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

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

Contents Volume 60, Issue 2, April 2016

88 Guest Editorial: The Importance of Characterisation Techniques By Peter Ash 90 Surface Selective 1H and 27Al MAS NMR Observations of Strontium Oxide Doped γ-Alumina By Nathan S. Barrow, Andrew Scullard and Nicola Collis 98 Erratum: The Effects of Hot Isostatic Pressing of Platinum Alloy Castings on Mechanical Properties and Microstructures By Teresa Fryé 99 “New Trends in Cross-Coupling: Theory and Applications” A book review by Victor Snieckus 106 Frontiers in Environmental Catalysis A conference review by Djamela Bounechada 110 7th International Gold Conference A conference review by Nicoleta Muresan and Agnes Raj 117 The Use of Annular Dark-Field Scanning Transmission Electron Microscopy for Quantitative Characterisation By Katherine E. MacArthur 132 Applications of Neutron Scattering in Catalysis By Stewart F. Parker and Paul Collier 145 “Industrial Tomography: Systems and Applications” A book review by Hugh Stitt 148 “Ball Milling Towards Green Synthesis: Applications, Projects, Challenges” A book review by Maria Elena Rivas 151 “Hierarchical Nanostructures for Energy Devices” A book review by Patrick Daubinger 158 In the Lab: Synchrotron Radiation Based Solid Phase Characterisation of Industrial Catalysts and Materials Featuring Tim Hyde 161 Johnson Matthey Highlights 164 Towards 3D-Electrical Capacitance Tomography for Interface Detection By Peter J. Clark, Giuseppe Forte, Mark J. H. Simmons and E. Hugh Stitt http://dx.doi.org/10.1595/205651316X690385 Johnson Matthey Technol. Rev., 2016, 60, (2), 88–89 JOHNSON MATTHEY TECHNOLOGY REVIEW www.technology.matthey.com

Guest Editorial The Importance of Characterisation Techniques

We have previously described some of Johnson varies widely between practitioners. Reliable access Matthey’s core competencies in modelling (1) and the to material properties such as particle size distribution control of advanced materials at the atomic scale (2). at nanoscale is key to enabling empirical optimisation The third of these competencies, and a vital component and opens the potential for control of both size and of the company’s strategy to develop high performance distribution. solutions to its customers’ problems, is characterisation Within the third category, the delivery of insight requires of materials. an internal connection between a compositional or Materials characterisation is a huge and diverse structural property and the behaviour of a material. fi eld. One place perhaps to start is at the beginning, Hence most “in situ” and “operando” approaches sit and the fi rst principle to consider is the depth to which within this area. Taking the Pt on alumina model further, characterisation supports new materials discovery: observation of nanoparticle sintering in response to • Measuring a material’s property allows experimental changing temperature can be achieved with commercially improvement in the property; available equipment. Direct observation conducted with • Making unique measurements allows differentiation by a time capture component yields signifi cantly more improvement in unique areas; insight than empirical approaches. • Insight into compositional and structural origins of material There is therefore another level in the characterisation properties enables rationally designed improvement. hierarchy to pursue: Taking a model heterogeneous catalyst, of a metal • Making measurements that yield temporally limited supported on an oxide support (such as platinum structure (reactive intermediates), kinetic or energetic on alumina), hydrogen chemisorption allows for values during operando studies enables the in empirical exploration of preparation strategies leading silico modelling of materials and the prediction of to increased surface area. As well as the support it is performance based on proposed materials structure. essential to control Pt content between preparations to There is much fertile research ground to explore in the ensure commercial effi ciency, thus even for a relatively gap between current modelling capability (1) and a full simple material the number of measurements needed description of a material integrated across length scales. to achieve empirical success increases rapidly. The “unique” measurements are measurement Characterisation Driving Innovation capabilities developed for a bespoke solution to a particular question or performed with instrumentation The above pressures would tend to suggest that or facilities which have a high barrier to access. characterisation demand will continually drive towards Returning to the model material, the optimum metal the delivery of the ultimate, since the richness of the surface area available depends upon nanoparticulate information has a direct impact on the ease with Pt dispersed over the alumina surface. Transmission which materials discovery can be accomplished. The electron microscopy (TEM) coupled with automated predictable trade-off is with cost and time. image processing can count and describe particle size, Figure 1 describes the dilemma. At the pinnacle of shape and dispersion. TEM is not a unique technique, the triangle is the delivery of intellectual property (IP) in yet developed know-how in sample preparation, data materials development. Empiricism (discovery through collection, image processing and data interpretation systematic exploration of the preparation variables) is

88 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X690385 Johnson Matthey Technol. Rev., 2016, 60, (2) furthest away from IP and requires signifi cantly more Rewarding Collaborations (albeit lower complexity) characterisation support. It is no surprise that, as characterisation complexity We already see this happening. Increasingly, academic increases, so does the stimulation towards insight and and industrial researchers are either demanding or designed improvement, hence one moves closer to beginning to develop characterisation methodologies the delivery of IP with fewer measurements. But cost to support solutions to their problems. The market is increases and the availability or throughput drops. shifting to meet the arising need. Characterisation Not implicit in the understanding of this model is the development is a rewarding area for collaborations: the bias from which it is observed. One challenging thought intellectual challenge to extract predictive information is that “characterisation” has traditionally operated from materials is high, and has high impact, yet the as a service activity to the synthetic researcher: i.e. exploitation of insightful methods can be separated from people make materials, then analyse, activity test and commercial materials development through use of the try to work out what the results mean to formulate the next experiment. This “perspiration approach” requires simple yet realistic model systems which are needed to a huge bias in delivery of capability in order to drive drive the cutting edge. innovation. Generally speaking, more of the picture of One area where this is already yielding a rich a material is needed before the further stage can be collaboration is the rapidly developing partnerships formulated and hence the demand for measurement centred at national research facilities. In Johnson provision is exponential. The exponent is exacerbated Matthey’s case, the Harwell campus in the UK by materials complexity, which increases with the incorporating Diamond Light Source, ISIS the neutron need to achieve new performance requirements. The source and the Lasers for Science facilities all have risk is that a gap between capacity to make materials, capabilities to enhance characterisation development. and the capacity for characterisation, limits the rate of The UK Catalysis Hub consortium is also on campus improvement. and is developing researchers with the capacity to The alternative turns this on its head: model-led develop characterisation methodologies. Working characterisation demands materials to confi rm in this environment, providing rich problems and hypotheses, enabling predictive optimisation. Fewer collaboration support opens up great promise for the materials are synthesised yet insightful measurement development of future solutions to the four objectives stimulates the inventive step (the “inspiration approach”). for characterisation. More research support is committed to development of characterisation methods which are consistent with So here we should celebrate this issue based on insight delivery for complex materials. characterisation methods. The thread that binds them is the development of methodologies seeking to acquire information about increasingly complex systems, towards fi nding answers to critical problems for real-world applications.

References

1. D. Thompsett, Johnson Matthey Technol. Rev., 2015, 59, (2), 72 2. S. Coles, Johnson Matthey Technol. Rev., 2016, 60, (1), 2

PETER ASH Technology Manager, Research Support Services Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, Fig. 1. Innovation through characterisation RG4 9NH, UK Email: [email protected]

Surface Selective 1H and 27Al MAS NMR Observations of Strontium Oxide Doped γ-Alumina

Surface binding sites of strontium oxide on γ-alumina identiﬁ ed for the ﬁ rst time

By Nathan S. Barrow*, Andrew Scullard and can be determined, allowing an optimal level of SrO

Nicola Collis doping that can saturate all the AlO5 sites. Importantly, Johnson Matthey Technology Centre, this provides a method of subsequently depositing

Blounts Court, Sonning Common, Reading, catalytically active elements on just the AlO4 or AlO6 RG4 9NH, UK sites, which can provide a different catalytic activity or

stability compared to the AlO5 binding site. Email: [email protected] 1. Introduction

High-surface area -alumina is industrially used as Over 80 million tons of alumina are produced annually, a catalyst support. Catalytically active elements which is used industrially for the production of aluminium are doped onto the support, where they can bind to and in applications requiring high melting point,

AlO4, AlO5 or AlO6 sites on the surface. Pretreating excellent mechanical strength, electrical resistivity or the surface with alkaline earth oxides can alter the chemical inertness (1). There are several polymorphs availability of these surface sites, hence affecting the of alumina and the thermal transformations between catalytic activity. The surface binding sites of strontium them are well known (2). -Alumina in particular has a oxide (SrO) on -alumina were previously unknown. favourable combination of high surface area, pore size Direct 27Al magic angle spinning nuclear magnetic distribution and acid/base characteristics, which makes resonance (MAS NMR) could not detect AlO5 sites at -alumina a common industrial catalyst support (3). 9.4 T, so 1H–27Al cross-polarisation (CP) MAS NMR was Speciﬁ c applications of -alumina in catalysis include used to preferentially select the surface environment as a binder for zeolite catalysts in ﬂ uid catalytic signals. We directly observed the three surface cracking (4), as iron/copper/potassium (FeCuK) environments on dehydrated -alumina as a function support for Fischer-Tropsch catalysis (5) and as a of SrO impregnation up to 4 wt% SrO. We found that platinum support for dehydrogenation of jet fuel (6) or 2+ Sr preferentially binds to AlO5 and AlO6 surface sites. carbon monoxide oxidation (7). Three-way catalysts 1H MAS NMR revealed SrO impregnation causes a (TWC), which are widely used on gasoline engines to reduction in the terminal (−0.3 ppm) and bridging reduce CO, nitrogen oxides (NOx) and hydrocarbon (2.2 ppm) hydroxyl environments, as well as the emissions, include -alumina as part of the mixed oxide promotion of a new peak in between these sites, support because of its high surface area and relatively at 0.5 ppm. By using 1H–27Al CP/MAS NMR the good thermal stability under hydrothermal conditions relative proportions of surface sites on -alumina (8, 9). Barium oxide (BaO) and lanthana are commonly

90 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X690943 Johnson Matthey Technol. Rev., 2016, 60, (2) used as stabilisers for alumina in TWC. An additional are terminated by hydroxyl groups. Knözinger and advantage of adding alkaline earth oxides to -alumina Ratnasamy extended the earlier work of Peri who is the improvement of NOx storage for TWC (10, 11). identifi ed fi ve types of hydroxyl environment on a Thus, the investigation of SrO structure on the surface hydroxylated ideal -alumina surface (28, 29). This of -alumina is one of practical importance. The matter model was again extended by Tsyganenko and is complicated as despite the massive production and Mardilovich who added a sixth environment, reproduced industrial use of -alumina, the surface topography in Figure 1. (Note that in this article we refer to AlIV as of -alumina is still under debate (12). Nevertheless, AlO4). there is clear agreement that three main aluminium Many workers have reported 1H NMR spectra of coordination environments exist on the surface: AlO4, -alumina and attempted to assign peaks to specifi c AlO5 and AlO6. hydroxyl environments. As-received -alumina Solid-state nuclear magnetic resonance (SSNMR) of contains a signifi cant amount of physisorbed water alumina is informative as the aluminium environments throughout its porous structure, which overwhelms the 27 1 give distinct Al peak positions; AlO4 ≈ 60 ppm, H NMR signal from the surface hydroxyls. Thus, all 27 1 AlO5 ≈ 30 ppm and AlO6 ≈ 0 ppm. As Al is a quadrupolar H NMR experiments on -alumina must necessarily nucleus (13), the observed chemical shifts vary as a be performed after the sample has been heated function of magnetic fi eld strength, but remain distinct above 250ºC to remove water and consequently the at all high fi elds (14). Studies on -alumina have been -alumina will be partially dehydroxylated. This is reported for the past 25 years, both using direct 27Al signifi cant as penta-coordinated environments such NMR (15–17) as well as using 1H to 27Al CP NMR (12, as OH5, OH65 and OH665, will be present that require 18, 19). More recently, dynamic nuclear polarisation assignment despite not having a particular Knözinger (DNP) has been used to enhance the 27Al NMR signals and Ratnasamy type (30–32). One limitation of this 1H from the surface of -alumina, calculations have shown NMR technique is the requirement for drying, as the that the 27Al CP signal is from the very fi rst surface environments formed might not be present during, for layer or primostrato (20, 21). This is in agreement example, a subsequent incipient wetness treatment to with a deuteration experiment that concluded the CP load the support with a catalytically active species. signal does not come from interstitial or sub-surface Initial studies reported peaks of around −0.3 ppm, hydrogens (18). Some computational studies disagree 2.4 ppm, 4.0 ppm and higher, which were assigned to as the surface 27Al nuclei are reported to have a large the most basic OH group (type Ib), a more acidic group quadrupolar coupling, which broadens the 27Al signal (type II), the most acidic (type III) and fi nally hydrogen- and makes CP ineffective (16, 22). However, the models bonded protons, respectively (33, 34). Later studies used for these calculations did not account for surface appreciated the presence of AlO5 with assignments of rearrangement, which will favour less strained symmetric −0.2 ppm to −0.5 ppm: OH4, 1.5 ppm to 2.5 ppm: OH45, environments, as has been seen in alumina (23). Penta-coordinated aluminium environments, i.e.,

AlO5, exist on the surface of -alumina and can be observed using CP NMR (19, 21) or directly using H H H O O O high-ﬁ eld NMR measurements (24–27). Interestingly, little or no AlO5 was observed by DNP NMR despite AlIV AlVI AlVI AlIV being a surface enhanced measurement, presumably Type Ia, OH4 Type Ib, OH6 Type IIa, OH64 because the alumina was mixed with an aqueous solution that reacted with the AlO5 sites (20, 21). These AlO sites have been shown to be the anchoring point H H H 5 O O O for platinum oxide (26). Concerning alkaline earth oxide doping, AlO5 has a one-to-one correlation against the AlVI AlVI AlVI AlVI AlVI AlVI AlVI AlIV number of BaO molecules impregnated on to -alumina Type IIb, OH66 Type III, OH666 OH664 (27). Hence, the surface AlO5 sites are of great interest to the catalyst scientist. Fig. 1. Six surface hydroxyl environments that can exist on Although no hydrogen atoms exist in the bulk of -alumina, labelled by Knözinger and Ratnasamy type and Tsyganenko and Mardilovich OHn nomenclature -alumina, the surfaces ({111}, {110} and {100})

2.5 ppm to 4.0 ppm: OHnnn and higher shifted hydrogen- radio-frequency (RF) fi eld gives selective spin-locking bonded protons (12, 35, 36). These assignments are in of the central transition (m = ½ to −½), but under MAS, agreement with the earlier studies, clearly showing the lineshapes will be distorted (41, 42). Proton decoupling trend of increasing chemical shift going from terminal did not improve resolution and was not used. (μ1) to bridging (μ2) to μ3 OH environments. 2.1 27Al Nuclear Magnetic Resonance To date, there have been no reports on the surface aluminium environments of any alumina that has Directly acquired 27Al spectra of the bulk are given in been impregnated by SrO. The catalytic activity of a Figure 2. Peaks corresponding to AlO4 and AlO6 were -alumina support, doped with an active species, will be clearly visible, present in an approximately 1:2 ratio affected by strontium doping that causes the surface by integration, which does not account for spinning environments to rearrange. For example, by blocking sideband intensity. The contribution of surface AlO5 the AlO5 sites, an active species can anchor to a weaker environments was undetectably small against the aluminium binding site (AlO4/AlO6), thus changing the background bulk signals. After normalising by sample catalytic activity. Accurate understanding of alkaline mass, there was no signifi cant variation in the bulk earth oxides on the surface of -alumina must explain aluminium environments. This indicates that the changes in the aluminium coordination populations as SrO impregnation did not affect the structure of the well as 1H NMR assignments. The scope of this article -alumina nor form a strontium-aluminate phase. is to present novel NMR spectra on a strontium alumina Primostrato 27Al CP spectra are given in Figure 3. system and suggest possible surface environments Focusing initially on the undoped alumina; in contrast to explain those spectra. These hypotheses could be to the bulk spectra, the presence of AlO5 was clearly tested by future infrared measurements as well as ab visible. The ratio of AlO4 and AlO6 sites showed a initio calculations. preference for AlO6 sites at the surface of -alumina. Although CP NMR spectra are not quantitative, the 2. Experimental Results quadrupolar parameters of AlO4 and AlO6 were similar enough to allow for qualitative comparison. This is 1 wt%, 2 wt% and 4 wt% SrO on -alumina (Sasol) strong evidence that a surface rearrangement has samples were prepared by incipient wetness taken place, i.e., the surface structure is not the same impregnation using strontium nitrate (Alfa Aesar) as the bulk structure. dissolved in suffi cient water. Following addition of the The effects of SrO doping are also pronounced, nitrate solution to the alumina, the samples were dried with variation seen in all three types of aluminium at 105ºC before being calcined at 500ºC for two hours. environment. With the addition of 1% SrO there was

Powdered samples were dried under vacuum at 350ºC an enhancement of AlO4 signal, no change to the AlO5 overnight, before being packed in air-tight zirconia signal and a reduction in AlO6 signal. For the -alumina rotors under an inert atmosphere and were spun at with 2% SrO the AlO4 signal was enhanced even more, 14 kHz in a static magnetic ﬁ eld strength of 9.4 T. A widebore Bruker 4 mm broadband (BB)/1H MAS probe was used. Spectra were normalised by mass and Directdirect number of scans and all acquired on a Bruker Avance III console using TopSpin 3.1 software. All experiments used a 2 s recycle delay. 1H spectra were acquired (256 scans) with one rotor period spin echo experiment (37) and referenced to the low-ppm peak of adamantane-d16 at 1.73 ppm. Pulse lengths were 3 μs (90º) and 6 μs (180º) at νRF = 83 kHz. γγ-alumina-alumina 27 ++1% 1% SrO SrO Al spectra were referenced to yttrium-aluminium ++2% 2% SrO SrO garnet (YAG) AlO6 at 0.8 ppm (38). Direct acquisition +4% SrOSrO AlO AlO AlO (128 scans) used a 1.5 μs (<30º) pulse at 44 55 66 ν = 50 kHz. CP spectra (7168 scans) used a 1.5 ms 150 100 50 0 0 −50 –50 –100−100 RF δδ ( (2727Al),Al) / ppm contact pulse. 27Al ν = 12 kHz and 1H ν = 36 kHz to RF RF Fig. 2. Normalised bulk 27Al solid-state MAS NMR spectra satisfy the Hartmann-Hahn condition (39, 40). The low

at 2.2 ppm and HO-μ3 at 4.3 ppm. Hydrogen-bonded cross-polarisation Cross-polarisation environments also existed at higher chemical shifts, but in a lesser amount to the main environments. Systematically doping -alumina with SrO produced an additional peak at 0.5 ppm whilst simultaneously reducing the terminal and bridging hydroxyl environments at −0.3 ppm and 2.2 ppm, respectively. The hydrogen-bonded and HO-μ3 environments γγ-alumina-alumina ++1% 1% SrO SrO appeared unaffected by the SrO doping. ++2% 2% SrO SrO As there are several specifi c types of surface hydroxyl +4%+4% SrO group and the peaks appeared asymmetric, the spectra AlOAlO4 4 AlOAlO5 5 AlOAlO6 6 were deconvolved. At least seven peaks were required 150 100100 5050 00 –50−50 –100−100 27 δ δ( (27Al),Al) /ppm ppm to obtain a good fi t, although the lack of resolution Fig. 3. Normalised primostrato 27Al solid-state CP/MAS meant that such a fi t was ambiguous. Two of the fi tted NMR spectra spectra are shown in Figure 5. The purpose of the fi ts is to show that a full structural model of the surface of -alumina must account for at least seven hydroxyl whilst there was a reduction in AlO5 and AlO6 signals. environments. Also, the fi ts show approximately how This trend continued for the 4% SrO sample. the various environments change upon doping with Generally, a reduction in CP 27Al signal could be caused SrO. by several factors: (a) an actual physical reduction in Further experiments were performed to help assign that type of environment; (b) the removal of nearby the additional peak at 0.5 ppm. A strontium hydroxide hydrogen to that environment (as the magnetisation (Sr(OH)2) species was ruled out, as measurements is transferred to aluminium from hydrogen) or (c) a on this material gave much higher chemical shifts. distortion of the aluminium site symmetry causing Both X-ray photoelectron spectroscopy (XPS) and extreme quadrupolar broadening and rendering the chemical testing (addition of acid caused carbon signal ‘invisible’. The likelihood of these different factors dioxide evolution) indicated the presence of strontium occurring upon Sr doping will be discussed below. carbonate (SrCO3), which being energetically stable, is a likely surface species. 2.2 1H Nuclear Magnetic Resonance Hence, the 4% SrO -alumina sample was dosed with

The processed spectra are given in Figure 4. For the CO2 overnight to promote a carbonate environment. The undoped -alumina three main environments were resulting 1H NMR spectrum is given in Figure 6. A loss seen, in agreement with previous studies (12, 33, 35). These were assigned as HO-μ1 at −0.3 ppm, HO-μ2

0.5

++4% 4% SrOSrO μ2 2.2 μ1 –0.3−0.3

μ3 γγ-alumina-alumina μ γγ-alumina-alumina ++1% 1% SrO SrO 4.3 ++2% 2% SrO SrO 15 10 5 0 −5 −10 + 4% SrO 15 10 5 0 –5 –10 +4% SrO δδ ((1H)H), / ppm 15 10 5 0 0 −5 –5 −10–10 1 1 δ (11H)H), / ppmppm Fig. 5. H solid-state MAS NMR spectra deconvoluted with seven half Gaussian, half Lorentzian peaks. Vertical Fig. 4. Normalised 1H solid-state MAS NMR spectra dashed lines are a guide to the eye

bonded hydroxyls would increase coordination causing 0.5 ppm 0.5 ppm the signal to appear elsewhere, which has not been observed. Another possibility is that the presence of

H SrO promotes a structural rearrangement of higher O O coordination aluminium to AlO4. This would be the 0.80.8 ppm ppm C opposite of condensation reactions typically seen OOO O when dopants are added to alumina and a plausible Al AlIVIV mechanism has not been forthcoming (43). 5.2 ppmppm Two possibilities remain. One is that the addition of γγ-alumina-alumina ++4% 4% SrO SrO SrO causes AlO4 sites to become more symmetric. +4% SrO w/ CO + 4% SrO with CO2 2 This reduces their quadrupolar broadening giving rise to a more effective magnetisation transfer from 1H to 15 10 55 0 –5−5 –10 −10 1 27 δδ (( H)H), / ppm Al. Thus, previously ‘invisible’ AlO4 sites appear in Fig. 6. Normalised 1H solid-state MAS NMR spectra of the primostrato spectra when SrO is added. Further -alumina doped with 4% SrO experiments at multiple ﬁ elds or using multiple quantum magic angle spinning (MQMAS) could be performed to determine the quadrupolar parameters and test this of intensity for the HO-μ1 environment was observed possibility. The other possibility is that SrO, replacing and a new peak at 5.2 ppm was gained. This peak AlO5 and AlO6 hydroxyls, could then prohibit the fast corresponds to an aluminium carbonate OH signal exchange of protons across the surface, facilitating CP

(12). Clearly the presence of Sr did not inhibit the from the remaining protons on AlO4 sites (16). 1 adsorption of CO2 on -alumina but signiﬁ cantly, this Next we discuss the changes in the H spectra upon mechanism did not occur for atmospheric amounts of SrO doping shown in Figure 4. As mentioned above,

CO2. This suggests the 0.5 ppm peak was not related the peak at 0.5 ppm grew in proportion with the to carbonate species. quantity of SrO doped, which suggests these hydroxyls In conclusion, SrO doping systematically affected are proximate to some Sr compound. Additional the proton environments on -alumina, modifying the experiments have ruled out the assignment of Sr(OH)2 1 2 previously identifi ed HO-μ and HO-μ hydroxyls and or SrCO3. Furthermore, the chemical shift is in between producing a new peak at 0.5 ppm. Although this new the typical terminal and bridging environments. environment has not been identifi ed, the correlation Combined with the unchanging environments above with SrO doping suggests the hydroxyls are proximate 4 ppm, these fi ndings lead us to conclude that the peak to some strontium compound, which will be discussed at 0.5 ppm is from a shifted OHn or OHnn site. below. As the most reactive/electronegative OH5, OH6, OH56 and OH66 environments have most likely been replaced 3. Discussion by SrO, these are probably not an accurate assignment for the 0.5 ppm peak. The presence of an OH44 We fi rst discuss the changes in the 27Al CP/MAS NMR environment is generally energetically unfavourable, so spectra upon SrO doping shown in Figure 3. The this too can be ruled out. This leaves us with modifi ed addition of SrO appears to preferentially replace the OH4 and/or OH46 environments. hydroxyls located at AlO5 and AlO6 sites. This preference One issue with assignment is that the Sr form is 2+ 2+ could be because of the energetic favourability of Sr currently unknown. SrCO3, SrO clusters or Sr ions towards the more electronegative AlO5 and AlO6 sites. are possibilities. These three particular possibilities are Consequently, the reduction in CP signal from the shown in Figure 7, next to an OH4 environment. As the

AlO5 and AlO6 sites is simply because of a reduction in deconvoluted proton spectrum in Figure 5 suggested nearby protons that the magnetisation would otherwise just one environment for the peak at 0.5 ppm, the be transferred from. Whether Sr is present as isolated likelihood is that just one of these mechanisms was atoms or in SrO clusters is unclear from the 27Al NMR. present in our system. 4 An increase in AlO4 CP signal upon SrO doping is Further evidence that favours the assignment of OH harder to explain. One possibility for enhanced signal comes from the CO2 dosing results shown in Figure 6. is to introduce additional hydroxyls. However, directly As it has been shown that CO2 preferentially goes

5. Acknowledgements SrO H H H H H We would like to thank our colleagues for their O O O O O O assistance during this investigation: Richard Smith and Al Al Al Al O O O Tuğçe Eralp Erden (XPS), Peter Ellis (CO2 dosing), O O CC – Jonathan Bradley (NMR), Janet Fisher (discussions). OOOO H H H Sr++ As well as Peter Ash, Sue Ellis and Dave Thompsett for O O O O O O facilitating this work. We appreciate the comments of Al Al Al Al three anonymous reviewers who have greatly improved O O this manuscript.

H H H H Sr H H O Sr O O O O Al Al Al Al Al Al References O O O O 1. K. Wefers and C. Misra, “Oxides and Hydroxides of Fig. 7. Potential mechanisms whereby an OH4 is modifi ed Aluminum”, Alcoa Technical Paper No. 19, Aluminum by proximity to a SrO cluster, a strontium carbonate species Company of America, USA, 1987 and a Sr ion 2. G. Busca, ‘Structural, Surface, and Catalytic Properties of Aluminas’ in “Advances in Catalysis”, ed. F. C. Jentoft, Volume 57, Academic Press, USA, 2014, pp. 319–404 to the OH4 sites (12), the dramatic intensity loss of the 0.5 ppm peak strongly suggests this was a OH4 3. M. Trueba and S. P. Trasatti, Eur. J. Inorg. Chem., 2005, (17), 3393 environment. 4. J. S. J. Hargreaves and A. L. Munnoch, Catal. Sci. Technol., 2013, 3, (5), 1165 4. Conclusion 5. K. Keyvanloo, J. B. Horton, W. C. Hecker and M. D. The need for further research into the nature of the Argyle, Catal. Sci. Technol., 2014, 4, (12), 4289 Sr environment and more comprehensive 1H peak 6. C. Lucarelli, S. Albonetti, A. Vaccari, C. Resini, G. assignments, is clearly indicated. This could be achieved Taillades, J. Roziere, K.-E. Liew, A. Ohnesorge, C. through higher resolution 1H NMR experiments as well Wolff, I. Gabellini and D. Wails, Catal. Today, 2011, 175, (1), 504 as 1H–27Al correlation experiments. NMR data can be combined with infrared measurements to probe 7. V. Novák, P. Kočí, M. Marek, F. Štěpánek, P. Blanco- the hydroxyls. Raman microscopy and transmission García and G. Jones, Catal. Today, 2012, 188, (1), 62 electron mi croscopy (TEM) may also be used to 8. H.-Y. Chen and H.-L. Chang, Johnson Matthey elucidate the Sr environment. Ideally, the experimental Technol. Rev., 2015, 59, (1), 64 data can all be used to create accurate surface models 9. J. Kašpar, P. Fornasiero and N. Hickey, Catal. Today, for density functional theory calculations to further 2003, 77, (4), 419 explain the mechanisms at work in this system. 10. C.-W. Yi, J. H. Kwak, C. H. F. Peden, C. Wang and J. In conclusion, 1H–27Al CP MAS NMR has shown Szanyi, J. Phys. Chem. C, 2007, 111, (41), 14942 that doping -alumina with SrO covers AlO5 and alters 11. C. Verrier, J. H. Kwak, D. H. Kim, C. H. F. Peden and 1 the surface AlO4:AlO6 ratio. H MAS NMR revealed J. Szanyi, Catal. Today, 2008, 136, (1–2), 121 SrO impregnation caused a loss or shifting, of the 12. M. Taoufi k, K. C. Szeto, N. Merle, I. D. Rosal, L. Maron, terminal and bridging hydroxyl environments, with a J. Trébosc, G. Tricot, R. M. Gauvin and L. Delevoye, new peak at 0.5 ppm hypothesised to be a modifi ed Chem. Eur. J., 2014, 20, (14), 4038 OH4 environment. These changes to surface chemistry 13. A. J. Vega, ‘Quadrupolar Nuclei in Solids’, in will strongly affect where catalytically active metals are “eMagRes”, ed. R. E. Wasylishen, John Wiley & Sons, subsequently deposited. Such tunability for a common Inc, Hoboken, New Jersey, USA, 2010 industrial catalyst support, like -alumina, is of great 14. M. E. Smith and E. R. H. van Eck, Prog. Nucl. Magn. interest to the catalyst scientist. Reson. Spectrosc., 1999, 34, (2), 159

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The Authors

Nathan Barrow is currently a Principal Scientist in the Advanced Characterisation department at the Johnson Matthey Technology Centre, Sonning Common, UK. He graduated with an MPhys in 2006 from the University of Warwick, UK, where he remained to gain a PhD in SSNMR. In 2010 Barrow was a Knowledge Transfer Partnership associate between the University of Warwick and Johnson Matthey, helping to install and run an SSNMR service. His current research focuses on applying advanced characterisation to materials such as zeolites, alumina, glasses and batteries.

