Concepts, Theories and Methodology

Professor Sir Hugh Taylor, after whom the Taylor conferences are named, was a pioneer in the study of chemisorption and catalysis on metals and metal oxides (3). As Professor Frank Stone (Emeritus Professor of Chemistry, University of Bath, U.K.) reminded us in his opening address, H. S. Taylor (as he was known in his time) was responsible for introducing the concepts of activated adsorption and of the active site, both of which were highly controversial when he first proposed them around 1930 (4), but which have become fundamental to our understanding of many catalytic phenomena.

Professor Gabor Somorjai (University of California, Berkeley, U.S.A.) developed the theme that progress in catalysis is stimulated by revolutionary changes in thinking. He predicted that, whereas in previous eras new catalysts were identified through an Edisonian approach (based on trial and error) or discovered on the basis of empirical understanding, future catalyst design will be based on the principles of nanoscience. He highlighted his idea of ‘hot electrons’ that are ejected from a metal by the heat of reaction produced at active sites, but which could become a potential energy source if they were generated by the absorption of light.

As described by Professor Richard Catlow (University College London, U.K.) and Stephen Jenkins (University of Cambridge, U.K.), quantum mechanical techniques for modelling many-electron systems lend themselves to the study of catalytic materials and catalytic reaction pathways. Professor Catlow's particular expertise lies in the study of defective metal oxides, and the way in which they interact with metal particles. In the case of palladium deposited on ceria, his models predict an increase in the concentration of Ce³⁺ species resulting from electron transfer from the metal to the metal oxide. Jenkins has been examining the likelihood of specific reaction steps taking place on the surface of supported metal catalysts. For both alkane synthesis and combustion, his calculations implicate a common formyl intermediate, which is not readily detected by spectroscopic techniques. However, Professor Charles Campbell (University of Washington, U.S.A.) cautioned against an over-reliance on surface modelling. Based on classical microcalorimetric measurements, he has shown that density functional theory (DFT) underpredicts the heat of adsorption for a variety of molecules (for example, carbon monoxide, cyclohexene and aromatics) on a range of surfaces (such as carbon, precious metals or metal oxides).

Taking a View

Professor Lynn Gladden (University of Cambridge) described how macroscopic and microscopic events can be tracked in an optically opaque system, such as a catalytic reactor. Using magnetic resonance imaging (MRI) – essentially the same technique as used diagnostically in medicine – she has been able to observe the liquid flow fields that develop in packed bed reactors. By combining the images with measurements from temperature sensors, detailed reaction profiles can be produced for steady-state and dynamic operating conditions.

On a different scale, Professor Chris Kiely (Lehigh University, U.S.A.) has used dark-field imaging techniques to detect the smallest metallic, bimetallic and metal oxide particles (less than 1 nm in diameter) by electron microscopy. In what may become a seminal study, he has correlated the high CO-oxidation activity of a specific gold/iron oxide (Au/Fe₂O₃) catalyst with the presence of two-layer, 0.5 nm-diameter gold clusters.

The importance of studying catalysis over a range of scales was emphasised by Professor Trevor Rayment (Diamond Light Source Ltd, U.K.). The new U.K. synchrotron light source is intended to provide understanding of ‘real catalysts, under real conditions, in real time’ (5). One of the ambitions is to increase the throughput for techniques such as X-ray absorption spectroscopy, by reducing the amount of non-productive beam time. Although the Diamond facilities are not expected to provide the tools for catalyst discovery, it is hoped that they can accelerate the development process by identifying the critical relationships between catalyst structure and performance.

Controlling Selectivity

Stressing a point made by Professor Somorjai that catalysis in the 21st century is all about selectivity, Chris Baddeley (University of St Andrews, U.K.) and Professor Andrew Gellman (Carnegie Mellon University, U.S.A.) separately described the complex dependence of enantioselective reactions on surface composition and structure. As explained by Professor James Anderson (University of Aberdeen, U.K.), in the context of alkyne hydrogenation, poor selectivity is often the result of heterogeneity in the exposed sites, even on apparently clean and compositionally homogeneous surfaces. Through targeted use of additives, such as bismuth in the case of palladium-based hydrogenation catalysts, specific non-selective sites can be deliberately blocked.

