Bond Making and Breaking Processes at Surfaces: Fundamentals of Adsorption and Catalysis

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From the platinum-rhodium catalytic converter in your car exhaust system, to the iron catalyst that turns atmospheric nitrogen into fertilizer, highly-reactive metals are key to many of the most important chemical reactions that drive the modern world. Noxious gases like carbon monoxide or nitric oxide, produced in quantity by the internal combustion engine, would naturally revert to less toxic materials in the atmosphere given enough time, but only after causing significant respiratory problems at ground level on our city streets. Similarly, simply mixing nitrogen and hydrogen at high enough pressures would eventually yield the ammonia essential for agriculture, but impossibly slowly. In each case, and in many, many others, the role of the metal catalyst is to speed up and/or re-direct the reaction, preventing environmental pollution or making important high-value chemicals out of uninteresting low-value ones. It is little wonder that the often rare metals involved, from the transition region in the centre of the periodic table, are amongst the most valuable elements in the world, nor that efforts to understand and to optimise their effects are keenly pursued.In many cases, the chemical reactions important in catalysis happen at the surfaces of solid transition metal particles. Passing molecules settle and stick upon the surface (adsorb), move around on the surface (diffuse) and eventually detach from the surface and float away (desorb); in between these basic steps, the molecules may fall apart on the surface (dissociate) or join together to make new molecules (associate). The detailed chemical interactions between the molecules and the metal surface are crucial in determining the relative rates of these five elementary types of process, meaning that each different metal, and indeed each different exposed facet of a metal crystal, may have different catalytic properties. In our work, we make sophisticated measurements of these processes on extremely well-characterised surfaces under highly-controlled conditions. By comparing these with results from our state-of-the-art theoretical calculations, we are able to build up a complete picture of the surface chemistry, and hence to predict better catalysts for future industrial and environmental use. Increasing use of novel alloys and nanostructured surfaces will be a characteristic of our planned work in this direction.The potential for our kind of fundamental surface science to make a significant impact in real-life situations is reflected in the funding we have attracted from industrial sponsors. In recent years, we have been working with Toyota on iridium-gold catalysts for removal of nitric oxide from automobile exhausts, and we have just begun a collaboration with Johnson Matthey looking at the catalytic activation of methane in the same automotive context. Meanwhile, our planned work with BP Alternative Energy is looking towards optimising the production of hydrogen for fuel cell technology, and Shell Research have just committed to work with us towards new routes for ethylene epoxidation.Looking beyond the current applications of surface catalysis, however, we are also focussing a substantial effort in the direction of so-called asymmetric catalysis. The biological molecules found in living organisms are often characterised by the fact that they are chiral, which is to say that they can exist in two inequivalent mirror-image forms. These mirror-image molecules are indistinguishable from each other by most chemical means, but can have radically different behaviour within the body; in many pharmaceutical contexts, therefore, it is vital that drugs be prepared in a pure chiral state. Our research aims to provide an efficient means to achieve this through the use of intrinsically-chiral metal surfaces, which are cut from their parent crystal in such a way as to induce chirally-selective surface chemistry.