Andrew Scullard joined Johnson Matthey in 2006 upon graduation from the University of Surrey, UK, where he studied Chemistry with Business. As part of this course he spent a year working in the pharmaceuticals industry for Eli Lilly. Since joining Johnson Matthey he has spent the majority of his time in the Low Carbon Technology Group working on customer funded and EU funded projects focused on greenhouse gas abatement, reforming and sulfur removal.

After graduating from the University of Southampton, UK, Nicola Collis joined Johnson Matthey working on catalyst preparation for fuel cells where she looked at generating robust metal alloy particles for proton exchange membrane fuel cell applications. Nicola subsequently transferred to emission control research where she specialises in metal solution chemistry and support surface chemistry, focusing on how to design catalyst particles and control metal-support interactions during catalyst preparation.

Erratum The Effects of Hot Isostatic Pressing of Platinum Alloy Castings on Mechanical Properties and Microstructures

It has come to our attention that an image in a recent article published in this journal (1) was incorrectly attributed. The image caption should read as follows. The authors sincerely apologise for the error in attribution.

TERESA FRYÉ TechForm Advanced Casting Technology, 5558-D SE International Way, Portland, Oregon 97222, USA Email: [email protected]

Reference 1. 1 T. Fryé, J. Tunick Strauss, J. Fischer-Bühner and U. E. Klotz, Johnson Matthey Technol. Rev., 2015, 59, Fig. 3. HIP Unit, courtesy ABRA Fluid AG, Switzerland (3), 207

“New Trends in Cross-Coupling: Theory and Applications”

Edited by Thomas Colacot (Johnson Matthey Catalysis and Chiral Technologies, USA), RSC Catalysis Series, No. 21, Royal Society of Chemistry, Cambridge, UK, 2015, 864 pages, ISBN: 978-1-84973-896-5, £175.00, US$290.00, €218.75

Reviewed by Victor Snieckus Individual authors teach the reader about the breadth Department of Chemistry, Queen’s University, of the cross-coupling landscape, the still incomplete Kingston, ON, Canada, K7L 3N6 mechanistic knowledge and the important practical characteristics of new areas: the rapid acceptance of Email: [email protected] continuous fl ow reactions, the increasing attempts to be ‘green’ and the requirements of metal detection in active pharmaceutical ingredients (APIs), the latter, a This massive volume with a most appropriate title will humbling read for the academic researcher. lead the user to say: ‘I read, I understand modern trends, The authors ask the reader to linger on mechanistic I will apply’. After perusing it, the reader is encouraged aspects before leaping into the less cerebral-requiring to give a quiz to their students or colleagues: what synthetic data. The present reviewer ponders that since is the largest scale Heck reaction to date? Why is a the advent of transition metal reactions, revision of reaction in water not a fully ‘green’ process? What are retrosynthetic strategy as taught by Corey and others is the problems of scale up fl ow reactions? How much required. As suggested by the Editor, this compendium palladium may there be in the over-the-counter drug will ‘rejuvenate the entire area of cross-coupling’; it ibuprofen? What are the major caveats for a chemist may be argued that ‘re’ is not required – the area is still using transition metal catalysis? among many other in its rapidly developing infancy. questions. Colacot has gathered thirteen academic and industrial Review of Fundamentals chemists with their respective coauthors who are at the forefront of the various reactions that comprise Following historically insightful brushstrokes on ‘how the cross-coupling theme to serve as a cornucopia of we got here’ in the foreword by Professor Barry Trost fundamental and practical knowledge. With forewords (Stanford University, USA) and in the preface by by three chemists, prominent in their own right in the Tom Colacot, the fundamentals of the broad canvas fi eld, this collection of chapters will bring up-to-date of cross-coupling are described. In Chapter 1, Carin all those interested in teaching and practicing the Johasson Seechurn (Johnson Matthey Catalysis and chemistry which has largely superseded the classical Chiral Technologies, UK), Andrew DeAngelis and reactions of the pre-1970 era. Tom Colacot (Johnson Matthey Catalysis and Chiral Although multi-authored, a uniformity of presentation Technologies, USA) defi ne the basic terminology, prevails, perhaps owing to the fact that all chapters have such as turnover number (TON) and valence bond been independently reviewed by two to three experts. picture, sketch the discovery tree with due recognition,

99 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X690925 Johnson Matthey Technol. Rev., 2016, 60, (2) including names of some widely unknown chemists, these reactions. The chapter concludes with valuable and set the stage for both industrial and academic tables of recommended precatalysts and their use in chemists’ palates to view the recent state (mainly specifi c cross-couplings. the last fi ve years) of transition metal-catalysed cross-coupling reactions and to taste the next courses Ligands of surprises in the near future. Chapter 2 by DeAngelis and Colacot serves In the hefty Chapter 4, Anthony Chartoire (Econic as a perfect primer for the rapid advances that Technologies, UK) and Steven Nolan (University have occurred in the last decade in the design of St Andrews, UK) focus on the NHCs, the new kid and development of ligands that have reached on the block in ligands. Appreciation is gained of the sophisticated levels of application in catalytic infl uence of pKa, sterics and electronic effects on cross-coupling reactions where Pd is king. In NHCs and a journey is provided of the cross-coupling agreement with Crabtree (1), who perceptually taught name reactions for which they are effective and in his textbook that ‘to pick suitable spectator ligands’ those as yet little studied (Kumada-Corriu, Hayashi, is the key to the art, the reader learns about steric, copper-free Sonogashira, discovered by Heck, electronic effects and bite angle consequences (a C–S coupling, α-arylation). For informed chemists, table of 29 ligands) and is then taken on a tour (for conspicuously absent so far in the literature are also example in industrial context, Figure 1) on useful the C–O coupling and the asymmetric Suzuki-Miyaura modern ligands: bulky Ar3P, chelating bisphosphines, reactions. biaryl monophosphines, N-heterocyclic carbenes In Chapter 5, Mark Stradiotto (Dalhousie University, (NHCs) and chiral ligands. Copious examples in C–C but Canada) summarises the impact of sterically also C–N and C–O coupling reinforce the schematically demanding phosphine ancillary ligands on achievement explained, mechanistically signifi cant points. of monoarylation in Pd-catalysed Buchwald-Hartwig Chapter 3 by Carin Johasson Seechurn, Hongbo Li amination and α-arylation reactions. The long-sought + and Tom Colacot, the last chapter of collaboration from NH2 equivalent in anionic aromatic chemistry is the very able Johnson Matthey chemists, authoritatively overcome by the amination reaction which also instructs in the rapidly emerging evolution of preformed supercedes the classical electrophilic aromatic

Pd catalysts (isolated, pure LnPdX ligands). A refresher substitution (SEAr) nitration–reduction route to course on the 18 electron rule, with examples and arylamines. Thus monoarylation of ammonia is especially exceptions is followed by a section on complex useful in the synthesis of bioactive heterocycles and that of stability which is photographically illustrated by the hydrazine gives a new twist to the Fischer indole reaction. decomposition of ‘tetrakis’ with time, events very The completely chemoselective synthesis of anilines, familiar in practicing laboratories. Mechanistic studies, bearing primary and secondary amines, through use still incomplete, promise to play a crucial part in the of Stradiotto’s heterobidentate ligands cannot be easily provision of improved precatalyst procedures on large achieved by classical methods. A useful feature of this scale that will undoubtedly help process economics of chapter is the brief outlines for each topic discussed.

Br Pd(AmPhos)2Cl2 0.1 mol% CO2H N Me N K3PO4 N N N N F O + F O Me Iso-propanol/H O, 80ºC Me (HO) B 2 F 2 CO2H 93%, 2.05 kg scale F

Fig. 1. Kilogram-scale Suzuki-Miyaura reaction using Pd(AmPhos)2Cl2 (Reproduced by permission of The Royal Society of Chemistry)

C–O and C–S, Suzuki-Miyaura Cross-Coupling numerous examples of synthetic utility (indoles via and Carbohalogenation gem-dihalo olefi n coupling stand out, Scheme I), including aspects of selective coupling of In Chapter 6, James Stambuli (Ohio State University, polyhalogenated systems, containing Handy’s USA) discusses C–O and C–S bond formation predictive guide, and density functional theory (DFT) reactions, areas that have progressed less than the corroboration of the experimental results. The chapter corresponding C–C and C–N coupling processes. As shows a fl ourish of considerable new synthetic potential. gleaned also from other chapters, copper catalysis The brief but instructive Chapter 8 by Alastair Lennox is making an impact on the C–O and C–S coupling and Guy Lloyd-Jones (University of Edinburgh, reactions. Best choices of ligands are defi ned, including UK) is the present best source for the mechanistic those that prevent thiolate attack on the Pd centre and understanding of the Suzuki-Miyaura cross-coupling, most recent replacements for Pd (manganese, cobalt, named the ‘gold standard’ for biaryl construction, nickel and rhodium) are noted. From this chapter, we which can be appreciated by the given examples appreciate that a long road has been travelled to give of ‘blockbuster’ drugs whose synthesis critically improvements and alternatives to the nucleophilic incorporates this reaction. A survey of 40,000 reactions aromatic substitution (SNAr) and ancient Ullmann in the preparation of this review is a sound basis for procedures. discussion of mechanistic aspects for which evidence Chapter 7 by David Petrone, Christine Le, Stephen is currently weak or not available, for example, for Newman and Mark Lautens (University of Toronto, the transmetalation step. The requirement for boron Canada) provides a detailed view of the C-halogen preactivation and the practical aspects of various boron reductive elimination process from transition coupling reagents, including the recently introduced metal complexes and then delves deeply into the and promising triisopropylborate and cyclic triol salts carbohalogenation reaction. Surprisingly, the reductive are features also to be appreciated. The categorisation elimination to form new or reform existing C-halogen of Suzuki-Miyaura couplings into three groups is bonds is a poor cousin to the equivalent oxidative tentative but practically useful. addition in terms of synthetic use and its mechanism is incompletely understood and controversial. The review Mizoroki-Heck and Beyond focuses on the Pd(0/II) species in these transformations as a background for the carbohalogenation reaction Chapter 9 (>100 pp) by Irina Beletskaya and Andrei to which Lautens’ group has made substantial Cheprakov (Moscow State University, Russia) is a contributions. Provided is a scheme displaying masterful, well organised and defi nitive review of the commendably the mechanistic divergences between Heck reaction that the authors use in a generic sense the Heck and the new carbohalogenation process, to cover the family of reactions, the main one of which is

NH2 Pd(OAc) (1 mol%), SPhos (2 mol%) Br 2 K3PO4·H2O (5 equiv.), PhB(OH)2 Ph N Br PhMe, 90ºC, 6 h H

Oxidative Intermolecular addition PhB(OH)2 Suzuki reaction

Br Intramolecular Oxidative C–N coupling addition Br [Pd] [Pd] Br N N Br NH2 H H

Scheme I. Synthesis of 2-substituted indoles via Pd-catalysed tandem C–N/C–C coupling of gem-dibromooleﬁ ns (Reproduced by permission of The Royal Society of Chemistry)

101 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X690925 Johnson Matthey Technol. Rev., 2016, 60, (2) of course the Mizoroki-Heck reaction (~50 pp). (For an useful intramolecular, including heterocyclic, versions. opinion of the value of the Heck reaction see Fuhrhop In spite of a partial mechanistic understanding, all of and Li: these methods provide different retrosynthetic thinking, especially for the construction of alkynones by the “Synthesis: the preparation of large molecules classical Grignard and Friedel-Crafts reactions. A litany from smaller ones by the formation of C-C bonds. of new reactions is schematically indicated with the The Wittig, Heck, and Diels-Alder reactions are following highlights: the use of aryldiazonium coupling all the methods a beginner needs to know. Most partners, the preparation of 11C and 13C labelled ketones compounds can be made by these reactions if (although lacking examples of industrial reactions), they are followed by intelligent functional group and the advantages of NHC complexes. Perhaps most interconversions (FGIs)” (2)). signifi cant are the results from the rapidly evolving C–H After a historical account, rightfully acknowledging activation-carbonylation process whose application chemists along the path to Heck’s discovery, the to the synthesis of substituted and heteroannulated diversity of this dominant C–C bond forming reaction aromatics will certainly see further advances. is unfolded, always by reference to mechanism Chapter 11 by Ben Glasspoole (Sigma-Aldrich, USA), which the authors are eminently qualifi ed to Eric Keske and Cathleen Crudden (Queen’s University, analyse. The differences between the Heck, general Canada) address work on the Suzuki-Miyaura cross-coupling, and C–H activation reactions is such cross-coupling reaction from perspectives of: an area of mechanistic ambiguity that it demands (a) stereospecifi c coupling of chiral organoborons; further studies. Carbonylative and decarbonylative (b) chiral electrophile partners for stereospecifi c (‘masked carbanion’, discovered by Blaser and couplings; and (c) racemic electrophiles in dynamic Spencer (3)) reactions are demystified. The resolution tactics. All three topics are expertly Mizoroki-Heck reaction is given a deserved tackled from which may be arguably underlined: the comprehensive presentation and includes many challenges (β-hydride elimination and their invisibility elements of theoretical and practical value, some for symmetrical nucleophiles, steric effects) faced in perhaps unappreciated, for example: mechanistic reactions of secondary aliphatic boranes (overcome pathways using mono- and bidentate ancillary ligands, in Crudden’s group); the coupling of racemic and the functioning of Pd nanoparticles as precatalysts, the enantioenriched allylic boranes (provided in tabular qualitative rules for ligand acceleration, the status of the data); the use of benzylic carbamates and esters Pd(II)/Pd(IV) mechanism and the new developments in to displace the need for enantiomerically pure alkyl copper-catalysed reactions. The increasingly used halides in coupling with organoboron derivatives; the oxidative Heck and Fujiwara-Moritani reactions power of Ni-catalysed radical-mediated asymmetric are given merited coverage. The most intriguing arylation of racemic α-, γ- and δ-chloroamides; and deborylative (Heck discovery, 1975), desilylative and the initial demonstration of arylation of tertiary halides. destannylative Heck reactions are remarkably evolving The chapter critically discusses the highs and the lows methods of new synthetic opportunities. In spite of the of current methods for stereo-selective and -specifi c authors’ modest concluding comment, this chapter sp2-sp3 and sp3-sp3 bond formation which are of growing provides the depth and the breadth of the Heck reaction signifi cance in new synthetic method development. that will be read with great benefi t for some time (for a personal account, see (4); for obituaries see (5, 6)). C–H Activation 456 References are listed and a SciFinder search for 2010–2015 reveals 3371 hits; therefore, this chapter is In Chapter 12, Upendra Sharma, Atanu Modak, Soham perceptively discriminating no doubt due to the authors’ Maity, Arun Maji and Debabrata Maity (Indian Institute command of the area. of Technology Bombay, India) discuss a topic of high Chapter 10 by Xiao-Feng Wu (Leibniz Institute for current activity – the direct arylation by C–H activation Catalysis, Germany) and Christopher Barnard (Johnson which, it should be noted, is formally not a cross-coupling Matthey, UK) concentrates on the Heck, Sonogashira, reaction. With a very good historical preamble on today’s Suzuki and C–H activated carbonylation reactions. named coupling methods using two prefunctionalised With respect to the Heck carbonylation, there is no substrates, presented critically is the moving frontier of overlap with Chapter 9; in fact, Chapter 10 presents the C–H activated arylation methods based on using one

102 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X690925 Johnson Matthey Technol. Rev., 2016, 60, (2) prefunctionalised system. The chapter has a strong is laid to rest by considering that it needs to be treated mechanistic orientation, for example Figure 2. The before release into the environment. Laudably, examples subdivision by metal (Pd, Rh, Ru, Ir, Cu) shows clearly for Pd, Cu and Fe catalysed fl ow reactions are presented that Pd is king of the catalytic highway. Examples of with comparison of cost and toxicity, as are comparisons illustrative reactions are outlined, mechanistic pictures of batch vs. fl ow procedures. The ‘metal-free’ reaction, are presented, and, notably, experimental needs, as currently often carelessly stated in the literature, costs, catalysts and ligands are examined. A glossary is given the proper caveat by need for fl uorescence of acronyms, for example chemical manganese dioxide or inductively coupled plasma mass spectrometry (CMD) appears early in the book but is not defi ned until (ICP-MS) metal detection that apparently may be carried later. Also, a table of directing groups, giving an eye out by a fi rst year student (see also review of Chapter 16 blink view of the section on directing scaffolds, would below). As this chapter emphasises, the benefi ts of fl ow have been valuable. Sections on direct arylation of will occur with increased attention to gas-liquid coupling heterocycles and of sp3 bond C–H activation represent reactions, more effi cient operation, increased engineering tips of the iceberg of current research, as are the initial automation, and introduction of fl ow chemistry courses in results of remaining challenges, such as erasable and curricula. traceless directing groups, which are described in this Chapter 14 by Kevin Shaughnessy (University of well formulated chapter. Alabama, USA), which takes up the ‘green’ theme in depth, will be valuable reading especially for academics. Flow and Green Cross-Coupling and APIs As stated in the introduction, the philosophy in industry has changed since passing of the Pollution Prevention The review (Chapter 13) on cross-coupling chemistry Act by the US government in 1990 from ‘how to deal in continuous fl ow by Timothy Noël and Volker Hessel with hazardous substance’ to ‘how to design processes (Eindhoven University of Technology, The Netherlands) of minimum waste and hazard’. The chapter focuses on begins with an instructive table on the pros and cons of using various solvents with emphasis on water, even this new technology over batch processes. Presented though its ‘green-ness’ is questioned (see also review on by two experts in this rapidly emerging fi eld that, of Chapter 13 above). Shaughnessy makes many important necessity, involves also chemical engineers, the reader points, some of which are very familiar to both academic receives step-by-step instruction on how to set up a and industrial chemists (for example, atom economy, fl ow experiment, be aware of technical diffi culties and protecting group avoidance, toxicity) and some not to hazards and be concerned about safety issues on academics (for instance environmental (E) factor = mass scale. The myth of reactions in water being ‘green’ of waste/mass of product, life-cycle assessment (LCA)

C–H activation I + O H OAc N OAc PhI IV H N II Pd H3C N Pd N O Pd N N Ph N L Ph

+ N H + coordination OAc trans H II N effect OAc OAc Cs CO N Pd I H 2 3 II II N Pd N N Pd OAc N H N PdOAc N H Csl AcOH N N N

Fig. 2. C-2 Arylation of pyridine using a Pd(OAc)2/1,10-phenanthroline catalyst system (Reproduced by permission of The Royal Society of Chemistry)

= a cradle to grave evaluation from raw materials to this evolving methodology. A discussion of preferred materials returned back to Earth). Although examples metals (currently Pd > Cu > Ni > Fe) and a glossary of of large scale reactions appear to be absent, aspects abbreviations terminate this welcome chapter. of cross-coupling in aqueous media are given thorough In the signifi cant and fi ttingly last chapter (Chapter treatment in which choice of hydrophilic or hydrophobic 16), Kazunori Koide (University of Pittsburgh, USA) metal-ligand combinations and surfactant-mediated defi nes the necessary standards for metal removal for reactions (original observation by Jefferey) are APIs (for Pd = 10 ppm, for example, 2 μg of Pd in a highlighted. Heterogeneous, recyclable catalysis 200 mg ibuprofen tablet) and the trials and tribulations such as nanoparticles, which are clearly of value in in this achievement which are a process chemist’s industry, and other green alternative solvents, such as nightmare since scrupulous metal removal can delay polyethylene glycol (PEG) and ionic liquids, as well as scale up campaigns. This major problem has led to novel conditions such as microwave and thermomorphic the development of new fl uorimetric and colorimetric techniques receive coverage. Although all chemists analytical methods, many in the Koide laboratories, are innately ‘green’, commandments such as solvent for the detection of trace amounts of Pd. These are minimisation, E factor and LCA which are currently not delineated by required criteria and are illustrated. given suffi cient attention, especially by academics, are The synthesis of Pd sensors and chemodosimeters vital and appreciated points from reading this chapter. are exemplifi ed which, rather appropriately, illustrates Chapter 15 by Javier Magano (Pfi zer Global Research cases involving Suzuki-Miyaura and Sonogashira & Development, USA) and Joshua Dunetz (Gilead couplings as the key steps. An informed synthetic Sciences, Inc, USA) is a well-placed wide-ranging organic chemist surveying this chapter may be tempted review, with 171 references, of recent large-scale to devise new ‘ideal metal detection’ molecules. cross-coupling reactions in service of the synthesis of pharmaceuticals. With a forceful table of statistics Conclusion regarding numbers of >100 mmol reactions carried out in the 2000–2013 period, the impact of the 2010 Nobel To conclude, the volume under review amassed by Prize in Chemistry is truly respected and is undeniably Colacot is a magnum opus of contemporary organic shown by a table of commercial drugs which are chemistry. The studious chemist of this volume, synthesised by C–C (mainly), C–N and C–S coupling should receive a 95% grade (as with yields, there is methods, for example Scheme II. There follow sections no 100%) on the quiz posed at the beginning of this on some of these household name reactions that offer review. Dipping into sections of interest will provide instructive details in schemes and commentary. The the practicing chemist with knowledge and insight value of organic synthesis is alive in this chapter for from scientists in the trenches of the areas discussed. sharing with students and laypersons alike. A brief Whether in a university or industrial lab, the chemist will section on C–H activation processes (only four to take information from this compendium into the lab (if date of the review) is evidence of rapid application of allowed) and immediately profi t.

Active Commercial Structure Type of Applicant Indication FDA Exclusivity ingredient name coupling approval expiration date name date (USA)

Losartan Cozaar Suzuki- Merck Angiotensin 28th April Generic potassium Cl Miyaura. II receptor 1995 (August 2009) N Also used for antagonist for OH structurally the treatment similar drugs of high blood n-Bu N N N + pressure NN– K

Scheme II. Losartan drug synthesised by Suzuki-Miyaura cross-coupling reaction (Reproduced by permission of The Royal Society of Chemistry)

References

1. R. H. Crabtree, “The Organometallic Chemistry of the “New Trends in Cross-Coupling: Transition Metals”, 5th Edn., Wiley, Hoboken, New Theory and Applications” Jersey, USA, 2009 2. J.-H. Fuhrhop and G. Li, “Organic Synthesis”, 3rd Edn., Wiley-VCH, Weinheim, Germany, 2003, p. 468 3. H.-U. Blaser and A. Spencer, J. Organomet. Chem., 1982, 233, 267 4. R. F. Heck, Synlett, 2006, 18, 2855 5. V. Snieckus, Nature, 2015, 527, (7578), 306 6. T. Colacot, Angew. Chem. Int. Ed., 2015, 54, (52), 15611

The Reviewer

Victor Snieckus was born in Lithuania and ﬂ ed with his parents to Germany during World War II. He obtained the BSc at the University of Alberta, Canada; graduate degrees at the University of California, USA, under the supervision of D. S. Noyce, and University of Oregon, USA, with Virgil Boekelheide and carried out postdoctoral studies at National Research Centre, Canada with O. E. Edwards. He taught at the University of Waterloo, USA, until 1998 and then moved to Queen’s University, Canada, as the inaugural Bader Chair of Organic Chemistry. He continues fundamental research as Bader Chair Emeritus as well as Director of Snieckus Innovations, an institute for the synthesis of small molecules for the pharmaceutical and agrochemical industries. He is a recipient of the American Chemical Society Cope Scholar Award, Arfvedson-Schlenk Preis of the Gesellschaft Deutscher Chemiker (GDCh), Canadian Society for Chemistry Bernard Belleau and Alfred Bader Awards, is a Lithuanian Academy of Sciences Laureate, and is a Royal Society of Canada Fellow.

Frontiers in Environmental Catalysis

Symposium in recognition of 20 years with the Competence Centre for Catalysis

Reviewed by Djamela Bounechada reading. The topics covered by the poster presentations Johnson Matthey Technology Centre, Blounts Court, will also be mentioned to give an overview of the current Sonning Common, Reading RG4 9NH, UK research at KCK.

Email: [email protected] 2. Oral Presentations 2.1 Twenty Years with KCK

An introductory talk to celebrate the history of KCK, an 1. Introduction interdisciplinary research centre within heterogeneous catalysis since 1995, was given by the director of KCK, This symposium was organised by Chalmers University Professor Magnus Skoglundh (Chalmers University of of Technology, Sweden, to commemorate the fi rst 20 Technology). The diversity of the centre was emphasised: years of research at Competence Centre for Catalysis it was originally composed of Chemical Engineering, (KCK). The Frontiers in Environmental Catalysis Chemical Reaction Engineering and Applied Physics conference was held on 24th September 2015 at departments at Chalmers University of Technology, Chalmers University of Technology. All previous and and recently it was further expanded by Eva Olsson’s current KCK employees were invited, together with microscopy group. The research is focused on catalysis representatives of KCK’s member companies and for emission control with a growing interest in energy few invited speakers. About 50% of the invited people related areas. KCK is fi nancially supported by Chalmers attended, resulting in about 120 participants. Among University of Technology, the Swedish Energy Agency and them were four Johnson Matthey delegates (including the member companies: AB Volvo, ECAPS AB, Haldor the present reviewer, having been a post-doctoral Topsøe A/S, Scania CV AB, Volvo Car Corporation AB fellow at KCK). Although Johnson Matthey is no longer and Wärtsilä Finland Oy. part of KCK, a collaboration project on the study On a similar note, Pär Gabrielsson’s (Haldor Topsøe, of methane oxidation is ongoing between Johnson Denmark) talk on ‘The Impact of KCK from an Industrial Matthey Technology Centre, Sonning Common, UK Perspective’ stressed the role played by KCK on students’ and Professor Per-Anders Carlsson (KCK, Sweden) education and how it prepares them for employment in the resulting in two poster presentations at the symposium. catalysis and aftertreatment industry. This is confi rmed The aim of this review is to summarise the oral by statistics, with 70% of PhDs employed by industry of presentations relevant for emission control applications which 45% are member companies. The remaining 30% and provide the reader with useful references for further of PhDs are employed by other organisations.

2.2 Automotive Aftertreatment correlations between a catalyst structure and its activity. The research of oscillatory catalytic reactions, Galen Fisher (University of Michigan, USA) presented such as carbon monoxide oxidation, was reported as ‘Progress and Challenges in Automotive Emission an example of successful application of this technique Control’. Fisher is one of the members of the KCK’s (5). In this case a link between catalyst activity and international advisory board. His talk concentrated shape of platinum nanoparticles has been found on current challenges facing the automotive emission (shape dynamics from squared to circular). Sintering control industry such as improving the low-temperature mechanisms and kinetics can also be investigated with activity of three-way catalyst (TWC) and nitrogen the same technique. oxides (NOx) traps. Moving the TWC design from Anders Nilsson (Stockholm University, Sweden) a supported metal to a metal core surrounded by presented ‘Fundamental X-ray Studies of Catalytic an inert shell was shown to prevent poisoning and Reactions’. The understanding of the nature of the metal sintering (1). Johnson Matthey was cited in interaction between water and metals, which is of relation to the development of palladium/cerium based interest in heterogeneous catalysis, was described low-temperature NOx adsorbers, characterised by (6). Professor Nilsson developed a library of X-ray lower nitrous oxide (N2O) production compared to absorption spectroscopy (XAS) and X-ray Raman platinum-based catalysts and preferential storage scattering (XRS) spectra that spans the liquid part of of nitrogen oxide (NO) vs. nitrogen dioxide (NO2) the water phase space and several important aqueous (2). Fisher discussed how the new aftertreatment solutions. This was used as a reference to identify technologies use layering/zoning strategies as well as hydroxyl species involved in catalytic reactions such as diffusion effects to decrease emissions. hydrogen oxidation in fuel cell applications. Jonas Jansson (AB Volvo, Sweden) presented Per-Anders Carlsson presented ‘Studies of Catalysts ‘Exhaust Aftertreatment for Heavy Duty Diesel at Work’. This talk focused on the use of synchrotron Engines – Current Trends and Future Challenges’. The facilities (European Synchrotron Radiation Facility evolution of the aftertreatment systems for heavy duty (ESRF), France; MAX IV, Sweden and PETRA III, diesel vehicles at Volvo was discussed. The original Germany) to examine catalytic reactions. Some confi guration diesel oxidation catalyst (DOC) + diesel examples were given of various spectroscopic particulate fi lter (DPF) + selective catalytic reduction techniques (extended X-ray absorption fi ne structure (SCR) has been recently substituted by close-coupled (EXAFS), high-energy X-ray diffraction (HEXRD), DOC + SCR on fi lter (SCRF®) (or passive NOx adsorber Fourier transform infrared (FTIR) and MS) to distinguish (PNA) + SCRF®), allowing an enhancement of the light- the adsorbed species and simultaneously coordinate off performances. The location of the catalysts and of catalytic activity to changes in the catalyst oxidation the urea tank depends on the geographical area where state (7, 8) (Figure 1). The possibility of exploiting the the vehicle is commercialised. structure gap, from ideal surfaces to big particles, small clusters and single atoms, to improve the understanding 2.3 Advanced Characterisation of catalytic reactions was also mentioned. Active areas Stig Helveg (Haldor Topsøe) presented ‘Electron of interest are total methane oxidation and partial Microscopy Advances for Catalysis’. Helveg introduced selective methane oxidation to methanol. the cutting-edge technology of in situ and operando 2.4 Reaction Engineering and Molecular transmission electron microscopy (TEM) imaging Modelling (3). A nanoreactor was designed at Haldor Topsøe in collaboration with TU Delft, The Netherlands, and Louise Olsson (Chalmers University of Technology) was functionalised with a micrometre-sized gas-fl ow presented ‘Studies of Aging Mechanisms for channel, transparent windows to allow the electron NH3-SCR Catalysts using Experiments and Kinetic beam to reach the sample and with a heating device Modelling’. Sulfur poisoning and regeneration on (4). This device allows the collection of images at copper-SSZ13 were studied and performed in 1 bar, solving the pressure gap issue typical of traditional collaboration with Cummins Inc. A three-site kinetic TEM imaging. The time resolved TEM images and model was necessary to describe ammonia storage measurement of the outlet gas composition by mass and oxidation during temperature programmed spectrometry (MS) results in a powerful way of studying desorption (TPD) experiments (9). A connection was

1.2 (a) 0.6 1.0 (b)

0.8 ), a.u. 0.4

0.6 –1 0.4 0.2 0.2

Log (R 0 0 Norm. Absorption 15 20 4000 10 3500 15 Time, min 3000 Time, min10 11.65 2500 –1 11.60 5 2000 5 11.55 0 1500 0 11.50 Energy, keV Wavenumber, cm

Fig. 1. (a) Ev olution of X-ray absorption near edge structure (XANES) Pt LIII-edge spectra; and (b) IR bands in the interval –1 1500–4000 cm for a 4% Pt/Al2O3 catalyst exposed to 1000 ppm CH4 in He while periodically switching the O2 concentration between 0 (60 s) and 1.5% (60 s) at 280ºC (Reprinted with permission from (7). Copyright (2011) American Chemical Society)

also found experimentally between the rate of SCR CH4 desorption, CH3 dissociation or CH4 dissociation reaction and Cu loading, that was introduced in the with increasing temperature. Different activation kinetic model. A sulfur exposure study showed that energies depending on the temperature were found sulfur dioxide (SO2) can be adsorbed on Cu sites also experimentally, which supports the modelling (S1-SO2), resulting in a decreased catalytic activity. The results. experimental observation of more NH3 stored in the presence of sulfur was elucidated with the formation of 3. Poster Presentations adsorbed ammonium-sulfur species (S1-(NH3)2-SO2). The gradual decomposition of these species leads to 29 posters were presented during the poster session, catalyst regeneration and also provides an alternative covering two main research areas: emission control route to the SCR reaction. This mechanism was and energy conversion. successfully included in a kinetic model. One of the main focuses in emission control research Henrik Grönbeck (Chalmers University of Technology) was the understanding of ageing and sulfur poisoning presented ‘Catalysis from First-Principles Calculations’. mechanisms as well as deactivation during operation, A recent work combining X-ray photoelectron for lean NOx trap (LNT), SCR and hydrocarbon oxidation spectroscopy (XPS) measurements and fi rst-principle catalysts. The SCR of NOx in the presence of excess calculations suggests that the most active phase for oxygen by NH3/urea (NH3-SCR) or hydrocarbons methane oxidation on Pd-based catalyst is a double (HC-SCR), and NOx storage reduction (NSR) using layered PdO(101) structure (10). Density functional mixed lean/rich operation were explored. Posters theory (DFT) calculations show that CH4 repulsion from on passive SCR and SCR coated DPFs were also the surface is quite strong when only one monolayer of presented. Another subject of interest is the catalytic PdO is present, but it can be overcome after a second oxidation of soot and methane in particular the inhibiting monolayer is created therefore explaining the better effect of H2O on the latter. activity observed experimentally. A micro-kinetic model Moving to the energy conversion theme, research for CH4 oxidation was also built from fi rst principles, is ongoing in the synthesis of methanol from direct able to predict reaction orders in methane, water, and selective oxidation of methane on Cu-zeolites as well oxygen as well as apparent activation energies in good as selective hydrogenation of oxygenates. agreement with a range of experimental fi ndings (11). In many cases activity studies were accompanied by Interestingly, different reaction steps were found to be kinetic modelling and/or fi rst principles calculations to limiting depending on the temperature: water inhibition, support the interpretation of the experimental results.