During the direct synthesis of hydrogen peroxide from hydrogen and oxygen, the combustion of hydrogen and the over-hydrogenation of hydrogen peroxide to water need to be suppressed. Professor Graham Hutchings (Cardiff University, U.K.) has shown that gold-palladium catalysts are among the most effective, but their performance can be sensitive to the support material used. In collaboration with Professor Kiely, he has found that the nature of the dispersed gold-palladium can vary, with core-shell particles (on titania and alumina) producing lower yields of H₂O₂ than palladium-rich alloy particles (on carbon). Both types of core-shell particle, those with a gold core and palladium shell and those with a palladium core and gold shell, were less active than the palladium-gold alloy.

Professor Masatake Haruta (Tokyo Metropolitan University, Japan) has found that small gold clusters can selectively catalyse some particularly challenging reactions. The outstanding example is the selective insertion of oxygen into propylene to form propylene oxide, which is currently produced by indirect processes that produce large quantities of waste byproducts. By reactively grinding a non-chloride Au(III) precursor with titanium silicalite (TS-1), Professor Haruta has dispersed the gold as 1.6 nm particles, which can activate propylene to react with O–O–H species formed from oxygen and water at the metal-support interface.

Promoting and Maintaining Activity

Vanadia supported on θ-alumina is one of the best catalysts for butane dehydrogenation, but the rate of reaction is very sensitive to the vanadia loading. Professor David Jackson (University of Glasgow, U.K.) reported that maximum activity coincides with the presence of a mainly polymeric form of vanadate species which covers most of the alumina surface. However, another key performance criterion is durability. During butane dehydrogenation, two forms of deactivation can be discerned: a short-term but reversible effect caused by deposition of carbon-rich species on the catalyst surface, and a longer-term effect associated with an irreversible phase change in the alumina.

In the Francois Gault Lecture, Professor Robbie Burch (Queen's University Belfast, U.K.) explained the challenges faced in developing and studying catalyst technology for removing nitrogen oxides (NOx) from diesel exhaust. Focusing on the use of silver for NOx reduction by direct reaction with some of the diesel fuel, he showed that its performance can be dramatically improved by the addition of hydrogen. As described in a presentation by Stan Golunski (Johnson Matthey Technology Centre, Sonning Common, U.K.) the hydrogen can be generated in situ through a process of exhaust gas reforming using a rhodium catalyst. Professor Burch explained how X-ray absorption fine structure (EXAFS) studies of silver have been used to refute one of the proposed roles of hydrogen, as a structural modifier, implying instead that it is directly involved in the NOx-reduction mechanism. Although several spectroscopic studies have been published (6) showing the presence of cyanide and isocyanate on the silver surface when hydrogen is present, kinetic measurements at Queen's University Belfast have ruled these out as reactive intermediates, suggesting that more transitory species (such as hydroxamic acid or ammonia) are involved.

Future Prospects

During his introduction to the postgraduate student and postdoctoral researcher presentations, Jack Frost (Johnson Matthey Fuel Cells, U.K.) compared and contrasted the academic process of research with the industrial activity of innovation. He used the example of vehicle emission control to show how the pressing need for improved local air quality led to the development of technology for catalytic aftertreatment using pgm catalysts (7). This highly effective technology does not, however, address the global problem of greenhouse gas emissions, which is now the prime motivator for the introduction of fuel cells.

Appropriately, there was an environmental theme running through many of the presentations in this section of the conference. For example, CO oxidation was covered by Sankaranarayanan Nagarajan (National Chemical Laboratory, Pune, India), who looked at oxygen mobility and the role of subsurface oxygen on palladium surfaces (8, 9), Figure 1. The subject was also covered by Kevin Morgan (Queen's University Belfast), who presented a temporal analysis of products (TAP) study showing that the addition of gold to CuMnO_x results in the availability of more surface oxygen and promotion of the Mars-van Krevelen oxygen transfer mechanism.

