes text to electrochemical energy devices which covers of metal oxides and conducting polymers to mitigate material considerations, historical developments the conductivity issues of the metal oxides and achieve of the technology and future prospects, spanning higher mass loadings for more practical devices. fundamental mechanisms to engineering challenges at The detailed discussion of carbon based a high level perspective. The supercapacitor section in supercapacitor electrodes and pseudocapacitor particular goes into much more detail of the materials. electrodes then leads onto asymmetric supercapacitors This text would be most useful for students studying an which combines the two together to give a higher introduction to electrochemistry course. energy density due to a wider operating voltage window. Reﬂ ecting on all of this, the section closes with a comparison of commercially available supercapacitors and the prospects of the device. “Electrochemical Power Photoelectrochemical Devices Sources: Batteries, Fuel Cells, and Supercapacitors” The closing chapter of the book covers electrochemical aspects of solar energy conversion. This gives a very brief review of the mechanisms of photoelectrochemical devices such as semiconductor solar batteries and dye-sensitised solar cells.

Conclusions

“Electrochemical Power Sources: Batteries, Fuel Cells, and Supercapacitors” is an excellent introductory

The Reviewer

Dr Billy Wu is a lecturer in the Dyson School of Design Engineering at Imperial College London where he works on electrochemical devices at a range of length scales from novel materials to engineering integration. He gained his PhD from Imperial College London in 2014 on PEMFCs, lithium-ion batteries and supercapacitors for automotive applications.

JOHNSON MATTHEY TECHNOLOGY REVIEW www.technology.matthey.com

Selective Removal of Mercury from Gold Bearing Streams Exploring the use of solid adsorbents to avoid the undesirable loss of gold

By James G. Stevens Introduction Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, In modern gold mining processes, typically it is RG4 9NH, UK necessary to extract gold from complex ores which comprise gold in addition to other metals, including Email: [email protected] mercury. A common technique for extracting gold from its ores is the cyanide process, wherein leaching of gold is achieved by the addition of cyanide at alkaline Many gold ore bodies contain high levels of mercury pH following the Elsner equation (Equation (i)) (1). which are co-extracted with the gold. This mercury Cyanide is a strong lixiviant for gold and so leaches then travels through the process circuit to pose health, the gold out of the ore into solution (2). The gold is environmental and technical issues. This article typically present in the leaching solution as a gold highlights a method to selectively remove the mercury cyanide complex. Silver can also be extracted from its whilst leaving the gold to be processed as normal. ores using a similar cyanide leaching process. The removal of mercury from the circuit mitigates the 4Au + 8CN– + O + 2H O → 4[Au(CN) ]– + 4OH– (i) need for retorting of the produced gold, reduces the 2 2 2 potential environmental impact of any waste solutions A problem with this process is that cyanide is an and decreases any potential mercury exposure to plant equally strong lixiviant for many other metals, including workers. mercury. Accordingly mercury, which is typically present The solid bound thiol species used have been in the ore along with gold or silver, is also leached into shown by inductively coupled plasma optical emission the solution. The mercury may be present in the leach spectrometry (ICP-OES) to reduce the mercury to solution as a variety of complex anions with the general x– undetectable levels whilst having no measurable formula [Hg(CN)2+x] (where x is 0, 1 or 2) depending effect on the gold concentration. The control of the on the ratio of mercury to cyanide ions. However, 2– cyanide concentration at the adsorption step has been typically it is present as [Hg(CN)4] . shown to be key to ensuring that the mercury removal The removal of mercury from mining waters is very is achieved selectively. This in turn ensures that no important, both on health and safety grounds and precious metal value is lost in the mercury removal on environmental grounds. In particular, mercury process. The process has been shown to be applicable volatilisation during extraction processes can be a to both batch and continuous operation which will allow threat to the health of plant workers and the presence the technology to be applied to a variety of flow rates of mercury in waste waters from mining is of significant and applications. environmental concern. Environmental legislation limits

322 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689487 Johnson Matthey Technol. Rev., 2015, 59, (4) the concentration of mercury permitted in waste waters Sartorius infrared balance. 15 ml of the metal solution to very low levels in many countries. Accordingly, was then added and allowed to stir for the required time effective removal of mercury from mining waters is period on a Radleys 12 position stirrer. After stirring of significant interest to the industry. However, itis the solution was filtered using a 0.45 µm syringe filter, important that mercury removal technologies do not the solution was analysed by ICP-OES and compared remove significant quantities of the gold or silver being against the initial solution. The percentage metal mined, to avoid undesirable loss of these products removed is calculated from the difference of the initial during processing. concentration to the final concentration divided by the A range of different methods have been employed initial concentration. for mercury removal in this field. Miller et al. Kinetic data was obtained by running multiple tubes in reviewed different technologies for the removal parallel with each tube being filtered after an appropriate of mercury, including precipitation with inorganic time. For the cyanide addition experiments an aliquot sulfides or sulfur-based organic compounds; of cyanide solution was added to the appropriate tubes adsorption with activated carbon or crumb rubber; after an appropriate initial period, the tubes were then solvent extraction by alkyl phosphorus esters or filtered as required. thiol extractants; ion exchange with isothiouronium Two samples of real mining process solutions groups or polystyrene-supported phosphinic acid; were analysed by inductively coupled plasma mass and electrochemical cementation all with varying spectrometry (ICP-MS). From this a model solution degrees of selectivity. They deem further work on concentration of 4.0 ppm gold and 1.0 ppm mercury was set; resins with thiol functionality necessary in order to this initial concentration was used unless stated otherwise. achieve the desired selectivity (3). The pH of the electrowinning (EW) pregnant and Dithiocarbamates form stable mercury precipitates barren was pH 12.6 and 12.0 respectively; samples which have been used to selectively precipitate of heap leach were pH 10.0. The pKa of cyanide is mercury from gold bearing solutions (4), this can reported as 9.2 (8); therefore, pH 9.2 is considered be carried out more efficiently by using colloidal the minimum safe operating pH as below this the hydroxides to cause coagulation which can then be conversion to hydrogen cyanide occurs and loss of removed by dissolved air flotation (5). Hutchison and cyanide as HCN(g) causes both experimental and Atwood further reviewed mercury remediation methods safety concerns. For these experiments a pH range of highlighting dimethyldithiocarbamate as an effective 10 to 13 was adopted. precipitation reagent. However the long term stability Column experiments were carried out by loading the has been questioned with suggestion that mercury adsorbent material into a glass column, rinsing with pH leaches from the precipitate over time; this combined adjusted deionised water and then pumping the test with its degradation into toxic byproducts means that solution through the column using a peristaltic pump. it is only applicable under certain circumstances (6). The flow rate is controlled proportional to the bed Alkyl thiols such as 1,3-benzenediamidoethanethiol volume (BV) with a standard flow rate of 6 BV h–1 being can also be used to precipitate mercury (7). utilised. Outlet samples were collected and analysed This paper explores the use of solid adsorbents and by ICP-OES and compared against the inlet solution. how they can be applied to the selective adsorption of mercury from gold cyanide bearing process streams Materials and Reagents such as those found within the gold mining circuit. Johnson Matthey produce a range of metal adsorbents Experimental Procedures for metal removal applications under the brands Smopex® (9), QuadraPure® and QuadraSil® which are Test solutions were made by dissolving the appropriate based on polymer fibres, polymer beads and silica salts in deionised water adjusted to the correct pH using spheres respectively with an additional industrial silica sodium hydroxide solution to prepare a stock solution. based Functional Silica (FS series) range. Further This stock solution was diluted by pipette to provide the details of the adsorbent materials are detailed in required concentration solution. Table I. From internal knowledge and experience and Batch adsorption experiments were carried out preliminary screening several materials were identified by weighing 0.5 dry wt% of the adsorbent into a to explore the selective adsorption of mercury. Initial 60 ml glass tube, dry weight was determined using a screening showed both thiol and quaternary amine

323 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689487 Johnson Matthey Technol. Rev., 2015, 59, (4) materials to remove mercury; however quaternary CN band to be shifted to a higher frequency. This shift amines strongly adsorbed gold therefore this selectivity relates to both the coordination number and the metal study concentrated on the thiol based materials. oxidation state (10). Several mercury cyanide species The following reagents were used as received: are known to exist with varying coordination numbers mercury cyanide (Sigma-Aldrich), potassium gold which depends on the cyanide concentration of the cyanide (Alfa Aesar), potassium cyanide (Sigma- solution. Gold cyanide does not vary its coordination Aldrich), sodium cyanide (Sigma-Aldrich), sodium but can exist as either gold(I) or gold(III); however, the cyanate (Sigma-Aldrich), sodium thiocyanate (Sigma- Elsner equation (Equation (i)) predicts the gold to exist Aldrich), sodium sulfate (Alfa Aesar), sodium thiosulfate only as gold(I) in cyanidation process solutions. (Fisher Chemicals), sodium hydroxide (Fisher Model solutions of gold and mercury cyanide were Chemicals). Deionised water was used from an Elga made and used to explore the likely species in a Purelab DV35 at 15 MΩ. cyanidation circuit. Varying the cyanide ratio with 2– mercury showed that the tetracoordinate [Hg(CN)4] Results and Discussion is formed easily once the required ratio of cyanide is added (Figure 1). Above this ratio additional free Speciation cyanide is observed. Comparison of dissolved gold Metal cyanide coordination complexes are well known with known standards of potassium gold(I) cyanide and Nakamoto discussed how the easily identifiable (KAu(CN)2) and potassium gold(III) cyanide (KAu(CN)4) CN stretching band at 2200–2000 cm–1 can provide demonstrates that no gold(III) is present under standard information on the structure of the complex, as conditions (Figure 2). This is in accordance with the coordination of cyanide to the metal centre causes the Elsner equation. Model adsorption testing was carried

Table I Details of the Tested Materials

Name Support Material properties Functional group FS1 Granular silica 250–710 µm, 90 Å pore size Alkyl thiol Smopex®-111 Polymer fibre 300 × 50 µm (length × diameter) Benzyl thiol Smopex®-112 Polymer fibre 300 × 50 µm (length × diameter) Alkyl thiol

1:0

KAu(CN)2 1:1 2342 2146 1:2 2193 Au in KCN 1:3 2169 2146 1:4 2160 KCN

Transmission, % Transmission, 1:10 2079 Transmission, % Transmission, 2142 2079 KOCN 2079 2500 2400 2300 2200 2100 2000 1900 Wavenumber, cm–1 2169 2500 2400 2300 2200 2100 2000 1900 Fig. 1. Speciation in a cyanidation circuit using model solutions Wavenumber, cm–1 of gold and mercury cyanide with varying ratios HgCl2:KCN. Formation of the 4 coordinate species is observed even at Fig. 2. Comparison of dissolved gold with known standards. 1:3 ratios indicating that its formation is favoured. At higher The gold powder dissolved in KCN shows formation ratios the free cyanide peak is observed at 2079 cm–1. Mercury of KOCN from the oxidation under air of KCN. Gold concentration 0.1 M concentration 0.1 M

out using KAu(CN)2 and mercuric potassium cyanide improved. Following this result, further testing focused ® (K2Hg(CN)4), made in situ (Equation (ii)): on the FS1 and the Smopex -112 which are both alkyl thiols in a more hydrophilic environment (Figure 4). 2KCN + Hg(CN) → K Hg(CN) (ii) 2 2 4 The amount of gold adsorbed by the materials generally increases with increasing pH whilst the Model Adsorption adsorption of mercury is unaffected. This is likely due to the pKa of thiols occurring at around 12 to 13 An initial comparison of the three materials (Figure 3) (11); therefore, over the pH range 10 to 13 the thiols at pH 11 showed that Smopex®-111 gave low removal will become increasingly deprotonated changing the rates of mercury compared against the other materials. behaviour of their adsorption. Mercury forms a strong Both Smopex®-112 and FS1 gave excellent removal of bond to sulfur and the adsorption is not affected by this mercury although in both cases some gold was removed. change from free thiol to thiolate. The lower performance of Smopex®-111 is probably Various salts can be present in mining solutions due to the hydrophobic nature of the material. When depending on the source of the ore. These could it was tested at a higher pH or when using the sodium potentially include sulfur containing salts such as thiolate form of the material then the performance was thiosulfate or sulfate from oxidation of sulfide minerals, excess cyanide from the heap leach solution or products of cyanide decomposition including thiocyanate or 100 Au cyanate. Cyanide is often used in the region of 100 to Hg 80 500 ppm (12), this decreases during the process as the cyanide becomes bound to both the desired metal 60 (gold) and undesired metals such as mercury, nickel and iron; it is also oxidised to the cyanate ion or reacts 40 with sulfides to form thiocyanate. Infrared spectroscopy of the EW samples (Supplementary Information) shows 20 Metal removed, % no free cyanide but does show peaks indicative of 0 cyanate and nickel cyanide. All of these species could FS1 Smopex®-111 Smopex®-112 potentially cause a change in the adsorption behaviour; particularly chelating species such as thiocyanide, Fig. 3. Comparison of the three materials for removing cyanide and cyanate. 4 ppm gold and 1 ppm mercury cyanide at pH 11. FS1 To explore the effect of these species, 100 ppm and Smopex®-112 show good mercury removal whilst of each was added to the model solution and the Smopex®-111 had poor wetting and showed low mercury removal adsorption was retested. Figure 5 shows that in all cases the addition of 100 ppm of the anions had

(a) (b) 100 100 Au Au 80 Hg 80 Hg

60 60

40 40

Metal removed, % 20 Metal removed, % 20

0 0 10 11 12 13 10 11 12 13 pH pH Fig. 4. Effect of pH on removal of 4 ppm gold and 1 ppm mercury cyanides using: (a) FS1; (b) Smopex®-112. As the pH is increased the amount of gold adsorbed increases. This presumably relates to the pKa of the thiol functional group. The higher adsorption with Smopex®-112 at pH 10 has not been explained but may relate to additional hydroxyl functionality in the material

325 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689487 Johnson Matthey Technol. Rev., 2015, 59, (4) little effect on the adsorption of mercury when using 100 FS1 with 99% removal being achieved in most cases Au and 97% removal with cyanide addition. For gold 80 Hg adsorption some significant differences were observed. 60 The addition of sulfate, thiocyanate or thiosulfate had 40 no effect; however both cyanate and cyanide caused a reduction in the removal of gold from 87% removal 20 Metal removed, % with no added anion to 65% and <1% removal with 0 cyanate and cyanide respectively. Lewis and Shaw None demonstrated that gold thiolate and gold cyanide are Sulfate Cyanide Cyanate Thiosulfate in equilibrium (as in Equation (iii)) (13). At low cyanide Thiocyanate concentrations the gold forms an insoluble gold Fig. 5. Effect on metal removal using FS1 when adding thiolate with the surface bound thiol and is removed various sodium salts (100 ppm) to a solution of 4 ppm gold from solution; at higher cyanide concentrations the and 1 ppm mercury at pH 12. The commonly encountered equilibrium is shifted to the soluble gold cyanide and ions show little change in the amount of gold removed from the gold stays in solution. Mercury more easily forms the system. Adding additional sodium cyanide meant that an insoluble complex with the thiol and the mercury the gold removal was completely prevented whilst mercury cyanide equilibrium is not strong enough to maintain removal was unaffected the mercury in solution.