4. Conclusions 5 S. B. Vendelbo, C. F. Elkjær, H. Falsig, I. Puspitasari, P. Dona, L. Mele, B. Morana, B. J. Nelissen, R. van The contribution by representatives from both Rijn, J. F. Creemer, P. J. Kooyman and S. Helveg, academia and industry to the symposium made it an Nature Mater., 2014, 13, (9), 884 overall successful event. The range of presentations 6 T. Schiros, K. J. Andersson, L. G. M. Pettersson, A. was diverse and covered the most relevant topics in Nilsson and H. Ogasawara, J. Electron. Spectrosc. environmental catalysis, with an emphasis on future Relat. Phenom., 2010, 177, (2–3), 85 challenges. The Chalmers’ KCK demonstrated once 7 E. Becker, P.-A. Carlsson, L. Kylhammar, M. A. Newton again to be one of the leading groups in environmental and M. Skoglundh, J. Phys. Chem. C, 2011, 115, (4), catalysis research. 944 8 D. Bounechada, S. Fouladvand, L. Kylhammar, T. References Pingel, E. Olsson, M. Skoglundh, J. Gustafson, M. Di Michiel, M. A. Newton and P.-A. Carlsson, Phys. 1 T. M. Onn, S. Zhang, L. Arroyo-Ramirez, Y.-C. Chung, Chem. Chem. Phys., 2013, 15, (22), 8648 G. W. Graham, X. Pan and R. J. Gorte, ACS Catal., 9 L. Olsson, K. Wijayanti, K. Leistner, A. Kumar, 2015, 5, (10), 5696 S. Y. Joshi, K. Kamasamudram, N. W. Currier and 2 J. E. Coulson, R. J. Brisley, O. Keane, P. R. Phillips and A. Yezerets, Appl. Catal. B: Environ., 2015, E. H. Mountstevens, Johnson Matthey Plc, ‘Thermally 174–175, 212 Regenerable Nitric Oxide Adsorbent’, World Appl. 10 N. M. Martin, M. Van den Bossche, A. Hellman, H. 2008/047,170 Grönbeck, C. Hakanoglu, J. Gustafson, S. Blomberg, 3 S. Helveg, J. Catal., 2015, 328, 102 N. Johansson, Z. Liu, S. Axnanda, J. F. Weaver and E. 4 J. F. Creemer, S. Helveg, G. H. Hoveling, S. Ullmann, Lundgren, ACS Catal., 2014, 4, (10), 3330 A. M. Molenbroek, P. M. Sarro and H. W. Zandbergen, 11 M. Van den Bossche and H. Grö nbeck, J. Am. Chem. Ultramicroscopy, 2008, 108, (9), 993 Soc., 2015, 137, (37), 12035

The Reviewer

Djamela Bounechada graduated in Chemical Engineering at Politecnico di Milano, Italy, in 2008

with a thesis combining experimental and modelling studies of SCR of NOx by ammonia (NH3- SCR) for automotive applications. She obtained a doctorate degree from the same university in 2012, with a thesis on strategies for enhanced methane oxidation in the exhausts of natural gas vehicles. After two years as a post-doctoral Fellow at Chalmers University of Technology, she joined Johnson Matthey Technology Centre (Sonning Common, UK) in 2014, as part of the reaction engineering research team.

7th International Gold Conference

Recent developments in gold research from heterogeneous catalysis to biomedical imaging

Reviewed by Nicoleta Muresan* and used as a catalyst in various reactions. Historically, Au Agnes Raj was known to be a transition metal rich in coordination Johnson Matthey Technology Centre, Blounts Court, numbers but compared with other metals, bulk Au was Sonning Common, Reading RG4 9NH, UK thought to be chemically much less reactive. However, this changed in 1973 when Bond et al. (1) reported the *Email: [email protected] hydrogenation of oleﬁ ns over supported Au catalysts; subsequently Haruta and Hutchings independently found that supported nanosized Au catalysts are surprisingly active for low temperature carbon Introduction monoxide oxidation and for the hydrochlorination of acetylene to vinyl chloride monomer (2, 3). The most The 7th International Gold Conference takes place important aspects of Au catalysts are the high activity every three years and in 2015 was hosted by the Cardiff at low temperatures, promotion by water and unique Catalysis Institute at Cardiff City Hall, UK, from 26th to selectivity. The catalytic activity of Au nanoparticles 29th July. Scientists from all over the world gathered (NP) can be further tuned by careful choice of the to discuss the latest advances in gold chemistry. Over support material. There are many examples of Au NP three days, the conference covered different aspects loaded on semiconductor materials to exhibit high CO from homogeneous and heterogeneous catalysis to oxidation activity at temperatures below 0ºC, activity biomedical imaging. The conference had more than which has not been reached by other metals (4–8). 200 attendees from academia and industry and was divided into three parallel sessions with 72 talks and Low Temperature CO Oxidation by Gold nearly 100 posters. This review will focus mainly on the industrially At the beginning of the 1980s, Haruta reported that relevant heterogeneous catalysis and to some extent Au NP deposited on semiconducting oxides such biomedical applications. as titanium dioxide (TiO2), iron(III) oxide (Fe2O3), cobalt(II,III) oxide (Co3O4) and nickel oxide (NiO) exhibit Gold in Catalysis markedly high catalytic activity for CO oxidation even at temperatures as low as 200 K (7). The contributing Catalysis by Au has rapidly increased importance in factors to this high activity are still disputed. Haruta chemistry since Masatake Haruta (Osaka National studied low-temperature CO oxidation with Au and

Research Institute (ONRI), Japan) and Graham platinum NP supported on TiO2 prepared by different Hutchings (Cardiff University) found that Au can be methods and found that activity is dependent on the

Mostafa A. El-Sayed Solution Metallic Catalysis on Plenary and Keynote Speakers (Georgia Institute of the Nanoscale Technology, USA) Masatake Haruta Simple Chemistry Catalyzed (Tokyo Metropolitan by Gold Nanoparticles Tatsuya Tsukuda Precise Synthesis and Non- University, Japan) (The University of scalable Oxidation Catalysis Tokyo, Japan) of Gold Clusters Luis M. Liz-Marzán Crystalline Facets in Gold (Bionanoplasmonics Nanocrystals Christopher Kiely Population Distributions and Laboratory, CIC (Lehigh University, Catalytic Hierarchy of Active biomaGUNE, Spain) USA) Species in Gold on Iron Oxide Catalysts for Low Temperature Catherine J. Murphy A Golden Age for Colloidal CO Oxidation (University of Nanoparticles Illinois at Urbana- Vivian Wing-Wah Luminescent Gold-Containing Champaign, USA) Yam Molecular Photofunctional (The University of Materials Dean Toste Homogeneous Gold Catalysts Hong Kong, Hong (University of for Selective Organic Kong) California, USA) Synthesis Mona Tréguer- Gold Nanoparticles of Unusual Peter Johnston The Development and Delapierre Morphologies (Johnson Matthey Commercialisation of Gold (University of Chemical Catalysts, Catalysts for VCM Production Bordeaux, France) UK) Robert T. Tshikhudo Gold Nanotechnology (DST/Mintek Innovation at Mintek: Creating preparation method. Interestingly, hemispherical Nanotechnology and Capturing Value in the Au particles adhering to the support, prepared by Biotechnology Sector Innovation Centre, deposition-precipitation methods, exhibited greater South Africa) activity than spherical particles having little interaction Malcolm Kadodwala Biosensing with a with the support prepared using impregnation and (University of Twist: Detection and photochemical methods. Pt catalysts did not show Characterization of Glasgow, UK) such variation in activity with the preparation method, Biomaterials with Sculpted EM Fields indicating that only the metal is involved in CO conversion. These data indicate that metal-support Chris J. Ackerson Jahn-Teller Effects in Gold interactions play a crucial role in Au-catalysed CO (Colorado State Nanoclusters oxidation at low temperature (9). Another important University, USA) aspect is the size of the NP. It has been found that the Ewa Kowalska Visible Light-induced optimal inside diameter range for maximum activity is Photocatalysis Through (Hokkaido between 0.5 and 5 nm (9), supporting Kubo’s prediction University, Japan) Surface Plasmon Excitation of Gold on Titania Surfaces ﬁ ve decades ago that small NP would behave differently from the bulk (10). Metal NP smaller than 2 nm show Cristina Nevado On Gold Mechanistic Studies a discrete energy band structure similar to those of (University of and Beyond semiconductors, Figure 1. Zürich, Switzerland) The promoting effect of moisture has been intensively Antonio M. Synthetic Transformations via studied and it has been found that it promotes CO Echavarren Gold-Carbenes oxidation at room temperature but is not indispensable (Institute of Chemical Research at temperatures below 273 K, depending on the support. of Catalonia, Spain) Semiconducting supports such as TiO2, manganese(IV) oxide (MnO2), Fe2O3, Co3O4, NiO, zinc oxide (ZnO), A. Stephen K. Efﬁ cient Homogeneous Gold zirconium dioxide (ZrO2) and cerium(IV) oxide (CeO2) Hashmi Catalysis exhibit high activity at temperatures below 273 K in the (Universität absence of water. However, insulating, non-reducible

Heidelberg, metal oxides such as aluminium oxide (Al2O3), silicon Germany) dioxide (SiO2) or titanium carbide (TiC) show moderate

activity and require a signiﬁ cant amount of moisture H2 at 550ºC. Transmission electron microscopy (TEM) (11). A detailed study on the inﬂ uence of moisture and showed that the average metal particle size increases NP size on CO oxidation over supported Au catalysts, with the temperature of activation and micro-energy including kinetic studies has been published (11). dispersive spectroscopy (EDS) performed on individual As an extension to low-temperature CO oxidation particles indicated that the metal particles become over supported Au catalysts, Professor Bert Chandler more bimetallic. Ultraviolet (UV)-visible spectroscopy (Trinity University, USA) presented a new process performed during in situ reduction indicated that the for eliminating CO from hydrogen feeds with minimal bimetallic particles formed gradually as temperatures hydrogen oxidation, the so-called preferential oxidation increased. The evolution of the surface composition

(PROX) process with Au/TiO2 and Au/Al2O3 catalysts. of the metal particles studied by X-ray photoelectron CO PROX over Au catalysts has been investigated spectroscopy (XPS) and CO diffuse reﬂ ectance infrared in two ways: ﬁ rstly, the fundamentals of CO oxidation Fourier transform spectroscopy (DRIFTS) revealed an are studied in the absence of H2 (at low conversion); increasing proportion of silver atoms during activation secondly the CO oxidation reaction is studied at under H2 (14). industrially useful conversions and temperatures (close to 80ºC) in the presence of H2 (PROX). In both cases, Other Industrially Relevant Processes with the reaction is greatly affected by the presence of water Gold Catalysts in the feed and formation of carbonates on the catalyst surface. In a recently published study, the Chandler Peter Johnston presented the latest progress in the group established that the CO oxidation reaction is development and commercialisation of Au catalysts for co-catalysed by weakly adsorbed water at the NP vinyl chloride monomer (VCM) production. He showed perimeter (12, 13). that Au supported on activated carbon as catalyst for the In situ characterisation of CO oxidation over bimetallic hydrochlorination of acetylene shows superior activity

Au-Ag/TiO2 catalysts was presented by Catherine Louis to the current mercury(II) chloride (HgCl2) on carbon. (Centre National de la Recherche Scientique (CNRS)- He explained the importance of the migration to the University Pierre et Marie Curie (UPMC), France). new, mercury-free process taking into consideration the

Bimetallic Au-Ag NP supported on TiO2 showed environmental and handling problems due to the toxic higher activity and stability for CO oxidation at room nature of the Hg catalyst. He described the progress temperature than monometallic Au and Ag catalyst. in developing ultra-low Au catalysts for this application

The best performances were obtained by Au-Ag/TiO2 (3, 15, 16). catalysts with an Au:Ag ratio of 1:0.8 after activation in Another important industrial process is glycerol oxidation. In recent years, the catalytic transformation of alcohols into fuels and other valuable chemicals has played an important role in the quest to more Terraces Edges and corners sustainable processes. It is well known that oxidation of glycerol is a complex process leading to a complex mixture of products including six potential C3 oxygenated products. Chunli Xu (Shaanxi Normal University, China) presented the effect of the Bulk Nanoparticle Cluster Atom acid/base nature of supports on the selectivity of <10 nm <2 nm glycerol oxidation over supported Au-Pt bimetallic

catalysts. Various supports, including acidic (TiO2 and Valence CeO ), basic (magnesium oxide (MgO), magnesium band 2 hydroxide (Mg(OH)2), magnesium carbonate (MgCO3), Mg(OH)2, ZnO and calcium carbonate (CaCO3)) and amphoteric (Al O and hydrotalcite), were reported. Continuous Discrete 2 3 The results showed that the acid-base properties of the Fig. 1. Major surface states and valence band structures of supports signiﬁ cantly affected product selectivity. The bulk, NP, clusters and atoms (9) (Reproduced by permission selectivity to glyceraldehyde was inversely proportional of The Royal Society of Chemistry) to the strength of basic sites on the supports while the

112 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691069 Johnson Matthey Technol. Rev., 2016, 60, (2) selectivity to tartronic acid, glycolic acid and glyoxalic Kinetic analysis and a Hammett plot suggest that acid was directly proportional to the strength of basic hydride transfer is involved and the cationic Au is sites of supports. In contrast, the selectivity to glyceric catalytically active. The Au/CeO2 catalysts were acid was neither proportional to the strength of basic initially evaluated in the oxidation of benzyl alcohol sites of supports nor proportional to the strength of acid to benzaldehyde showing around 99% selectivity for sites of supports (17). benzaldehyde (22, 23). Two other talks on alcohol oxidation were presented Another important catalytic process is by Emiel J. M. Hensen (Eindhoven University of low-temperature C–C bond activation. Hidehiro Technology, The Netherlands) studying the synergy Sakurai (Osaka University, Japan; and Japan Science + between Au NP and Cu in MgCuCr2O4-spinel supports and Technology Agency) has shown that bimetallic in the selective aerobic oxidation of alcohols and Au/Pd alloy nanoclusters stabilised by a hydrophilic oleﬁ ns. He showed that Au/MgCuCr2O4 catalyst is polymer, poly(N-vinylpyrrolidone) (PVP), have unusual highly active and selective for the aerobic oxidation of catalytic activity towards the activation of the C–Cl ethanol to acetaldehyde. TEM, XPS and extended X-ray bond at room temperature under aqueous conditions. absorption ﬁ ne structure (EXAFS) characterisation The reaction does not occur with either Au or Pd single results show that the synergy involves close interaction metal clusters, indicating that a synergistic alloy effect between metallic and surface Cu+ species stabilised in plays a key role. In addition, alloy clusters exhibited the support. MgCuCr2O4 is more stable under reaction unique reactivity when aryl bromide was used as conditions than MgCuAl2O4. The use of MgCuAl2O4 coupling partner. With Pd-rich catalyst the reaction leads to segregation of Cu and formation of copper(II) occurs through conventional catalysis by leaching of oxide (CuO). Au/MgCuCr2O4 is also an active catalyst Pd from alloy clusters (24–26). for the selective oxidation of propylene to acrolein The selective hydrogenation of acetylene to ethylene

(selectivity close to 90%). Density functional theory over SiO2-supported Au alloyed Pd single-atom (DFT) study of the reaction mechanism explained the catalyst (SAC) was discussed by Tao Zhang (Chinese dynamic nature of Au atoms at the interface of the Au Academy of Sciences, China). This industrially clusters with the support. Adsorption of propylene leads important process uses mainly Pd-based catalysts to the formation of an isolated Au atom that strongly but suffers from poor selectivity. The speaker showed binds propylene. The reaction proceeds by activation that conversion of acetylene to ethylene can be of the allylic C–H bond by an adsorbed O2 molecule. achieved with high selectivity using Au alloyed Pd Detailed analysis of the electronic structures of the SAC on SiO2 support. In situ DRIFT spectra of CO reaction intermediates in the catalytic cycle show that adsorption and EXAFS results give evidence of the the critical role of Cu is the decrease of the desorption formation of Au-Pd interaction. The data ﬁ tting results energy of acrolein (18–21). gave only Pd-Au interactions in the AuPd0.025/SiO2 On the same subject of selective oxidation of alcohols catalyst, providing direct evidence for the isolation of to aldehydes or ketones, Feng Wang (Waterloo Pd atoms. This new catalyst improved the catalytic Institute for Nanotechnology, Canada) discussed the performance of the acetylene conversion by 95% effect of crystallinity of Au NP supported on CeO2 for compared to the monometallic Au/SiO2 catalyst the conversion of alcohols to aldehydes or ketones and ethylene selectivity by 1000% compared to the and amines to imines via oxidative dehydrogenation. monometallic Pd catalyst (27). Both types of reactions achieve moderate to excellent Two other relevant talks were delivered by Florencia yields. Importantly, regardless of Au size and surface C. Calaza (Fritz-Haber-Institut der Max-Planck- area of CeO2 support, the study reveals that the active Gesellschaft, Germany) and Wilm Jones (The UK plane for C–H and N–H bond activation is the {110} Catalysis Hub, UK; and Cardiff University, UK) on plane on nanorod ceria. Characterisations by Raman carbon dioxide activation by negatively charged and XPS show that positively charged Au dominates two-dimensional (2D) Au clusters; and photocatalytic in nanorod Au/CeO2 possibly via the interaction with H2 production using core-shell AuPd promoters with oxygen vacancy sites. Au NP supported on CeO2(110) controlled shell thickness, respectively. The adsorption crystal planes were more reactive than on CeO2(111) and activation of CO2 on various Au clusters grown and (100) in the oxidative dehydrogenation of alcohols. on Ag(001)/supported MgO ﬁ lms were reported using

XPS, temperature programmed desorption (TPD) and MW-CNT Modiﬁ ed Electrodes for Glycerol infrared reﬂ ection absorption spectroscopy (IRAS). Electroanalytical Detection and Transformation’

It has been found that the adsorption of CO2 in the was presented by Laura Prati (University of Milan, presence of 2D Au clusters with negatively charged Italy). She mentioned that carbon nanotubes could perimeters is reversible and that these species are be decorated with Au NP using sol immobilisation or stable up to slightly above room temperature. When deposition precipitation. More details can be found in heated beyond 380 K, the clusters appear to take the literature (36). on more three-dimensional (3D) morphologies. The negative charge on the 2D clusters is responsible for Gold in Clinical Diagnostics and Treatments the activation of CO2. Furthermore, isotope labelled 13 18 18 compounds were used ( CO2, CO2 and O2 for Using nanotechnology-based systems for diagnosing labelling the oxide surface) to identify the presence of and treating speciﬁ c diseases is increasing in popularity different species on the surface (28). Au NP supported in recent decades and it is particularly relevant for on TiO2 followed by the deposition of thin layers of personalised medicine, allowing a disease to be one and two monolayer equivalent of Pd was studied detected at an early stage and a suitable therapy to be for photocatalytic H2 production. Deposition of Pd directed toward the target tissue. The unique properties was performed by a photodeposition method as well of Au play an important role in this area. A series of as sodium borohydride (NaBH4) reduction (SR). The presentations were delivered by experts including Laura photocatalytic performance was evaluated using Maiore on synthesis, characterisation and evaluation the reforming of ethanol as a model. TEM analysis of Au complexes as antimicrobial compounds (37) demonstrates that all metal NP were deposited on the and Meike Roskamp (Midatech Pharma Plc, UK) on

TiO2 homogeneously for all samples. Analysis by XPS Au anticancer drugs. Midatech Pharma are using confi rmed the presence of metallic Au and Pd with ultra-small targeted sugar-coated Au NP as delivery the lack of any signifi cant PdO. The absence of PdO agents and have their own manufacturing facility for suggests the Pd is associated with Au, as segregated large scale NP under good manufacturing practice Pd would likely oxidise. The association of Pd with Au (GMP) conditions (38). In the same area of drug delivery, was also supported by EXAFS results which show Pd Federica Scalleti delivered a talk on the fabrication of Au existing in a Au-rich environment with no signifi cant nanorods for incorporation into nanocapsule systems Pd–O and Pd–Pd interactions (29). (39). Another presentation in the same fi eld was by Sebastian Kopitzki (Midatech Pharma Plc) on targeted Gold in Sensing Applications treatment of liver diseases. This study focuses on attaching the targeting and chemotherapeutic agents One of the sessions focused on sensing applications to ultra-small glycol-coated Au NP. It has been shown using Au as the active material. Malcolm Kadodwala that after administration and circulation in the body (Glasgow University, UK) delivered a particularly the targeted Au NP accumulate in the liver, where the interesting talk titled ‘Biosensing with a Twist: Detection small NP show increased tumour penetration potential and Characterization of Biomaterials with Sculpted EM (40). In the diagnostics area, Yasuro Niidome (Kyushu Fields’, on ultrasensitive detection and characterisation University, Japan) presented a study on liver tissue of biomolecules using Au to create plasmonic mass spectrometry imaging using Au NP. He showed nanomaterials (30–32). ‘Metal Oxide Supported Gold that the highly stable chemical nature of Au and the Nanoparticles for Gas Sensing Applications’ delivered high sensitivity of mass spectrometry enabled the Au by Katia Fajerwerg (Paul Sabatier University, France), nanorods to serve as a mass marker in body tissue focused on Au/ZnO and Au/TiO2 for CO and propane (41, 42). detection (33, 34). Another presentation on sensing with Au was delivered by Emilie Lebon-Taillhades Conclusion (Laboratoire de Chimie de Coordination, France; and Sciences et Technologies pour l’Aéronautique This review presents only a fragment of the et l’Espace, France). She introduced a new strategy presentations and posters disclosed. The increasing for nitrate monitoring in aqueous media using Au popularity of the conference, combined with the large nanostructured electrodes (35). ‘Gold-decorated number of key stakeholders present, highlighted the

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The Reviewers

Nicoleta Muresan is a Senior Scientist working in Core Science studying the development of precious metal precursors and catalyst preparations at Johnson Matthey Technology Centre, Sonning Common, UK. She completed her degree in Chemistry at the University of Bucharest, Romania, and PhD in Organometallic Chemistry at the University of Bochum, Germany, followed by Post-Doctoral Research positions in Organometallic and Coordination Chemistry at the Max Planck Institute for Bioinorganic Chemistry, Germany, and Renewable Energy at the University of Cambridge, UK.

Agnes Raj is a Senior Scientist, compressed natural gas (CNG) project leader and global coordinator at Johnson Matthey Emission Control Technologies, Sonning Common, UK. She obtained MSc and MPhil in Chemistry from the University of Madras, India and PhD in Material Science from Imperial College London, UK. Since joining Johnson Matthey in 2007, her research activities have been focused on platinum group metal-based catalysts for diesel/CNG aftertreatment systems.

The Use of Annular Dark-Field Scanning Transmission Electron Microscopy for Quantitative Characterisation

A review of techniques developed for a high resolution study of bimetallic catalysts and other materials

By Katherine E. MacArthur*§ University of Oxford the cost of the platinum loading which currently Parks Road, Oxford OX1 3PH, UK limits their wide-scale use. To help the search for better catalysts, with higher catalytic activity and *Email: [email protected] specifi city and lower cost, steady improvement §Present address: Ernst Ruska Centrum, in characterisation abilities is essential. Since Forschungszentrum Jülich GmbH, 52425 Jülich, the technique’s infancy (1), ADF STEM has been Germany utilised for the study of catalysts due to the strong dependence of signal intensity on atomic number (see Figure 1). This so-called ‘Z-contrast’ has enabled Small metallic nanoparticles used for polymer differentiation of individual heavy atoms from lighter, exchange membrane fuel cells (PEMFC) represent a and even crystalline, supports well before atomic characterisation challenge. Electron microscopy would resolution was even possible, as fi rst demonstrated seem the ideal technique to analyse their structure by Crewe, Wall and Langmore (1) and later by Nellist at high resolution. However, their minute size and and Pennycook (2) among others. STEM suffers sensitivity to irradiation damage makes this diffi cult. In from the inherent problem of many other high this review, the latest techniques for overcoming these resolution techniques, where the particularly small limitations in order to provide quantitative structural area investigated may not be representational of the and compositional information are presented, focusing whole sample. However the technique does provide specifi cally on quantitative annular dark-fi eld (ADF) the local atomic scale information which is essential scanning transmission electron microscopy (STEM) to completely understand catalysts at the same and quantitative energy dispersive X-ray (EDX) length scales as their chemical reactions. analysis. The implications for the study of bimetallic This paper will introduce the key advantages and fuel cell catalyst materials are also discussed. limitations of STEM for materials characterisation. Some of the more recent advances and developments 1. Introduction in the fi eld of quantifi cation, where the machines are beginning to be used much more as analytical tools It is well established that catalyst nanoparticles rather than high resolution cameras, will also be are critical to the success of PEMFC; indeed it is discussed.

similar to scanning electron microscopy (SEM) (3) except that here the transmission signal is collected and it is customary to use an electron transparent transmission electron microscopy (TEM) thin foil, producing signifi cantly improved resolution because beam-spreading or particle scattering events are reduced in thin samples. There are a variety of signals emitted by the sample due to excitation from the high energy electron probe (see Figure 2(b)). One of the major advantages of STEM is the ability to detect several of these signals in parallel. The collection of several signals in parallel is particularly benefi cial for beam sensitive samples, such as catalysts, as they will often damage or reconstruct during analysis due to the high energy provided by the electron beam. 20 nm This means that sequential images will not present the same sample structure, especially at atomic resolution. Fig. 1. Example of a low magnifi cation ADF STEM image A small on-axis detector, with an outer collection angle of Pt(Pd) core-shell catalyst nanoparticles which show up typically less than 5 mrad, produces a bright-fi eld (BF) much more brightly than the low atomic number carbon STEM image. The origin of the image contrast is the black support. Image was taken on a Jeol atomic resolution microscope (ARM) interference between overlapping Bragg disks (these disks are green in Figure 2(a)). Due to the theory of reciprocity (5) BF-STEM, with a convergent illumination 2. The Fundamentals of Scanning Transmission and a small on-axis detector, is analogous to BF high Electron Microscopy resolution TEM, with a small point source and a large STEM is a process where pre-specimen lenses focus detector collecting the resultant scattered beams. the beam into a small probe that is scanned in a raster A disadvantage of both methods is that they suffer pattern across the sample (see Figure 2(a)). This is contrast inversions with defocus and thickness, and

(a) (b)

Electron source Incident high- Objective aperture Backscattered kV beam Secondary electrons (BSE) electrons (SE) Characteristic Objective lens X-rays Auger electrons Visible light Focused probe at specimen ‘Absorbed’ Electron-hole electrons pairs Overlapping Bragg discs Bremsstrahlung Specimen X-rays Elastically Inelastically scattered scattered Direct electrons electrons ADF detector beam Bright fi eld detector Fig. 2. (a) Schematic of i mage formation in a STEM, showing the on-axis small bright-fi eld detector and the larger annular dark-fi eld detector, pink; (b) signals generated when a high-energy beam of electrons interacts with a thin specimen. The directions shown for each signal do not always represent the physical direction of the signal but indicate, in a relative manner, where the signal is the strongest or where it is detected. Image (b) was redrawn from Williams and Carter (4)

118 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691186 Johnson Matthey Technol. Rev., 2016, 60, (2) diffi culties associated with image interpretation of a the possible additional detectors for microanalysis. The coherent signal (4). A recent approach aims to produce most recent solution is an aberration corrector, where a more incoherent BF image by using an annular a series of non-round lenses are used to counteract detector, referred to as annular bright-fi eld (ABF). The the aberrations, similar to the lenses in glasses for theory is that ABF is more sensitive to lighter elements eyesight improvement. In STEM this results in not only like oxygen or lithium (6–8), although the exact origin a reduced probe size (improving resolution) but also of the image contrast is still a matter of debate so an increase in the current density within this corrected whether or not quantitative intensity information can be probe (16), causing an enhanced signal-to-noise ratio extracted is uncertain. (SNR) during imaging and signifi cant improvement in Using an annular detector to only collect the electrons counts for microanalysis. Aberration correctors allow scattered to higher angles, typically larger than microscopes to keep their large pole piece gaps and are 80 mrad, often referred to as high angle annular now commercially available, existing in microscopes dark-fi eld (HAADF), produces a coherency loss in the around the world (16–19). signal detected. Originally, Howie proposed (9) that the cause of this coherency loss was thermal diffuse 3. Quantifi cation of Annular Dark-Field Images scattering (TDS) at larger detector angles becoming the more dominant scattering mechanism over elastic ADF STEM images are easy to interpret qualitatively or Bragg scattering. However, it was later demonstrated due to the Z-contrast nature of the technique and the that the process of integration of the signal over a absence of any contrast inversions with thickness. large annular detector, and therefore a large range of The information they can provide when images are scattering angles, is the cause of the coherency loss. treated as data sets and analysed quantitatively is Any dynamical elastic diffraction of electrons scattered only just beginning to be explored. Often what people onto the detector will have no effect on the fi nal image mean by ‘quantifi cation’ is a comparison with simulated as the intensity is only redistributed elsewhere in the images. Anderson et al. (20) scaled simulated images detector and is still integrated into the total collected to fi t with experimental data rather than acquire them signal (10). ADF STEM produces an image with on an absolute scale. Darji and Howie (21) have also atomic number sensitivity or Z-contrast following discussed the necessity of correcting experimental power law relationship Zα, where α is between 1 and 2 data; in this case for additional scattering from a depending on the angular range of electrons collected crystalline substrate, as they believed this was the origin (11) and relative Debye-Waller factor between atoms of the mismatch between experiment and simulation. among other things. Many early attempts at intensity Meanwhile for TEM there is a widely accepted mismatch quantifi cation have relied on accurately characterising between simulation and experimental intensity known this exponent. as the Stobbs factor (22). 2.1 Aberration Correction The earliest attempts on quantifi cation were by Retsky Unlike optical lenses, electromagnetic lenses contain (23) and Isaacson et al. (24) in the 1970s, comparing inherent aberrations that are unavoidable (4, 12, 13). the integrated intensity of heavy metal atoms or small When aberration correction is discussed it normally clusters to those values calculated from fi rst principle refers to correction of the positive spherical aberration, quantum mechanics. Work on small metallic clusters has often assumed that the integrated ADF intensity of Cs or C3, in which the rays furthest from the optic axis are focused more by the lens fi eld. In early a cluster scales linearly with the number of atoms it instrumentation, namely TEM, resolution was improved contains (25, 26). Therefore if the intensity of a single by using higher accelerating voltages (14) in order to atom is known, dividing the total intensity of a cluster minimise the sample interaction volume, a process by this value yields the cluster size in atoms. It is an which inevitably had its own limitations due to the acceptable approximation for amorphous and off-axis resulting high damage rates. Later it was combatted clusters where it can be assumed that no channelling by using a so-called high-resolution or narrow gap pole (see below) is occurring. However, it is overly simplistic piece with a much smaller Cs of 0.7 mm (15). However, to assume such linearity for on-axis crystalline particles the narrow gap produces large restrictions on specimen and neglecting coherency effects in calculations is holders, including tilt ranges and in situ cells and limits likely to cause signifi cant errors.