A number of researchers from Cardiff University presented work on selective oxidation reactions. For example, Jonathan Counsell has been studying the effect of adding gold to a supported palladium acetoxylation catalyst. He has found that the gold suppresses carbon formation on the palladium surface by preventing dehydrogenation. Kara Howard described her work on modelling oxygen dissociation on gold clusters supported on iron oxide, and showed that the iron oxide stabilises dissociated oxygen atoms. Dyfan Edwards presented a surface science study of the synergy between the individual metal oxides in iron molybdate catalysts, which are used for oxidising methanol to formaldehyde. The selectivity of iron oxide changes with the level of coverage by molybdenum, from total combustion when no molybdenum is present, to partial oxidation (to CO) at low coverage, and finally selective oxidation (via a methoxy intermediate) at high coverage. Dr Jennifer Edwards has been examining the effect of preparation and pretreatment variables on the performance of gold-palladium catalysts for the direct synthesis of hydrogen peroxide. Acid pretreatment of the support material has resulted in catalysts with lower activity for the unwanted consecutive hydrogen peroxide-hydrogenation reaction, leading to very impressive hydrogen peroxide yields.

The influence of the iron:cobalt ratio in an Fe₂O₃-Co₃O₄ catalyst, for converting ethanol to hydrogen, has been studied by Abel Abdelkader (Queen's University Belfast). Fe₂O₃ catalyses ethanol steam reforming, and Co₃O₄ the water-gas shift reaction, so that a 1:1 ratio produces the optimum yield. In the field of syngas and hydrogen utilisation, Poobalasuntharam Iyngaran (University of Cambridge) presented a study of the effect of potassium promoters on ammonia synthesis over iron, which showed that stepwise hydrogenation of nitrogen surface adatoms is unaffected by the presence of potassium. Sharon Booyens (Cardiff University) is interested in DFT modelling of CO adsorption on iron surfaces, in the context of Fischer-Tropsch catalysis. The models predict that surface carbon causes a weakening of the Fe–CO interaction, and therefore CO dissociation becomes less favourable. Andrew McFarlane (University of Glasgow) presented his work on C5 olefin hydrogenation over 1% Pd/Al₂O₃. He suggested that reaction of the cis-pentene isomer must proceed via formation of the trans isomer before hydrogenation can occur.

Finally, there were several very topical presentations concerned with clean synthesis of chemicals and fuels. Lee Dingwall (University of York, U.K.) has been synthesising and working with a bifunctional heterogeneous catalyst that combines an active ruthenium organometallic centre with a polyoxometallate cage, Figure 2. This provides acid sites and also confers great stability, and the catalyst structure displays high activity for C–C bond formation. Ceri Hammond (Cardiff University) described the reaction of glycerol with urea over zinc, gallium or gold supported on zeolite. Glycerol carbonate can be obtained with high selectivity in a one-step solvent-free process over these catalysts, though there is some question over their stability.

The prize for the best student presentation was awarded to Janine Montero (University of York) who is researching the use of heterogeneous catalysts for biodiesel synthesis by transesterification, as a replacement for the liquid catalysts currently in use. She has shown, by Auger electron spectroscopy, that high-temperature calcination of nanoparticulate magnesium oxide results in increased surface polarisability, and therefore higher Lewis basicity. Her results show that there is a linear relationship between polarisability and transesterification activity over these MgO catalysts.

Summary

In summing up the conference, Professor Wyn Roberts (Emeritus Professor, Cardiff University) recalled that when he began his Ph.D. he had to make the choice between studying clean surfaces (i.e. single crystals) or real catalysts. As many of the presentations highlighted, this distinction is no longer useful, with the so-called ‘material’ and ‘pressure’ gaps in catalysis, between results obtained from surface science studies, usually using idealised surfaces under high vacuum, and those from real catalyst materials at ambient or high pressures (10), having gradually narrowed. Frost had earlier commented on a similar convergence, between research and innovation. As he pointed out, though, these activities need to remain distinct, because they fulfil quite different functions. However, with their shared values of insight, integrity, creativity and professionalism, they will be increasingly directed in parallel at our most urgent challenges in catalysis and in society: sustainability and environmental protection.

The fifth Taylor Conference is scheduled to take place in Aberdeen in 2013 (11).