– – – RS – RS – Au CN [ NC Au CN ] [ RS ] – [ RS Au SR ] CN– CN

CN– (iii) RS Au

Adsorption Kinetics (instrument detection limit) within 5 minutes, less than 1 minute for Smopex®-112. Once adsorbed the mercury Simple adsorption kinetics (Figure 6) show that mercury was tightly bound to the material and is not eluted under is rapidly adsorbed with both FS1 and Smopex®-112. standard conditions, moderate pH or temperature. From the initial mercury concentration of 1 ppm both Gold is adsorbed more slowly, particularly with materials reduced the solution concentration to 0.01 ppm Smopex®-112. This is not expected as Smopex® usually

(a) (b) 5 5 Au Au 4 4 Hg Hg 3 3 2 2 1 1 0 0 Metal concentration, ppm Metal concentration, ppm 0 20 40 60 0 20 40 60 Time, min Time, min Fig. 6. Adsorption kinetics of 4 ppm gold and 1 ppm mercury cyanides at pH 12 using: (a) FS1; (b) Smopex®-112. Both materials give rapid removal of the mercury with only 0.04 ppm mercury detectable after 1 minute and 0.01 ppm after 5 minutes. Gold adsorption is moderate with FS1 being reduced to 0.57 ppm (85% removed) after 15 minutes. Smopex®-112 had not yet reached equilibrium with gold after 1 h indicating gold adsorption is slow with that material

exhibits rapid kinetics due to its small particle size; 0 b.Ce.Q therefore, the poor gold adsorption kinetics maybe q = (iv) A 1+b.Ce indicative that the adsorption process is strongly equilibrium limited. Further kinetic measurements were carried out Both materials show a similar maximum capacity (Q0) whereby an aliquot of cyanide was added at 30 from the Langmuir fitting at 28.8 mg g–1 and 31.2 mg g–1 minutes, these are shown in Figure 7. This shows how for FS1 and Smopex®-112 respectively. However, the upon cyanide addition the gold is rapidly desorbed from adsorption with FS1 is much more favourable with an the resin. This provides further evidence for the gold adsorption parameter (b) of 5.1 l mg–1 compared to thiolate ⇌ cyanide equilibrium and its effect on the gold 0.50 l mg–1 for Smopex®-112. adsorption. The addition of cyanide has little effect on the mercury which remains adsorbed onto the resin, Column Adsorption Trials confirming that the insoluble mercury thiolate is strongly favoured over the soluble mercury cyanide species. Due to the scale of liquid flow in a mining application the final process must run continuously; therefore columns Adsorption Isotherms are preferred. To test the feasibility column trials were run with a model solution containing 5 ppm gold and The mercury-only adsorption isotherms were measured 5 ppm mercury at pH 10 with no added cyanide. for both FS1 and Smopex®-112 at an initial mercury Figure 9 shows that the outlet mercury concentration concentration of 100 ppm (Figure 8); with mercury was below the instrument detection limit (0.01 ppm) in the form of the K2Hg(CN)4 salt. The data were during the entire test period; gold was initially adsorbed fitted to a Langmuir isotherm, shown in Equation (iv), but then partially displaced as indicated by the outlet –1 where: qA = mg adsorbate per g adsorbent (mg g ), concentration going above the inlet concentration –1 b = adsorption parameter (l mg ), Ce = equilibrium ([M]/[M]0>1). Figure 10 shows the effect of adding concentration (mg l–1) and Q0 = maximum capacity (mg 100 ppm cyanide to the test solution. In this case no g–1), which is deemed to be more suitable due to the gold was adsorbed by the material whilst mercury expected chelation mechanism. adsorption was maintained; with the outlet mercury

4 –1 30 Au 3 Hg 25

2 20

1 15 FS1 0 10 Smopex®-112 Metal concentration, ppm Mercury adsorbed, mg g 0 20 40 60 5 Time, min 0 Fig. 7. Adsorption kinetics of 4 ppm gold and 1 ppm mercury 0 20 40 60 80 100 cyanides at pH 12 using FS1. After 30 minutes an aliquot Mercury equilibrium concentration, ppm of sodium cyanide is added equivalent to 100 ppm. The full lines/symbols show the concentration for the standard Fig. 8. Graph showing adsorption isotherm of mercury as adsorption; the dashed line/open symbols show the solution K2Hg(CN)4; initial mercury concentration 100 ppm, with concentration with cyanide addition. This clearly shows that the dashed lines showing the Langmuir isotherm fitting upon addition of cyanide the gold is rapidly desorbed from (anomalous data point with open symbol ignored for the resin whilst mercury remains adsorbed Langmuir fitting)

1.2 1.2 Au 1.0 Hg 1.0 0 0.8 0.8 0.6 0 0.6 0.4

[M]/[M] Au 0.4 0.2 [M]/[M] Hg 0 0.2 0 3 6 9 12 15 18 0 Bed volumes 0 3 6 9 12 15 18 Fig. 9. Flow test of adsorption of 5 ppm gold and 5 ppm Bed volumes mercury by FS1 at a flow rate of 6 BV –1h . Mercury Fig. 10. Flow test of adsorption of 5 ppm gold, 5 ppm concentration was below the instrument detection limit mercury and 100 ppm cyanide by FS1 at a flow rate of 6 (0.01 ppm) during the entire test period; gold was initially BV h–1. Mercury concentration was below the instrument adsorbed but then partially displaced (outlet concentration detection limit (0.01 ppm) during the entire test period; above inlet concentration) gold outlet concentration was at the inlet concentration immediately (ignoring any initial sample dilution effect) concentration being below the detection limit of the cyanide concentration was increased to 1000 ppm. This analysis (0.01 ppm). The low gold concentration of provides further evidence for the equilibrium nature of the the first bed volume is due to the dilution effect of the gold-thiolate interaction which can easily be perturbed by column pre-rinse. increasing the free cyanide concentration. Meanwhile the The material loaded in Figure 9 was subjected to mercury-thiolate interaction is considerably more stable. washing with an increasing concentration of cyanide solution at pH 10, shown in Figure 11. Initially, Real Stream Adsorption when washed with dilute sodium hydroxide the gold concentration rapidly decreased as the load solution Due to the complexity of the real solution streams the was rinsed from the packed bed (BV 1 to 6); when batch adsorption was repeated using a real sample 100 ppm cyanide was added to the wash solution of heap leach pregnant solution containing 1.22 ppm (BV 7 to 12) then the gold was rapidly eluted. The gold and 0.31 ppm mercury at pH 10.0; both FS1 and mercury was retained by the solid even when the Smopex®-112 were tested as adsorbents. The solution was tested both as received and with cyanide added at 100 ppm in order to assess whether the addition of cyanide would improve the selectivity of adsorption in a real feed. Gold adsorption was observed from 5 1000 Cyanide concentration, ppm the as received test solution with both materials Au 4 Hg 800 (Figure 12), with 7% and 20% gold removal with CN FS1 and Smopex®-112 respectively. In any real world 600 3 application non-selective adsorption would represent 2 400 loss of product and is therefore unacceptable. When the cyanide was added to the received material 1 200 the gold removal was reduced to less than 1% with

Metal concentration, ppm 0 0 Smopex®-112 and to undetectable levels with FS1. 0 6 12 18 24 30 Both materials reduced the mercury concentration from Bed volumes 0.31 ppm to below the detection limit of the instrument Fig. 11. Elution of loaded resin from Figure 9. The material (<10 ppb), this was regardless of whether cyanide was was washed with deionised water at pH 10 with increasing added to the solution. This represents the ideal result cyanide concentration. The initial gold concentration is a for the desired application whereby a minor modification dilution effect of the load solution. As 100 ppm cyanide or monitoring of the stream allows highly selective is added to the wash water the gold is rapidly eluted. adsorption of mercury from a gold bearing stream. No mercury is detected (<0.01 ppm) even at a cyanide concentration of 1000 ppm The process is currently being piloted in Nevada, USA (shown in Figure 13).

(a) (b) 100 100 Au Au Hg Hg 80 80

60 60

40 40 Metal removed, % Metal removed, %

20 20

0 0 As received Plus 100 ppm As received Plus 100 ppm cyanide cyanide

Fig. 12. Batch adsorption from real feed (heap leach pregnant solution) containing 1.22 ppm gold and 0.31 ppm mercury at pH 10.0 using: (a) FS1; (b) Smopex®-112. Both adsorbents show unacceptable levels of gold adsorption with the as received samples whilst adding 100 ppm of cyanide led to less than 1% of the gold being adsorbed with Smopex®-112 and undetectable gold adsorption with FS1

Fig. 13. 70 l pilot unit in Nevada, USA, operating at flow rates in the order of tens of litres per minute

Conclusions could be achieved when using thiol based adsorbents. The adsorbents were effective regardless of whether the In this study infrared spectroscopy was used to show thiol was bound to a silica or polymer based support. The 2– – that [Hg(CN)4] and [Au(CN)2] are the most likely theory was then applied to a real sample from a mining species to occur within a mercury containing gold circuit, with the real stream it was found that when the cyanide stream, as would be found in a gold mining adsorbents were applied to the solution as received circuit where cyanide is used as the lixiviant. These then an unacceptable level of gold adsorption was species were then used to conduct model adsorption obtained in conjunction to the mercury adsorption. When tests in order to identify a method of selective mercury cyanide was added to the received solution then gold adsorption. adsorption was completely prevented whilst mercury By adding free cyanide to the system (or ensuring that adsorption was maintained, with the adsorbent reducing free cyanide exists in the solution) it was found that the the mercury concentration to below the detection limit selective adsorption of mercury in the presence of gold (0.01 ppm) of the analytical equipment.

The adsorbed mercury is strongly bound to the resin 4 M. Misra and J. A. Lorengo, Board of Regents of the as an insoluble complex without leaching under mild University and Community College System of Nevada, conditions; therefore the material can be more easily ‘Method of Removing Mercury From Solution’, US handled or stored than many mercury complexes. Patent 5,599,515; 1997 Alternatively, it is envisaged that the material could be 5 F. Tassell, J. Rubio, M. Misra and B. C. Jena, Min. regenerated with strong concentrated acids (14). Eng., 1997, 10, (8), 803 The concept was also tested using a flow system which 6 A. R. Hutchison and D. A. Atwood, J. Chem. Cryst., is likely to be the final application method. Here it was 2003, 33, (8), 631 also found that the addition of cyanide to the solution led 7 M. M. Matlock, B. S. Howerton, M. A. Van Aelstyn, F. L. to the prevention of gold adsorption whilst maintaining Nordstrom and D. A. Atwood, Environ. Sci. Technol., a mercury outlet concentration below the detection limit 2002, 36, (7), 1636 of the equipment (0.01 ppm) during the test period. 8 R. M. Smith and A. E. Martell, “Critical Stability Additionally, the solution was run with no free cyanide Constants: Volume 4: Inorganic Complexes”, 3rd and a dilute cyanide solution was then used to rinse Edn., Springer Science+Business Media, New York, any adsorbed gold from the material. This shows that USA, 1976 alternative process solutions can be used depending 9 S. Phillips and P. Kauppinen, Platinum Metals Rev., on the individual process economics with both methods 2010, 54, (1), 69 being based on the same scientific concept. 10 K. Nakamoto, “Infrared and Raman Spectra of Inorganic and Coordination Compounds”, 3rd Edn., References Wiley and Sons Inc, New York, USA, 1978 11 M. M. Kreevoy, E. T. Harper, R. E. Duvall, H. S. Wilgus 1 L. Elsner, J. Prakt. Chem., 1846, 37, (1), 441 III and L. T. Ditsch, J. Am. Chem. Soc., 1960, 82, (18), 2 J. S. MacArthur, R. W. Forrest and W. Forrest, 4899 ‘Improvements in Obtaining Gold and Silver From Ores and Other Compounds’, British Patent 14,174; 1888 12 “Fact Sheet - Cyanide and its Use by the Minerals Industry”, Minerals Council of Australia, Australian 3 J. D. Miller, E. Alfaro, M. Misra and J. Lorengo, Capital Territory, Australia, 2005 ‘Mercury Control in the Cyanidation of Gold Ores’, in “Pollution Prevention for Process Engineers”, eds. P. 13 G. Lewis and C. F. Shaw III, Inorg. Chem., 1986, 25, E. Richardson, Frank Lanzetta Jr and B. J. Scheiner, (1), 58 Engineering Foundation, New York, USA, 1996, pp. 14 A. Arencibia, J. Aguado and J. M. Arsuaga, Appl. Surf. 151–64 Sci., 2010, 256, (17), 5453

The Author

James Stevens is a Senior Scientist at the Johnson Matthey Technology Centre, Sonning Common, UK. His work is focused on the development and application of solid adsorbents to the removal of metals from process water. Previously he gained his PhD at the University of Nottingham, UK, under the supervision of Martyn Poliakoff. His thesis focused on the hydrogenation of biorenewables in supercritical carbon dioxide.

In the Lab Uranium Capture From High Sulfate and Nitrate Waste Streams with Modiﬁ ed Silica Polyamine Composites

Johnson Matthey Technology Review features new laboratory research

Edward Rosenberg is a Professor of Chemistry at the University of Montana, USA. His research interests are About the Researcher in the areas of the applications of composite materials for metal ion removal, separation and concentration from aqueous systems.