3.1 Channell ing dopant atoms within an atomic column based on their For direct interpretation of images and accurate contribution to the image intensity (34–37). quantifi cation, on a column by column basis, cleanly 3.2 Dete ctor Calibrations resolved atomic resolution images are required. This necessitates viewing the sample down a low order The more common method of ADF STEM quantifi cation zone-axis (27). When atoms are aligned in this way, utilises a detector normalisation method pioneered parallel to the incident electron probe, they provide by Singhal, Yang and Gibson (26), and LeBeau and a small lensing and therefore focusing effect on the Stemmer (38), as such it is essential to be able to beam (28) (see Figure 3). The subsequent atoms in understand and map the effi ciency of the STEM the atomic column then experience a more focused detectors. Commercially available ADF detectors probe than the fi rst atom; resulting in the amount of consist of a scintillator, a glass light pipe and a scattering to the detector initially increasing faster than photomultiplier tube (PMT). The scintillator is normally linearly with respect to the number of atoms. Therefore either a powder Y2SiO5 doped with cerium, a single the contribution of the second atom in a column to crystal made of yttrium aluminium garnet (YAG) or, the integrated intensity (the summed value of all the more commonly, yttrium aluminium perovskite (YAP). pixels within the image which represent one feature) or Electrons hitting the scintillator are converted into a the total scattering cross section (29) of the column is photon cascade. The photons are directed to a PMT larger than the contribution of the fi rst and so on. Along through the light pipe and converted into an electrical a longer column, oscillations in intensity are seen (30) signal, which can be controlled to vary the ‘contrast’ and the process is often referred to as channelling. in the fi nal image. A constant voltage (direct current TDS simultaneously leads to a reduction in the electron offset) may be added by a preamplifi er, controlling intensity along a column which is often referred to as the ‘brightness’ in the fi nal image. Finally the output is absorption. Channelling has considerable implications digitised by an analogue to digital converter and sent for ADF STEM if not taken correctly into account to the computer. When aiming for a truly quantitative (27, 29, 31, 32). Heavier atoms provide a stronger comparison with simulations it is important to map and lensing effect than lighter atoms, which means that in understand the detector effi ciency, as most simulation columns containing a mixture of atom types the specifi c software packages normally assume a perfect detector, sequence of the atoms in the column will affect the modelled by a uniform effi ciency mask. resulting intensity scattered out to the detector (33). The more common method of detector mapping In certain circumstances channelling can be exploited relies on using a confocal arrangement, referred to as to expose subtle sample changes; for example it STEM ‘alignment’ mode. The post specimen lenses has been used advantageously to map the height of are adjusted to translate an image of the probe at the specimen plane onto the detector plane (see Figure 4(b)). This produces a fi ne convergent beam which can be scanned over the detector; recording the detector output with respect to probe position First atom acts as provides a map of the detector effi ciency. The gain and a lens DC offset are optimised from the detector map such that the maximum range of signal is achieved without saturating the detector or clipping any of the pixels in the vacuum (38). Scattering from The alternative ‘pencil beam’ method of detector second atom is stronger scanning (26) may provide a more realistic map in angle space (because of how the beam passes through the post specimen optics); the beam is travelling tilted from the optic axis. This was the more traditional method of detector mapping used in dedicated STEMs Fig. 3. Schematic of how a colu mn of atoms focuses the where no post specimen lenses were present due STEM probe producing a stronger scattering and therefore a higher intensity than the atoms would produce individually to the way in which the beam passes through the post-specimen optics. The convergence angle of a

for imaging nanoparticles or other low dimensional (a) (b) materials. Working from the premise that real detectors are asymmetric in their collection effi ciency (43, 44) there needs to be a way to compensate for this to improve matching with simulations. Rosenauer et al. (39) use a two-dimensional (2D) profi le estimate of their detector asymmetry to simulate images with an asymmetric detector; however this approach is only possible with certain simulation packages. The fl ux weighting method developed by Martinez et al. (45) aims to improve the detector normalisation by accommodating for this asymmetry in the experimental images rather than adjusting the simulations.

Fig. 4. (a) Ray diagram of a STEM with the electron fl ux 3.3 Recent Advances in STEM Quantifi cation seen by the detector due to scattering from the sample; (b) LeBeau and Stemmer are generally acknowledged scanning confocal geometry where the image of the probe as the originators of a resurgence of interest in at the specimen plane is transferred to the detector plane detector calibrations and recording images on an absolute intensity scale (see Figure 5) (38). Their typical STEM probe is around 20 mrad, whereas the method relies on a full characterisation of microscope angle of scattering out to the detector can be up to imaging parameters and detector responses in order ten times that. These scattered electrons will interact to achieve direct comparison with simulations (46), with post specimen lenses at a very large tilted angle although defocus has been fi tted empirically from where lens aberrations are normally worse, making it selecting the simulated image which fi ts most closely benefi cial to map the detector in angle space. However, to experimental images. it is more diffi cult to set up and thus its use so far has Quantitative comparison between experiment been limited to dedicated STEM machines. and simulation has only become possible due to When using detector normalisation for quantitative improvements in parallelisation of simulation software, analysis it is important to be aware of the linearity of the fi rst on central processing units (CPU) and then detector response (38–40). In particular, altering the graphical processing units (GPU). Such parallelisation detector contrast does not produce a linear variation makes it possible to produce accurate simulation (41) in the detector signal, which is why contrast and reference libraries in reasonable time-scales using the brightness settings are normally kept constant between more accurate (but more computer intensive) frozen detector mapping and experimental images. phonon approach rather than absorptive potential (47). The majority of image intensity detector normalisation From a detector effi ciency map like the one presented methods rely on the detector responding linearly with in Figure 6(b), the average pixel intensity in the the number of incident electrons. Care should be taken active region (the area coated with the scintillator, not to rely too heavily on this linearity alone. During predominantly blue in Figure 5(b)) can be considered experimental image acquisition the detector will see to represent one hundred percent of the total electron a diffuse electron fl ux (see Figure 4(a)) at a much beam, whilst the average intensity of the background lower intensity, whereas during detector mapping region (anywhere in the image detector image the whole beam is scanned over it in a fi ne probe. considered not to be the active region) represents To combat this it is prudent to drop the probe current no-beam and is also known as the black-level. by a known amount during the detector mapping (42) Normalising experimental images using these maps and subsequently incorporate a ratio of the probe results in images being scaled as a fraction of the currents into the quantifi cation method. Lowering the incident beam which has been scattered out to the current also helps to minimise damage to the detector detector. and makes a greater contrast range available in the An improvement to this has been developed by fi nal experimental image, which is particularly useful calculating the scattering cross-section (29) or

(a) (b) Fraction of average intensity 0 0.2 0.4 0.6 0.8 1.0 1.2

0.5 nm 0 2 4 6 8 10 12 14 16 18 Percentage of incident probe intensity Fig. 5. (a) One of the ﬁ rst ADF STEM images normalised to the incident beam. The sample is a gold wedge and the white numbers represent the assigned atom counts based on direct comparison with simulation; (b) the associated detector efﬁ ciency map in units of fraction of the average intensity of the active region. (Reprinted with permission from (46). Copyright (2010) American Chemical Society)

9 (a) (b) (c) 1870 8 7 6 5 1850 4 ICL criterion ICL Frequency 3 2 1830 1 0 100 200 300 400 500 600 700 800 0 5 10 15 Scattered intensities, au Number of Gaussians

Fig. 6. (a) Image of a silver nanoparticle embedded in an aluminium matrix, the dots represent the estimated position of the atomic columns with those assigned as silver marked in red; (b) histogram of the scattered intensities of the Ag columns. The black solid curve shows the estimated mixture model; the individual components are shown as dashed curves; (c) the ICL criterion evaluated as a function of the number of Gaussians in a mixture model. The minimum at 10 components represents the optimum number of components (Reprinted by permission from Macmillan Publishers Ltd: Nature (59), copyright (2011))

Gaussian volume (31, 48) of each atomic column of electrons interacting with that column and scattering within an image. Scattering cross-sections are out to the ADF detector, much in the way that other calculated by integrating the normalised intensity over cross-sections behave in particle physics. Gaussian one atomic column, using a Voronoi polygon (49, 50) or volumes are mathematically identical to cross-sections otherwise, and multiplying by pixel area. This creates (51), only instead of integrating the intensity in a a value with area units that represents the probability polygon method, a 2D Gaussian curve is ﬁ tted to the

122 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691186 Johnson Matthey Technol. Rev., 2016, 60, (2) image intensity of each column and the integrated combining several images for incorporation into the volume under this curve is used. Using these volumes histogram (31). or cross-sections for quantifi cation still requires 3.4 Three-Dimensional Reconstruction comparison with simulations but is a lot more robust to defocus, tilt, source size and other aberrations (29, The traditional method for three-dimensional (3D) 32, 48). Although the robustness of such an integration reconstruction is electron tomography (61), developed or averaging over a unit cell has been commented from X-ray tomography (62). A 3D structure is on before (20, 39, 52), it was not until the scattering reconstructed from a tilt series of images using one cross-section work (29, 32, 48, 51) that it was of a variety of possible reconstruction algorithms (63, mathematically proven and robustly tested through a 64). The main requirement is for the micrographs to series of simulations. Scanning distortions may still affect be ‘true projections of the structure’ (65) such that the quantifi cation accuracy; recent advances in non- image intensities are a monotonic function of sample rigid image registration may provide the solution to this thickness. Therefore the use of TEM or BF-STEM is (53–55). not desirable because Fresnel fringes and diffraction An alternative technique to direct comparison with contrast are a signifi cant problem. Tomography was simulation is one based on statistical parameter initially developed for biological samples (66, 67) and estimation theory (56), pioneered by Van Aert et al. (57). is regularly used to determine the location of catalysts The theory relies on the inherently quantised nature of a particles on their supports (68–73). Once the 3D shape has been reconstructed important information such histogram of integrated intensities from an experimental as surface area, volume and thickness distributions image (58). For a single element scenario, for example can be measured for particles (74). Carrying out the silver particle in Figure 6, the integrated intensities tomographic reconstructions by this method subjects should naturally be quantised in the histogram due to the sample to high amounts of electron dose from the the fact that each column contains an integer number hundred images required for a single reconstruction of atoms. These discrete values become somewhat and the dose infl icted during any tilting and re-centring smeared by Poisson noise and other random errors; processes between frames. however, it should still be possible to decompose the The more recent method of discrete tomography histogram of all the integrated intensities into a series (75) signifi cantly reduces the number of necessary of Gaussian components through statistical estimation projections to less than 10, even in the presence of and a maximum likelihood criterion. The critical noise or defects, through the use of ‘prior knowledge’. component of this algorithm is a ‘cost function’ to Atomic resolution discrete tomography (59) uses the minimise the total number of Gaussians incorporated, prior knowledge that the particles have crystalline otherwise the best solution would be an infi nite number structure, contain no voids and surface steps and kinks of Gaussians. The integrated classifi cation likelihood are minimised. This is primarily carried out by defi ning (ICL) criterion is used for this purpose and provides a each crystallographic point as a boxed region and then local minimum at the optimum number of components only one atom may lie within each box. (Figure 4(c)). Van Aert et al. were the fi rst group to publish atomic The main advantage of ICL is that it does not require resolution discrete tomography data on their embedded the experimental images to be intensity normalised. A silver nanoparticle (59) and also the core of core-shell systematic error or scaling can easily be incorporated or semi-conductor nanocrystals (76). In their research alternatively it can be used as an independent method they found that two or three quantifi ed HAADF STEM of atom counting. The limitations of the ICL approach images down crystallographic orientations (for example have also been covered by Van Aert (51) and De Backer [100], [010] and [110]) gave suffi cient information to et al. (60). In particular the fi eld of view and therefore reconstruct the particle, provided the atoms are restricted the number of observations per Gaussian component to a particular, in this case face-centred cubic (fcc), greatly affect the ability of the ICL criterion to estimate crystalline structure. Their quantitative STEM method the correct number of components. This is critical for (57) was used on each projection image to estimate the investigations of nanomaterials where their fi nite size number of atoms before carrying out a reconstruction. It severely limits the number of observations available is, however, important to note that both of the example per component, but could potentially be overcome by particles used to demonstrate this method had been

123 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691186 Johnson Matthey Technol. Rev., 2016, 60, (2) embedded in another material, thereby limiting the information. A signifi cant advantage to this approach surface mobility and improving the dose tolerance for is the requirement of only one experimental image for such reconstructions to be carried out. 3D information. This not only provides reduced dose A more recent alternative to tomography has been capabilities but also increases throughput, leading to designed specifi cally for the study of free standing the ability to reconstruct a time series of images (77) or catalysts where the mobility of surface atoms between several particles in one sample (47, 78). sequential images will be high. Using the method of quantitative STEM with scattering cross-sections, 4. Composition Analysis an automated procedure has been developed for peak fi nding (to fi nd each atomic column location The STEM image intensity contains both composition within an image) and subsequently estimating the and thickness information which can be diffi cult number of atoms they contain (77). Armed with the to differentiate. A particular example of this is atomic column locations and the number of atoms it acid leached platinum cobalt nanoparticles; the becomes possible to estimate the 3D structure from increased ADF image intensity of the heavy metal only one experimental image (see Figure 7). The same shell is swamped by the loss in intensity due to the assumptions described above for atomic resolution large reduction in sample thickness at the particle discrete tomography are combined with atomic spacing surface (79, 80). The challenge facing quantifi cation in the beam direction predicted from bulk crystal of bimetallic systems is how to incorporate additional

(a) (b)

0.02 0.06 0.1 0.14 0 10 20 30 40 Fraction of incident beam Scattering cross section, Mb

Fig. 7. Particle 3 – norma lised experimental ADF image (a) of a Pt/Ir particle with the calculated cross-section map; (b) the 3D reconstructed particle is shown parallel to the beam direction; (c) perpendicular to the beam direction at from two angles 90º from each other (d) and (e). For ease of comparison with a Wulff construction the (d) projection has had the faces coloured in, {111} purple, {100} blue, {110} red

124 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691186 Johnson Matthey Technol. Rev., 2016, 60, (2) information or assumptions in order to separate out prove challenging to use for mapping of several the thickness and composition contributions from elements at the same time. This is especially true for the ADF STEM signal. For PEMFCs in particular it is heavy elements like Pt (89) where the ionisation edge desirable to understand how much Pt there is and its is very high energy. Microanalysis is a region where location on the particle surfaces. STEM demonstrates advantages over TEM due to A combination of sequential high resolution the focused probe providing localised information and STEM images and multi-slice simulation has been the ability to collect ADF and microanalysis signals successfully implemented by Ortalan et al. (81) on simultaneously. iridium-rhodium clusters, using intensity ratios to predict Aberration correction has produced a signifi cant possible combinations of Ir and Rh in each column. advancement for microanalysis, particularly in the This procedure, however, was only possible due to the realms of 2D elemental mapping due to the signifi cant small size of the clusters investigated (containing a increase in current density within the fi ner corrected maximum of three atoms per column); at larger particle probe. However for samples prone to beam damage sizes with an exponential increase in the number of this high current density is a problem. For mapping possible combinations this approach quickly becomes in particular, one must be aware of the damage unrealistic. Carlino and Grillo (82, 83) demonstrated being caused by the long acquisition times in order that it is possible to use a section of sample with to improve total counts (90). From experience (47) it known composition to interpolate the thickness where has been found that this damage can be minimised composition is unknown to estimate composition through using sequential fast scanning, where several changes on a relative scale. Rosenauer et al. have fast acquisitions of a line scan or map are combined, extended this approach at atomic resolution analysis to reduce the dwell time per pixel per scan while still (49, 84). However, this technique is rather limited to maintaining the total live time necessary for high the semiconductor multilayer systems for which it counts. Ideally sub-pixel scanning is also used so the was designed, where areas of known composition beam is never stationary over the sample. It is unclear are present in the same image. Molina (85, 86) and why such an approach yields lower damage unless Hernández-Maldonado et al. (87) have demonstrated the lower dose rate per pixel allows suffi cient time for approaches using a series of carefully controlled the charge or heat to dissipate away from the analysis reference samples with accurately known composition, region into the remainder of the sample. although this comes with its own problems of being Even without aberration correction Prestvik et al. able to manufacture such standards. carried out some of the earliest investigations into With these restrictions to quantitative ADF STEM for STEM-EDX analysis of bimetallic nanoparticles where compositional analysis it is therefore useful to look at the individual platinum-rhenium particles sized 0.5–2.5 nm other available signals which an electron microscope were imaged and the percentage Pt was plotted can provide. Elemental analysis of catalyst particles against particle size (91). EDX mapping has been began as early as 1983 (88). The advantage of using used to demonstrate the localisation of sulfur on the so-called microanalysis in the electron microscope nanoparticles in ‘poisoned’ catalysts (90). In more recent is the ability to achieve compositional information at years EDX of 5 nm core-shell nanoparticles mapping the same length scale as the electron micrographs has been able to differentiate the layered structure very thereby providing invaluable local microstructural effectively (89); however the atomic columns are not information of a sample. Here the focus is on EDX, resolved due to the trade-off between resolution and one of the two microanalysis techniques which can be counts. Deepak et al. (89) have demonstrated the use carried out within a TEM/STEM. EDX analysis relies of multivariate statistical analysis (92) to improve their on the excitation and ejection of an inner shell electron quantifi cation, mining their data cube for independently within an atom in the sample. An outer shell electron varying compositions. fi lls the subsequent hole whilst emitting an X-ray of The new generation of EDX detectors has provided a characteristic energy. The EDX signal has signifi cantly huge advancement for the fi eld of TEM microanalysis. worse SNR than ADF but the elemental information The improved detector design of these silicon drift is much more readily separable. The alternative detectors (SDD) (93–95) allows for larger devices and technique, electron energy loss spectroscopy (EELS), therefore increased solid angles for X-ray collection. provides higher count rates but in many cases can They are also often windowless which allows them

125 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691186 Johnson Matthey Technol. Rev., 2016, 60, (2) to be located closer to the sample thereby increasing the primary method for EDX quantifi cation due to its the improved solid angle of collection (up to 1.3 srad, incorporation into the majority of commercial analysis whereas previous generation detectors only reached software. Most EDX software allows a quantifi cation up to 0.3 srad). The improvement in X-ray count rates option, however it is a rather ‘black box’ approach is suffi cient that atomic resolution maps (96–99) and meaning it is not necessarily clear which methods of X-ray tomography (100) are now both regularly possible count extraction are being used. in reasonable time scales. Tran et al. are among the fi rst The k-factors used in software have normally been to apply these new detectors to nanoparticles (101), calculated from fi rst principle quantum mechanics. allowing them to map their copper-gold nanoparticles These theoretical k-factors have a minimum estimated ranging 1–10 nm in diameter. No comment or attempt systematic error of around 10% (103) but this could be is made towards quantifi cation of the alloy ratio apart as high as 20% (104). The only way to reduce such from establishing their particles as random alloys. an error is to calculate an experimental k-factor by analysing a set of known samples with very similar 5. Quantifi cation of Energy Dispersive X-ray density and thickness at a range of compositions to create a calibration curve (~±1% error is achievable). SDD have opened up a new era in microanalysis. Unfortunately, this is time consuming and there is Combining their improved collection effi ciency with an inherent diffi culty in manufacturing such samples the higher current densities provided by aberration where the composition is homogenous on the correction (16, 19) yields a huge increase in X-ray nanoscale and corresponds to the macroscopic counts leading to the potential for improved quantitative chemical composition determined by other methods. analysis. This is particularly relevant when investigating An alternative to the k-factor approach is the catalyst nanoparticles where only a small number of ζ (zeta)-factor (104, 105) method fi rst proposed in counts are generated from the low mass of sample. 1996 by Watanabe et al. (105). Designed for thin fi lm Quantifi cation through direct comparison with specimens it makes use of pure element standards simulations (98) must take into account both X-ray which are rather more readily available than the alloy absorption and subsequent re-fl uorescence. This alternatives, Equation (ii): requires knowledge of, or estimation of, local thickness Ix and density (although this can be neglected for thin t = ζx C D (ii) specimens such as nanoparticles). The detector x e geometry and detection effi ciency must also be ζx is the factor connecting Ix, the raw X-ray counts, to included in quantifi cation calculations, including both ρt, the total density multiplied by sample thickness. Cx the solid collection angle and the take-off angle. is the weight fraction of x, with a similar relationship holding independently for other elements in the 5.1 k-Factors and ζ-Factors system. It assumes that the characteristic X-ray

The original ratio approach to EDX quantifi cation in intensity from element A, IA, is proportional to the TEM thin fi lms was fi rst proposed by Cliff and Lorimer mass-thickness of the sample. This holds well nearly forty years ago (102). The Cliff-Lorimer or as long as X-ray absorption and fluorescence are k-factor, kAB, relates the atomic fractions, CA and CB, of negligible, which is the case for thin specimens; for constituent elements A and B to their measured X-ray thicker specimens an absorption correction should intensities, IA and IB (Equation (i)). also be incorporated (106), (Equation (iii))

CA IA De = Neip (iii) = kAB (i) CB IB De is the total number of electrons seen by the In the technique’s infancy a ratio approach was sample during acquisitions defined in terms of Ne, essential due to the mechanical and electrical the number of electrons in a unit electric charge instabilities present in the early analytical electron (or 1/e), the probe current, ip, and the acquisition microscopes and therefore the low X-ray counts time, . The ζ-factor, therefore, is independent detected. Electron microscope capabilities are now of acquisition time, beam current, composition considerably improved, especially the beam current and mass-thickness providing it with units of kg stability. Despite this the k-factor approach remains electron m–2 photon–1.

It is vital to know the beam current at the time of quantifi cation should only be carried out with the K-lines analysis by a direct measurement (for example, as the L-lines also begin to drop below the safe limit. Faraday cup) in order to calculate the dose. Whilst this Often the degree of absorption occurring within the method may seem less trivial as it requires thickness sample can be estimated through the K-line to L-line calibration and current measurements it does have height ratios because the L-lines begin to be absorbed the signifi cant advantage that pure element thin fi lms much sooner, thereby changing the peak height ratios. can be used as standards (107), which are often 5.3 Energy Dispersive X-ray Cross-sections more routinely available. This method can prove more accurate than the k-factor method and is an absolute The most recent method for quantifi cation is by value rather than a ratio based on other elements calculating cross-sections. EDX partial cross-sections within the system. It can also be used as a method (79) provide a robust measure which is easy to compare of instrumentation comparison, to evaluate which with the scattering cross-sections used for ADF STEM microscopes have better analytical capabilities. quantifi cation (23, 29, 77) and ionisation edges in EELS (111–113). In suffi ciently thin samples, not aligned 5.2 Accuracy Limitations along a low order zone-axis, so that channelling, The key problem with STEM EDX of nanoparticles X-ray absorption and fl uorescence can be neglected, (108–110) is the limited count rate because of the the number of X-ray counts is linearly proportional small number of atoms excited. For example, in early to sample thickness. The EDX partial cross-section maps produced by Lyman (88) in 1987 the number of of a single atom can therefore be determined from a counts from a background area pixel was 7 whilst the polycrystalline pure element wedge sample with known maximum number of counts from a Pd pixel was only wedge angle using Equation (iv) (79): 36 (giving a N error of 18.5%). The problem with EDX e dlx is fi nding the balance between high beam currents for σx = · (iv) i n tan(θ) dx improved analytical resolution which increases the x probe diameter or keeping the probe diameter small where e is unit electronic charge, i is the probe current, for improved spatial resolution but which results in  is the total pixel dwell time, nx is the atomic density poor counting statistics. E et al. (42) confi rmed that of the element x and  is the wedge angle. dIx is the /dx the limiting factor for EDX of nanoparticles will be in gradient extracted from a line profi le showing integrated achieving high enough counts of characteristic X-rays X-ray counts plotted against distance into the sample. where beam sensitive samples place restrictions on the Further details about this calibration method and its length of acquisition possible. A low number of counts accuracy can be found in the literature (79). results in poor statistics, reducing the reliability of Cross-sections are specifi c to a given microscope quantifi cation. There is a thickness dependent error in setup, including the solid-angle and collection effi ciency quantifi cation (80) because a thicker region of sample of the EDX detector, as well as the microscope will produce more raw X-ray counts and so have a operating voltage, making them a good comparison smaller statistics error. tool between analytical TEMs. Such calibrations are Another fundamental limitation for accurate STEM also dependent on the specifi c X-ray peaks used, as EDX quantifi cation is absorption of X-ray photons well as the chosen energy window and background before they are able to escape the sample, although subtraction method, but this is also true for k and this becomes less of a problem for thinner samples ζ-factors. The cross-section approach can provide like nanomaterials. The absorption rate is element atomic counts on an absolute scale which negates the specifi c, so could cause an error in the quantifi cation need for knowledge of sample thickness, particularly results. Work by Watanabe and Williams (104) has useful for nanoparticles where the exact thickness can demonstrated the effects of both the sample thickness be diffi cult to determine. and X-ray line selection on EDX quantifi cation. For the EDX cross-section quantifi cation has been applied to very small sample thicknesses of nanoparticles any K acid leached PtCo nanoparticles in order to determine or L lines will provide suffi cient quantifi cation accuracy; the depth of the Pt enrichment produced by the leaching however the M-lines may still provide small absorption process (79, 80). With Pt(Pd) core-shell nanoparticles errors and should therefore be avoided where possible. it is possible to characterise the non-uniformity of the For elements with an atomic number lower than 30, Pt shell (see Figure 8). The intended two-monolayer

(a) (b)

HAADF PdL 2 nm PtL

0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Fraction of Pt Fraction of Co

Fig. 8. A 6–7 nm particle de monstrating the more usual Pt capping of the particles, with overall composition of 45.4% Pt. (a) The cumulative STEM image shows a clear step contrast variation towards the particle surface; (b) the composite image; (c) the fractional Pt and (d) Pd maps show distinct Pd core with a Pt rich cap covering two thirds of the particle edge. The Pt shell cannot cover where the particle is sitting on the carbon black support

coverage actually results in a thicker than two-monolayer information is invaluable in the work towards catalyst coverage over part of the particle surface. The Pt shell understanding and design. The beam sensitive is applied after the particles have been deposited onto nature of catalyst nanoparticles puts restrictions on the carbon support which limits the amount of coverage the acquisition time for microanalysis. However in which can occur due to the position of the particles’ STEM multiple signals can be collected in parallel. contact with the particle support. Therefore creating a cross-section method for quantification which is comparable across multiple 6. Conclusions signals opens the way for combining the information they provide to yield more fruitful results. The new developments in STEM characterisation now make it possible to pursue sub-particle and 7. Acknowledgments even atomic resolution compositional and structural K. MacArthur gratefully acknowledges the financial information about nanomaterials in a genuinely quantitative manner. Whilst this approach should support of the Engineering and Physical Sciences still be combined with more broad-beam techniques, Research Council (EPSRC) and Johnson Matthey such as X-ray or infrared spectroscopy, with much towards her DPhil. She is also indebted to the better statistical representation, the movement support of both research groups at the University of towards automation is beginning to allow for Oxford and her supervisors Dogan Ozkaya, Peter several particles to be analysed. Such atomic-scale Nellist and Sergio Lozano-Perez.