About the Research

The objective of this research is to fi nd a solid phase 2+ adsorbent that is selective for uranyl cation (UO2 ) in the types of waste streams found on the Navajo • Name: Edward Rosenberg reservation in the southern part of the USA. The technology could potentially be used to remove this ion • Position: Professor from ground and surface waters and to remove trace • Department: Chemistry and Biochemistry uranyl from drinking water supplies on the reservation. There are several ion exchange materials on the • University: University of Montana market that report effective removal of uranium from • Street: 32 Campus Drive water. Silica polyamine composites (SPC) are patented • City: Missoula, 59812 and commercialised ion exchange materials currently being developed for use in water remediation by • Country: USA Johnson Matthey Water Technologies division. SPC • Email Address: [email protected] are made by the coating of amorphous silica gel particles with functionalised silane, which are further reacted with polymeric amines to provide the parent composites WP-1 and BP-1 (Figure 1). with related functional groups using equilibrium batch The parent SPC can then be modifi ed to make them studies. The results of this exciting work are expected more specifi c to a given metal or group of metals to be submitted for publication shortly. (Figure 2). Much work remains to be done before the group can Preliminary results on the removal of uranyl cation from go forward to applying this technology to remediation solutions that mimic the contamination profi les of waste on the Navajo reservation. Breakthrough studies are streams on the Navajo reservation have been obtained currently underway as well as more direct comparisons using a range of SPC materials. The most effective with other materials. Most importantly cycle testing must SPC is then compared with a polystyrene material be done with actual waste stream samples from the

NH2 H N 2 NH2 H2N N N N N N N N H N N N N N H H2N H2N H2N HN HN H2N CH3 CH3 Poly(ethyleneimine) MW = 23 K Si O Si O Si O Si or O O O O H2N H2N H2N H2N H2N Si O Si Si O Si O

Poly(allylamine) MW = 11–15 K WP-1

or + NH2 H N NH2 NH2 2

CH3SiCl3:ClCH2CH2CH2SiCl3 7.5 1.0 HN HN H N H2N 2 + CH3 CH3 O Si O Si O Si OOSi OH OH OH OH O O O O Si O Si O Si O Si Si Si O O O Si O O O O Si

Si O Si Si O Si BP-1 O Acid washed and humidiﬁ ed amorphous silica gel

Fig. 1. Synthesis of the SPC materials

CuSelect (P = PAA) selective for Cu2+ WP-2 (P = PEI), BP-2 P = PAA): WP-4 (P = PAA): selective for over Fe3+ at low pH pH dependent selectivity for Fe3+, Ni2+ and Ga3+ over Al3+ divalent transition metals at pH~2 OH N P N P N H P N HN CH2CO2H H Made by reaction with picolyl chloride or by Made by reaction with X hydramination of chloroacetic acid pyridine-2-carboxaldehyde Made by Mannich Reaction with CH2O (X = H, Cl, SO3H) BPAP (P = PAA): selective for BPED (P = PAA), WPED (P = PEI) trivalent over divalent metals. 2+ 2+ BP-NTA (P = PAA), WP-NTA (P = PEI) Highly charged metals can be Very selective for Ni over Co , 2+ 2+ 2+ 2+ 2+ Very selective for Ni over Co , Fe immobilised for anion capture Fe and Zn and Zn2+ P P O P O HN OH HN C HN C P O CH2 CH2 OH NCH2CH2NCH2CO2H N CH2CO2H HO CH C HO CH C Made by Mannich Reaction 2 2 CH2CO2H 2 2 CH2O + phosphorous acid Made by reaction with EDTA Made by reaction with anhydride nitrilo-acetic anhydride

Fig. 2. Ligand modiﬁ ed SPC and their applications to date

Y. O. Wong, P. Miranda and E. Rosenberg, J. Appl. Polym. reservation to evaluate the usable lifetime of the solid phase adsorbents under real working conditions. The Sci., 2010, 115, (5), 2855 results obtained to date are certainly worth following up M. A. Hughes, J. Wood and E. Rosenberg, Ind. Eng. Chem. with pilot scale runs as the next milestone. Res., 2008, 47, (17), 6765

Acknowledgements M. A. Hughes and E. Rosenberg, Sep. Sci. Technol., 2007,

Ranalda Tsosie is a graduate student in chemistry and 42, (2), 261 environmental studies at the University of Montana and E. Rosenberg and R. J. Fisher, University of Montana, her work on this project is acknowledged. ‘Materials and Methods for the Separation of Copper Ions and Ferric Iron in Liquid Solutions’, US Patent Selected Publications 7,008,601; 2006 E. Rosenberg, P. Miranda, Y. O. Wong, ‘Oxine Modiﬁ ed E. Rosenberg and D. C. Pang, University of Montana, Silican Polyamine Composites for the Separation of Gallium from Aluminium, Ferric from Nickel and ‘System for Extracting Soluble Heavy Metals from Copper from Nickel, US Patent 8,343,446; 2012 Liquid Solutions’, US Patent 5,997,748; 1999

JOHNSON MATTHEY TECHNOLOGY REVIEW www.technology.matthey.com

New Smopex® Ion Exchange Materials for the Removal of Selenium from Industrial Effluent Streams Material characterisation, modelling and process implementation

By Carl Mac Namara*, Javier Torroba# and 1. Introduction Adam Deacon Johnson Matthey Plc, PO Box 1, Billingham, Selenium is a non-metal, trace element with crucial Cleveland, TS23 1LB, UK roles in animal and plant biology, although it becomes highly toxic at relatively low levels. The European Email: *[email protected]; Union suggests a maximum Se concentration of #[email protected] 10 mg l–1 in drinking water (1) while the guideline in USA was set at 50 mg l–1 (2). Selenium forms different water-soluble ions that can be found in aquatic This article discusses new Smopex® ion exchange environments from both natural and industrial origins. Fossil materials which have been developed by Johnson fuel and mining related activities, agriculture and glass Matthey Water Technologies, and highlights their manufacture constitute the most significant contributors to performance relative to other commercially available anthropogenic sources of selenium contamination (3). materials for the removal of selenocyanate, selenate Selenium remediation has been widely investigated and selenite ions from aqueous solutions. The ion during the past few decades, although today few exchange mechanisms by which these materials sorb technologies are being applied on a commercial scale these ions are also explained and modelled in order (3). An efficient process must deal with the difficult to highlight the additional benefits that these materials task of removing selenium from large volumes at offer that non ion-exchange materials do not, such low concentrations (although above the regulated as the ability to achieve the full material exchange levels), the complex speciation of selenium, and the capacities at feed concentrations lower than 1 mg l–1 presence of high concentrations of sulfur. For example, selenium. The unique characteristics of these fibrous wastewater from oil refineries can contain selenium in type materials are also discussed, including fast the order of few parts per million (ppm, mg l–1), typically sorption kinetics, facile regeneration and enhanced as a mixture of selenocyanate (SeCN–) and selenate 2– 2– selectivity for selenium ions against competing sulfate (SeO4 ) ions (4). Sulfate (SO4 ) would be found in the ions. Finally, the performance of these materials in a same feeds at more than 10 times that concentration continuous stirred tank reactor setup is demonstrated, (1). Treated effluents from flue-gas-desulfurisation showing that performance levels as high as in fixed bed (from coal combustion power plants) can contain ppm processes can be achieved, due to the high selectivity levels of selenium, most commonly as selenate, but as and mass transfer kinetics of Smopex® materials. much as 1 g l–1 of sulfur in the form of sulfate (5).

Current technologies include chemical and biological functionalisation is generally based on fixed quaternary methods. Amongst the first group it is easy to find amines (ammonium cations), while weak-base type processes based on the reduction of selenium ions materials are based on alkyl and/or aromatic amines. using reagents such as zero-valent iron (6, 7) or sodium The advantage of strong-base materials is that, thanks sulfide (8). Although relatively cost-effective techniques, to having fixed positively-charged groups, they can they are extremely dependent on pH and temperature act as anion exchangers in wider pH ranges than conditions and require vast quantities of non-reusable weak-base type materials. In this work we investigate reactants. Biological methods still demand rigorous in detail the performance of some materials within control of pH, salinity and temperature, although they this class: the polymeric fibres Smopex®-103 and consume smaller amounts of additional chemicals (3, Smopex®-269, the silica-polymer composites WP11 9). Anaerobic tank reactors or packed bed systems can and WP13, and two standard polystyrene resins from be designed to accommodate bioreduction of selenium Dow Chemicals, AmberliteTM IRA 900 and DowexTM ions by specific bacteria strains. Treatment of large 1x2 100–200. All these materials have fixed positively- volumes of slurry waste is necessary to separate the charged groups and mobile chloride ions as the solid elemental selenium. exchangeable anions. In particular, Smopex®-103 Physical separation methods have also been explored bears trimethylammonium groups (similar to Dow’s for selenium remediation, but only reverse osmosis and resins) while the functionality of Smopex®-269 is based nanofiltration seem to decrease the concentration of on the aromatic base benzyl-pyridinium (Figure 1). selenium ions below the acceptable discharge levels. Smopex® (Figure 2) is a unique type of material where Both technologies present high operating costs and the binding functionality lies on side chains grafted require efficient pretreatment of feeds to avoid fouling onto 0.3 mm long polymeric fibres (olefin or natural) and degradation of membranes (10). (14), in contrast with standard spherical porous resin Ion exchange technology is widely used for many beads used in common adsorption or ion exchange different municipal and industrial wastewater processes. This structure grants an efficient, fast treatments. All kinds of natural and synthetic sorbents recovery of the target species with very high loading have been investigated as potential materials for capacity. On the other hand, Johnson Matthey’s silica- selenium remediation at bench scale, but there is no polymer composites could seem, in principle, a very reference to full scale processes being implemented different kind of material (Figure 2) (15), formed by (3). Materials from varied groups such as resins, a silica particle core coated with a polymer bound to carbon-based adsorbents and metal oxides have been the silica chemically. However, the active functionality found to show potential activity to adsorb Se ions from is homogenously distributed onto the surface of the water media. Amongst them, only a few synthetic coating polymer, allowing an excellent interaction with materials were reported as having the necessary the liquid medium and thus enhancing properties such selenium capacity to be considered potential solutions as kinetics and loading capacities, as in the case of (3). Unfortunately, most of them are unable to achieve Smopex®. good levels of selectivity for the removal of selenium in In the next sections we discuss experimental results the presence of sulfur or other contaminants, limiting and modelling work carried out in order to determine the prospects of any potential applications. Advanced the principles governing the sorption of selenocyanate – 2– 2– selective ion exchange technology could help in (SeCN ), selenite (SeO3 ) and selenate (SeO4 ) ions mitigating these disadvantages. onto the different strong-base sorbents investigated. A In this article we discuss our investigations of the special attention is paid to the removal of selenium ions 2– fundamentals of selective sorption of selenium ions from in the presence of the competing ion sulfate (SO4 ). aqueous solutions using strong-base functionalised materials. Exploratory tests highlighted the potential of Johnson Matthey’s synthetic scavengers for the (a) (b) Cl– removal of inorganic selenium species. Strong-base N+ Cl– N+ type materials were identified as the best candidates (11), although weak-base type materials sometimes ® also show high affinities for inorganic anions such Fig. 1. Chemical functionalisation of: (a) Smopex -103; and (b) Smopex®-269 as phosphate (12) and arsenate (13). Strong-base

(a) (b) (c)

(d)

20 mm

Fig. 2. (a) and (b) photographs of Smopex® fibres; (c) SEM of Smopex® fibres; (d) photograph of silica-polymer composite material

The effects caused by other potentially competing Selenocyanate-containing samples were kept at ions such as nitrate, phosphate or bicarbonate are not basic pH (addition of 0.1 ml of 1 M NaOH) to prevent covered in this work. decomposition until just before the analysis. Chloride levels were analysed by ion chromatography. Dilution 2. Experimental steps were included, when necessary, to adjust sample concentration to calibration window in the equipment. All chemicals were purchased from Alfa Aesar and Results were compared to certified external standards. scavenging materials were used as supplied by Johnson 2.2 Materials Matthey, Finland. Sodium selenite (Na2SeO3·5H2O), ® sodium selenate (Na2SeO4·10H2O) and potassium The Smopex materials are polypropylene fibres, selenocyanate (KSeCN) were used as selenium having a trilobal shape and a typical length of 300 mm sources for the model solutions, while sodium sulfate and diameter 50 mm (see Figure 2). The polypropylene

(Na2SO4 anhydrous) was used for sulfur containing fibres are cut to this length during the manufacturing feeds. Sodium chloride (NaCl, 99% min) solutions stage, prior to functionalisation. The bulk density of were used as eluent. Solutions were prepared at the dry Smopex® fibres is approximately 275–1 gl . room temperature using demineralised water. First, The dry content (mass of dry material per mass of an adequate amount of the corresponding salt was the supplied material which includes moisture) of dissolved in water in a beaker and then diluted to the the supplied Smopex® is approximately 60%. The final volume in a volumetric flask. Further dilutions were spherical silica polymer composite materials have carried out in order to prepare the very dilute solutions. a particle diameter range of 250–750 mm and bulk pH was adjusted using small volumes of HCl 1 M or densities for the dry material of 500 to 600 g l–1. The NaOH 1 M when necessary. silica polymer composite materials are supplied dry. Please note that concentrations expressed in w/v Properties of the AmberliteTM IRA-900 and DowexTM terms in this document are based on elemental 1x2 can be readily found online, however the dry concentration, not on molecular concentration. For contents of these materials as supplied were measured example, a KSeCN solution with Se concentration of at 61% and 75%, respectively. Bulk densities of the dry 200 mg l–1 would actually contain 365 mg l–1 of KSeCN. material were measured at approximately 373 g l–1 and 468 g l–1, respectively. All of the materials were used in 2.1 Analytical the experimental trials as supplied. Selenium and sulfur concentrations in the samples were determined by elemental analysis by ICP-MS 2.3 Adsorption Tests (Perkin Elmer Elan 6100 DRC) or ICP-OES (Thermo Different procedures were followed to carry out tests, Scientific iCAP 7600 Radial). All samples were depending on the nature of the experiment. They are acidified with 0.1 ml of 69%3 HNO prior to analysis. based on standardised procedures used by Johnson

Matthey Water Technologies. In this investigation all isotherms with sulfate were run using Se:S equimolar tests were done at room temperature. solutions with 150 mg l–1 of Se and 60 mg l–1 of S (1.9 mmol l–1). 2.3.1 Determination of Batch Kinetic Profiles These tests were carried out at neutral pH and 25ºC. 2.3.3 Continuous Flow Column Tests Scavenger masses of 0.150 g (based on dry content) The general procedure is based on passing a feed were put in contact with 15.0 ml of selenate, selenite of known Se or Se/S concentration through a column or selenocyanate solutions with 1.0 mg l–1 of Se (Figure 4) containing a known amount of scavenging (0.013 mmol l–1). The mixtures were allowed to react material, generally between 1 and 2 g (dry content). for different lengths of time under gentle stirring The fixed-beds in these experiments had volumes (60 rpm), typically for 1, 2, 5, 10, 30, 60 and 120 between 3 and 7 ml. Flow rates were kept constant at minutes. Sorption tests with 30 seconds of contact time 36 ml h–1 while the lengths of the experiments were were also performed for Smopex® materials. Once the altered depending on the case. Initial Se concentration reaction time had passed, the solutions were filtered was adjusted to 1.0 g l–1 (12.7 mmol l–1) for Smopex® out from the suspensions and analysed by ICP-MS. fibres and Dow resins and to 0.50 g l–1 (6.3 mmol l–1) for silica-polymer composites. Lower initial concentrations 2.3.2 Determination of Adsorption Isotherms were used for the latter materials in order to optimise The procedure requires using fixed concentration and the length of the experiment, as they have generally volumes of solution and varying masses of scavenger lower maximum materials concentration than Smopex® material in parallel tubes in order to cover the expected and Dow materials. range of loadings and equilibrium concentrations For competitive tests with sulfate, a Se:S mass (Figure 3). These tests were carried out at room ratio of 1:12 was used (1:30 molar ratio) with initial temperature and allowing enough reaction times to Se concentrations of 0.50 and 0.25 g l–1 (6.3 and reach equilibrium conditions (generally overnight). 3.2 mmol l–1) and S levels of 6.0 and 3.0 g l–1 respectively The Se and S initial concentrations were kept fixed (187 and 93.6 mmol l–1). for a given isotherm trial but varied across different Desorption studies were carried out in a similar trials, and ranged from 10 to 500 mg l–1 (Se: 0.13 to way, but passing NaCl solutions through the column 6.33 mmol l–1; S: 0.31 to 15.6 mmol l–1). The required at similar rates (36 ml h–1), containing in this case masses of the materials were calculated based on an about 1.0 g of scavenger pre-loaded with selenium ‘expected maximum loading’, i.e. a mass range of material ions. Eluents with different chloride concentration was added such that the equilibrium concentrations at were used, ranging from 0.06 mol l–1 (0.2 wt%) to the end of the trial were expected to range from 10% 2.82 mol l–1 (10 wt%). to 90% of the initial concentration value. Competitive

Fig. 3. Experimental apparatus (temperature controlled multi-vial carousel) for batch kinetics test and isotherms Fig. 4. Experimental apparatus (column and fraction collector) for the continuous flow column tests determination studies

Samples were collected downstream at regular not usually be elucidated from simple charts and with intervals using an automated fraction collector. which they can think up new interesting experiments. It must be emphasised that while modelling in this 2.3.4 Continuous Stirring Tank Reactor paper appears to be the representation of sorption For this test a baffled cylindrical glass reactor,fabricated mechanisms through mathematical equations, the bulk in house,was used (Figure 5) which allowed continuous of the modelling exercise undertaken here is rather the flow (at 100 ml h–1) into the reactor of a SeCN– feed understanding of the real sorption mechanisms taking having a concentration 450 mg l–1 (5.7 mmol l–1). The place so that the correct mathematical expressions can reactor liquid level was controlled using a glass dip then be applied. Special focus throughout the paper will tube. Both the inlet tube at the bottom of the reactor and thus be given to highlighting the mechanisms involved dip tube were fitted with glass frits to prevent Smopex® in the sorption processes and how they influence the exiting the reactor. 30 g (dry content) of Smopex® choice of mathematical models. 103 was vigorously mixed with the reactor solution for Modelling the removal of ions from solution by ion the duration of the test. The operating volume of the exchange involves both mass transfer and equilibrium reactor was 0.64 l. Samples were manually collected equations (16). Typical models used to represent downstream at regular intervals (every hour). The sorption equilibrium are the Langmuir, Freundlich and experimental results from a single continuous stirring mass action law models (17–18). The Langmuir and tank reactor (CSTR) are presented in Section 4.4. Freundlich models both consider a material as having The model was validated for the single CSTR system adsorption sites with equal or different adsorption and deemed to be suitable for accurately predicting potentials for solution ions, and are often used to performance in the two CSTR system, therefore only model the sorption isotherms of both adsorption and the predicted performance for a two CSTR in-series ion exchange systems as the fit to experimental data in system are shown with no experimental results to verify both cases is often very good (19–21). Several authors these predictions. (22–23) have already discussed the unsuitability of these models in accounting for all the mechanisms at play in ion exchange systems, leading to inaccurate and unreliable predictive models for scale-up and design of ion exchange processes. The effect of these mechanisms on the engineering of ion exchange processes is further discussed and featured in this study. The mass action law has been used to model the specific mechanisms of ion exchange systems, such as electroneutrality in the material, non-ideality of the solution and material phases as well as the effect of the ion released from the material during ion exchange (24). Equilibrium is represented in the mass action model by Equation (i):

γ q*Z j γ C*Zi mii sjj Kij− = (i) ® *Zi *Z j Fig. 5. CSTR reactor, shown without Smopex in the reactor γ mjq γ C j sii for clarity where i and j refer to two distinct ionic species in

the system, Ki–j is the thermodynamic equilibrium are the activity coefficients in the 3. Modelling constant, ɣsi and ɣsj liquid phase, ɣmi and ɣmj are the activity coefficients on * * The modelling of the ion exchange mechanisms the material, q i and q j are the concentrations in the –1 * * discussed in this paper is an important task as it not only material phase at equilibrium (eq g ), C i and C j are enables the rigorous scale-up of material behaviour the concentrations in the liquid phase at equilibrium –1 from lab to plant scale, but also further informs material (eq l ) and zi and zj are the absolute values of each researchers with additional information which would species’ valency. The sum of the concentrations on the

338 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689694 Johnson Matthey Technol. Rev., 2015, 59, (4) material phase is equal to the real exchange capacity differential equations. The film diffusion mass transfer (REC) on the material, as given by Equation (ii): equation is given by Equation (iii) (28):

RECq=+**qq+…+ * (ii) dq 3 12 n i =−kC()C * (iii) ρ fbiis dt r Where n is the number of ionic species in a given system, including the species initially present and then where r is the material particle radius (m), ρ the particle –1 released from the material and with which ion exchange density (g l ), kf the film mass transfer coefficient –1 can occur. The REC is determined experimentally and (m s ), Cbi the bulk solution concentration of species i –1 * –1 is usually lower than the theoretical exchange capacity (eq l ) and C si the concentration of species i (eq l ) at (TEC) of a material which is measured by chemical the surface of the material, at equilibrium with the ion * * analysis of the material (23). concentration in the material qi. C si is equivalent to C i In order to estimate activity coefficients for a given in Equation (i). The bulk mass transfer in the CSTR is system, the Bromley method (25) has been used given by (Equation (iv) (28)): for estimating the liquid phase coefficients and the dCb m dq QCfb()− Cf Wilson method (26) for the material phase coefficients. ii=− i + i (iv) dt V dt V Estimating the values of additional parameters in the Wilson model and the values of the equilibrium constants in Equation (i) is achieved by using nonlinear where m is the mass of material in the system (g), V is optimisation methods to minimise predicted and the volume of liquid (l), Qf is the feed flow rate into the -1 experimental equilibrium data (for example sorption CSTR (l s ) and Cfi is the concentration of species i in isotherms) from binary systems. A full description on the feed (eq l–1). the use of these methods has been well described by In modelling the ion exchange sorption in the CSTR other authors (24) and will not be repeated here. process (see Section 4.4), the following assumptions The equilibrium models are then combined with were made: mass transfer models to predict the dynamic sorption • The only resistance to mass transfer is film behaviour of adsorption and ion exchange processes. diffusion resistance As most commercially available ion exchange resins • Ion exchange at the liquid/solid interface, i.e. are typically macroporous polymeric resin beads the material surface is instantaneous and with chemical functionality located both within the the equilibrium between both phases can be pores of the resin and on the surface (27), bulk mass represented by the mass action law transfer models are combined with film diffusion and • The process occurs under isobaric and isothermal intra-particle diffusion models to predict the material conditions concentration profiles as a function of time (23). • Physical properties of the ion exchanger and liquid Furthermore as commercial processes usually consist are constant of fixed-beds through which fresh feed is continuously • As Smopex® materials are not spherical, the introduced, concentration profiles which vary along the radius used in Equation (iii) corresponded to a axis of the column must be predicted by the model as sphere with equivalent surface area to a typical well as axial dispersion effects, ultimately resulting in a Smopex® particle (approximately 60 mm). large set of partial differential and non-linear equations Equations (i)–(iv) were solved using the Athena which must be solved simultaneously. visual studio (29) software to obtain the predicted The system under study here is somewhat simplified liquid and material phase concentrations as a as first of all, the Smopex® materials have all of their function of time of all ionic species involved in the functionality located on the surface of the material ion exchange. Additional model parameters, such and thus only bulk and film diffusion mass transfer as the particle radius r and particle density ρ, were equations need be considered. Secondly, the obtained from experimental measurements or Smopex® behaviour has specifically been studied in a material characterisation techniques, while the film

CSTR setup, where perfect mixing was assumed and mass transfer coefficient kf was estimated from the the dynamic behaviour can be represented by ordinary experimental bulk liquid concentration data.

4. Results and Discussion and material concentrations of chloride, which also 4.1 Material Selectivity change as selenocyanate concentrations change, due to ionic exchange of chloride and selenocyanate In Figures 6, 7 and 8, the sorption isotherm results for between the material and liquid solution. The sorption selenocyanate, selenate and sulfate ions, respectively, isotherms experimentally measured here are only are plotted for the four functionalised materials. The valid for systems having equal levels of chloride points represent experimental measurements while initially present on the material. Thus a material having the dashed lines are fitted lines with no physical more or less functionality and hence differing initial significance but which assist in interpretation of the concentrations of chloride on the material will lead to a material sorption behaviour. different selenocyanate sorption isotherm. In the case In Figure 6, Smopex®-269 is the most ‘selective’ of sorption by adsorption or chelation, there would material for selenocyanate, since it has the highest be no exchanged ion from the material and so the material concentration at low liquid concentrations, isotherm itself would only depend on those ions initially while the highest material concentration is achieved on in solution. Thus models such as the Langmuir or Smopex®-103, at approximately 1.7 mmol per gram of Freundlich isotherms are not applicable for modelling dry Smopex®-103. Interestingly, this concentration is this system, even though they would probably fit the approximately equal to the measured nitrogen content on experimental data very well. Smopex®-103 of 1.8 mmol g–1, i.e. the TEC, suggesting In Figure 7, Smopex®-103 is now the material with that nearly all of the functionality on the material is both the highest material concentration and selectivity available for sorption. This is likely linked to design for selenate. The maximum material concentration of the Smopex® material where all of the functionality has approximately halved from 1.7 mmol g–1 for is located on the surface of the polymeric fibres, in selenocyanate to approximately 0.85 mmol g–1 for contrast to typical sorption materials where most of selenate. This is because the selenate ion is divalent the functionality is located within the porous structure so two chloride ions must be exchanged in order to of the material. WP11 has the overall lowest material preserve electroneutrality in the material. The same concentrations but appears to be more selective than decrease in capacity is observed for all four materials. WP13. The maximum material concentration of WP13 This behaviour would not typically occur with materials was not reached in these experiments but is likely to where sorption occurs through chelation or adsorption, be similar to that of Smopex®-269 at approximately 1.2 as there is no exchanged ion and thus valency is not mmol g–1. important in determining material concentrations. The It must be pointed out that there is one critical selectivity and maximum material concentrations of omission from these results: the equilibrium liquid Smopex®-269 and WP13 are approximately equal for selenate. Contrasting Figures 6 and 7, it is clear that Smopex®-269 is no longer the material with highest 2.0 – 1.8 SeCN –1 1.6 1.0 1.4 SeO 2– 0.9 4 1.2 –1 0.8 1.0 0.7 0.8 ® 0.6 0.6 Smopex -103 Smopex®-269 0.5 0.4 Equilibrium material WP11 0.4 ®

concentration, mmol g 0.2 WP13 0.3 Smopex -103 0 Smopex®-269 0.2 0 0.5 1 1.5 2 2.5 3 Equilibrium material WP11 –1

concentration, mmol g 0.1 Equilibrium liquid concentration, mmol l WP13 0 Fig. 6. Selenocyanate sorption isotherm. ‘Equilibrium 0 0.5 1 1.5 2 2.5 3 material concentration’ is the concentration of the sorbed Equilibrium liquid concentration, mmol l–1 species on the material, expressed in mmol of species per – 2– gram of dry material. The initial SeCN concentration was Fig. 7. Selenate sorption isotherm. The initial SeO4 –1 approximately 6 mmol l–1 and volume of solution per sample concentration was approximately 3 mmol l and volume of was 200 ml solution per sample was 200 ml