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The Author

Katherine MacA rthur is currently a Helmholtz Postdoctoral Fellow at the Ernst Ruska Centre, Forschungszentrum Jülich, Germany. Her Doctorate was sponsored by Johnson Matthey at the University of Oxford, UK, which she completed in 2015. Dr MacArthur continues to look at developing techniques for improved high-resolution characterisation of catalyst nanoparticles.

Applications of Neutron Scattering in Catalysis

Where atoms are and how they move

By Stewart F. Parker* is probably for two main reasons: there is a lack of ISIS Facility, STFC Rutherford Appleton Laboratory, understanding of what the technique can provide Chilton, Didcot, OX11 0QX, UK and it is seen as being the province of purely and academic research, largely for condensed matter UK Catalysis Hub, Research Complex at Harwell, physics. These perceptions are not correct: there STFC Rutherford Appleton Laboratory, Chilton, Didcot, is a wealth of information that can be provided to Oxfordshire OX11 0FA, UK the chemist, surface scientist and other specialisms within both academic and industrially focused Paul Collier§ research. There are a small number of centres for Johnson Matthey Technology Centre, Blounts Court, neutron scattering worldwide, since the production Sonning Common, Reading, RG4 9NH, UK of sufficient neutrons for useful science requires highly specialised facilities. The aim of this review Email: *[email protected]; is to show that even within the constraint of a limited §[email protected] number of facilities, neutron scattering can usefully contribute to many areas of catalysis that range across structure determination at the micro- to the Neutron scattering is a severely underused technique nanoscale and atomic and molecular dynamics from for studies of catalysts. In this review we describe how the femtosecond (vibrational spectroscopy) to the and why neutrons are useful to catalysis. We illustrate sub-microsecond (diffusion) timescale. the range of systems that have been studied by both elastic and inelastic neutron scattering. These range Neutron Properties and their Applications from structural studies of adsorbates in zeolites to determination of the structure of surface adsorbates, The usefulness of neutron scattering arises from the characterisation of nanoparticles, the measurement properties of the neutron, which is an uncharged and mechanism of diffusion and spectroscopic particle of mass 1.00866 amu (almost the same as characterisation of adsorbed species. We conclude the hydrogen atom 1H, 1.00782) with a magnetic with how to access neutron facilities and some future moment. Neutrons are scattered by the atomic nuclei prospects for the application of these techniques to of the sample and, since the nucleus of an atom is only industrially useful materials. ~0.1% of the diameter of the atom, to a neutron most of a material is empty space. Hence neutrons only Why Neutron Scattering? interact weakly with matter and as a result are highly penetrating; for example the attenuation at 1.81 Å Neutron scattering is undoubtedly a severely (which corresponds to the peak ﬂ ux from a research underutilised capability in catalysis research. This reactor) by 1 mm of aluminium is less than 1%.

For neutron diffraction studies it is straightforward that decreases with increasing scattering angle. Thus to study light elements in the presence of heavy the smaller d spacing information is relatively better ones. The scattering cross section is both atom and determined in a neutron experiment compared to a isotope dependent, Figure 1, and is not a monotonic similar X-ray experiment. This results in more precise function of atomic number as it is for X-ray scattering. atomic thermal parameters being determined from Hydrogen-in-metal systems are perhaps the most powder neutron data than from the powder X-ray data. important example of systems which are ideal for study The most striking difference in cross section using neutron techniques, but it is also a valuable way occurs for 1H and 2H, hydrogen and deuterium: 1 2 1 to determine the precise location of oxygen in heavy coh = 1.76 ( H), 5.59 ( H) barn, inc = 80.27 ( H), 2 metal oxides in fi elds ranging from high-temperature 2.05 ( H) barn (coh = coherent scattering cross section, superconductors to oxide ion conductors in fuel cells. inc = incoherent scattering cross section). Hence Bond distances are generally more accurate from hydrogen is overwhelmingly an incoherent scatterer neutron scattering data than from X-ray techniques, while deuterium is primarily a coherent scatterer. particularly for bonds involving hydrogen, since the Coherent scattering gives information on long range maximum electron density is not located at the atomic properties such as structure while incoherent scattering position but is displaced towards the heavier atom. Thus is a local probe. The neutron may be scattered without a C–H and O–H distances measured by X-ray diffraction change in energy (elastic scattering) as in diffraction, or are typically 0.1 Å shorter than those measured by there may be exchange of energy between the sample neutron diffraction. and the neutron (inelastic scattering). The latter may be It is also possible to distinguish elements that are coherent or incoherent, but for any material containing adjacent in the periodic table, such as Al3+ and Si4+, hydrogen, the incoherent scattering dominates. Since as commonly found in microporous materials. These much of catalysis involves the transfer of hydrogen elements have identical scattering power for X-rays from reactant to product, the sensitivity to hydrogen (since they have the same number of electrons) but is a major advantage of the technique. However, different neutron cross sections, 1.503 vs. 2.167 barn for structural studies it may be necessary to use (1 barn = 1  10–28 m2) respectively, hence providing deuterated materials, since the incoherent scattering contrast. from hydrogen results in a signifi cant background that The atomic nucleus acts as a point-like scatterer, complicates the data analysis. For inelastic scattering, unlike the electron cloud, which is of fi nite size it is only the exceptionally large value of inc that makes compared to the wavelengths used. The consequence catalysis studies feasible. of this is that the neutron cross section is independent In short, neutron scattering can be summarised as: of the scattering angle, 2θ, in contrast to the X-ray “where atoms are and how they move”. case where the X-ray cross section has a form factor Elastic Scattering – Where Atoms Are

80 H Elastic scattering of neutrons covers length scales that 60 span from 1 to 5000 Å. This ranges from the atomic 40 Sc Xe scale for structure determination to typical sizes of 20 Ni colloids. The investigation of structure is the major use of Cl neutrons in this area, particularly by powder diffraction. N Fe The location of n-heptane in silicalite-1 (1) provides an 10 example of what can be achieved. This system shows D

Cross section, barn an infl ection in the adsorption isotherm at a loading near four molecules per unit cell that has been ascribed to a 0 commensurate freezing of n-heptane in the sinusoidal 0 10 20 30 40 50 channels of silicalite-1. To investigate the unusual Atomic number behaviour, the adsorption of C7D16 was followed in situ by powder neutron diffraction. (Deuterated heptane was Fig. 1. Neutron scattering cross section as a function of atomic number used to reduce the incoherent background). The results show that the straight channels are fi rst fi lled, then near

133 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691230 Johnson Matthey Technol. Rev., 2016, 60, (2) four molecules per unit cell, the molecules also fi ll the (a) sinusoidal channels, Figure 2. Thus the diffraction data 300 show that the infl ection point in the isotherm is due to 1.2 250 self-blocking of the channels at junctions. 1.0 Bed temperature, ºC Diffraction methods are well-suited to operando 0.8 200 measurements and this is already utilised for neutron 0.6 diffraction studies of batteries (2) and proton conductors CuO(11-1) 150 (3). Studies of catalysts are also feasible as shown 0.4 Cu(111) T, ºC 100 by a recent study of the activation and subsequent 0.2 working of a commercial Cu/ZnO/Al O methanol 2 3 0 50 synthesis catalyst (4, 5). Due to their pyrophoric nature, –0.2 commercial Cu/ZnO/Al O catalysts are prepared in 0 2 3 (b) the completely oxidised form as CuO/ZnO/Al2O3 and the fi rst step in methanol synthesis is activation of Normalised integral intensity the catalyst by reduction of CuO to metallic Cu. The m/z = 4 activation process is shown in Figure 3 following the m/z = 20 reduction of CuO and the growth of Cu. It has been proposed that defects play an important role in the active Cu phase but Cu would be expected to be mobile enough under the reaction conditions, T = 493–573 K, P = 3.5–10 MPa, for these to anneal out. In situ neutron Normalised ion current, au diffraction was able to show that the nanostructured 50 100 150 200 250 Cu generated by reduction of the CuO precursor has Time, min a high concentration of stacking faults and also that Fig. 3. Reduction procedure of a commercial Cu/ZnO/Al2O3 they were stable under industrial methanol synthesis catalyst from 301 K to 523 K in D2. Normalised integrated conditions (5). intensities of the CuO(11–1) and Cu(111) peaks correlated Pair distribution function (PDF) analysis or total with: (a) the catalyst bed temperature; and (b) the effluent gas composition (m/z = 4 = D m/z = 20 = D O) during scattering analysis (6) is a well-established technique 2, 2 isobaric reduction. (The missing ion–current between 130 for the structural characterisation of poorly- or and 140 min is an experimental artefact). Reproduced from non-crystalline materials including nanoparticles. (4) with permission of Elsevier Figure 4 shows a comparison between the

(a) (b)

c b

Fig. 2. Locations of C7D16 molecules within silicalite-1 obtained from Rietveld reﬁ nement of neutron diffraction patterns. At low loading, <4 molecules per unit cell only the straight channels are occupied (black molecules) while at high loading >4 molecules per unit cell both the straight and the sinusoidal channels are occupied (black and green molecules respectively). (a) View along the [010] direction; (b) view along the [001] direction. Reprinted with permission from (1). Copyright (2007) American Chemical Society

PdO·H2O, (a model system for CO oxidation by an automotive three-way catalyst) and the data generated As shown in Figure 6, inelastic scattering covers a vast from a simulation consisting of ~18 Å nanoparticle of range of length and time scales. For catalysis, the most

PdO (7). The alternative formulation as Pd(OH)2 is relevant are quasielastic neutron scattering (QENS) clearly untenable and the simulation shows that the which probes whole-body motions (such as diffusion) material has a core of poorly crystalline PdO. on and within materials and neutron vibrational The PDF approach has been extended to enable the spectroscopy or inelastic neutron scattering (INS), local structure of adsorbed species in zeolites (8) and which probes the motion of atoms and molecules within on surfaces (9) to be determined. The advantage of the a material. method is that it does not require long range order and Quasielastic scattering (10) is a very low energy that bond distances can be obtained by inspection of inelastic process which usually manifests itself as a the radial distribution function. However, the method broadening of the elastic line and is most commonly the greatly beneﬁ ts from input by ab initio methods as was result of diffusional (translational or rotational) motion done for acetylene on nickel ion exchanged zeolite of atoms. Translational and rotational diffusion occur NaY (8) and hydrogen on Raney nickel (9). Figure 5 simultaneously but on different timescales. The energy shows a comparison of the experimental data for resolution of a spectrometer, ΔE, and the timescale, , hydrogen on Raney Ni and the radial distribution of the motion are related by the Heisenberg uncertainty function generated from a periodic density functional principle: ΔE  ≈ h/2. Thus resolution and timescale theory calculation of hydrogen on Ni(111) (for an fcc are inversely related and the slower the motion, the metal, the 111 face is the lowest energy surface so is higher the required resolution. QENS spans the range expected to predominate on nanoparticles). It can be 10–11 ≤  ≤ 10–7 s, which requires 10–5 ≤ ΔE ≤ 10–8 cm–1. seen that the agreement is excellent. The extension of the method to more complicated systems involving chemisorbed organic species is an active area of 0.2 investigation.

0.1 ), –1 ) Q R ( ( N i D 

steradian 0

–1 0

0 5 10 15 20 Distinct scattering, barns atom –0.1 Momentum transfer, Q, Å–1 0 1 2 3 4 Distance, Å Fig. 5. Difference pair distribution function, ΔD(R), for hydrogen on Raney nickel (purple) at room temperature. This is the Fourier transform of the normalised difference of the catalyst before and after addition of hydrogen. (The peak at ~1.00 Å is non-physical and results from a truncation error in the Fourier transform.) Also shown is the pair distribution D(R) function (red) generated from a periodic-DFT calculation of H on Ni(111). The spectrum is Fig. 4. The neutron diffraction differential correlation function, abscissa scaled so that the large peaks at 2.54 Å have the DN(r), for as received hydrous palladium oxide (red). The blue same amplitude. No ordinate scaling has been applied, all curve is a simulated correlation function based on a model of a the distances come directly from the ab initio calculation 18 Å nanoparticle of PdO. Reprinted with permission from (7). (9). Reproduced by permission of the Royal Society of Copyright (2010) American Chemical Society Chemistry

Elastic peak Quasi-elastic Electronic Vibrational )

 (optic mode) , Q (

S Phonons (acoustic mode) Recoil Rotational

Energy transfer, cm–1 100 101 102 103 104 105 106 Timescale, s 10–7–10–11 10–12 10–13 10–14 10–15 10–16 Length probed, Å >30 10 1 0.1 0.001 0.0001 0.00001

Fig. 6. The time and energy ranges spanned by inelastic neutron scattering

No single spectrometer can access all of this range and typically three are needed: direct-geometry time- 100 of-ﬂ ight ( ~ 10–11 s), backscattering crystal analyser –10 ( ~ 10–9 s) and neutron spin echo ( ~ 10–7 s).

Understanding the diffusion behaviour of molecules × 10 –1 10 s in microporous materials is important for the design 2 of membranes for separations and for catalysts, particularly for applications in the petrochemical 1 cient, m industry. QENS is ideally suited for the measurement fi of diffusion constants in technologically relevant 0.1 materials (11). The QENS timescales also match those accessible by classical molecular dynamics (MD) Diffusion coef Diffusion simulations which can provide insight into the diffusion 0.01 mechanism. However, until very recently (12), the 8 10 12 14 16 Chain length agreement between observed and calculated diffusion constants could differ by as much as two-to-three Fig. 7. Diffusion coeffi cients of n-alkanes in silicalite: (♦) MD orders of magnitude, Figure 7. The improvement is the simulation using simpler models (united atom, rigid lattice); (▲) MD simulations at infi nite dilution; (*) hierarchical result of using fully atomistic models for both adsorbate simulations (united atom, coarse-grained framework); (○) and framework, adsorbate loadings that match those fully atomistic MD simulation with fl exible lattice; and (□) used experimentally and a fl exible framework that experimental (QENS) (12). Reproduced by permission of deforms as the molecule moves through the channel. the PCCP Owner Societies The diffusion of branched alkanes has been much less studied than the n-alkanes. However, an understanding compared with fi ts for 3D and 1D diffusion models. of the diffusion of branched alkanes is crucial to This immediately shows that 2-methylpropan e is able understand the effect of molecular shape on both to explore both the straight and sinusoidal channels, in catalytic cracking and molecular sieving in zeolites. contrast to previous results that indicated the molecule 2-methylpropane (isobutane) is the simplest branched was confi ned to the straight channels. The MD results alkane and its diffusion in silicalite has been studied by support this conclusion and show that from an initial a variety of experimental and computational methods. confi guration where the 2-methylpropane molecules The diffusion is suffi ciently slow that the neutron were placed in the large cages at the intersections of the spin echo technique and long MD runs are required. straight and sinusoidal channels, the system evolves to Figure 8(a) shows the neutron spin echo results (13) have both types of channel populated, Figure 8(b).

(a) (b)

0.6  

,0) +

Q 0.4 )/I( t ,

Q * I( 0.2

0 0.1 1 10 Time, ns

Fig. 8. Diffusion of 2-methylpropane in silicalite: (a) Comparison between experimental and calculated normalised intermediate scattering functions at 491 K, for (▼) Q = 0.2 Å−1 and (◊) 0.3 Å−1. The solid lines are calculated with a 3D (red and blue) and a 1D (green and gold) diffusion model. (b) MD simulation at 450 K showing molecules in the straight (+ and Φ) and sinusoidal (* and Δ) channels. Reprinted with permission from (13). Copyright (2015) American Chemical Society

The conversion of methanol to hydrocarbons over acid zeotype catalysts is an attractive route to increased value products. Industrially, the process uses a SAPO H-ZSM-5 or ZSM-5. The mechanism of the process is still debated but an early step is the formation of methoxy by reaction of methanol with an acid site. The mobility of methanol in the catalyst is clearly important and was the focus of a QENS study that compared the mobility of methanol in the industrial catalyst, H-ZSM-5, and zeolite HY which has the same Si:Al ratio (=30) (14). HY ), arbitrary units

Figure 9 compares the results of the two systems  , Q obtained with a backscattering spectrometer crystal ( S analyser. It can be seen that the two systems behave completely differently. Zeolite HY shows a signiﬁ cant broadening at room temperature, consistent with simple Fickian diffusion through the material. In contrast, in H-ZSM-5 methanol is immobile on the timescale of the –4 –2 0 2 4 spectrometer. Wavenumber, cm–1 To attempt to understand the striking difference in Fig. 9. QENS spectra of methanol in zeolites H-ZSM-5 and behaviour, INS spectra of the same samples that were HY at Q = 0.9 Å–1 at 298 K; (…) represents the resolution used for the QENS measurements were recorded. INS data taken at 5 K. Reproduced from (14) with permission spectroscopy has no selection rules, hence all modes from The Royal Society of Chemistry are allowed. However, the intensity of a mode depends on the incoherent cross section and the amplitude of For HY, Figure 10(a) and 10(b), it can be seen motion of the atoms in the mode. Both of these are that the characteristic spectral features of methanol: 1 –1 large for normal hydrogen ( H), see Figure 1, thus the C–O–H deformation at 750 cm , CH3 rocking mode spectrum is dominated by modes that involve hydrogen at 1150 cm–1, C–H bends at 1500 cm–1, C–H stretches displacement. This has a corollary that most elements at 3000 cm–1 and O–H stretch at 3200 cm–1 are largely that occur in catalysis such as transition metals, metal preserved on absorption into HY. The O–H stretch oxides or carbon are largely invisible, so the spectra do broadens consistent with hydrogen bonding to the not exhibit cut-offs due to lattice absorption (as occurs framework hydroxyls, which downshift to ~3400 cm–1 with many supports with infrared spectroscopy) and the from 3650 cm–1 in the clean zeolite. This indicates a entire 0–4000 cm–1 range is available. relatively weak interaction between methanol and HY,

137 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691230 Johnson Matthey Technol. Rev., 2016, 60, (2) allowing free diffusion as seen by QENS. In contrast, and/or fl uorescence. These are irrelevant to INS in H-ZSM-5, Figure 10(c) and 10(d), while the C–H spectroscopy and it has been used to study the related modes (rock, bend, stretch) are all present, the overlayers on Ni/Al2O3 methane reforming catalysts C–O–H and Si–O–H deformations and the methanol (17–19). As part of the work, a method to quantify and and framework O–H stretch all disappear upon speciate hydrogen on the surface of the catalyst was adsorption of methanol. This indicates that reaction developed (17). between methanol and the H-ZSM-5 hydroxyls has An even more challenging system for conventional occurred to generate bound methoxy, Equation (i): spectroscopies is the Fischer-Tropsch synthesis of alkanes from synthesis gas. Historically, this has used CH OH + Si–O–H  Si–O–CH + H O (i) 3 3 2 an iron-based catalyst, although supported cobalt The reaction occurs at room temperature, suggesting catalysts are becoming widely used. Cobalt is in limited a very low activation barrier, with water removed in the supply, so iron catalysts are still of interest. Figure 11(a) He/methanol gas stream used to prepare the samples. shows the INS spectrum (20) of a technical-grade On almost any working catalyst that involves iron-based Fischer-Tropsch catalyst taken from the reaction of hydrocarbons, over time there is a Sasol Ltd coal-to-liquids plant at Secunda, South Africa, build-up of a hydrocarbonaceous overlayer, which whilst the reactor was operating under steady-state may be detrimental to the catalyst activity by blocking conditions. The most surprising feature of the spectrum access to the active sites or may be integral to its is that it shows distinct bands, i.e. it is molecular-like. operation by controlling the fl ow of reactants to the The spectrum exhibits bands characteristic of both surface. The overlayers are diffi cult to study with aliphatic and aromatic functionalities. This is most light-based spectroscopies due to strong absorption clearly seen in the C–H stretch region, Figure 11(b),

(a) (b) HY HY ), arbitrary units ), arbitrary units   , , Q Q ( ( S S

500 1000 1500 2000 2500 3000 3500 4000 400 600 800 1000 1200 1400 Wavenumber, cm–1 Wavenumber, cm–1

Fig. 10. INS spectra of solid methanol (green line), the empty dehydrated zeolite (blue line) and methanol loaded into the zeolite (purple line) at incident energies of: (a) and (c) 5240 cm–1; and (b) and (d) 2016 cm–1. The arrow in (a) shows the downshift of the framework hydroxyls on hydrogen-bonding to methanol. (The spectrometer is described in (15, 16)). Reproduced from (14) with permission from The Royal Society of Chemistry

same intensity per C–H, unlike infrared spectroscopy (a) where aliphatic and aromatic C–H stretch modes can have very different extinction coefﬁ cients making quantitative comparison problematical. From Figure 11(b) it can be seen that there is an approximately equal number of hydrogen atoms bonded to sp2 and sp3 hybridised carbon.

), arbitrary units As a model system for Fischer-Tropsch catalysis, CO  , Q

( methanation (Equation (ii)) has been investigated: S

3H2 + CO  CH4 + H2O (ii) 0 1000 2000 3000 4000 –1 Wavenumber, cm Starting from a pre-catalyst (hematite, α-Fe2O3), under –1 (b) reaction conditions (6 hours, 623 K, He 600 ml min , H2 150 ml min–1, CO 75 ml min–1, 1.1 bar) the active catalyst evolves in the same way as the industrial material. For INS studies of catalysts, sample quantities are in the range 10–50 g, as compared to the usual microreactor scale of 10–50 mg. To generate the 1000-fold larger samples required, a large scale gas handling facility ), arbitrary units  , with online mass spectroscopy has been developed, Q (

S Figure 12 (21) together with a range of cells for the different pressure and temperature regimes that are 2200 2600 3000 3400 3800 encountered, Figure 13. Figure 11(c) shows the C–H Wavenumber, cm–1 (c) stretch region after six hours methanation (21, 22), the 0.15 3061 similarity to the commercial system is evident. This approach provides a tractable method to study a very complex process. 2942 In Figure 4 it was shown that hydrous palladium 0.10 oxide, PdO·H2O, has a core of nanocrystalline A PdO. This leaves the role of water unresolved. Figure 14(a) compares the INS spectra of high ), arbitrary units 

, density amorphous ice (Figure 14(a),lineA) with Q ( 0.05 B PdO·H O (Figure 14(a),lineB) (7). The similarity S 2 of the spectra suggests that the water is present as a disordered shell on the PdO nanoparticle. 2000 2500 3000 3500 4000 However, there is an additional peak at ~1000 cm–1 Wavenumber, cm–1 that is more apparent after drying at 373 K under Fig. 11. (a) INS spectra of an iron-based Fischer-Tropsch vacuum (Figure 14(a),lineC). This is assigned to a catalyst taken from an operating plant. The sample was Pd–O–H deformation mode (the corresponding O–H Soxhlet extracted in toluene for 24 hours to remove stretch can be seen with a different spectrometer) hydrocarbon product; (b) same sample as (a) showing equal numbers of aliphatic and aromatic C–H bonds; (c) a model and indicates a layer of hydroxyls at the surface. iron-based Fischer-Tropsch catalyst, A before and B after This system was modelled with periodic density six hours methanation (21, 22). Reproduced from (20) and functional theory as a two layer slab of PdO capped (21) with permission of Wiley-VCH Verlag GmbH & Co and with hydroxyls and a layer of water, Figure 14(b). It the Institute of Physics, respectively can be seen that the INS spectrum generated from this model (Figure 14(a),lineD) shows reasonable where bands below (aliphatic) and above (aromatic) agreement with the experimental data (Figure 14(a), 3000 cm–1 are seen. The intensity of an INS mode is line C), although it does not capture the disorder in directly proportional to the number of oscillators, thus the water layer. The structure that emerges from the aliphatic and aromatic C–H stretch modes have the combined diffraction, spectroscopy and modelling

Fig. 12. A photograph of the gas handling apparatus (blue panel), that is housed within a walk-in fume cupboard. The furnace is located to the left hand side of the gas panel and is mounted on a heavy-duty lab-jack. A cell is in the furnace, which is connected to the gas panel by heated lines that terminate in high-temperature valves, which may be used to isolate the cell. The reaction may be followed by online mass spectrometry using the Hiden RGA on the trolley at the left of the picture. Toxic gas (H2 or CO) monitors (black boxes on the walls on the extreme left and right of the picture) are integral to automatic safety shutdown safeguards. A more complete description is given elsewhere (21)

Fig.13. INS cell designed to carry out high-temperature and high-pressure (600ºC, 20 bar) reactions. The cell body is made of Inconel™. Similar designs are used for less severe operation: 316 stainless steel (300ºC, 5 bar), aluminium (100ºC, 1 bar) and TiZr (100ºC, 1 bar). The background scattering from the cell decreases in the order given. TiZr is not suitable for use with hydrogen due to rapid embrittlement above ~50ºC (21)

(a) (b)

C ), arbitrary units  , Q (

S D

0 400 800 1200 1600 2000 Wavenumber, cm–1

Fig. 14. (a) INS spectra of: A high density amorphous ice, B hydrous palladium oxide, C hydrous palladium oxide after drying at 373 K and D generated from a periodic functional theory calculation of the structure shown; (b) the structure used for DFT calculation. Reprinted with permission from (7). Copyright (2010) American Chemical Society

study is that hydrous palladium oxide is not Pd(OH)2 but that it is best described as ~18 Å diameter particles of nanocrystalline PdO with a monolayer or so of hydroxyls capped by 4–7 layers of water. Transmission electron microscopy (TEM) shows that A the very small nanocrystallites are agglomerated into larger clusters which reduces the effective surface area. B ), arbitrary units  It was pointed out that Pearlman’s catalyst is , Q

( C

commonly formulated as palladium(II) hydroxide S supported on carbon, Pd(OH)2/C. This is a versatile reagent that is effective for hydrogenation, 0 500 1000 1500 2000 –1 hydrogenolysis and C–C coupling reactions. In view Wavenumber, cm of the results for hydrous palladium oxide, there Fig. 15. Comparison of the difference spectra of the dried was a strong suspicion that the commonly accepted samples of Pearlman’s catalyst: A ([20 wt% Pd/C] – C), formulation of Pearlman’s catalyst was incorrect. B ([5 wt% Pd/C] – C) with that of C hydrous palladium oxide Knowledge of the state of the palladium is important in dried at 100ºC (7). Reproduced from (23) with permission from the PCCP Owner Societies trying to understand the mechanism of the reactions since recent work has suggested that the catalyst functions by leaching of palladium into solution. 5 wt% and 20 wt% Pearlman’s catalyst were described as carbon supported hydrous palladium measured by INS in the as-supplied wet state and after oxide. Interestingly, the TEM and X-ray photoelectron drying in vacuum (23). In the wet state, only water was spectroscopy (XPS) study showed a minor fraction, visible. Vacuum drying removed most of the water ca. 13–17%, of nearly metallic palladium. Since and allowed the spectral features to be observed. Pearlman’s catalyst is generally used in non-aqueous Figure 15 compares the INS spectra of the dried environments this species may simply be a spectator catalysts after subtraction of the spectrum of the bare species in the C–C coupling reactions that are a major carbon with that of dried hydrous palladium oxide. The use of the catalyst. This supports work that showed similarity is obvious: TEM showed lattice spacings that Pd/C is a much less effective catalyst for C–C typical of PdO hence Pearlman’s catalyst is better coupling reactions than Pearlman’s catalyst.

How to Access Neutron Scattering Techniques

Neutron scattering facilities occur across the globe, Figure 16, (24) but the pre-eminent facilities are: ISIS in the UK (25), the Institut Laue Langevin (ILL) in France (26), the Spallation Neutron Source (SNS) in the USA (27) and J-PARC in Japan (28). All of these, and most of the smaller centres, operate a system of “free at the point at use”. Generally, beamtime is awarded via a competitive proposal system with application dates in the spring and autumn. If time is awarded, then the user is responsible for travel, subsistence and the preparation of samples but there is no charge for the actual beamtime. (Depending on the nationality of the user, the facility may partially or wholly cover these costs). This access mechanism requires that the work is made publicly available, generally by publication. The facilities also allow beamtime to be purchased and the work is then subject to a confi dentiality agreement. A proposal generally consists of a two page science case plus experimental requirements and safety considerations. For new or inexperienced users, it is essential to discuss the proposal with an established user or the instrument scientist to avoid failure of the experiment. This is especially the case for catalysis experiments. The requirements include: • Suffi cient sample quantity (g not mg are generally needed) • A large number of active sites (the minimum for current inelastic instruments is 1 mol% of hydrogen in the beam) • At least one of the surface species must be hydrogenous. Even for experienced users, the fi rst experiment at the start of a new project is often unsuccessful. In this case, the problem may be that the physical characteristics are different on the gram scale (for example, different space Fig. 16. The location of neutron scattering centres in velocities or plugging). However, the experienced user 2015, coloured by continent. Reproduced from (24) with learns from the problem and subsequent experiments permission of Neutronsources.org are generally successful.