340 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689694 Johnson Matthey Technol. Rev., 2015, 59, (4) selectivity, as it was for selenocyanate. It is believed ® that the sorption sites of Smopex -269 are hindered by 1.0 Smopex®-103, pH 11 2– bulkier molecular groups when compared to the other 0.9 SeO3 –1 ® 0.8 Smopex -103, pH 7 materials, which mitigates the sorption of the selenate ® 0.7 Smopex -103, pH 3 ion, while the smaller and linear shaped selenocyanate 0.6 ® ion is unaffected. Thus while Smopex -269 still exhibits 0.5 good recovery for divalent selenate ions, optimal use of 0.4 its particular functionality is achieved when monovalent 0.3 0.2 ions are recovered. Equilibrium material

concentration, mmol g 0.1 In Figure 8, WP13 has the highest selectivity but 0 Smopex®-103 has again the highest overall maximum 0 0.5 1 1.5 2 2.5 3 –1 material concentration. The relative selectivity of both Equilibrium liquid concentration, mmol l ® ® 2– Smopex -103 and Smopex -269 for sulfate over Fig. 9. Selenite sorption isotherm. The initial SeO3 selenocyanate has now decreased relative to the concentration was approximately 2.5 mmol l–1 silica-polymer materials, with Smopex®-269 having the overall lowest selectivity for sulfate. Since sulfate recovery is generally undesired, this lower selectivity measured at equilibrium, where they were found to is a beneficial property of Smopex® materials, as have not significantly varied. The results found matched selenium removal will be favoured in feeds containing expectations and ion exchange theory. Sorption both selenium and sulfate ions. Due to their design, performances of selenocyanate and selenate were not the functionality of both Smopex® materials have a affected by changes in pH within the range 3–10 (acidic higher ionic character than those of the silica-polymer conditions for selenocyanate were not investigated as materials, which in turn leads to a stronger sorption the ion decomposes under those conditions). On the interaction with the selenate, due to the selenate other hand, the performance for selenite was found having a higher charge density on its oxygen atoms to be highly influenced by the pH of the solution. The than sulfate. Note that the scale of the equilibrium liquid strong-base functionality on Smopex®-103 remains concentration axis is different in Figure 8 due to the unaltered in a broad pH range, and neither the lower molecular weight of sulfate over selenium and monovalent selenocyanate nor the divalent selenate thus overall the selectivity of all materials for sulfate ion change their protonation states in the pH range has decreased relative to selenium ions. investigated (conjugated bases of very strong acids: – In Figure 9 the effect of equilibrium pH on the pKa(HSeCN) < 1, pK1(H2SeO4) = –3.0, pK2(HSeO4 ) = sorption perfomance of selenite on Smopex®-103 is 1.7). On the other hand, selenous acid is a considerably depicted. The solution pH of each experimental trial weaker acid than the other species (pK1(H2SeO3) = 2.6, – was adjusted prior to the addition of material and then pK2(HSeO3 ) = 8.1). Thus, the optimum performance for the adsorption of selenite ions takes place at pH 10, where almost all selenium is in the form of the 1.0 2– ® 2– fully deprotonated SeO3 (ca. 99% of the total selenium Smopex -103 SO4 0.9 ® –1 Smopex -269 content). A selenite concentration on the material of 0.8 WP11 –1 0.7 0.7 mmol g was achieved at pH 10. Under neutral WP13 – 0.6 conditions, ca. 93% of the selenium present is HSeO3 2– 0.5 with the rest being in the form of SeO3 (ca. 7%). Thus, 0.4 a significant decrease of the selectivity is observed at 0.3 pH 7, as reflected in the shift to higher equilibrium liquid 0.2 Equilibrium material concentrations for the exchange isotherm, although

concentration, mmol g 0.1 0 maximum material concentration remains similar to that 0 3 6 9 12 15 at pH 10. A major decline in performance is observed Equilibrium liquid concentration, mmol l–1 under acidic conditions, with a further shift to higher 2– Fig. 8. Sulfate sorption isotherm. The initial SO4 equilibrium liquid concentrations and a significant concentration was approximately 15 mmol l–1 and volume of solution per sample was 25 ml reduction in material concentrations. At pH 3, the – major Se species is HSeO3 (ca. 70%), with significant

presence of fully protonated, selenous acid (H2SeO3, moving out of the material into solution by diffusion. 2– ca. 30%) and nothing of the fully deprotonated SeO3 . However, since electroneutrality with the fixed cations In these conditions, a moderate loading of 0.3 mmol g–1 in the material must be preserved, the chloride was measured, most likely originated by the uptake of ions are pulled back into the material by an electric – available monovalent HSeO3 ions. potential difference, the strength of which increases These results indicate that the Smopex®-103 material as the relative electric potential between material and 2– presents a greater selectivity for divalent SeO3 than solution increases (for example as solution dilution – for monovalent HSeO3 , and that it is incapable of increases). For a given electric potential difference, scavenging the neutral species H2SeO3, consistent the force with which it acts on an ion also increases with the ion exchange nature of the process. Different with ionic charge. Divalent selenate ions are thus sorption mechanisms are required to efficiently more strongly attracted by this electrical potential than remove selenous acid under acidic conditions, as the monovalent chloride ions and hence favoured by demonstrated by Awual et al. by using silica based the material. As dilution increases and the electric material functionalised with chromogenic Schiff bases potential force increases, this relative preference of the (30, 31). selenate over chloride ions increases further, resulting In Figures 10 and 11, the effect of initial selenium in the observed increase in ‘apparent selectivity’. The concentration in solution on the sorption isotherms is only suitable model for predicting this behaviour must shown. Again, in the case of adsorption or chelation, a thus include as parameters the selenium and chloride fixed isotherm would be expected with changing initial concentrations as well as the valency of the respective concentration. In the case of ion exchange, a varying ions and activity coefficients to account for the non- isotherm can be observed depending on the relative ideal sorption behaviour due to the Donnan potential ionic valency of the ions exchanged. In Figure 10, for the effects. case of selenocyanate and chloride exchange, sorption Figures 12 and 13 show the selenium sorption isotherms at both initial selenium concentrations isotherms for solutions having both selenocyanate and overlap, as the valency of each ionic species is one sulfate initially present in solution, for Smopex®-269 and hence the isotherm does not depend on initial and WP11 respectively. Only the results for these concentration. In Figure 11, the sorption isotherm materials are shown as Smopex®-269 was observed of the divalent selenate ion changes as a function of (Figure 7) to have the highest overall selectivity for initial selenate concentration, with ‘apparent selectivity’ selenocyanate and lowest for sulfate, while WP11 had increasing with decreasing initial concentration. the lowest selectivity for selenocyanate. In Figure 12 These effects are attributed to a phenomenon known the selenocyanate concentration on Smopex®-269 as the Donnan potential (18). The material initially is very selective over sulfate, with sulfate desorbing has a higher chloride anion concentration than the from the material as liquid concentrations increase. A surrounding solution which results in the chloride ions maximum 5:1 ratio of equilibrium concentrations for

2.0 SeCN– 1.0 1.8 2– –1 0.9 SeO4 1.6 –1 1.4 0.8 1.2 0.7 1.0 0.6 0.8 0.5 0.6 0.4 ® –1 0.4 Smopex -103, 0.25 mmol l 0.3

Equilibrium material ® –1 ® –1 0.2 Smopex -103, 0.10 mmol l Equilibrium material concentration, mmol g 0.2 Smopex -103, 6.0 mmol l

0 concentration, mmol g 0.1 Smopex®-103, 3.0 mmol l–1 0 0.5 1 1.5 2 2.5 3 0 Equilibrium liquid concentration, mmol l–1 0 0.5 1 1.5 2 2.5 3 Equilibrium liquid concentration, mmol l–1 Fig. 10. Selenocyanate sorption isotherm, two different initial Fig. 11. Selenate sorption isotherm, two different initial – solution concentrations. The initial SeCN concentrations solution concentrations. The initial SeO 2– concentrations –1 4 were approximately 0.25 and 6.0 mmol l were approximately 0.1 and 3 mmol l–1

1.6 0.6 ® – 2– ® 2– 2– Smopex -269, Equimolar SeCN /SO4 Smopex -103, Equimolar SeO4 /SO4 –1 1.4 –1 0.5 1.2 SeCN– 0.4 1.0 2– SO4 0.8 0.3 0.6 0.2 0.4 2– 0.1 SeO4 Equilibrium material 0.2 Equilibrium material 2–

concentration, mmol g concentration, mmol g SO4 0 0 0 0.5 1 1.5 2 0 0.5 1 1.5 2 –1 Equilibrium liquid concentration, mmol l–1 Equilibrium liquid concentration, mmol l

Fig. 12. Competitive sorption isotherms for selenocyanate Fig. 14. Competitive sorption isotherms for selenate and and sulfate on Smopex®-269, equimolar initial sulfate on Smopex®-103, equimolar initial concentrations. – 2– 2– 2– concentrations. The initial SeCN and SO4 concentrations The initial SeO4 and SO4 concentrations were were approximately 1.9 mmol l–1 approximately 1.9 mmol l–1

0.6 0.6 WP11, Equimolar SeO 2–/SO 2– WP11, Equimolar SeCN– /SO 2– 4 4 4 –1 0.5 –1 0.5 0.4 0.4 0.3 0.3 0.2 0.2 2– – 0.1 SeO4

SeCN Equilibrium material 0.1 2– Equilibrium material 2– concentration, mmol g SO4 SO4 concentration, mmol g 0 0 0 0.5 1 1.5 2 0 0.5 1 1.5 2 Equilibrium liquid concentration, mmol l–1 Equilibrium liquid concentration, mmol l–1 Fig. 13. Competitive sorption isotherms for selenocyanate Fig. 15. Competitive sorption isotherms for selenate and and sulfate on WP11, equimolar initial concentrations. The sulfate on WP11, equimolar initial concentrations. The initial – 2– 2– 2– initial SeCN and SO4 concentrations were approximately SeO4 and SO4 concentrations were approximately 1.9 mmol l–1 1.9 mmol l–1 selenocyanate:sulfate is achieved. A maximum 3:1 in selectivity for both ions. Smopex®-269 achieved ratio was achieved with Smopex®-103. By contrast, in an approximate 2:1 ratio, greater than Smopex®-103, Figure 13, there is no discernible difference in selectivity however the overall maximum material concentration is for selenocyanate versus sulfate on the WP11 material. still greater for Smopex®-103. As mentioned previously, The strong base Smopex® materials are thus well suited the higher ionic character of both Smopex® materials to selectively removing selenocyanate from solution in likely leads to a stronger interaction with the selenate. process streams containing both selenocyanate and sulfate. 4.2 Material Kinetics Figures 14 and 15 show the selenium sorption In Figure 16 the sorption of selenate kinetics for isotherms in solutions having both selenate and the different materials is shown. For all materials, sulfate initially present in equimolar concentrations, approximately 90% of the selenate concentration is for Smopex®-103 and WP11. In this case, there is transferred to the materials within ten minutes. For the still a significant selectivity of selenate over sulfate on two Smopex® materials, this material concentration Smopex®-103 (an approximate ratio of 1.5:1), which is reached within one minute. Typical ion exchange is interesting considering the fundamental similarities materials would not exhibit such fast kinetics, as they between these ions. WP11 again exhibits no difference are porous resins with most of the functionality located

1.0 16 ® 2– Smopex -103 0.9 SeO4 14 0.8 12 0.7 ® Smopex -103 0.6 Smopex®-269 –1 10 0.5 WP11 8 0.4

WP13 mmol l 0.3 6 – 0.2 4 SeCN 2– 0.1 SeO4

Liquid concentration, scaled 2 Column outlet concentration, 2– 0 SeO3 0 100 200 300 400 500 600 0 Time, s 0 1 2 3 4 5 6 Selenium fed to column, mmol Fig. 16. Kinetics of selenate sorption. Liquid concentrations are scaled relative to the initial liquid selenate Fig. 17. Smopex®-103 column outlet concentration profile 2– concentrations. The initial SeO4 concentration was versus selenium fed to the column for selenocyanate, –1 approximately 0.013 mmol l selenate and selenite feeds

within the material pores. The ions in solution must 1.2 thus diffuse slowly through the resins in order to reach SeCN– the functionality. In contrast, all of the functionality on 1.0 Smopex®-103 ® these silica-polymer resins and the Smopex® materials Smopex -269 0.8 WP11 is located on the particle surface and hence is much WP13 easier for the liquid ions to access. Only diffusion 0.6 TM

scaled Amberlite through the stagnant film surrounding the material DowexTM 0.4 particles needs to be considered in kinetic modelling of sorption for these materials. The Smopex® sorption 0.2 kinetics are believed to be so fast relative to the Column outlet concentration, 0 silica-polymer materials, due to the rod shaped 0 0.2 0.4 0.6 0.8 1 geometry of Smopex® fibres as well as their small Material concentration, scaled particle size, resulting in a generally higher surface Fig. 18. Column material concentration versus column outlet area per volume of material than the larger silica- concentration profile for a selenocyanate feed solution. The polymer resins. Similar trends in the rates of sorption material concentrations are scaled relative to the maximum were observed for selenocyanate and selenite. material concentrations. The column outlet concentrations are scaled relative to the feed concentrations. The feed 4.3 Continuous Flow Through Fixed Bed SeCN– concentration was 12.66 mmol l–1 for the Smopex® Columns and Dow materials and 0.66 mmol l–1 for the silica polymer composite materials In Figure 17 the column outlet concentrations of selenocyanate, selenate and selenite are shown as a function of the mass of selenium fed to each fixed polystyrene ion exchange resins, DowexTM 1x2 bed column of Smopex®-103. For all selenium species 100–200 and AmberliteTM IRA 900 is plotted using the column outlet concentration is 0 mmol l–1 then the alternative format. The maximum concentrations increases sharply to the feed selenium concentration achieved for the Smopex® and silica-polymer resins value of approximately 12 mmol l–1. Data from fixed bed are also in agreement with the values seen in the experiments is typically represented in this manner, sorption isotherms (Figure 6). An important factor however an alternative format has been adopted in the design of fixed bed ion exchange systems is in this study for ease of performance comparison the material utilisation factor, defined here as the between different material types, and will be used in all fraction of the maximum material concentration which subsequent column results figures. has been reached at the point where a detectable In Figures 18 and 19 the performance of Smopex®, concentration of the targeted ion for removal from silica-polymer resins and two commercially available solution (for example selenocyanate) appears in the

contact with the solution and decreases the internal 3.0 particle distance for porous diffusion, both likely to –1 SeCN– result in faster sorption kinetics and a smaller mass 2.5 transfer zone. The silica-polymer materials exhibit TM 2.0 similar utilisation factors to Dowex while having a much larger average particle size of 500 mm, reaffirming ® 1.5 Smopex -103 the idea that having the functionality on the surface of Smopex®-269 the material and hence exposed to the solution is very 1.0 WP11 WP13 beneficial to material performance in fixed beds. Larger 0.5 AmberliteTM particle sizes are also important for limiting the pressure TM Dowex drop across fixed beds in full-scale applications with Material concentration, mmol g 0 0 2 4 6 8 high feed flow rates. Selenium fed to column, mmol In Figures 20 and 21 the fixed bed performance Fig. 19. Column material fed concentrations versus charts are shown for a feed solution containing selenate selenium fed to the columns, for a selenocyanate feed ions. Again the Smopex® materials achieve the highest solution utilisation factors, approximately 0.9 for both materials, again with maximum values consistent with the sorption isotherms in Figure 7. In this case WP11 has the column outlet solution. For full-scale ion exchange lowest material concentrations and utilisation factor. processes, the drive is often to minimise column size The AmberliteTM material achieves the highest material while maintaining high (greater than 0.9) material concentrations but again a low utilisation factor. utilisation factors, as column size dictates material In Figures 22 and 23 the fixed bed performance for (i.e. capital) costs. In Figure 18 the material utilisation a feed containing both selenocyanate and sulfate, factors are approximately 0.75, 0.85 and 0.5 for the both present at a molar concentration ratio of 1 to 30, Smopex®, silica-polymer and DowexTM resin, and respectively is shown. This low selenocyanate:sulfate AmberliteTM materials respectively. Also considering ratio was selected in order to study sorption the maximum material concentrations shown in performance under realistic conditions, similar to those Figure 19, the Smopex®-103 is reaching the overall in industrial wastewaters having a highly competitive highest material concentrations before selenocyanate environment for selenium removal. Considering starts to appear in the column outlet. The WP11 this, low material concentrations of selenocyanate material achieves the lowest material concentration. would be expected due to the competitive sorption The AmberliteTM material presents the highest material concentration overall, but as mentioned above it also has the lowest utilisation factor. This reflects the low 1.2 2– selectivity and slow sorption kinetics this material likely SeO4 ® has for the selenocyanate species, resulting in a larger 1.0 Smopex -103 ® ‘mass transfer zone’ in the fixed bed, i.e. the length of Smopex -269 0.8 WP11 the bed over which exchange of selenocyanate ions WP13 TM 0.6 TM

from liquid to the material occurs. As Amberlite is scaled Amberlite DowexTM also a standard polystyrene type resin, most of the 0.4 functionality is likely located within the particle and thus the sorption rate decreases significantly as the material 0.2 Column outlet concentration, concentration increases since more time is required for 0 the selenium to diffuse into the particle to reach the 0 0.2 0.4 0.6 0.8 1 available functionality. Material concentration, scaled The DowexTM material exhibits a similar maximum Fig. 20. Column material concentration versus column outlet material concentration to Smopex®-269, with a higher concentration profile for a selenate feed solution. The feed 2– –1 ® utilisation factor. In the case of DowexTM this is believed SeO4 concentration was 12.66 mmol l for the Smopex and Dow materials and 0.66 mmol l–1 for the silica polymer to be predominantly due to its low particle size of 100 composite materials to 200 mm which increases material surface area in

1.6 2.0

–1 2– –1 – 2– SeO4 SeCN , (SO4 ) 1.4 1.8 1.6 1.2 1.4 1.0 1.2 0.8 ® 1.0 Smopex -103 Smopex®-103 ® 0.6 Smopex -269 0.8 Smopex®-269 WP11 0.6 WP11 0.4 WP13 WP13 TM 0.4 0.2 Amberlite AmberliteTM TM 0.2 Dowex DowexTM Material concentration, mmol g 0 Material concentration, mmol g 0 0 1 2 3 4 5 0 1 2 3 4 Selenium fed to column, mmol Selenium fed to column, mmol Fig. 21. Column material concentrations versus selenium Fig. 23. Column material concentrations versus selenium fed to the columns, for a selenate feed solution fed to the columns, for a mixed selenocyanate and sulfate feed solution

1.2 – 2– SeCN , (SO4 ) over sulfate, while it is also the material with the lowest 1.0 Smopex®-103 decrease in capacity, approximately 12%, when sulfate Smopex®-269 is introduced to the feed. The DowexTM, AmberliteTM 0.8 WP11 WP13 and Smopex®-103 materials all show an approximate TM 0.6 Amberlite TM 30% decrease in maximum material concentration scaled Dowex when sulfate is present in the feed. Overall, considering 0.4 the absolute maximum material concentrations and 0.2 utilisation factors, Smopex®-103 is the best material for Column outlet concentration, selenocyanate removal in this competitive feed solution 0 0 0.2 0.4 0.6 0.8 1 as it has the highest material concentration at the point Material concentration, scaled when selenium appears in the column outlet. At even higher sulfate to selenocyanate ratios however, it is Fig. 22. Column material concentration versus column outlet likely that the Smopex®-269 would perform better than concentration profile for a mixed selenocyanate and sulfate ® feed solution. The sulfate in brackets signifies that sulfate is Smopex -103 due to its higher relative selectivity of present in the feed but is not plotted in the figure. The feed selenocyanate over sulfate. – 2– –1 SeCN and SO4 concentrations were 6.3 mmol l and Another important factor to consider is the absolute 187 mmol l–1 respectively, for the Smopex® and Dow values of the selenocyanate and sulfate concentrations –1 –1 materials and 3.2 mmol l and 93.6 mmol l respectively, in the feed, where these experiments were carried out for the silica polymer composite materials with selenocyanate and sulfate feed concentrations of 6 and 200 mmol l–1, respectively. If the same ratio was maintained but absolute concentrations were of sulfate on the materials. The maximum material decreased, a different fixed bed performance would be concentrations of selenocyanate on Smopex®-103 and observed. This is again due to the nature of the ion Smopex®-269 are indeed lower than in the case of a exchange sorption mechanism, where the ‘apparent feed with selenocyanate only, but for Smopex®-103 the selectivity’ of the sulfate increases with decreasing maximum material concentration has only decreased to initial concentration due to the Donnan potential effect. approximately 1.2 mmol g–1 from the maximum observed Conversely, at even higher absolute concentrations, value of 1.8 mmol g–1 with only selenocyanate present higher selenocyanate sorption performance is to be in solution, while for Smopex®-269 the maximum expected due to lower ‘apparent selectivity’ of sulfate. material concentration is approximately 1.05 mmol g–1, To highlight the effect of this mechanism on fixed down from the maximum of 1.2 mmol g–1. Furthermore, bed performance, a further experiment was carried Smopex®-269 was shown in the sorption isotherms to out at the same selenocyanate to sulfate ratio but at have the highest relative selectivity for selenocyanate absolute feed concentrations of 0.013 mmol l–1 and

0.37 mmol l–1, respectively (equivalent to 1 mg l–1 of likely to perform better than the Smopex®-103 due to selenium and 12 mg l–1 of sulfate). These results are its greater relative selectivity for selenocyanate over shown in Figures 24 and 25. Due to the laborious sulfate. Choosing the best material for selenocyanate nature of these tests (more than 14 weeks of continuous sorption is thus a complex choice as it depends on flow being required to reach maximum material many factors, hence the correct choice of model to concentrations on only 1 g of material in the column), predict material performance at differing absolute feed only results for Smopex®-103 were obtained. concentrations is critical. Contrasting Figures 25 and 23, maximum material Also shown in Figures 24 and 25 is the column concentration of selenocyanate on Smopex®-103 for performance for a feed containing only selenocyanate, the feed with selenocyanate to sulfate concentration but at the lower feed concentration of 0.013 mmol l–1, ratio of 1:30 has decreased from approximately compared to 6 mmol l–1 used previously (Figure 22). 1.2 mmol g–1 to 0.35 mmol g–1. This is due to the A maximum material concentration of approximately increased ‘apparent selectivity’ of the sulfate at these 1.7 mmol g–1 was reached, very close to the observed lower feed concentrations. During the experiment, once maximum in the selenocyanate sorption isotherm this maximum material concentration for selenocyanate (Figure 6). was reached, the feed selenocyanate concentration Considering the frequently employed adsorption was increased such that the selenocyanate to sulfate models, for example Langmuir or Freundlich, the reader concentration ratio was decreased to 1:15. Maximum might be surprised that the approximate total exchange material concentration for selenocyanate subsequently capacity of the material was reached with such a low increased to approximately 0.45 mmol g–1. The feed selenocyanate concentration in the feed. According to selenocyanate concentration was further increased these sorption models, for a given feed concentration to lower the concentration ratio to 1:2.5. Maximum the corresponding material concentration, as given by material concentration subsequently increased to the sorption isotherm, is also the maximum material approximately 1.2 mmol g–1. Selenocyanate sorption concentration that should be attainable experimentally, versus sulfate has thus clearly been affected by the since equilibrium between the material and solution is absolute feed concentrations of each ion in the feed. reached at these conditions and no further ions can At the higher absolute feed concentrations, the be loaded without an increase in feed concentration. Smopex®-103 was the overall best choice of material for In the case of ion exchange however, the situation is selenocyanate sorption, however at the lower absolute different as there is also an ion from the material which feed concentrations, the Smopex®-269 material is is exchanged and released into solution. In the case of these particular experiments, the chloride released from

1.2 ® – 2– Smopex -103, SeCN , (SO4 ) 1.8 –1 ® 1.0 – 2– Smopex -103, SeCN , (SO4 ), 1:30 1.6 – 2– – 2– SeCN , (SO4 ), 1:15 SeCN , (SO4 ) – 2– 1.4 0.8 SeCN , (SO4 ), 1:2.5 – 2– SeCN , no SO4 1.2 0.6

scaled 1.0 0.4 0.8

0.6 – 2– 0.2 SeCN , (SO4 ), 1:30 0.4 – 2– Column outlet concentration, SeCN , (SO4 ), 1:15 – 2– SeCN , (SO4 ), 1:2.5 0 0.2 – 2– SeCN , no SO4 0 0.2 0.4 0.6 0.8 1 Material concentration, mmol g 0 Material concentration, scaled 0 1 2 3 4 Selenium fed to column, mmol Fig. 24. Column material concentration versus column outlet concentration profile for a mixed selenocyanate and sulfate Fig. 25. Column material concentrations versus selenium feed solution at low concentrations (0.013 mmol l–1 SeCN– fed to the columns, for a mixed selenocyanate and sulfate –1 2– –1 – and 0.37 mmol l SO4 ). Results are also shown for a feed feed solution at low concentrations (0.013 mmol l SeCN –1 2– solution containing only selenocyanate at low concentration and 0.37 mmol l SO4 ). Results are also shown for a feed (0.013 mmol l–1) solution containing only selenocyanate at low concentration

347 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689694 Johnson Matthey Technol. Rev., 2015, 59, (4) the material is also subsequently washed out of the fixed however this is only valid when the material is initially bed due to the feed flow through the fixed bed. Thus concentrated with chloride ions. When concentrated chloride concentrations inside the fixed bed are always with selenocyanate ions (having higher molecular decreasing and equilibrium is continuously pushed in weight than chloride ions) the mass of the material is favour of more chloride releasing into solution,which in subsequently increased which reduces the maximum turn requires selenocyanate to be exchanged into the material concentration per gram of dry material to material. In summary, provided there is no chloride in 1.6 mmol g–1. This value is in agreement with the the feed itself then in a fixed bed the selenocyanate maximum achieved selenocyanate desorption for material concentration should always reach the the experiment having feed chloride concentration highest maximum material concentration observed of 2.82 mol l–1, indicating complete desorption of experimentally in sorption isotherms, regardless of the the selenocyanate from the material. Even for the absolute feed selenocyanate concentration. experiment having fifty times less chloride in the feed, This phenomenon also highlights another point, that i.e. 0.06 mol l–1, a significant desorption of approximately if only chloride ions were present in the feed, then 1.0 mmol g–1 selenocyanate was achieved after 30 bed regardless of their concentration, complete desorption volumes of flow through the fixed bed. of loaded selenocyanate from the material should be Considering adsorption theory, such a significant achievable, even though the selenocyanate ion has a desorption of selenocyanate using a low concentration much greater selectivity for the material than chloride feed of a competitive ion such as chloride would not be ion. The results of fixed bed trials carried out to expected. The material concentration of chloride ions investigate this are shown in Figures 26 and 27. on the material would quickly reach equilibrium with Figures 26 and 27 both show that significant the low feed concentration, mitigating the exchange of selenocyanate desorption is achievable regardless further chloride ions into the material. In Figure 27, it of the feed chloride concentration. While not all can also be seen that the low concentration chloride the experiments were continued until a maximum feed is also the most effective at desorbing selenocyanate desorption was achieved, in all selenocyanate, per mass of chloride that has flowed experiments the selenocyanate desorption trend is still through the fixed bed, with efficiency decreasing as increasing when flow to the fixed bed was stopped and feed chloride concentration increases. This is believed heading towards an approximate desorption value of to be predominantly due to increases in the viscosity of 1.6 mmol g–1 selenocyanate. It was previously the feed solution as chloride concentration increases, mentioned that the total material capacity of Smopex®-103 mitigating the diffusion rate of chloride ions to the is approximately 1.8 mmol g–1 of dry material, surface of the Smopex® material, rather than any ion

1.8 –

–1 SeCN 1.6 1.8 –

–1 SeCN 1.4 1.6 1.2 1.4 1.0 0.06 mol l–1 Cl– 1.2 0.8 0.17 mol l–1 Cl– 1.0 0.06 mol l–1 Cl– 0.6 0.28 mol l–1 Cl– 0.8 0.17 mol l–1 Cl– 0.4 1.41 mol l–1 Cl– 0.6 0.28 mol l–1 Cl– 0.2 –1 – 0.4 –1 –