Future Prospects low temperature (<50 K) to obtain the best results. However, recent work (29) has shown that in favourable It will be obvious that most of this review concerns cases it is possible to obtain time resolved results at inelastic scattering. Partly this reﬂ ects the bias and ambient temperature or above. The time resolution is interests of the authors but it is also a reﬂ ection of the very modest (10s of minutes) but this will improve as current landscape. Inelastic scattering is more widely the instruments’ sensitivities improve. used even though it is less sensitive than elastic Elastic scattering by contrast has been massively scattering. This arises because the energy analysis underused in studies of catalysts. The technique methods all involve rejecting most of the neutrons in is well-suited to operando measurements: realistic either the incident or scattered beam. QENS studies temperatures and pressures are accessible, the are routinely carried out at varying temperatures to penetrating nature of neutrons means that there is a yield activation energies as well as diffusion constants wide choice of materials for cell design, time resolution and insight into the diffusion mechanism (Fickian, jump of minutes or better is available and hydrogenous or immobile). To date most INS studies have used a reactants and products give strong signals. An example ‘react and quench’ approach (14, 17–22) to provide where neutron diffraction studies are potentially very a snapshot along the reaction coordinate. This is useful, would be to follow the evolution of an iron-based largely because of the need to measure spectra at Fischer-Tropsch catalyst from the hematite pre-catalyst

142 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691230 Johnson Matthey Technol. Rev., 2016, 60, (2) to the working catalyst which includes other iron a resource that are much simpler to access than is oxides, iron carbides, metallic iron and carbon, under generally believed, new users with interesting problems operando conditions. Neutrons are sensitive to the are always welcome! light elements as well as the heavy ones, Figure 1, so phase identifi cation should be more certain. Although Acknowledgements it has not been discussed here, small angle neutron The STFC Rutherford Appleton Laboratory is thanked scattering (SANS) covers the size range typical of for access to neutron beam facilities. The UK Catalysis supported catalysts and the pores in microporous Hub is kindly thanked for resources and support materials. Real time study of pore fi lling is clearly provided via our membership of the UK Catalysis feasible. Hub Consortium and funded by EPSRC (grants Neutron scattering is a fl ux limited technique and EP/K014706/1, EP/K014668/1, EP/K014854/1, there is a continual push to improve sensitivity by EP/K014714/1 and EP/M013219/1). increasing the detector coverage, increasing the incident fl ux by better neutron transport from source to References sample and by building more powerful facilities. The latest facilities, SNS (27) and J-PARC (28), are nearly 1. N. Floquet, J. P. Coulomb, J. P. Bellat, J. M. Simon, a factor of 10 brighter than the previous generation. G. Weber and G. Andre, J. Phys. Chem. C, 2007, 111, The European Spallation Source (ESS, (30)) under (49), 18182 construction in Sweden will offer at least another 2. M. Bianchini, F. Fauth, E. Suard, J.-B. Leriche, C. factor of 10. Masquelier and L. Croguennec, Acta Cryst. B, 2015, The other way that more information will be extracted 71, 688 from the data is via better modelling. Throughout 3. F. G. Kinyanjui, S. T. Norberg, I. Ahmed, S. G. Eriksson this paper the link with atomistic modelling has been and S. Hull, Solid State Ionics, 2012, 225, 312 demonstrated; neutrons are particularly strong here 4. T. Kandemir, D. Wallacher, T. Hansen, K.-D. Liss, R. because neutron observables are readily calculable N. d’Alnoncourt, R. Schlögl and M. Behrens, Nucl. as a result of the simple interaction between the Instrum. Meth. A, 2012, 673, 51 neutron and the sample. Neutrons and modelling are 5. T. Kandemir, F. Girgsdies, T. C. Hansen, K.-D. Liss, I. a synergistic couple; the neutron data is an exacting Kasatkin, E. L. Kunkes, G. Wowsnick, N. Jacobsen, test of the model and the modelling allows deeper R. Schlögl and M. Behrens, Angew. Chem. Int. Ed., insight into the process. QENS and MD simulations 2013, 52, (19), 5166 such as shown in Figure 8 are a good illustration of 6. T. Egami and S. J. L. Billinge, “Underneath the Bragg the synergy. Peaks: Structural Analysis of Complex Materials”, 2nd edition, Pergamon Materials Series, Volume 16, Elsevier Ltd, Amsterdam, The Netherlands, 2012 Conclusions 7. S. F. Parker, K. Refson, A. C. Hannon, E. Barney, S. Neutron scattering is a very versatile tool that J. Robertson and P. Albers, J. Phys. Chem. C, 2010, is underused for studies of catalysts. While it is 114, (33), 14164 applicable to most elements it has a particular focus 8. J. F. C. Turner, C. J. Benmore, C. M. Barker, N. on light elements, especially hydrogen. Thus it is highly Kaltsoyannis, J. M. Thomas, W. I. F. David and C. R. complementary to synchrotron studies with X-rays that A. Catlow, J. Phys. Chem. B, 2000, 104, (32), 7570 excel for the heavier elements. Neutron scattering 9. S. F. Parker, D. T. Bowron, S. Imberti, A. K. Soper, provides observables that are readily calculable, K. Refson, E. S. Lox, M. Lopez and P. Albers, Chem. thus modelling and neutron scattering are a natural Commun., 2010, 46, (17), 2959 partnership. This review has highlighted the detailed 10. M. Bée, “Quasielastic Neutron Scattering”, Adam information that can be extracted from QENS and INS Hilger, Bristol, UK, 1988 studies. Elastic neutron scattering has much to offer 11. H. Jobic, Curr. Opin. Solid State Mater. Sci., 2002, 6, catalysis, particularly in the areas of operando studies (5), 415 of catalyst activation and deactivation, pore fi lling of 12. A. J. O’Malley and C. R. A. Catlow, Phys. Chem. micro- and mesoporous materials and the location Chem. Phys., 2015, 17, (3), 1943 of hydrogen or hydroxyls in materials. Neutrons are 13. A. J. O’Malley, C. R. A. Catlow, M. Monkenbusch

and H. Jobic, J. Phys. Chem. C, 2015, 119, (48), 21. R. Warringham, D. Bellaire, S. F. Parker, J. Taylor, R. 26999 A. Ewings, C. M. Goodway, M. Kibble, S. R. Wakeﬁ eld,

14. A. J. O’Malley, S. F. Parker, A. Chutia, M. R. Farrow, M. Jura, M. P. Dudman, R. P. Tooze, P. B. Webb and D. Lennon, J. Phys.: Conf. Ser., 2014, 554, 012005 I. P. Silverwood, V. García-Sakai and C. R. A. Catlow, Chem. Commun., 2016, 52, (14), 2897 22. R. Warringham, A. R. McFarlane, D. A. MacLaren, P. B. Webb, R. P. Tooze, J. Taylor, R. A. Ewings, S. F. 15. P. C. H. Mitchell, S. F. Parker, A. J. Ramirez-Cuesta Parker and D. Lennon, J. Chem. Phys., 2015, 143, and J. Tomkinson, “Vibrational Spectroscopy with 174703 Neutrons: With Applications in Chemistry, Biology, 23. P. W. Albers, K. Möbus, S. D. Wieland and S. F. Parker, Materials Science and Catalysis”, Series on Neutron Phys. Chem. Chem. Phys., 2015, 17, (7), 5274 Techniques and Applications, Volume 3, World Scientiﬁ c, Singapore, 2005 24. Neutronsources.org, Neutron Centres: http:// neutronsources.org/neutron-centres.html (Accessed 16. S. F. Parker, D. Lennon and P. W. Albers, Appl. on 17th February 2016) Spectrosc., 2011, 65, (12), 1325 25. ISIS, Science and Technology Facilities Council, 17. I. P. Silverwood, N. G. Hamilton, C. J. Laycock, J. Z. Rutherford Appleton Laboratory, UK: http://www.isis. Staniforth, R. M. Ormerod, C. D. Frost, S. F. Parker stfc.ac.uk (Accessed on 17th February 2016) and D. Lennon, Phys. Chem. Chem. Phys., 2010, 12, 26. Institut Laue-Langevin, Grenoble, France: www.ill.eu (13), 3102 (Accessed on 17th February 2016) 18. A. R. McFarlane, I. P. Silverwood, R. Warringham, E. 27. Oak Ridge National Laboratory, Spallation Neutron L. Norris, R. M. Ormerod, C. D. Frost, S. F. Parker and Source, US Department of Energy: http://neutrons. D. Lennon, RSC Adv., 2013, 3, (37), 16577 ornl.gov/sns (Accessed on 17th February 2016) 19. A. R. McFarlane, I. P. Silverwood, E. L. Norris, R. M. 28. Japan Proton Accelerator Research Complex, Ibaraki, Ormerod, C. D. Frost, S. F. Parker and D. Lennon, Japan: j-parc.jp/index-e.html (Accessed on 17th Chem. Phys., 2013, 427, 54 February 2016) 20. N. G. Hamilton, I. P. Silverwood, R. Warringham, 29. S. F. Parker, Chem. Commun., 2011, 47, (7), 1988 J. Kapitán, L. Hecht, P. B. Webb, R. P. Tooze, S. F. 30. European Spallation Source, Tunavägen, Lund, Parker and D. Lennon, Angew. Chem. Int. Ed., 2013, Sweden: www.europeanspallationsource.se/ 52, (21), 5608 (Accessed on 17th February 2016)

The Authors

Stewart Parker is the ISIS Facility Catalysis Scientist. After earning his PhD at the University of California at Santa Barbara, USA, he carried out postdoctoral research at the University of East Anglia, UK, before joining the Analytical Division of the BP Research Centre at Sunbury-on-Thames, UK. In 1993 he moved to the ISIS Facility. He is currently responsible for promoting the use of neutrons in catalysis research. He interacts with a wide range of scientists from academia and industry studying areas that range from Fischer-Tropsch catalysis to methanol-to-hydrocarbons to selective hydrogenation catalysts using a variety of structural and spectroscopic neutron methods.

Paul Collier is a Research Fellow at the Johnson Matthey Technology Centre, Sonning Common, UK. He is responsible for organising Johnson Matthey’s collaborations at the Harwell site, 25 km from the Johnson Matthey Technology Centre, which hosts the world class facilities such as the UK’s synchrotron (Diamond) and the ISIS neutron spallation source. He is interested in heterogeneous and homogeneous catalysis, metal organic frameworks (MOFs), platinum group metals, oxidation, synchrotron, neutron diffraction, neutron spectoscopy, lasers, zeolite catalysts, methane and alkanes.

“Industrial Tomography: Systems and Applications”

Edited by Mi Wang (University of Leeds, UK), Woodhead Publishing Series in Electronic and Optical Materials, No. 71, Woodhead Publishing, an imprint of Elsevier Ltd, Cambridge, UK, 2015, 744 pages, ISBN: 978-1-78242-118-4, £230.00, US$365.00, €260.00

Reviewed by Hugh Stitt are steadily being overcome. There are now a Johnson Matthey Technology Centre, number of companies that specialise in supplying the PO Box 1, Belasis Avenue, Billingham, TS23 1LB, UK equipment or services into the process industries. Geir Anton Johansen (Bergen University College, Norway) Email: [email protected] for example highlights three different commercially available gamma tomography-based offerings. Equally there are competitive offerings for electrical resistance and capacitance tomography, purpose-developed This book represents the latest magnum opus in a line for process tomography. Process tomography is no of multi-author books on process tomography, with longer a pet research area and a research tool, but the fi rst dating from 20 years ago. Following in the rather it is now becoming an important measurement tradition of Beck and Williams (1) this book presents approach for industry. This book is therefore timely, a comprehensive overview of process relevant coming at a time when industrial activity is markedly tomographic modalities, reconstruction techniques increasing and while the pace of academic progress and industrial applications. The editor Professor is being maintained. Mi Wang has done an excellent job in obtaining contributions from many leaders in the fi eld. Modalities The technical scope and application of process tomography has grown enormously since the The book is broken down into three sections: landmark Beck and Williams book (1) from a modalities, image reconstruction and applications. technology of signifi cant potential to one of substantial The section on modalities is meticulous, including impact. While it may be true that medical tomography well established and exploited routes such as is notably leading, the challenges of applying soft fi eld electrical capacitance and impedance tomographic approaches at meaningful scale for the (including resistance) tomographies and hard fi eld process industries and the complications of using it X-ray and gamma ray techniques (Figure 1) as well on manufacturing plant or even offshore, all within the as electromagnetic induction, magnetic resonance tighter cost constraints of the manufacturing industry and radioisotope methods. Attention is also given to

Applications The fi nal section of the book presents eleven chapters on different applications in process tomography. They vary a little in their thrust and aspect according to their titles. Some are quite specifi c in terms of modality and subject of interrogation: for example a chapter each on X-rays and electrical capacitance tomography for gas-solid fl uidised beds. Others are particular only in the application but wide ranging in the modalities used: general chapters on tomography for bubble columns and trickle bed reactors, reaction engineering (mixing processes) and microreactors. The fi nal category Fig. 1. The Tracerco DiscoveryTM tool for sub-sea pipe inspection by gamma-ray tomography (Image courtesy of of chapter is the general industry sector summaries: Tracerco) tomography in mineral transportation, oil and gas industry (two chapters, the fi rst of which actually focuses on oil fi eld and well imaging, and explains the otherwise incongruous inclusion of the earlier infrared (IR)-based chemical species tomography and chapter on sensor networks), food inspection and the (electrical impedance-based) spectro tomography. nuclear industry. There is also a stand-alone chapter Overall this section is well balanced with the chapters on tomographic velocimetry; which seems somehow for the most part being of similar length and depth out of place dealing as it does with the basic principles and also largely concentrating on describing the of the methods and may better have been included in fundamentals of the technique and its relevance to the opening section. Some of the chapters are however process tomography while remaining distinct from the slightly more restrictive in their subject matter than outright applications focused chapters later in the book. their titles may suggest. The report on food inspection The chapter on imaging in sensor networks is a bit of actually considers only X-ray based systems. an outlier in the sense that its relationship to process While it is easy to be critical of the unevenness of this imaging seems a little tenuous. Nonetheless it is well fi nal section it should not be seen to detract from the written and a thought provoking addition. overall impact; the collation provides a wide-ranging re view of the application of various tomographic Image Reconstruction modalities to a number of different measurement problems and operations relevant across the process The second section, on image reconstruction is industries such as fl ows, fi ltering, mixing, drying however less thorough and somewhat less balanced. processes and chemical reactions inside vessels and The chapters on mathematical concepts and statistical pipelines. The editor has assembled a broad cross- techniques are well considered as two of the basic section of applied process tomography written by approaches to reconstruction. Both are effectively leading practitioners. written and structured, and serve as very useful guides and introductions. Between them lie dedicated Summary chapters on hard fi eld tomography (mainly on X-ray tomography although clearly more widely applicable) In conclusion this compilation presents a broad and electrical capacitance tomography; both are coverage of tomographic techniques that can be informative. For the other modalities the image applied to process measurements. The method reconstruction is covered within the earlier chapters. sections feature a diverse range of modalities and It is not evident why only these two are singled out provide in depth analysis of their respective techniques, for special treatment. This makes the book less useful equipment and data processing. There is detailed as a general reference for image reconstruction study of some aspects of image reconstruction and techniques. an extensive investigation of process tomography in

applications. The new book thus offers an up to date “Industrial Tomography: Systems compendium that will be invaluable to those involved and Applications” in process tomography research and those looking to utilise tomographic methods for process measurement either as a research tool or for industrial application.

Reference 1. “Process Tomography: Principles, Techniques and Applications”, eds. R.A. Williams and M. S. Beck, Butterworth-Heinemann Ltd, Oxford, UK, 1995

The Reviewer

E. Hugh Stitt is a Scientiﬁ c Consultant at Johnson Matthey Technology Centre, Billingham, UK. He is a Visiting Professor at the University of Birmingham, UK, and at Queen’s University Belfast, Fellow of the Institution of Chemical Engineers and a Fellow of the Royal Academy of Engineering. He has 25 years of industrial research experience across a variety of themes related to catalytic reaction engineering and catalysts with over 100 refereed publications.

“Ball Milling Towards Green Synthesis: Applications, Projects, Challenges”

Edited by Brindaban Ranu (University of Jadavpur, India) and Achim Stolle (University of Jena, Germany), RSC Green Chemistry Series, No. 31, Royal Society of Chemistry, Cambridge, UK, 2015, 303 pages, ISBN: 978-1-84973-945-0, £155.00, US$255.00, €193.75

Reviewed by Maria Elena Rivas of different milling equipment (Figure 1), which may Johnson Matthey Technology Centre, be of interest for general research on the technique. Blounts Court, Sonning Common, Reading RG4 9NH, The main reactions described in this book are UK related to the following processes: carbon–carbon and carbon–heteroatom bond formation, oxidation-reduction, Email: [email protected] organocatalytic reactions, dehydrogenative coupling, synthesis of peptides and polymeric materials. One key point of the book is that it highlights how ball milling can be used not just for particle size reduction applications The aim of this book “Ball Milling towards Green but for creating chemical reactions since the energy Synthesis” is to highlight the importance of ball induced by the mechanical treatment is high enough to milling as a potential route to produce organic induce transformations. In all chapters the authors point materials. The book was published by the Royal out that mechanochemical reactions lead to economic Society of Chemistry and edited by Brindaban advantages over existing technologies providing the Ranu and Achim Stolle. In this book, applications, same products. projects, advantages and challenges related to ball The content of this book is arranged in ten chapters, milling for specific organic syntheses are reviewed. which are based on different organic chemistries. In principle the book should interest researchers Each chapter is divided into several sections related working in general mechanochemistry: however to speciﬁ c organic syntheses or reactions. All chapters in this reviewer’s opinion most of the chapters are show how successfully the technique can be used to aimed more at the organic chemist. The book is very obtain speciﬁ c organic products, and information is well structured for researchers focused on organic provided about experimental procedures and yields as synthesis, allowing them to go directly to a specific well as a comparison with conventional routes such as subject. Also, the book has a very formulaic structure; solution chemistry, microwave and ultrasound methods. every chapter systematically describes specific organic syntheses, with experimental procedures, Bond-forming Processes yields and advantages of using ball milling over other techniques. Each chapter describes the ball Chapter 1, ‘Carbon-Heteroatom Bond Forming milling conditions in detail, including specifications Reactions and Heterocycle Synthesis under Ball

high energy mills, with different milling times, media size and media materials, giving increased selectivity compared to solution based techniques.

Solvent-Free Synthesis

Chapter 3, ‘Oxidation and Reduction by Solid Oxidants and Reducing Agents using Ball-Milling’ by Giancarlo Cravotto and Emanuela Calcio Gaudino (University of Turin, Italy) deals with several oxidation and reduction reactions in the solid state by application of ball milling, generally under solvent-free conditions. Again the authors emphasise the effi ciency and scalability of the ball milling process as well as the absence of solvents and the low energy consumption. The different examples shown in this chapter confi rm that technical parameters such as volume, size of the reactor, material and number of milling balls Fig. 1. FRITSCH Planetary Mill PULVERISETTE 5 classic have a clear effect on the fi nal product. Vibration or line (Image courtesy of FRITSCH GMBH) revolutions, speed and milling time also needed to be optimised in order to improve yields. Chapter 4, ‘Asymmetric Organocatalytic Reactions Milling’ by Brindaban C. Ranu, Tanmay Chatterjee under Ball Milling’ by Elizabeth Machuca and Eusebio and Nirmalya Mukherjee (Indian Association Juaristi (Centro de Investigación y de Estudios for the Cultivation of Science, India) covers Avanzados del Instituto Politécnico Nacional, Mexico) carbon–heteroatom (C–N, C–O, C–S, C–Cl, C–Br) highlights some advantages of the application such bond formation and synthesis of heterocycles under as the ability to carry out reactions in the absence ball milling. The authors show examples of the of solvent, with an immediate reduction in costs and quantitative formation of different organic materials handling procedures. A very interesting fi nding is that of commercial interest such as imines, azines, by-products and potentially toxic wastes are frequently hydroquinone and hydrazine. It is pointed out that reduced. The shorter heating time, implying energy chemical reactions with fullerenes in solution are savings, is once again highlighted, as well as the diffi cult to perform. Ball milling gives the opportunity frequent observation of reduced reaction times. to carry out the synthesis in the solid state. It was Chapter 5, ‘Cross Dehydrogenative Coupling found that this method provides better results than Reactions by Ball Milling’ by Jingbo Yu, Zhijiang Jiang performing the reaction in solution. and Weike Su (Zhejiang University of Technology, Chapter 2, ‘Carbon–Carbon Bond Forming by Ball China) confi rms how ball milling can promote cross Milling’ by Katharina Jacob, Robert Schmidt and dehydrogenative coupling (CDC) and asymmetric CDC Achim Stolle (Friedrich-Schiller University Jena, reactions. This method effi ciently provides C–C bonds Germany), shows how carbon–carbon bonds can be directly from C–H bonds. formed by ball milling to produce compounds such as diphenylacetylene or biphenyl derivatives, which are Protection and Synthesis used in medicine, drug design and electronics. The advantages of this method over conventional routes In Chapters 6, 7, 8 and 9 the authors demonstrated such as microwave or ultrasound-assisted synthetic that different processes such as amino acid derivation routes are highlighted. Shorter reaction times and or protection, peptide synthesis, polymers and previously unknown molecular transformations have cellulose processing could greatly benefi t from been reported. Reactions between carbon electrophiles mechanochemical synthesis. According to the authors and organometallic nucleophiles, known as it is expected that further studies with ball milling cross-coupling reactions, were carried out in different on these areas will lead to signifi cant advances.

Chapter 7, ‘Ball-milling Mechanochemical Synthesis costs and increasing the effi ciency of existing of Coordination Bonds: Discrete Units, Polymers reaction processes. Using different examples, the and Porous Materials’ by Tomislav Friščić (McGill book highlights how mechanochemistry can replace University, Canada) discusses the importance of conventional processes for the preparation of organics the in situ analysis of mechanochemical reactions. and green materials for various applications. This It is well known that this has been largely limited book is not a basic text about ball milling or organic to temperature and pressure measurements on synthesis; it is aimed at people with experience mechanically induced self-sustaining reactions or in either organic synthesis or ball milling. Several monitoring pressure changes in reactions adsorbing examples confi rm that technical parameters such as or releasing gas, but some progress has been made volume, size of the reactor, material and number of by the use of penetrating synchrotron X-ray radiation milling balls have a clear effect on the fi nal product. All studies. this is in line with our fi ndings in the fi eld of dry milling for inorganic materials synthesis. An important point Technical Implications described in this book is related to in situ analysis. Due to mechanical forces and equipment confi guration, Chapter 10, ‘Technical Implications of Organic in situ analysis has been largely limited and milling Synthesis in Ball Mills’ by Achim Stolle explains that reactions have been mainly followed by temperature ball mills are characterised by different parameters, and pressure measurements on mechanically induced regardless of the type of process. This information is self-sustaining reactions or by monitoring pressure of value not just for organic synthesis but for general changes in reactions adsorbing or releasing gas. applications. The author lists a large number of Some progress on highly penetrating synchrotron infl uencing variables including chemical parameters, X-ray radiation is reported. Overall the book is a technological parameters such as the milling media positive review about ball milling for organic materials and process parameters such as temperature or development. pressure. The chemical parameters include all those variables that are directly related to the chemical “Ball Milling Towards Green Synthesis: Applications, Projects, reactions taking place in the mill. Challenges”

Summary

In general the book is a summary of the preparation of several organic materials by ball milling. The book would be a very good reference for scientists focused on organic synthesis who are interested in reducing

The Reviewer

Maria Elena Rivas obtained a BSc in Chemistry from the Central University of Venezuela, and a PhD in Chemistry (Heterogeneous Catalysis) from the University Complutense of Madrid, Spain. She joined Johnson Matthey in 2012, working in a European Union-funded project (CAtalytic membrane REactors based on New mAterials for C1–C4 valorisation (CARENA)) about catalyst development for membrane reactors. Currently she is working as a Core Scientist in the New Applications group. In this project she is focused on mechanochemistry as an alternative route for the development and improvement of new and current materials.

“Hierarchical Nanostructures for Energy Devices”

Edited by Seung Hwan Ko (Seoul National University, Korea) and Costas P. Grigoropoulos (University of California, Berkeley, USA), RSC Nanoscience & Nanotechnology Series, No. 35, Royal Society of Chemistry, Cambridge, UK, 2015, 308 pages, ISBN: 978-1-84973-628-2, £165.00, US$270.00, €206.25

Reviewed by Patrick Daubinger today’s society. In the past decade, research associated Johnson Matthey Piezo Products GmbH, with energy devices has concentrated on developing Bahnhofstr. 43, 96257 Redwitz, Germany new materials for these applications. As progress has become sluggish, there is a stronger focus on the Email: [email protected] three-dimensional arrangement of nanostructures. Hierarchical nanostructure designs in various applications are introduced, together with their advantages compared to zero-dimensional (0D), Introduction one-dimensional (1D) or two-dimensional (2D) nanostructures. Typical improvements are the “Hierarchical Nanostructures for Energy Devices” is increased surface-to-volume ratio and diffusion part of the Royal Society of Chemistry Nanoscience optimisation for chemical species. In general, & Nanotechnology Series. The editors Seung Hwan hierarchical structures strongly increase the effi ciency Ko and Costas P. Grigoropoulos have published more of many physical and chemical devices. than 60 articles together with a strong focus on laser Chapters 1 and 2 introduce the topic with processing of nanomaterials and hierarchical surface fundamental considerations and the physics of coatings. nanomaterials. Chapter 3, ‘Nanotechnology’s Wonder This book highlights the advantages of hierarchical Material’ describes carbon nanotubes and their arrangements for energy conversion, and particular growth process. Chapters 4 to 11 are each dedicated attention is paid to nanowire-based materials and to one application of hierarchical structures: solar their manufacturing techniques. Inspiration is found cells, fuel cells, thermoelectric devices, piezoelectric in nature, where hierarchical structures are present energy harvesting, photoelectrochemical cells (PEC), whenever energy transfer processes have to be supercapacitors, fi eld electron emission devices and optimised. Divided into 13 chapters, the book shows sensors. Chapter 12 shows other applications like relevant applications as well as physical fundamentals hierarchical adhesives, superhydrophobic surfaces and the unique characteristics of nanostructured and metal nanowire percolation networks. The fi nal materials. chapter summarises the book and gives an outlook Developing and commercialising sustainable energy on the great potential of hierarchical designs in future conversion systems are among the major issues of applications.

This review focuses on the applications of hierarchical band of the semiconductor. By using hierarchical TiO2 nanostructures rather than the fundamentals of or ZnO nanostructures (Figure 2) on the anode, the nanotechnology and quantum effects, which however performance can be boosted: higher carrier mobility should always be considered when developing (nanowire structures lead to less recombination), large nanomaterials. Figure 1 shows the change in melting surface area to adsorb more dye molecules and capture temperature of gold nanoparticles compared to more sunlight (compare the principle of a tree) and light bulk material, exemplary for changing properties of scattering layers to efﬁ ciently capture more sunlight by nanostructures. multiple scattering.

Fuel Cells Melting point bulk 1300 In November 2014, Toyota presented the world’s ﬁ rst commercially available fuel cell car at the Los Angeles Auto show (4). Fuel cells are seen as one of the most 1000 promising future technologies to replace combustion engines due to their much higher efﬁ ciency. Besides transportation, fuel cells can also be used in combined heat and power (CHP) or power-to-gas systems to

Melting temperature, ºK 500 convert and store energy effi ciently. Chapter 5, ‘Hierarchical Nanostructures for Fuel Cells 300 and Fuel Reforming’, explains the two most common 0 50 100 150 200 Diameter, Å types of fuel cells: polymer electrolyte membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC). The Fig. 1. The graph shows the melting temperature as main advantages of SOFC are their high conversion a function of the gold particle size. The melting point effi ciency and direct conversion of hydrocarbons instead strongly decreases when the particle size falls below 5 nm (Reprinted with permission from (1). Copyright (1976) by the of hydrogen. Furthermore, these systems do not require American Physical Society) expensive precious metals on the electrodes. SOFC are mainly used for stationary power generation due to their high operating temperature and the related lengthy Solar Cells heat-up process. PEMFC is mostly used for automotive applications due Chapter 4, ‘Hierarchical Nanostructures for Solar Cells’ to its low operating temperature. These fuel cells usually examines dye-sensitised solar cells (DSSC, Grätzel cell). use hydrogen (anode reaction) and oxygen from air Solar cells are typically divided into three generations. (cathode reaction) to run the system, Equation (i): The fi rst generation is the widely used crystalline silicon H + 0.5O → H O (i) solar cell (mono- or multi-crystalline), which account 2 2 2 for more than 90% of the worldwide market. Thin-fi lm Another approach is to directly convert alcohols, which solar cells are referred to as the second generation; their leads to potentially higher effi ciencies and increased market share is around 8%. energy densities compared to hydrogen fuel cells. The

A third generation of solar cells is currently on the rise conversion of a primary fuel into H2 can be avoided. due to its low manufacturing costs and higher ﬂ exibility These types of fuel cells are a sub-category of PEMFC compared to conventional silicon solar cells, namely and are usually referred to as direct alcohol fuel cell DSSC or organic photovoltaic (OPV). These solar (DAFC); typically the fuel is methanol or ethanol. cells usually involve a platinum catalyst at the counter However, the complete dissociation from ethanol to electrode to utilise a redox reaction, where the yield can CO2 remains challenging. The complete reaction of be improved by nanostructures (2). The anode is typically methanol to CO2 is shown in Equation (ii): made from titanium dioxide (TiO ) or zinc oxide (ZnO), 2 CH OH + H O → CO + 6H+ + 6e– (ii) both of which are wide band-gap semiconductors. The 3 2 2 electrons emerging from the reaction of the photosensitive In simple terms, the efﬁ ciency of fuel cells can be dye with sunlight are transferred into the conduction improved by increasing the catalytic activity of the

(a)

3 μm 3 μm

(b)

3 μm 3 μm

(c)

3 μm 3 μm

(e) (d) Dark 8 Vertical nano-tree Short branched nano-tree –2 Long branched nano-tree 6

0 Current density, mA cm mA Current density, –2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Voltage, V

Fig. 2. Scanning electron microscopy pictures of the ZnO nanowire hierarchical nanostructures for DSSC: (a) vertically aligned nanowire carpet; (b) short branched nano weeping willow tree; (c) long branched nano weeping willow tree; (d) pseudo-coloured SEM picture of the long branched nano weeping willow tree. Magenta coloured nanowire represents backbone nanowire after ﬁ rst growth and purple coloured nanowires represent branch nanowires after second growth; (e) current-voltage curve of DSSC made from the long branched nano weeping willow trees of ZnO nanowires (3) (© IOP Publishing. Reproduced with permission. All rights reserved)

153 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691221 Johnson Matthey Technol. Rev., 2016, 60, (2) electrodes. Hierarchical structures strongly increase conductivity is unaffected thus leading to a more the active surface area and hence the number of effi cient thermoelectric device (5). catalytically active sites. They also enhance the diffusion of the fuel to the electrode site and raise the Piezoelectric Energy Harvesting mass activity since only the top layer consists of the catalytically active material (core-shell principle). Chapter 7, ‘Piezoelectric Energy Harvesting Nanofi bers’ reports key accomplishments in nanofi bre generators Thermoelectric Materials and Devices composed of polyvinylidene fl uoride (PVDF) and lead zirconate titanate (PZT). In the last few years, Chapter 6, ‘Thermoelectric Materials and Devices’, autonomous piezoelectric power generators have introduces the materials, their applications and explains gained much attention due to the miniaturisation of the benefi ts of nanostructures in thermoelectric devices. electrical devices and medical implants. Independence Thermoelectric devices convert thermal energy into from a space consuming battery and eliminating the electrical energy and vice versa. The effect is based need to recharge or replace these systems are the on differences in the Fermi levels at the point of contact main driving forces in developing energy harvesting of two metals or semiconductors. The Fermi level solutions with piezo materials. is a measure of the affi nity of the electrons to leave Research is devoted to three different kinds of a material: the electrons fl ow from the material with a piezoelectric nanogenerators: fi lm-based, nanowire- higher Fermi level to the material with the lower Fermi based and nanofi bre-based. Film-based piezoelectric level. When a temperature gradient between the two materials, for example aluminium nitride (AlN) or PZT materials creates an electrical potential, it is referred (6, 7) are commonly applied by thin-fi lm deposition or to as the Seebeck effect. The reverse effect, where an spin-on methods. Nanowire-based piezoelectric electrical potential is applied to create a temperature materials are typically made of semiconductors like ZnO, difference, is called the Peltier effect. zinc sulfi de (ZnS), gallium nitride (GaN) or cadmium Practical applications are found in power generation, sulfi de (CdS). By coupling their semiconducting and energy harvesting, cooling and heating. Typical materials piezoelectric properties, mechanical strains are directly used in thermoelectric devices are bismuth telluride converted into electrical energy.