Selenium desorbed, mmol g 2.82 mol l Cl 1.41 mol l Cl 0 0.2 –1 –

Selenium desorbed, mmol g 2.82 mol l Cl 0 10 20 30 40 50 0 Material bed volumes of flow, ml ml–1 0 100 200 300 400 500 Fig. 26. Selenocyanate desorption versus material bed Mass of chloride flow through the column, mmol volumes of chloride solution flowed through the fixed bed, for varying concentrations of chloride in the feed solution. Fig. 27. Selenocyanate desorption versus mass of chloride A bed volume is defined as the volume flowed through the flowed through the fixed bed, for varying concentrations of column divided by the resin bed volume chloride in the feed solution

348 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689694 Johnson Matthey Technol. Rev., 2015, 59, (4) exchange related phenomena. A 2.82 mol l–1 chloride backbone polymer, a fixed bed of Smopex® can be solution has a viscosity of approximately 0.002 Pa s easily compressed and at high feed flow rates (for at 25ºC, compared to approximately 0.001 Pa s for example greater than 0.001 m s–1 superficial velocity) water at 25ºC, a two-fold increase. Finally, while the the pressure drop across a fixed bed can be significantly low concentration chloride feed was the most efficient higher than if a silica or polystyrene ion exchange were for selenocynate desorption, average selenocyanate used (often in excess of 5 bar per metre of fixed bed concentrations in the outlet stream are also lowest for length). For such applications, an alternative process that feed. If a concentrated stream of selenocyanate is configuration is required. In this study, the CSTR has desired from the desorption process, then the higher been explored as a viable process configuration for chloride feed concentrations are required. Smopex® materials. In contrast to fixed bed reactors, Selectivity of selenate over sulfate was also material utilisation factors in CSTR are generally lower, investigated in continuous flow experiments for since concentration gradients between the bulk liquid Smopex®-103, the silica polymer composite WP13 and the material surface are lower than in fixed beds. and the Dow resins. Under the conditions tested, Many CSTR connected in series are often required to with a selenium:sulfur molar ratio of 1:30 and initial achieve comparable material utilisation factors to fixed feed selenate concentration of 6.3 mol l–1, maximum beds. material concentrations did not exceed 0.12 mmol g–1 However, as will be shown from the trials carried (1 wt% Se) for any of the materials. Even though these out here, comparable material utilisation factors to results may seem very poor, a viable commercial fixed beds can be achieved using Smopex® and only process could be designed by implementing frequent two CSTR in series, due to the high selectivity and regeneration cycles of the material once this low exchange kinetics of Smopex® materials. maximum concentration is reached (32). In Figures 28 and 29 the experimentally measured

® and predicted reactor liquid and material concentrations 4.4 Process Implementation of Smopex are shown over the duration of the experiment using a Smopex® materials have been shown to outperform all single CSTR. The chloride liquid concentrations were other tested materials in terms of kinetics, selectivity only measured for the first 50 hours,in order to validate and maximum material concentrations. However due the predicted values. The experimental and model to the structure of Smopex®, where functionalised values are in agreement, likely due to the use of ion polymeric chains are attached to a relatively small exchange models. Adsorption models would not have

6 SeCN– 5

–1 2.0 –

SeCN 4 1.8 –1 SeCN– (expt.) – 1.6 SeCN– (pred.) 3 SeCN (pred.) – Cl– (expt.) 1.4 Cl (pred.) – mmol l 2 Cl (pred.) 1.2 1.0 1 0.8

Reactor liquid concentration, 0 0.6 0 50 100 150 200 250 0.4 Time, h 0.2 Fig. 28. Continuous stirred tank reactor liquid selenocyanate 0 Material concentration, mmol g 0 50 100 150 200 250 and chloride concentrations over time. Experimentally Time, h measured (expt.) and model predicted (pred.) values are ® shown. The volume of solution in the reactor was 0.64 l, the Fig. 29. Continuous stirred tank reactor Smopex -103 feed flow rate 100 ml h–1 and feed SeCN– concentration was selenocyanate and chloride material concentrations over 5.7 mmol l–1. The coefficient of determination (R2) value for time. Only predicted values are shown as experimental the predicted versus experimental SeCN– concentration was material concentrations are estimated values based on the calculated to be 0.97 liquid solution concentrations

349 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689694 Johnson Matthey Technol. Rev., 2015, 59, (4) included the effects of the chloride ion in this system and thus would not give an accurate prediction.

–1 2.0 SeCN– Comparing these results to Figure 22, a much 1.8 lower material utilisation of 0.25 has been reached, 1.6 with a material concentration of only 0.5 mmol g–1 at 1.4 the point in time when a significant concentration of 1.2 1.0 –1 selenocyanate (< 0.1 mmol l ) is present in the reactor 0.8 liquid solution (and hence the reactor outlet stream). 0.6 SeCN– (rct. 1) 0.4 SeCN– (rct. 2) As expected, the maximum material concentration of – ® –1 0.2 Cl (rct. 1) the Smopex -103 (i.e. 1.8 mmol g ) was eventually Cl– (rct. 2) 0 reached, as the exchanged chloride ions are again Material concentration, mmol g 0 50 100 150 200 250 continuously flowed out of the reactor, driving the Time, h equilibrium towards further selenocyanate exchange. Fig. 31. Continuous stirred tank reactor liquid selenocyanate Figures 30 and 31 show the predicted performance and chloride concentrations over time, predicted values for for the same system as Figures 28 and 29, but with two reactors in series (rct. 1 = Reactor 1; rct. 2 = Reactor 2) the inclusion of a second reactor in series with the same solution volume and mass of Smopex® as the first reactor. In this system, the outlet concentration CSTR process configurations, depending on the feed from the second reactor would exceed 0.1 mmol g–1 flow rates. at approximately 125 hours. By this time, the material ® concentration of Smopex -103 within the first reactor 5. Conclusions would reach approximately 1.8 mmol g–1. A material utilisation factor of approximately 1.0 could thus be In this study it has been shown that Smopex® reached for the Smopex® in the first reactor, with only a materials, i.e. functionalised polymeric chains attached single additional reactor required. Even for commercial to polypropylene fibre backbones, are very effective for fixed bed systems, two fixed beds in series (i.e. lead- the removal of selenocyanate, selenate and selenite lag operation) are commonly required and utilised to ions from solution. Their particular structure has been achieve such high utilisation factors. Thus predicted shown to give very fast rates of ion exchange when performance in this CSTR system is comparable to compared to resin bead ion exchangers (DowexTM fixed bed performance. 1x2 100–200 and AmberliteTM IRA 900 were also For the case of selenate or selenite, and competitive tested in fixed bed trials), comparable with resin beads removal of these ions versus sulfate, the ion exchange in the spherical diameter range of 100 to 200 mm. models can similarly be employed to engineer process Furthermore, since all of the material functionality is solutions for their removal, either in fixed beds or readily exposed to solution and not contained within restricted pore spaces (as is the case for typical ion exchange resin products), the material utilisation factors achieved in fixed bed trials regularly exceeded 6 0.9. In competitive selenocyanate/sulfate feeds, the SeCN– Smopex® materials were shown to outperform all other 5 materials.

–1 The sorption mechanisms by which these ions are SeCN– (rct. 1) – 3 SeCN (rct. 2) removed from solution were also studied. It was shown Cl– (rct. 1)

mmol l – that ion exchange clearly dominates the sorption 2 Cl (rct. 2) process. Knowledge of this mechanism was shown to 1 be very important in modelling and understanding how

Reactor liquid concentration, 0 changes in initial ion concentrations and ionic species 0 50 100 150 200 250 can affect the sorption performance. It was shown for Time, h example that, in contrast to typically employed models Fig. 30. Continuous stirred tank reactor liquid selenocyanate such as Langmuir, the relative selectivity of two ions and chloride concentrations over time, predicted values for such as selenocyanate and sulfate can change as a two reactors in series (rct. 1 = Reactor 1; rct. 2 = Reactor 2) function of their initial concentrations and that sorption

350 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689694 Johnson Matthey Technol. Rev., 2015, 59, (4) performance cannot be accurately predicted without References considering the chloride ion exchanged from the 1. European Chemicals Agency (ECHA) A. Selenium – material. It was also shown that maximum material Ecotoxicological Information: http://echa.europa.eu –1 concentrations (for example 1.8 mmol g for the case (Accessed on 1st October 2015) ® of Smopex -103 and selenocyanate) can be reached 2. “External Peer Review Draft Aquatic Life Ambient for feeds having selenium concentrations as low as 1 Water Quality Criterion for Selenium”, United States –1 –1 mg l (0.013 mmol l ), whereas such performance Environmental Protection Agency, USA, 2014, EPA- would rarely be seen when adsorption or chelation is 820-F-14-005 the sorption mechanism. 3. S. Santos, G. Ungureanu, R. Boaventura and The ion exchange models were then employed to C. Botelho, Sci. Total Environ., 2015, 521–522, 246 predict the performance of Smopex® materials in one 4. “Evaluation of Selenium Species in Flue Gas and two in-series continuous stirred tank reactors. The Desulfurization Waters”, Electric Power Research single reactor results were validated experimentally. It Institute, USA, 2009, Product ID: 1015586 was shown that due to their excellent selectivity and 5. H. L Reyes, S. García-Ruiz, B. G. Tonietto, J. M. kinetics, material utilisation factors approaching unity Godoy, J. I. García Alonso and A. Sanz-Medel, are achievable using only two reactors in series, i.e. J. Braz. Chem. Soc., 2009, 20, (10), 1878 full capacity of the material can be reached before 6. L. Ling, B. Pan and W.-x. Zhang, Water Res., 2015, outlet concentrations from the reactor system rise 71, 274 significantly. This level of performance is comparable 7. Y. H. Huang, P. K. Peddi, H. Zeng, C.-L. Tang and to fixed bed performance, where typically two fixed X. Teng, Water Sci. Technol., 2013, 67, (1), 16 beds in series (lead-lag operation) are employed in 8. E. Shah, P. Soni Hemant, 'An Improved Process for order to achieve such high material utilisation factors in Removal of Heavy Metals', Indian Patent Appl. 11/ commercial setups. MUM/2013, IPC: C10C003-02 Smopex® materials are currently undergoing 9. T. M. Pickett, Y. Ma, J. Sonstegard, J. S. Kain, D.-C. redevelopment so that their unique performance Vuong and D. B. Fraser, General Electric Co, ‘Selenium benefits are available without the pressure drop Removal using Chemical Oxidation and Biological limitations attributed to their current morphology. Reduction’, US Patent Appl. 2013/0,270,181 Additionally, other reactor configurations are also 10. L. A. Richards, B. S. Richards and A. I. Schäfer, J. under investigation to bring together the benefits Membrane Sci., 2011, 369, (1–2), 188 of the Smopex® materials with the ease of use and performance efficiency of fixed bed setups. 11. S. W. Colley, P. M. Kauppinen, M. A. Lincoln and J. Torroba, Johnson Matthey Plc, ‘Selenium Removal’, Acknowledgements World Appl. 2015/036,770 12. M. R. Awual, A. Jyo, S. A. El-Safty, M. Tamada and N. The authors want to thank S. Colley, P. Kauppinen and Seko, J. Hazard. Mater., 2011, 188, (1–3),164 J. Stevens for their support and useful comments and M. Lincoln for his assistance in some experiments. 13. M. R. Awual, S. Urata, A. Jyo, M. Tamada and A. Special thanks also to D. Scott and J. Clarke for Katakai, Water Res., 2008, 42, (3), 689 provision of crucial analytical services. 14. Johnson Matthey Advanced Ion Exchange: http://www.jmadvancedix.com/ (Accessed on 1st October 2015)

Johnson Matthey’s Advanced Ion eXchange (AIX) 15. J. Allen, M. Berlin, M. Hughes, E. Johnston, V. business is committed to providing simple, robust Kailasam, E. Rosenberg, T. Sardot, J. Wood and C. metal removal solutions for API purification. To allow Hart, Mater. Chem. Phys., 2011, 126, (3), 973 easier access to these products, the business is 16. V. J. Inglezakis and S. G. Poulopoulos, “Adsorption, introducing their ThioSep kit. If you are interested in Ion Exchange and Catalysis: Design of Operations targeting Palladium removal during API and HPAPI and Environmental Applications”, Elsevier BV, The Netherlands, 2006 manufacturing, we’d be keen to hear from you. Contact our scavenging team on: 17. “Perry’s Chemical Engineers’ Handbook”, 7th Edn., [email protected] or +44(0)1763254640 eds. R. H. Perry, D. W. Green and J. O. Maloney, The McGraw-Hill Companies, Inc, USA, 1999

18. F. Helfferich, “Ion Exchange”, Dover Publications Inc, 27. J. C. Crittenden, R. R. Trussell, D. W. Hand, K. J. Howe New York, USA, 1995 and G. Tchobanoglous, “MWH’s Water Treatment: 19. A. A. Hekmatzadeh, A. Karimi-Jashni, N. Talebbeydokhti Principles and Design”, 3rd Edn., John Wiley & Sons, and B. Kløve, Desalination, 2013, 326, 125 Inc, New Jersey, USA, 2012 20. T. R. Ferreira, C. B. Lopes, P. F. Lito, M. Otero, Z. Lin, 28. R. B. Garcia-Reyes and J. R. Rangel-Mendez, J. Rocha, E. Pereira, C. M. Silva and A. Duarte, Chem. Bioresource Technol., 2010, 101, (21), 8099 Eng. J., 2009, 147, (2–3), 173 21. I. C. Ostroski, M. A. S. D. Barros, E. A. Silva, J. H. 29. W. E. Stewart and M. Caracotsios, “Computer-Aided Dantas, P. A. Arroyo and O. C. M. Lima, J. Hazard. Modeling of Reactive Systems”, John Wiley & Sons, Mater., 2009, 161, (2–3), 1404 Inc, New Jersey, USA, 2008 22. I. C. Ostroski, C. E. Borba, E. A. Silva, P. A. Arroyo, 30. M. R. Awual, M. M. Hasan, T. Ihara and T. Yaita, R. Guirardello and M. A. S. D. Barros, J. Chem. Eng. Micropor. Mesopor. Mater., 2014, 197, 331 Data, 2011, 56, (3), 375 31. M. R. Awual, T. Yaita, S. Suzuki and H. Shiwaku, J. 23. Inamuddin and M. Luqman, “Ion Exchange Technology I: Theory and Materials”, Springer, The Netherlands, Hazard. Mater., 2015, 291, 111 2012 32. F. Mohammadi, P. Littlejohn, A. West and A. Hall, 24. C. E. Borba, G. H. F. Santos and E. A. Silva, Chem. ‘Selen-IXTM: Selenium Removal from Mining Affected Eng. J., 2012, 189–190, 49 Runoff Using Ion Exchange Based Technology’, 7th 25. L. A. Bromley, AIChE J., 1973, 19, (2), 313 International Symposium on Hydrometallurgy, Victoria, 26. G. M. Wilson, J. Am. Chem. Soc., 1964, 86, (2), 127 BC, Canada, 22nd–25th June, 2014

The Authors

Carl Mac Namara is a Process Engineer within the Johnson Matthey Water Technologies group, Chilton, UK. He obtained his MEng in Chemical Engineering from Cork Institute of Technology, Ireland, and an Engineering Doctorate from the University of Birmingham, UK. His doctorate and post-doctoral projects were based in Procter & Gamble’s Newcastle Innovation Centre, UK, where he carried out fundamental research on textile cleaning processes. His current role is focused on modelling and developing new water treatment technologies all the way from R&D through to commercial stages.

Javier Torroba is a Research Scientist at Johnson Matthey Technology Centre, Chilton, UK. He obtained his degree and doctorate in Chemistry from Universidad de La Rioja, Spain, and enjoyed post-doctoral stays at Universidad Complutense de Madrid, Spain, and University of York, UK, before joining Johnson Matthey. He developed his research work around the areas of Coordination Chemistry and Materials Science, in topics such as metal organic frameworks and organometallic liquid crystals. He now investigates the fundamentals of selectivity in sorption processes for water purification technologies.

Adam Deacon is a Research Technician at the Johnson Matthey Technology Centre. Before joining his current position, he obtained his degree in chemistry from Manchester Metropolitan University, UK. His interests and expertise include the synthesis of new sorbent materials and the underlying chelating properties of ligands. His work is now focused on the discovery and development of the next generation of products for water purification.

Johnson Matthey Highlights

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

EMISSION CONTROL TECHNOLOGIES FINE CHEMICALS

Increased NO2 Concentration in the Diesel Engine Functional Thin Film Coatings Incorporating Gold Exhaust for Improved Ag/Al2O3 Catalyst NH3-SCR Nanoparticles in a Transparent Conducting Fluorine Activity Doped Tin Oxide Matrix W. Wang, J. M. Herreros, A. Tsolakis and A. P. E. York, C. K. T. Chew, C. Salcianu, P. Bishop, C. J. Carmalt and Chem. Eng. J., 2015, 270, 582 I. P. Parkin, J. Mater. Chem. C, 2015, 3, (5), 1118

The authors investigate the creation of higher NO2 Gold nanoparticles (AuNPs) and F-doped SnO2 concentration and its performance in the Ag/Al2O3 composites were combined by layering, making unique catalyst for the SCR route of eliminating NOx at low fi lms which display interesting optoelectronic properties exhaust gas temperatures under real engine operation. such as high visible transparency and electrical The availability of NO2 concentration was increased conductivity. Aerosol assisted chemical vapour for the SCR route with: (a) adding various NH3 and H2 deposition (AACVD) was used to deposit both layers mixtures upstream of the SCR catalyst and/or (b) using onto heated glass substrates. The authors produced a Pt-based diesel oxidation catalyst (DOC) in front of and analysed four sets of fi lms: AuNPs, fl uorine-doped the Ag/Al2O3-SCR catalyst. H2 improves the production SnO2 (FTO), a layer of AuNPs on FTO and an FTO of NO2 on the Ag/Al2O3 catalyst therefore the “Fast- layer on AuNPs. Changing the precursor concentration SCR” like reaction is promoted by using the accessible could alter the sizes of the AuNPs. Layered Au:FTO NH3 primarily at low reaction temperature. The same composite fi lms were blue from the surface plasmon effect was shown by the integration of the DOC in front resonance of the AuNPs but demonstrate high of the Ag/Al2O3 as the NO2 availability was enhanced transparency in the visible region and are electrically for the SCR process. conducting. These are comparable to commercial FTO. The Effect of Pt:Pd Ratio on Light-Duty Diesel Oxidation Catalyst Performance: An Experimental and Modelling Study Au3+ J. Etheridge, T. Watling, A. Izzard and M. Paterson, SAE Int. J. Engines, 2015, 8, (3), 1283 Au(s) This article represents a section of a two-part investigation on the effect of Pt:Pd ratio at a constant total Pt+Pd loading of 120 g ft–3 on the catalytic activity C. K. T. Chew, C. Salcianu, P. Bishop, C. J. Carmalt of a DOC for light-duty operations. In this study a one- and I. P. Parkin, J. Mater. Chem. C, 2015, 3, (5), dimensional model able to estimate the effect of Pt:Pd 1118 (Reproduced by permission of The Royal Society of ratio on DOC activity was developed. This model was Chemistry) based on an earlier model and certain parameters are changed to take into consideration the variation in Pt:Pd ratio. A function to aid the interpolation to any Pt:Pd ratio was used to describe the difference in each kinetic A Convenient Palladium-Catalyzed Azaindole Synthesis parameter with ratio. The NEDC test data with optimised R. De Gasparo, P. Lustenberger, C. Mathes, T. Schlama, kinetic parameters was used to develop the model and to G. E. Veitch and J. J. M. Le Paih, Synlett, 2015, 26, (2), obtain the best fi t to measured data for each ratio. Good 197 estimates of post-catalyst CO, THC and NO2 emissions over the NEDC across the entire range of Pt:Pd ratios Azaindoles are diffi cult to access but of interest for were given by this model. promising pipeline drug candidates. A reaction cascade

353 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689685 Johnson Matthey Technol. Rev., 2015, 59, (4) involving enamine formation followed by intramolecular Optimal ADF STEM Imaging Parameters for Tilt-Robust Heck reaction was investigated as a possible route. Image Quantifi cation Palladium based catalyst systems and reaction systems K. E. MacArthur, A. J. D’Alfonso, D. Ozkaya, L. J. Allen were screened. The fi rst generation XPhos catalyst was and P. D. Nellist, Ultramicroscopy, 2015, 156, 1 selective for a single regioisomer. Ultimately a one-pot synthesis was devised which provides direct access to ADF STEM can be used to obtain useful qualitative data azaindoles from amino-halopyridines and ketones. about the atomic scale structure of materials including catalysts. The present study used the cross section NEW BUSINESS: FUEL CELLS approach, a statistical method of counting atoms with the advantage that it is not affected by image parameter Effect of Particle Size and Operating Conditions on errors. An fcc Pt nanocube was analysed in order to Pt3Co PEMFC Cathode Catalyst Durability demonstrate that small detector angles are helpful in M. Gummalla, S. C. Ball, D. A. Condit, S. Rasouli, K. avoiding problems caused by inaccurate tilt due to Yu, P. J. Ferreira, D. J. Myers and Z. Yang, Catalysts, rotation of the nanoparticle samples under the beam. 2015, 5, (2), 926 Optimised experimental parameters were devised and the balance between thermal diffuse scattering and The stability and performance of Pt catalysts has been elastic scattering is explained. proven to depend on particle size. To fi nd out the effect of alloying and particle size in alloy catalysts, Pt3Co catalysts with approximately the same Pt:Co:carbon NEW BUSINESS: WATER TECHNOLOGIES ratio and three different mean particle sizes (4.9 nm, Structure and Properties of Highly Selective and Active 8.1 nm, and 14.8 nm) were prepared by heat treatment. Advanced Ion Exchange (AIX) Materials A higher degree of ordering was found in the larger S. W. Colley, P. Kauppinen, J. Stevens and C. Mac particles. Systematic tests were carried out. The cathode Namara, Chim. Oggi, 2014, 32, (5), 72 based on 4.9 nm catalyst exhibited the highest initial electrochemical surface area (ECA) and mass activity, There is a substantial loss of precious metal while the cathode based on 8.1 nm catalyst showed catalysts from active pharmaceutical ingredient better initial performance at high currents. Accelerated (API) manufacturing processes into waste water performance loss testing using electrochemical decay streams. New composite materials for the recovery protocols showed similar trends to previous Pt studies and purifi cation of precious and base metals from with higher initial performance for smaller particles but API production, platinum group metals refi ning, higher durability for larger particles. Intermediate sized base metal mining and metal processing industries particles of ~8 nm provided the best balance of lifetime have been developed. These materials are created performance for Pt3Co catalysts. either by grafting active adsorption sites on the outer surface and large pores of silica or joining Performance Measurements and Modelling of the ORR polymeric chains of active adsorption sites to non- on Fuel Cell Electrocatalysts – the Modifi ed Double porous polymer fi bres. The new AIX materials and Trap Model conventional polystyrene resins are compared and M. Markiewicz, C. Zalitis and A. Kucernak, Electrochim. the benefi ts are discussed. Acta, 2015, doi: 10.1016/j.electacta.2015.04.066 Targeted Metal Purifi cation by Scavenging Results for the ORR in perchloric acid for ultra-low S. Phillips, Spec. Chem. Magazine, 2015, 35, (5), 12 loading Pt/C electrodes have been experimentally obtained for various ORR mechanisms which Transition metal catalysts used in any API were accomplished as a function of temperature manufacturing process must be reduced to an (280–330 K), oxygen partial pressure (0.01–1) and approved impurity limit in the fi nal product. Highly potential (0.3–1.0 V vs. RHE). The results confirm potent APIs such as kinase inhibitors for cancer the reaction exponent for oxygen of 1 ± 0.1 through treatment pose a specifi c problem due to the the potential range of 0.3–0.85 V vs. RHE and low dosage required and the fact that they are show that as the overpotential rises the surface synthesised using metal-catalysed aryl-aryl couplings becomes progressively blocked towards ORR. or A-X couplings. Scavengers can remove metals This was not taken into account in the double trap and overcome problems such as the length of time model therefore the present authors have created required, meeting minimum contamination levels, an alternative version to include the formation of avoiding product loss and solvent use. A large scale OOHad intermediates. At higher overpotentials the project to remove metal from APIs including kinase OOHad intermediates block the electrode and lead to inhibitors was carried out using Johnson Matthey’s a reduction in electrocatalyst performance compared patented Sealed Flow Cartridge System. The main to a Tafel type approximation. Hydrogen peroxide considerations were regulatory compliance (quality), can also be formed by these intermediates at high cost and time. Implementation was tested from lab to overpotentials and is poorly described by models. plant scale and reduced the time taken to recover the

354 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X689685 Johnson Matthey Technol. Rev., 2015, 59, (4) metal by a factor of 24, allowing the plant to reach its reactivity of the oxygen carriers must be understood. designed capacity. The authors have measured the redox reactivity

of CuO/Al2O3 and NiO/CaAl2O4 particles at high PRECIOUS METAL PRODUCTS: ADVANCED pressures in a pressurised high-temperature magnetic GLASS TECHNOLOGIES suspension balance. Pressure has an adverse effect A Combined Single Crystal Neutron/X-Ray Diffraction on the reactivity and this effect is kinetically controlled. and Solid-State Nuclear Magnetic Resonance Study of This may be caused by the decline in the number of oxygen vacancies at elevated pressures. The reactant the Hybrid Perovskites CH3NH3PbX3 (X = I, Br and Cl) T. Baikie, N. S. Barrow, Y. Fang, P. J. Keenan, P. R. gas fraction is an important parameter and may Slater, R. O. Piltz, M. Gutmann, S. G. Mhaisalkar and T. possibly be associated to the contest between various J. White, J. Mater. Chem. A, 2015, 3, (17), 9298 species for adsorption on the oxygen carrier surface. A kinetic model was proposed taking these effects into Hybrid perovskites such as methylammonium lead consideration. A particle model which acknowledges halide perovskites, CH NH PbX (X = I, Br and Cl), 3 3 3 diffusion limitations and kinetics was used to study are interesting as potential materials for photovoltaic these results on packed bed CLC applications with devices. Variable temperature 1H and 13C magic angle bigger oxygen carrier particles. It was concluded that spinning nuclear magnetic resonance (MAS-NMR) spectra were recorded for poly- and single crystalline at high pressure the diffusion limitation decreases due + to reduced reaction rates and a rise in diffusion fl uxes samples of the perovskites. The CH3NH3 units were found to undergo dynamic reorientation due to tumbling caused by Knudsen diffusion. of the organic component within the perovskite cage. Continuous Catalytic Upgrading of Ethanol to n-Butanol Only the amine end of the CH NH + group was shown 3 3 and >C Products Over Cu/CeO Catalysts in to interact with the inorganic network. Impedance 4 2 spectroscopy showed that the conductivity changes Supercritical CO2 signifi cantly at the phase transition temperature, with J. H. Earley, R. A. Bourne, M. J. Watson and M. Poliakoff, implications for the performance of the photovoltaic Green Chem., 2015, 17, (5), 3018 device at higher temperatures. The optical band-gaps n-Butanol (BuOH) has advantages over EtOH as a of each perovskite were determined using UV-visible biofuel as it can transported and used in a gasoline spectroscopy confi rming that they absorb strongly across the visible spectrum. engine with little or no modifi cation, has a higher energy content, lower water miscibility and better gasoline PROCESS TECHNOLOGIES compatibility. This paper uses a Cu-catalysed Guerbet reaction to investigate a more sustainable source of Reactivity of Oxygen Carriers for Chemical-Looping BuOH compared to the industrial OXO process, by Combustion in Packed Bed Reactors under Pressurized upgrading EtOH. Six Cu catalysts on different supports Conditions were prepared and tested. Supercritical CO2 was the H. P. Hamers, F. Gallucci, G. Williams, P. D. Cobden solvent and was used in a continuous fl ow reactor. and M. van Sint Annaland, Energy Fuels, 2015, 29, (4), The high surface area CeO2 support provided the best 2656 activity and gave over 30% yield and good selectivity.

In order to effectively design, scale-up and optimise Increasing CO2 pressure was found to improve the pressurised packed bed reactors for chemical-looping performance in this reaction possibly due to its effect combustion (CLC) the inﬂ uence of the pressure on the on the support’s redox cycle.

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