(Bi2Te 3), lead telluride (PbTe) and silicon-germanium Ceramic PZT and polymeric PVDF are the two (SiGe). All these materials are semiconductor alloys, most promising materials in making piezoelectric which were found to be more effi cient than insulators nanofi bres. The key manufacturing techniques to or metals. The ideal thermoelectric material has a high produce these nanofi bres are either near-fi eld or electrical and a low thermal conductivity. far-fi eld electrospinning. Besides the pioneering work from the US National PZT is known for its exceptional piezoelectric Aeronautics and Space Administration (NASA) in the properties; however the manufacturing process via 1950s to power their spacecraft with thermoelectric electrospinning requires solvents, which lower the energy, the biggest commercial success to date is the density and the overall power effi ciency. PVDF has lower thermoelectric refrigerator. These are mainly used in piezoelectric performance, but has the advantages small solid state devices in order to cool down the hot compared to PZT of fl exibility, biocompatibility, fi bre spots in a central processing unit (CPU). length and weight. High-temperature annealing is also The possibility of generating energy from waste heat not required for PVDF. is another hot topic and is often referred to as energy Piezoelectric energy harvesting is still in the early harvesting. However, the low energy conversion stages of research due to its limited power output. effi ciency, which is typically in the range 3% to 10%, is Success will depend on low energy systems and often too low for the required energy. advanced microelectronic technologies. Recent studies Nanostructures are being widely investigated to with multiple layers of nanofi bres have the potential to increase the conversion effi ciency. The mean free produce enough energy to drive miniaturised electrical pathway of an electron is in the range of 10 nm, devices. New approaches, using real mechanical whereas a phonon has a mean free pathway of around actuation sources like the human heartbeat also show 100 nm. When using structures smaller than 100 nm promising results for practical uses. Another interesting the thermal conductivity is reduced but the electrical approach is the support of energy storage systems

154 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691221 Johnson Matthey Technol. Rev., 2016, 60, (2) like batteries, where an energy harvester could potentially enhance the overall lifetime of, for example implantable devices. A possible breakthrough in nanoﬁ bres is predicted by the chapter’s author in Primary energy source the ﬁ eld of wearable electronics (smart watches, displays), where these devices are directly powered by human motions. Discharge Recharge time time Photoelectrochemical Cells

Chapter 8, ‘Hierarchical Nanostructures for Maxwell ultracapacitor Photo-Electro-Chemical Cells’ is closely related to the previous chapter on solar cells. The most famous PEC is the aforementioned Grätzel cell (DSSC). Besides photovoltaics, PEC are often used for hydrogen Discharge Recharge generation by splitting of water. time time In the beginning of the chapter the importance of hierarchical arrangements for catalysis and photogenerated electrons is emphasised. The cell Fig. 3. A comparison between ultracapacitors and primary effi ciency was enhanced by a factor of 4 to 5 through energy sources like batteries (Courtesy of Maxwell Technologies, Inc) hierarchical structures compared to non-hierarchical nanowire structures. Hierarchical structures enable higher light harvesting as well as a longer effective path for photons to be absorbed by improving scattering and electrolytic capacitors and batteries. The power density trapping. It strongly increases the chemical reactions of supercapacitors is signifi cantly higher than that of and the diffusion of species to the electrode sites. lithium-ion batteries and the time needed to charge Another important fact is that hierarchical structures are such a device is much shorter. Supercapacitors also usually constructed by hybridising two or more different allow a large amount of charge and discharge cycles. materials. For example, the integration of a material The main disadvantage, however, is the much lower with different band structures can increase the range energy density in supercapacitors. of photon absorption. In terms of catalysis, different Supercapacitors usually complement a primary materials can trigger different reactions at the same energy source like fuel cells, batteries or combustion potential and, for example, avoid electrode poisoning. engines. Typical applications are power harvesting In the following sub-chapters several fabrication from regenerative braking systems, power back-up, strategies for hierarchical structures are introduced. providing burst power for heavy operations and voltage The chapter fi nishes with theoretical considerations stabilisation in start/stop systems of automotive and examples of hierarchical structures for PEC vehicles. applications. Hierarchical nanostructures increase the energy that can be stored by a supercapacitor per geometrical Supercapacitors surface area. The hierarchical designs enable these devices to be more compact and effi cient. Chapter 9, ‘Hierarchical Nanostructures: Application to Supercapacitors’ considers two types of supercapacitors Sensors (sometimes referred to as ultracapacitors, Figure 3): electrical double-layer capacitors (EDLC) and Chapter 11, ‘Sensors’ introduces three types of pseudocapacitors, and the different nanomaterials sensors: gas sensors, biosensors and optical surface for supercapacitor electrodes such as activated enhanced Raman spectroscopy (SERS) sensors, carbon and carbon-based nanomaterials like carbon where hierarchical designs play an important role. nanotubes and graphene. Supercapacitors are Sensors are analytical components that convert electrochemical capacitors that bridge the gap between physical or chemical input signals into appropriate

155 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691221 Johnson Matthey Technol. Rev., 2016, 60, (2) electronic output signals. In order to increase sensor have spread to many other technology areas. The performance and obtain high surface-to-volume ratios, book is ideal for people working in catalysis and/or nanostructures are widely used in chemical sensor energy related fi elds and want to fi nd inspiration for applications. Limitations of nanostructures are found new developments. in the lower nanometre scale due to degradation and aggregation effects of the structures. References Hierarchical designs increase the surface-to-volume 1. Ph. Buffat and J.-P. Borel, Phys. Rev. A, 1976, 13, (6), ratio without decreasing the size of the nanostructures. 2287 Additionally diffusion and mass transport of chemical 2. L.-L. Li, C.-W. Chang, H.-H. Wu, J.-W. Shiu, P.-T. Wu species to the surface are improved. This leads to and E. W.-G. Diau, J. Mater. Chem., 2012, 22, (13), higher sensitivity, better resolution and faster response 6267 times of sensor elements. Figure 4 shows a picture of 3. I. Herman, J. Yeo, S. Hong, D. Lee, K. H. Nam, J. hierarchical platinum nanostructures, which are applied Choi, W. Hong, D. Lee, C. P. Grigoropoulos and S. H. to increase sensitivity and selectivity of non-enzymatic Ko, Nanotechnology, 2012, 23, (19), 194005 glucose sensors. 4. J. Voelcker, ‘Hydrogen Fuel-Cell Vehicle News From Los Angeles Auto Show’, 8th December, 2014: http:// www.greencarreports.com/news/1095804_hydrogen- fuel-cell-vehicle-news-from-los-angeles-auto-show (Accessed on 26th February 2016) 200 nm 5. C. Gould and N. Shammas, ‘A Review of Thermoelectric MEMS Devices for Micro-power Generation, Heating and Cooling Applications’, in “Micro Electronic and Mechanical Systems”, ed. K. Takahata, InTech, Croatia, 2009 6. D. Gloess, S. Barth, H. H. Bartzsch, P. Frach, T. Herzog, S. Walter, H. Heuer, G. Suchaneck, G. Gerlach, J. Juuti and J. Palosaari, “Splutter Deposition of Piezoelectric AIN and AlScN Films for Ultrasonic 1 μm and Energy Harvesting Applications”: http://www. fep.fraunhofer.de/content/dam/fep/de/documents/ Poster/2013/Sputter_deposition_of_piezoelectric.pdf Fig. 4. Hierarchical platinum nanostructures to improve the effi ciency of glucose biosensors and other catalytic (Accessed on 26th February 2016) applications (Image courtesy of the Department of 7. R. H. Wilke, R. L. Johnson-Wilke, V. Cotroneo, W. Microsystems Engineering (IMTEK), University of Freiburg, N. Davis, P. B. Reid, D. A. Schwartz and S. Trolier- Germany) McKinstry, Appl. Opt., 2013, 52, (14), 3412

Conclusions

The book gives a great introduction into important energy conversion technologies, where the efﬁ ciency “Hierarchical Nanostructures for is increased by using hierarchical nanostructures. Energy Devices” Most of the inventions are related to improved electrochemical reactivity, but other areas like piezo and energy harvesting technology are also investigated. In pacemakers or neurostimulators applications, hierarchical (or fractal) structures to increase the energy transfer have been state-of-the- art for more than ten years and the ideas behind this

The Reviewer

Patrick Daubinger works for Johnson Matthey Medical Components in technical sales and business development. He obtained his Masters degree from the University of Freiburg, Germany, in 2013, where he was conducting research on hierarchical platinum nanostructures for biosensor applications. His work on hierarchical nanostructures resulted in various publications and he holds a patent in this ﬁ eld.

Johnson Matthey Piezo Products

Johnson Matthey Piezo Products, based in Germany, has been developing, producing and selling piezoelectric ceramics, components, atomizers and piezo modules for more than 45 years. Piezos can be combined with electronics and mechanics and tailored in accordance with customers’ individual requirements for various applications world-wide. The products are used in industry, in textile machines, in automobiles, in medical and other applications. Johnson Matthey is a globally leading supplier of bending actuators.

The Piezo Bending Actuator, Sensor and Energy Harvester: Working Principles

When two piezoelectric ceramic plates are bonded together with a supporting material and counter- Johnson Matthey’s piezo benders usually have a actuated, this results in a pronounced deformation working life of more than a billion cycles. of the composite similar to the case of a bimetal. Its design enables deﬂ ections of several millimetres. Forces up to several Newton and a short cycle time Contact of a few milliseconds can be achieved. Therefore, the piezo bending actuator can be employed as a high performance and fast-reacting control element. Johnson Matthey Piezo Products GmbH Piezo ceramic benders can also be used as sensors. Bahnhofstrasse 43 Bending generates a charge or voltage on both D-96257 Redwitz ceramic layers. Connecting both ceramic layers in Germany parallel results in adding their charges together. Thus Tel.: +49 (0) 9574 81 453 they are suitable for measuring both large and small Fax: +49 (0) 9574 81 98453 movements, vibrations and accelerations and for Email: [email protected] energy harvesting. Web: www.piezoproducts.com

In the Lab Synchrotron Radiation Based Solid Phase Characterisation of Industrial Catalysts and Materials

Johnson Matthey Technology Review features new laboratory research

Tim Hyde is a Principal Scientist in the Advanced Characterisation Department at the Johnson Matthey About the Researcher Technology Centre, Sonning Common, UK. His research interests focus on the development of synchrotron radiation-based characterisation using in particular X-ray absorption spectroscopy (XAS) on a range of applications in environmental catalysis, process technologies and battery research. He has co-authored over 50 papers and four patents.

About the Research • Name: Timothy I. Hyde Beginning in the 1960s and 1970s, the advantages • Position: Principal Scientist of using synchrotron radiation for spectroscopy and • Department: Advanced Characterisation diffraction were realised by an ever-growing scientifi c community. While the original accelerators were • Organisation: Johnson Matthey Technology built for particle physics applications, their relevance Centre to materials science, biology and medicine soon • City: Sonning Common, Reading became apparent. Synchrotron light has since become an ideal tool for many types of research including • Country: UK numerous industrially related applications. Some of • Email Address: [email protected] the experimental techniques utilised at synchrotron beamlines include: pair distribution function (PDF) structural analysis of crystalline and amorphous to the oxidation state, local coordination geometry and materials; macromolecular X-ray crystallography; small the nature of ligands present in the fi rst coordination angle X-ray scattering; XAS; tomography; imaging and sphere, while EXAFS provides information about the photoemission spectroscopy. radial distribution of localised atoms surrounding One specifi c characterisation technique utilised by a central absorbing atom of interest. By analysing Johnson Matthey is XAS. This technique consists of these oscillations, the local structural environment of various data regions, among which X-ray absorption a specifi c absorbing atom can be revealed without near edge structure (XANES) and extended X-ray hindrance from other atoms present and is irrespective absorption fi ne structure (EXAFS) are the most useful of the chemical complexity of the system. Since in obtaining chemical information. XANES is sensitive EXAFS requires a tuneable X-ray source, data are

158 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691366 Johnson Matthey Technol. Rev., 2016, 60, (2) always collected at synchrotron beamlines such as temperature. Specifi c Cr K-edge XANES pre-edge Diamond Light Source, Harwell, UK, or the European features and EXAFS Cr-O bond lengths confi rmed and Synchrotron Radiation Facility (ESRF), at Grenoble, quantifi ed both octahedral Cr3+ and tetrahedral Cr6+ in France, that are specially optimised for the purpose. the 500ºC calcined material. Benefi ts of the XAS technique include: Bimetallic palladium-mercury alloys are formed (a) atom specifi city since each element has an unique at elevated temperatures of 204ºC as Hg is absorption edge; adsorbed during coal gasifi cation over supported (b) does not depend on long-range structural order, so Pd-based sorbents. Different adsorption capacities were that any liquid or solid material can be investigated; proposed on alumina and silica supports as signifi cant and differences were found in the nature of the alloy formed (c) does not depend on concentration of the absorbing following Hg exposure on the respective sorbents. element, so even concentrations at ppm levels can By EXAFS analysis, on exposure of the supported

be studied. Pd sorbents to Hg-containing fuel gas the Pd/Al2O3 Hence XAS has proven an ideal technique to provide sorbent produced a single homogeneous solid solution structural insight into catalytic systems and materials of Pd–Hg. The Pd/SiO2 produced an alloy of varying of technological relevance to Johnson Matthey. composition with particles containing predominantly a Applications in areas such as environmental catalysis, Pd core and an Hg–Pd alloy at the periphery of the supported bimetallics and in situ systems have been particle. This was explained as due to lower dispersion studied and are now discussed. of Pd on the SiO2 support and subsequently restricted Given recent emerging concerns about the Hg diffusion through the particle. bioavailability and toxicity of anthropogenic platinum High synchrotron beam intensities allow for time- compounds emitted into the environment from sources and temperature-resolved in situ studies, where including vehicle emission catalysts (VEC), the solid characterisation techniques can often be combined to phase platinum species present in a variety of unused provide additional insight under industrially relevant (fresh) and used (road aged) gasoline three-way reaction conditions. The formation of nano-sized zinc catalysts (TWC) and light and heavy duty diesel VECs oxide from zinc peroxide was followed through the ZnO2 were investigated. The localised Pt environments decomposition temperature range of 180–250ºC by in present in catalysts from both North American and situ XAS and X-ray diffraction (XRD) techniques and European origins were explored by Pt L2- and the structure and nature of the PdO-Pd transformation L3-edge XAS. EXAFS studies at these edges revealed supported on alumina was followed from ambient to the presence of mainly oxidic species in the fresh 900ºC and subsequent cooling using in situ XAS and catalysts while metallic and bimetallic components PDF techniques. The phase compositions of the Pd were found to be the dominant species in the respective components were established at various temperatures road aged catalysts. Importantly, supporting Cl K-edge by quantitative analysis of the PDF data. Above 850ºC X-ray absorption spectroscopy confi rmed the absence it was found that PdO had converted to metallic Pd, of chlorine associated with platinum. Hence from these with the extent of reduction of the order ca. 70% Pd studies the presence of environmentally signifi cant metal and ca. 30% PdO. Complementary in situ XANES quantities of chloroplatinate species in VECs could be and EXAFS experiments supported and quantifi ed discounted. the observations that residual PdO was detected at In another study, the location and quantifi cation of elevated temperatures. Hysteresis in the transformation 6+ 3+ Cr and Cr in Cr-doped Fe2O3 high temperature shift upon cooling was confi rmed from XAS studies where (HTS) catalysts were determined by ex situ XAS and reoxidation occurred below 680ºC (Figure 1). laboratory XPS. Levels of Cr6+ were found to increase Through such non-proprietary studies, Johnson on samples heated to 400ºC. At 500ºC the material Matthey has now established expertise in synchrotron crystallised to form haematite, and concomitantly radiation-based structural methodologies. This is being the Cr6+ became unstable and reduced to Cr3+. XPS implemented over a diverse range of systems from indicated that the residual Cr6+ after 500ºC calcination fundamental characterisation studies of materials was located at the surface and was probably formed to characterisation-based innovation of proprietary from reoxidation of Cr3+ during cooling to room catalysts.

(a) (b) 1.0 1 0.8 C B 0.8 0.6 A 0.6 PdO 0.4 1.2 Pd metal 0.4 0.2 0.8 Pd foil PdO Phase fraction 0.4 Normalised absorption, a.u. 0 0.2 Normalised

RT absorption, a.u. 0 Cooling 24320 24360 2440 900ºC Energy, eV 0 Heating 212290 417 504 602 700 831 900 881 784 682580 478376274 173 71 55 42 32 RT 24300 24350 24400 24450 Temperature, ºC Energy eV

Fig. 1. (a) In situ XANES plots from the heating and cooling of 4%Pd/Al2O3 catalyst. Insert shows the XANES of PdO and Pd foil reference materials. Three regions over the heating and cooling are observed. A The catalyst has PdO structure with heating to 900ºC. B The incomplete reduction to metallic Pd takes place on cooling from 900ºC to 580ºC. C The reoxidation to PdO has taken place. (b) Plot detailing the relative amounts of metallic Pd and PdO species present over heating and cooling cycle obtained using the ﬁ rst shell coordination number, N, from EXAFS analysis. (Reprinted with permission from J. Keating, G. Sankar, T. I. Hyde, S. Kohara and K. Ohara, Phys. Chem. Chem. Phys., 2013, 15, (22), 8555. Copyright Royal Society of Chemistry)

Selected Publications Verlag, Berlin, Heidelberg, Germany, 2015 T. I. Hyde, P. W. Ash, D. A. Boyd, G. Randlshofer, K. M. R. Ward, T. I. Hyde, E. D. Boyes and P. L. Gai, Rothenbacher and G. Sankar, Platinum Metals Rev., ChemCatChem, 2012, 4, (10), 1622 2011, 55, (4), 233 V. Martis, R. Oldman, R. Anderson, M. Fowles, T. Hyde, P. W. Ash, D. A. Boyd, T. I. Hyde, J. L. Keating, G. R. Smith, S. Nikitenko, W. Bras and G. Sankar, Phys. Randlshofer, K. Rothenbacher, G. Sankar, J. J. Chem. Chem. Phys., 2013, 15, (1), 168 Schauer, M. M. Shafer and B. M. Toner, Environ. Sci. S. Poulston, T. I. Hyde, H. Hamilton, O. Mathon, C. Technol., 2014, 48, (7), 3658 Prestipino, G. Sankar and A. W. J. Smith, Phys. Chem. Chem. Phys., 2010, 12, (2), 484 T. I. Hyde and G. Sankar, ‘Solid State Platinum Speciation from X-ray Absorption Spectroscopic Studies of T. Daley, E. Raj, S. Ramos, G. Cibin, A. Dent, T. I. Hyde Fresh and Road Aged Three Way and Diesel Vehicle and G. Sankar, J. Phys: Conf. Ser., 2013, 430, (1), Emission Control Catalysts’, in “Platinum Metals in the 012080 Environment”, eds. F. Zereini and C. L. S. Wiseman, J. Keating, G. Sankar, T. I. Hyde, S. Kohara and K. Ohara, Environmental Science and Engineering, Springer- Phys. Chem. Chem. Phys., 2013, 15, (22), 8555

Johnson Matthey Highlights

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

EMISSION CONTROL TECHNOLOGIES Understanding the Gas Transport in Porous Catalyst Layers by Using Digital Reconstruction Techniques V. Novák, M. Dudák, P. Kočí and M. Marek, Curr. Opinion Chem. Eng., 2015, 9, 16 Internal transport limitations are studied in supported porous catalytic layers to understand their effect on the performance of a catalytic reactor, specifi cally an automotive catalytic converter. The results are considered generally applicable. 3D digital reconstruction and complex mathematical modelling were applied and case studies are presented to illustrate their use in optimising transport properties in C. I. Hiley, J. M. Fisher, D. Thompsett, R. J. Kashtiban, J. a porous supported catalyst. Sloan and R. I. Walton, J. Mater. Chem. A, 2015, 3, (24), 13072 (Reproduced by permission of The Royal Society of 2+ Incorporation of Square-planar Pd in Fluorite CeO2: Chemistry) Hydrothermal Preparation, Local Structure, Redox Properties and Stability C. I. Hiley, J. M. Fisher, D. Thompsett, R. J. Kashtiban, J. Jansson, Å. Johansson, H. Sjovall, M. Larsson, G. J. Sloan and R. I. Walton, J. Mater. Chem. A, 2015, 3, Smedler, C. Newman and J. Pless, SAE Technical (24), 13072 Paper 2015-01-0997, 2015 Low-temperature hydrothermal synthesis was used to This paper, presented at SAE 2015 World Congress prepare a number of nanocrystalline Pd-substituted & Exhibition, reviews emission control systems for heavy duty diesel (HDD) applications to meet ceria-type fl uorites of the form Ce1–xPdxO2–δ (0.05 ≤ x ≤ 0.15). Efforts to attain a higher value of Pd post-EuroVI/US10 legislation. It focuses on systems substitution results in PdO being formed as a secondary containing SCR and covers the optimisation of the phase. X-ray absorption near edge structure revealed system for both fuel consumption and catalyst effi ciency. that Ce and Pd were in the +4 and +2 oxidation states Pros and cons are considered and it is concluded that respectively, therefore proposing oxide defi ciency as all current legislative requirements may be met using the mechanism of charge balance. Pd2+ has local optimised SCR containing systems. square-planar coordination which was confi rmed by the extended X-ray absorption fi ne structure (EXAFS) Thermal Decomposition of Bulk and Supported Barium analysis at the Pd K-edge. A structural model, Nitrate consistent with earlier modelling investigations, can C. J. Bardwell, R. I. Bickley, S. Poulston and M. V. be fi tted in which the mean fl uorite structure was Twigg, Thermochim. Acta, 2015, 613, 94 kept but with Pd2+ sitting in the square faces of oxide The decomposition of anhydrous bulk barium nitrate to ions available in the local cubic geometry of Ce. Pd produce O and NO plus small quantities of NO , N and is well dispersed in the nanocrystalline ceria particles 2 2 2 N O was found to take place principally at 630ºC. Lower as shown by TEM analysis and in situ powder XRD 2 indicated that upon heating in air the samples continue temperature decomposition took place after dispersing to be stable up to 800ºC. on Al2O3 and TiO2. This procedure also increased the complexity of the decomposition products as well as Heavy Duty Emission Control System Analysis and causing increased N2O production due to reaction of Optimization for Future Demands adsorbed gases on the supports.

FINE CHEMICALS: CATALYSIS AND CHIRAL region and the features correlated to functions of the + TECHNOLOGIES H2 and H concentration, anion concentration, potential scan limit and temperature. A Halogen- and Hydrogen-bonding [2]catenane for Anion Recognition and Sensing PRECIOUS METAL PRODUCTS J. M. Mercurio, A. Caballero, J. Cookson and P. D. Beer, RSC Adv., 2015, 5, (12), 9298 A Density Functional Study of Oxygen Vacancy Formation on α-Fe O (0001) Surface and the Effect of An anion templated Grubbs’ II-catalysed RCM clipping 2 3 Supported Au Nanoparticles mechanical bond forming process was used to synthesise a mixed halogen- and hydrogen-bonding S. W. Hoh, L. Thomas, G. Jones and D. J. Willock, Res. hetero-[2]catenane. The interlocked catenane host Chem. Intermed., 2015, 41, (12), 9587 was able to bind and sense anions, and form strong The PBE + U calculations of Au nanoparticles supported associations with acetate and dihydrogen phosphate on the on α-Fe2O3(0001) surface were investigated as shown by 1H NMR spectroscopy and fl uorescence and the authors found that the periphery atoms of Au10 titration experiments. particles became oxidised via the dissociation of O2 at the metal-oxide interface. The metal particle was shown NEW BUSINESSES: BATTERY TECHNOLOGIES to signifi cantly decrease the defect formation energy for surface oxygen vacancies in the oxide support Transforming Anatase TiO2 Nanorods into Ultrafi ne Nanoparticles for Advanced Electrochemical especially when the nanoparticle is in a semi-oxidised Performance state. The defect formation energy depends on the distance of the vacancy from the metal particle so the D. Bresser, G.-T. Kim, E. Binetti, M. Striccoli, R. smallest defect formation energies are calculated for Comparelli, S. Seidel, D. Ozkaya, M. Copley, P. Bishop, oxygen vacancies formed under the Au nanoparticle. E. Paillard and S. Passerini, J. Power Sources, 2015, For Au /α-Fe O (0001) forming the vacancy under the 294, 406 10 2 3 cluster involves a defect formation energy of 2.13 eV Although the smaller diffusion pathways for ions (relative to ½O2(g)) and this falls to 0.86 eV when the and electrons are frequently the main reason for perimeter of the particle is oxidised. These values are nanostructuring lithium-ion active materials, there are substantially less than that for the bare α-Fe2O3(0001) also other, less-expected phenomena happening when of 3.04 eV. leaving the microscale to go into the nanoscale. The authors investigate one of these phenomena – the PROCESS TECHNOLOGIES thermally induced fragmentation (i.e. ‘chopping’) of oleic acid-capped anatase TiO2 nanorods perpendicular NiO/CaAl2O4 as Active Oxygen Carrier for Low to the [001] direction. As a result of the fragmentation, Temperature Chemical Looping Applications ultrafi ne TiO2 nanoparticles with improved (001) facets J. A. Medrano, H. P. Hamers, G. Williams, M. van Sint are formed. These ultrafi ne nanoparticles have a Annaland and F. Gallucci, Appl. Energy, 2015, 158, 86 fantastic rate performance and cycling stability, even for cathodic cut-off potentials as low as 0.1 V, due to the A commercial Ni-based catalyst has been tested for altered surface facets ratio and the benefi cial utilisation chemical looping to capture CO2 more effi ciently from of carboxymethyl cellulose as binder. chemical processes at low temperature. Its behaviour at low temperature has been investigated and it was found NEW BUSINESSES: FUEL CELLS to require pre-treatment at higher reduction temperature as an activation step. The oxygen carrying capacity Properties of the Hydrogen Oxidation Reaction on Pt/C is thereby increased, improving reactivity. Redox Catalysts at Optimised High Mass Transport Conditions kinetics were found to be adequate for low temperature and its Relevance to the Anode Reaction in PEFCs and applications up to 200 cycles. The kinetic rates for Cathode Reactions in Electrolysers gas-phase and gas-solid reactions are also available to C. M. Zalitis, J. Sharman, E. Wright and A. R. Kucernak, use for prediction of performance in such applications. Electrochim. Acta, 2015, 176, 763 Effect of Mill Type on the Size Reduction and Phase A model is developed for use in simulation studies of Transformation of Gamma Alumina fuel cell and electrolyser systems using Pt/C catalyst S. R. Chauruka, A. Hassanpour, R. Brydson, K. J. with ultra-low loadings (0.84–3.5 μg cm−2, Johnson Pt Roberts, M. Ghadiri and H. Stitt, Chem. Eng. Sci., 2015, Matthey’s HiSPEC 9100). The dependence of the 134, 774 hydrogen reaction on pH and H2 concentration may be understood using this model. Using a high mass Air jet milling, planetary ball milling and single ball milling transport fl oating electrode technique allowed features were employed to investigate the effects of stress modes of the HOR and HER to be elucidated for the fi rst time. on particle size reduction in γ-alumina, a frequently Fine structure has been resolved in the H2 adsorption used catalyst support. The size reduction achieved was

greatest for air jet milling, from a d90 of 37 μm to 2.9 μm. Characterising the Rheology of Non-Newtonian Fluids Single ball milling produced a size reduction to 10.5 μm. using PFG-NMR and Cumulant Analysis The planetary ball milling procedure gave the least size T. W. Blythe, A. J. Sederman, J. Mitchell, E. H. Stitt, A. reduction to 30.2 μm and also caused phase transition P. E. York and L. F. Gladden, J. Magn. Reson., 2015, to unwanted α-alumina. The shear rates are known to 255, 122 be higher in planetary ball milling compared to other A new method to rheometry using pulsed fi eld gradient types of milling, and this is thought to account for the (PFG) NMR which increases the application of MR differences. rheometry to single-axis gradient hardware was A Kinetic Analysis Methodology to Elucidate the Roles proposed by the present authors. In this study the use of Metal, Support and Solvent for the Hydrogenation of of fl ow propagators was diffi cult due to the introduction of artefacts during Fourier transform which occur when 4-phenyl-2-butanone over Pt/TiO 2 realistic sampling procedures are limited by experimental S. K. Wilkinson, I. McManus, H. Daly, J. M. Thompson, and hardware constraints and especially when specifi c C. Hardacre, N. S. Bonab, J. ten Dam, M. J. H. Simmons, spatial and temporal resolution are needed. In this C. D’Agostino, J. McGregor, L. F. Gladden and E. H. paper, the authors outline a process involving the Stitt, J. Catal., 2015, 330, 362 cumulant analysis of the acquisition data directly, The choice of solvent affects the rate and selectivity therefore, avoiding the introduction of artefacts and shortening data acquisition times. A model-dependent (ketone vs. aromatic ring) of the hydrogenation of method was developed taking into account the 4-phenyl-2-butanone over a Pt/TiO catalyst. A crucial 2 pipe-fl ow characterisation of fl uids showing kinetic model for this multi-phase reaction, taking into non-Newtonian power-law rheology, which includes account statistical analysis, has been proposed. A 2-site using an analytic expression illustrating the fl ow model was found to be the most suitable, illustrating propagator in terms of the fl ow behaviour index. This aromatic hydrogenation (over a Pt site) and ketone process was found to be resilient to the signal-to-noise hydrogenation (at the Pt/TiO2 interface). The selected ratio (SNR). The simulated results were validated by an solvent has little effect on the ketone hydrogenation experimental investigation on shear-thinning aqueous rate constant, however it strongly infl uences aromatic xanthan gum solutions where the rheology could be hydrogenation due to solvent-catalyst interaction. correctly characterised using a power-law model across Reaction selectivity was also corresponded to a fi tted the experimental shear rate range of 1–100 s–1. The product adsorption constant parameter. The role of calculated fl ow behaviour indices were found to be within solvents in affecting reactant adsorption and reaction 8% of those acquired using spatially-resolved velocity selectivity was shown by the kinetic analysis method. imaging and within 5% of conventional rheometry.

Towards 3D-Electrical Capacitance Tomography for Interface Detection

Thresholding method improves accuracy for real-world 3D process monitoring

Peter J. Clark* and Giuseppe Forte measurement density towards the centre of the ECT Johnson Matthey Technology Centre, Billingham, sensor results in reduced accuracy. A thresholding UK, TS23 1LB; and School of Chemical Engineering, method, previously applied to binary systems to University of Birmingham, Edgbaston, UK, B15 2TT improve the imaged accuracy of a hard boundary between two separate phases, has been applied to the *Email: [email protected] 3D-ECT tomograms that represent the PE phantom. This approach has been shown to improve the accuracy Mark J. H. Simmons of the acquired image of a cylinder of air within a School of Chemical Engineering, University of non-conductive PE tube. Birmingham, Edgbaston, UK, B15 2TT 1 Introduction E. Hugh Stitt Johnson Matthey Technology Centre, Billingham, UK, The ability to accurately and quickly monitor the TS23 1LB evolution of physical and chemical changes within products during manufacturing has been the subject of much interest in both academic and industrial research The application of three-dimensional electrical communities. Enhanced process monitoring allows an capacitance tomography (3D-ECT) for the in situ engineer to implement better control thereby increasing monitoring of a hard boundary or interface has been efﬁ ciency, improving material and energy usage and investigated using imaged phantoms that simulate increasing product quality. Electrical tomography has real-life processes. A cylinder-in-tube phantom been developed over the last few decades with the manufactured from polyethylene (PE), a low di-electric purpose of delivering real time, in situ measurements and non-conductive material, was imaged using the of phase distributions within a process vessel or linear back projection (LBP) algorithm with the larger pipe. It provides spatially resolved information on tube immersed at varying intervals to test the ability of the distribution of material properties (permittivity, the technique to image interfaces axially through the conductivity, density) within a cross-sectional plane sensor. The interface between PE and air is clearly through the sensing zone represented by a pixel grid imaged and correlates to the known tube penetration known as a tomogram. In the majority of cases to date within the sensor. The cylinder phantom is imaged in two-dimensional (2D) images have been sufﬁ cient to the centre of the sensor; however, the reduction in provide the required process information for the user;

164 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691537 Johnson Matthey Technol. Rev., 2016, 60, (2) however, 3D tomography provides spatial information indeed due to the hazardous nature of the chemicals in both radial and axial planes yielding greater process used any potential for containment risk is unacceptable. insight. The compromise when applying 3D methods Liquid hold up within a gas/liquid packed bed has as opposed to 2D is increased computing time, greater been investigated using twin-plane ECT for application complexity of hardware and requirement for advanced within chemical reactors. Initial measurements using reconstruction algorithms. single plane ECT were conducted to determine liquid Electrical tomography is categorised as a ‘soft-fi eld’ drainage dynamics and determine their relationship with method. The distribution of the electric fi eld within the the gas velocity and packing particle size. Extensive sensor is unknown thus forming a non-linear relationship calibrations were performed using twin plane ECT so between the measured capacitances at the boundary as to accurately map bed hydrodynamics across the of the sensor and the material distribution within the dual sensing zones and the use of tracer led residence sensor. ECT is a specifi c form of electrical tomography time distribution (RTD) combined with ECT analysis applicable to systems with a non-conductive continuum to obtain hydrodynamic performance data was also and an insulating process vessel wall. It utilises a investigated (4). concentric ring of uniformly spaced electrodes fi xed Image refi nement of 2D tomograms of high dielectric to the outside of a non-conductive vessel wall. The materials using commercially available ECT equipment signal-to-noise ratio (SNR) of the system is dependent has previously been addressed by the present authors upon the area of each electrode therefore studies have (5, 6). The focus of that study was to vary the threshold been conducted that determine the optimal size of ECT value and optimise the image based upon image error electrodes (1). Guard electrodes are also required to as a function of the threshold. Two known methods for prevent process materials outside the sensing zone determining image error were compared: pixel-by-pixel having an effect upon the electric fi eld distribution (PbP) (Equation (i)) and areal (Equations (ii) and (iii)): (2). The number of electrodes used within the sensor N ring is a compromise between the required size of 100 (μimage – μreal) ePbP = (i) electrode to deliver the required SNR, the diameter N μreal of the electrode array (directly related to the diameter i = 1 of the process vessel) and the desired reconstructed image accuracy, with a greater number of electrodes where ePbP is the error using the PbP approach, N is the delivering increased spatial resolution. total number of pixels, μimage is the imaged pixel value of There are two critical parameters when designing pixel i in the tomogram and μreal is the theoretical pixel an ECT sensor: the ratio of permittivity of continuous value of pixel i in the real system. The areal analysis is and dispersed phases and the thickness of the sensor given as two stages; the fi rst calculates the area of the liner (3). When either of these phases has a high image occupied by the phantom: relative permittivity there is an increased electrical fl ux defl ection at the boundary of the ECT sensor that results μimage Aimage = AC (ii) in the relationship between material permittivity and N μimage 15. To image these where Aimage is the area of the image and AC is the materials a new sensor was developed that applies cross-sectional area of the circular sensor chamber. internal rather than external electrodes to measure The second stage calculates the error as a ratio of the the boundary inter-electrode capacitance. This system difference between the areas of the imaged and real has been used to image an air/de-mineralised water object to the area of the real object: phantom and a water continuous oil-in-water dispersion designed to mimic process equipment in the oil industry Aimage – Aphantom eA = (iii) such as separators and extraction units (3). Whilst the Aphantom confi guration of the sensor solves the boundary linearity problem, it removes the non-intrusive advantage that where eA is the image error calculated using the areal traditional ECT has over competing methods. In the approach and Aphantom is the area of the phantom. majority of the process industries it is unlikely that a These two methods were compared to determine pipe or unit will be taken offl ine for electrode fi tting, and the sensitivity of image error to threshold value. The

165 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691537 Johnson Matthey Technol. Rev., 2016, 60, (2) important conclusions were: (a) the optimal threshold accuracy of this method for 3D-ECT applications (12). values calculated using each method are consistent, The same group then used this approach to investigate demonstrating robustness; and (b) the calculated the dynamics of spiral bubble plume motion in the image error using the areal approach is much lower as entrance zone of bubble columns and three-phase it is not constrained to the pixel resolution. fl uidised beds (13). The NN-MOIRT approach was A similar study was conducted with the aim of again combined with volumetric ECT to investigate a defi ning ‘limited region tomography’ (7, 8). This is a jet of air, issued from a single orifi ce, within a poppy method based upon defi ning a region to which the seed bed of internal diameter 50 mm. The ECVT reconstruction is applied using a narrowband pass fi lter. results were thresholded to yield optimal images and The accuracy of the image is increased by limiting the then compared directly with magnetic resonance image area, thereby reducing the number of unknown imaging (MRI) data showing consistency at high air pixels and enhancing the resolution. This approach fl ow rates, although the ECVT was less sensitive than has so far been applied predominantly for detection of MRI to changes at lower air fl ow. Standard deviation deposits and scaling in pipes, although the potential measurements across the ECVT images gave 7 mm in applications stretch to any purpose whereby imaging of the radial plane and 8 mm in the axial plane, at least an the whole tomographic plane is not essential. However, order of magnitude higher than MRI. From this it may this method is currently not a commercially available be concluded that signifi cant further improvement is technology. required for ECT to be considered an accurate imaging The defi nitions of 3D and ‘pseudo-3D’ tomography technique (14). are confusing and often misapplied in the existing In another study, moving objects were imaged using literature on this subject. Until recently there was a 3D-ECT. Images of a rod and a ball were separately lack of precision hardware and capable software that moved down the central axis and periphery of the could obtain full 3D-ECT data, that is to say acquire sensor (15). A Gauss-Newton reconstruction algorithm and reconstruct electrode measurements in the axial, was used, another approach that is not yet commercially radial and angular dimensions (assuming cylindrical available and is largely confi ned to academia, yielding coordinates). Prior to suitable commercial equipment an image error of approximately 10% although this being available, 2D tomograms were stacked to obtain increased by 2% when the imaged objects were at ‘pseudo-3D’ images of a process. The term ‘pseudo’ is the sensor periphery. Interestingly, a parallel study used as there are no electrode measurements across was conducted that arrived at the same conclusion in the axial plane (9). that the image error at the wall was greater than in the An electrical capacitance volume tomography (ECVT) centre of sensor. The reasoning for this observation is approach has been used to monitor the hydrodynamics that the incidence of convergence of electric fi eld lines of the drying of wet granules with the purpose of being is greater in the sensor centre than at the periphery used in the pharmaceutical industry. From the 3D leading to greater measurement integrity in the centre data gained the granule moisture content and drying (5). An iterative non-linear reconstruction algorithm has medium velocity were calculated (10). also been applied to 3D-ECT measurement data and The majority of work so far in applied 3D-ECT compared with traditional linear back projection; suffi ce research has used back-projection approaches to say the non-linear algorithm showed signifi cantly for simplicity, however there has been some work improved results (16). done on a volumetric 3D approach using alternative The academic group responsible for the previously reconstruction algorithms. A neural-network mentioned ‘limited region tomography’ (7, 8) also multi-criterion optimisation image reconstruction investigated using a novel ECT sensor coupled with a technique (NN-MOIRT) algorithm has been developed resolution analysis method to determine the sensitivity that effectively applies the optimisation approach of image error and resolution to electrode and sensor across a Hopfi eld neural network (11). The results have design (17). This study provides a step change in the been compared favourably with existing reconstruction protocols used to design 3D-ECT sensors and certainly algorithms such as the iterative fi ltered back projection gives greater impetus to the argument for increased technique (IFBPT) and the iterative linear back use of this technology in industry. projection (ILBP) algorithm. The NN-MOIRT algorithm Given the usefulness of the information gathered, was then applied to a three phase bubble column so as there is potential for 3D-ECT to be integrated into to image multi-phase fl ows and showed the enhanced engineering processes (18). However, so far there

166 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691537 Johnson Matthey Technol. Rev., 2016, 60, (2) have been few works published using a commercial investigate the capability of a commercially available instrument. 2D-ECT has so far been applied to a wide ECT instrument to deliver 3D images of a transient range of engineering processes, however the majority system including an evolving interface. of published work has been carried out on academic 2.1 Cylinder-in-Tube Phantom Confi guration instruments in either single or dual plane confi guration. Few ECT instruments are commercially available and In order to show the development of a hard interface the capability for 3D image reconstruction amongst in the radial plane moving axially through the sensor these is very low, the main reason being a lack of a series of phantom experiments were designed. image accuracy which is problematic for real-world Phantoms are objects of known dimensions that industrial applications. Indeed the implementation of a simulate a desired process or system. A triple plane reconstruction package into a useable graphical user ECT sensor was used comprising a polyether ether interface (GUI) has only recently been published (19). ketone (PEEK) cylinder of inner diameter 156 mm The present paper addresses this need by applying with three planes of eight electrodes implanted to the commercially available 3D-ECT technology to image outside of the wall and encased in stainless steel. A a series of mobile phantoms. The phantom used is a cylinder-in-tube PE phantom was used to simulate the simple cylinder-in-tube setup that demonstrates both a interface. The PE tube had diameter 142 mm with an moving object and a hard boundary, in this case the annular core of diameter 30 mm, tube wall thickness interface between polyethylene and air. A methodology, 62 mm and length 300 mm. The PE cylinder was initially developed on 2D-ECT images, that improves 30 mm in diameter and 300 mm long with a fl anged image accuracy using optimised binary thresholds is base having a diameter of 160 mm. The schematics for then applied to the 3D-ECT tomograms. these phantoms are given in Figure 1. The small cylinder (Figure 1(b)) and sensor were 2 Materials and Methods for 3D-ECT Imaging raised above the large tube (Figure 1(a)) and imaged 1 at penetration intervals of /8 of the length of the The objectives of this work were to determine the small cylinder. Once the whole of the large tube had usefulness of ECT as an online measurement tool for been removed, the sensor was imaging just the small interface detection within a 3D sensing zone and to cylinder with a large air gap between the sensor wall

(a) (b) 40 mm 40 mm 160 mm 140 mm 160 mm 300 mm 300 mm

Fig. 1. Schematics for cylinder-in-tube phantom experiments: (a) thick-walled PE tube showing annulus; (b) PE cylinder with ﬂ ange base

1 and the phantom. The /8 penetration intervals were tomography data is normalisation of the capacitance measured and marked onto the polyethylene cylinder data according to Equation (iv): shown in Figure 2(b). C – Cl Figure 2(a) shows the large tube housed within the Cn = (iv) C – C PEEK-lined ECT sensor. There is an air gap of 1 mm h l between the PE large tube and the wall of the sensor. Where Cn and C are the normalised and measured As this is present during the sensor calibration it has no capacitance values respectively, and Ch and Cl are 1 impact on the reconstructed ECT data. The /8 intervals capacitance values from the high permittivity and of the length of the small cylinder at which the large low permittivity reference frames respectively. The tube was imaged are shown in Figure 2(b) as markings reference frames are taken prior to measurement and on the small cylinder. for both experimental series the conditions for these are given in Table I. 2.2 ECT Setup and Conﬁ guration The low permittivity reference frame is taken prior The ECT instrument used for this work was an Industrial to the high permittivity reference frame and once Tomography Systems (ITS) m3c 24 input channel unit. both are acquired process data can be recorded. The Individual electrodes were connected from the sensor phantom experiment was imaged for 30 frames at to the instrument ports using Bayonet Neill-Concelman each of the large tube holding intervals thus building (BNC) cables. The ﬁ rst step in processing of electrical a series of images of penetration depth of the small cylinder.

(a) Co-axial cable housing (b)

ECT sensor PE cylinder

1 /8 interval mark

Pipe annulus

PEEK pipe Flange base

Fig. 2. Individual components of cylinder-in-tube phantom: (a) thick-walled PE tube within PEEK-lined ECT sensor; (b) 40 mm diameter PE cylinder with ﬂ anged base

Table I High and Low Permittivity Reference Frames: Material Properties for Both Phantom and Drying Bed Experimental Series High permittivity reference framea Low permittivity reference framea Experimental series material r material r Phantom PE 2.2–2.4 Air 1.5 a r = Relative permittivity

A reconstruction algorithm is required to calculate the has been created to allow the comparison between spatially resolved material data from electrical boundary the reconstructed tomograms and the real object, this measurements. In commercial ECT equipment there image is given in Figure 3. has previously been just one algorithm available to use: The PbP approach, deﬁ ned as voxel-by-voxel in the LBP algorithm. This is a single-step algorithm that 3D tomography, compares the corresponding voxels requires less computing power due to its simplicity and in the reconstructed and in the theoretical images delivers fast results; however the reconstructed images (Figure 3) and is described by the same equation are inaccurate (5). It assumes a linear relationship as PbP analysis (Equation (i)). The areal approach between the capacitance and permittivity vectors as (Equations (ii) and (iii)) is extended to the third given in Equation (v): dimension by evaluating the volume occupied by the phantom in the image, Equation (vi): g = STλ (v)

μimage where g is the normalised permittivity vector, ಓ is Vimage =VC (vi) N μimage

S (20). This equation is solved by discretisation where Vimage is the volume of the image and VC is the across a pixel grid containing 1024 pixels that is total volume of the sensor chamber. The volumetric reduced to 812 pixels as the corners are fi lleted error is calculated by evaluating the ratio of the for circular images. Although LBP has several difference between the volumes of the imaged and drawbacks, it is the most commonly used method in the real object and the volume of the real object, industry and is therefore used for reconstructing data Equation (vii): within this paper. Vimage – Vphantom 2.3 3D-ECT Tomogram Image Processing ev = (vii) Vphantom A true 3D-ECT measurement acquires charge data in both the radial and axial direction for each stimulus, where ev is the image error calculated using the yielding an output of a 3D tomogram composed of volumetric-areal approach and Vphanton is the volume 32 × 32 × 32 voxels (corresponding to pixels in 3D of the phantom. space). Each voxel represents a volume portion within the sensor and is assigned a value in the interval between 0 and 1. In order to identify the geometrical shape placed in the sensor, a binary gating method is applied. This method, widely applied in MRI and in 2D-ECT, consists of dividing the tomogram into a number of zones equivalent to the number of phases. In this study one zone represents the volume occupied by the phantom and the second is composed of the surrounding gas phase (5). Once the image has been thresholded it is possible to compare it with a previously built reference to evaluate the image error. The advantage of using phantoms is that their exact geometry is known and therefore a digitalised reference image as basis for comparison for error analysis is accurate and simple to construct. The data processing methods applied in the refi nement of 2D images have been applied also to the 3D-ECT tomogram analysis. The objective of the gating method is to fi nd the optimum threshold value Fig. 3. Pixelated refer ence image of fully withdrawn PE cylinder from PE tube for comparison in voxel-by-voxel to allow generation of the binary matrix that describes analysis the object with the minimum error. A reference image

3 Results and Discussion with the PE/air interface being the top of the PE large tube. The tomograms are shown in Figure 4. The phantom experiments use PE, with a low relative The images displayed in Figure 4 are cross-section permittivity, to simulate an interface so the 3D-ECT tomograms whereby the electrode measurements reconstruction can be trialled using the basic LBP from the three planes of uniform electrodes are algorithm to ascertain its effi cacy before application reconstructed and subsequently discretised to give to high di-electric media. Therefore these experiments 16 image planes. In pseudo-3D tomography this form are used solely to examine whether 3D-ECT is capable of image is strongly discouraged as the lack of axial of monitoring an interface. inter-electrode capacitance data nullifi es any assumptions required to use an inter-plane interpolation 3.1 3D-ECT Reconstruction using LBP approach. This is not the case in 3D-ECT as inter- The ECT three-plane sensor is attached to the fl anged electrode capacitances on different planes have base of the small cylinder therefore the phantom is been measured. Figure 4(a) shows the system at defi ned as the large tube being inserted into the sensor full immersion, that is to say the small cylinder is fully

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

0 0.25 0.5 0.75 1.0 Relative permittivity

7 Fig. 4. Cross-section s liced 3D tomograms of cylinder-in-tube phantom experiment: (a) fully immersed phantom; (b) /8 3 5 1 3 1 1 immersed; (c) /4 immersed; (d) /8 immersed; (e) /2 immersed; (f) /8 immersed; (g) /4 immersed; (h) /8 immersed; (i) fully withdrawn

170 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691537 Johnson Matthey Technol. Rev., 2016, 60, (2) immersed in the large tube. The colour scheme of these was clearly shown to progress down the sensor. charts indicate low permittivity regions as being blue Figure 5(d) shows the presence of the small cylinder and high permittivity regions as being red, however the at the top of the image, indicating that although the 5 corners of the square pixel grid are included in these large tube was /8 immersed in the sensor and the reconstructed images and coloured blue. These pixels small cylinder occupied the centre of the void behind are present in all the cross-section slice tomograms, the interface, it was not clearly detected by the sensor. 1 however they are not representative of the circular When the large tube reached /4 immersed sensor (in 2D-ECT these are typically removed from (Figure 5(g)) the small tube was imaged as a rounded the image) and are therefore excised from all analyses. cone and the interface of the large tube and air was The large tube is withdrawn from the ECT sensor at clearly present at the bottom of the sensor. The imaged 1 intervals of /8 penetration. It can be seen in Figure 4(b) shape of the small cylinder was consistent from that a reduction in the mean conductivity across the Figure 5(e) to Figure 5(h) and was only imaged as electrode plane occurs and continues through the other a cylinder once the large tube was fully removed from images within Figure 4 as the phantom moves through the sensor in Figure 5(i). This result is symptomatic of the sensor. The small cylinder is present inside the a number of limitations of the technique in its current cylinder at all times even as the large tube is removed state: the limit of the software to drive a maximum from the sensing zone. The fi rst evidence of the of three planes; the reconstruction algorithm when presence of the small cylinder phantom is in Figure 4(e) imaging hard boundaries; and the low permittivity where a series of voxels in the centre of the top ratio between the two phases (air = 1,PE = 2.3). ECT cross-section planes of the 3D image register an therefore images the permittivity difference between increase in the permittivity. Moving from Figure 4(e) phases by keeping this difference high and the ability of to Figure 4(h) it is clear this area of high permittivity the instrument to detect a hard boundary theoretically spreads down the 3D tomograms in this series. This increases. has to be attributed to the presence of the small cylinder 3.2 Binary Image Refi nement of 3D-ECT in the centre of the sensor, however the shape of the Tomograms high permittivity region is not clear which is unsurprising given that the air gap between the cylinder and the Following on from the evaluation of accuracy of sensor wall is 51 mm. The air gap affects the linearity 2D-ECT tomograms, a similar error analysis has assumption used in the sensitivity matrix. However, it is been applied to the cylinder-in-tube experiment. The surprising that the small cylinder is affecting the inter- reference image for the voxel-by-voxel analysis was electrode electric fi eld given the size of this air gap. The shown in Figure 3. The voxel values are extracted from images given in Figure 4 have been smoothed to a high the tomographic data and processed using MATLAB® degree that gives the impression of soft edges in the (MathWorks® Inc). The voxel-by-voxel error and the process. volumetric error are calculated for several values in the To better represent the interface between low and interval between 0.2 and 0.7 in order to evaluate the high permittivity phases iso-surface plots of the same optimal value in terms of minimum error. experiment are presented in Figure 5. Iso-surface As shown in Figure 6 in both cases it is possible to plots were calculated from volumetric data (given in reach very low errors and the optimum threshold value Figure 4) and represent constant scalar data within does not change substantially. It is 0.501 according to a scalar 3D distribution. In the case of the phantom the voxel-by-voxel analysis (Figure 6(a)) and 0.547 by experiments this is represented as the interface applying the volumetric analysis (Figure 6(b)). This between PE and air. Initially, in Figure 5(a), the sensor characteristic is an important sign of the robustness of was at the equivalent of a high permittivity reference the reconstruction and leads to the conclusion that the frame as the large tube was fully immersed in the results do not change substantially if the threshold value sensor. The four rectangular regions at the sensor is fi xed for either error analysis methodology. In fact, boundary are image artefacts and enclose the high using the voxel-by-voxel method to compare the errors permittivity region; they are able to be excised from when choosing the threshold value at the minimum of the images but have been retained to give clarity Figure 6(a) and Figure 6(b), gives values of 0.0306 as regards to the phase distribution. As the large and 0.0316 respectively. This confi rms the strength of PE tube was removed from the sensor the interface the volumetric analysis which unlike the voxel-by-voxel

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

7 3 Fig. 5. Iso-surface plots of PE cylinder-in-tube phantoms: (a) fully immersed large tube phantom; (b) /8 immersed; (c) /4 5 1 3 1 1 immersed; (d) /8 immersed; (e) /2 immersed; (f) /8 immersed; (g) /4 immersed; (h) /8 immersed; (i) fully withdrawn

(a) (b) 0.12 100 0.11 0.10 10–1 0.09 0.08 10–2 0.07 Image error Image error 0.06 –3 0.05 10 0.04 0.03 10–4 0.2 0.3 0.4 0.5 0.6 0.7 0.2 0.3 0.4 0.5 0.6 0.7 Threshold value Threshold value

Fig. 6. Binary sensitivity to thresholds of 3D-ECT tomograms of polymethyl methacrylate (PMMA) cylinders: (a) voxel-by-voxel error analysis; (b) volumetric analysis

172 © 2016 Johnson Matthey http://dx.doi.org/10.1595/205651316X691537 Johnson Matthey Technol. Rev., 2016, 60, (2) analysis does not need precise geometric information 3D tomograms and a similar analysis was adopted. It has relative to the position of the phantom or objects within been shown that commercially available ECT equipment the sensor. has the capability to image a mobile object and yield In Figure 7 the iso-surface plots display the information on the location of a hard boundary in 3D reconstructed interface between the phantom and space. the gas phase using the threshold value suggested The images were optimised using a previously by the voxel-by-voxel analysis (Figure 7(a)) and the published method whereby the threshold value of the volumetric analysis (Figure 7(b)). image was optimised by correlation with image error. A The obtained images from both analyses are very voxel-by-voxel approach to calculating image error was similar with both unable to display a geometric compared with a volumetric approach demonstrating the cylindrical shape, a limitation of the spatial resolution robustness of the latter as it does not require precise provided by single-step reconstruction algorithms. geometric information to provide improved tomographic The permittivity difference between the air and PE is accuracy. The image accuracy of ECT is a function of approximately two thereby reducing the SNR of the the spatial resolution that is dependent upon both the measurement and leading to lower image accuracy. software (reconstruction algorithm and data processing) The data post-processing improves the image accuracy and hardware (electrodes and data acquisition system as the images in Figure 7 represent the cylindrical (DAS)) used in the measurement. When these are phantom more closely than those in Figure 5(i). To improved the quality of the reconstructed images reduce image error the two available options with the improves thereby making this method more applicable to current ECT instrumentation are to use an alternative chemical processes. reconstruction algorithm or to improve the phase permittivity difference. 5 Acknowledgements

4 Conclusions Peter Clark was an Engineering Doctorate (EngD) student and Giuseppe Forte is a current EngD student, This paper examines recent innovations in the ﬁ eld of 3D both at the University of Birmingham, UK, and funded electrical capacitance tomography and the application jointly by Johnson Matthey and the Engineering & to 3D-ECT of previously used 2D-ECT post-processing Physical Sciences Research Council (EPSRC) UK. methods that improve tomogram accuracy of imaged The authors would like to thank Dr Phil Robbins, geometric phantoms. Firstly, development work of the Director of Studies at the University of Birmingham, for data processing techniques applied to 2D-ECT was his contributions to the binary optimisation techniques examined and shown to be effective, then using a similar and Dr Kent Wei of Industrial Tomography Systems Plc, phantom system the 3D-ECT method was used to acquire UK, for his assistance in data acquisition and processing.

(a) (b)

Fig. 7. 3D-ECT iso-surface plots with applied optimal threshold value and dashed lines showing object location: (a) voxel-by-voxel threshold value = 0.501; (b) volumetric threshold value = 0.547

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The Authors

Peter Clark is a Process Engineer working at Johnson Matthey within Platinum Group Metal Services. Peter obtained a MEng (Hons) in Chemical Engineering at the University of Birmingham, UK, and then undertook an Engineering Doctorate within the Johnson Matthey Technology Centre, Chilton, UK, where he applied advanced characterisation tools, including but not exclusively process tomography, to a range of chemical engineering applications.

Giuseppe Forte is an engineering doctoral student at the University of Birmingham, whose research project is jointly funded by the Engineering and Physical Sciences Research Council (EPSRC) and Johnson Matthey. His current research, based at Johnson Matthey Technology Centre, Chilton, concerns the development of on-process measurement techniques for monitoring formulated products. Before joining his current position, he obtained his MEng in Chemical Engineering from the University of Pisa, Italy. His interests and expertise focus on industrial electrical tomography, acoustic spectroscopy and image processing.

Professor Mark Simmons is Professor of Fluid Mechanics and Head of the School of Chemical Engineering at the University of Birmingham. His research interests encompass how ﬂ ows can be utilised or exploited in processes which are used to manufacture complex multiphase products, including catalysts, foods and fast moving consumer goods (e.g. shampoos, detergents). He has applied his expertise in ﬂ ow measurement, sensing and rheology in a variety of projects funded by EPSRC, Biotechnology and Biological Sciences Research Council (BBSRC) and industry: notably Johnson Matthey, Unilever and Rolls-Royce. Professor Simmons is a Fellow of the Institution of Chemical Engineers, has published over 100 peer-reviewed papers and has graduated over 32 PhD and EngD students.

E. Hugh Stitt is a Scientiﬁ c Consultant at Johnson Matthey Technology Centre, Chilton, UK. He is a Visiting Professor at the University of Birmingham, UK, and at Queen’s University Belfast; Fellow of the Institution of Chemical Engineers and a Fellow of the Royal Academy of Engineering. He has 25 years of industrial research experience across a variety of themes related to catalytic reaction engineering and catalysts with over 100 refereed publications.

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