The significance of the oxidation state of gold in heterogeneous catalysis

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We aim to construct a detailed picture of the processes, intermediates and active sites involved in the hydrochlorination of ethyne over gold chloride nanoparticles supported on carbon surfaces. The hydrochlorination of ethyne provides an alternative route to vinyl chloride which at present is manufactured by a two step process involving the oxychlorination of ethene to form 1,2-dichloroethane followed by cracking. Vinyl chloride is polymerised to polyvinyl chloride (PVC) an important polymer with widespread applications in the construction and electronics industries. Sixty years ago vinyl chloride was almost exclusively made by the direct hydrochlorination of ethyne, which derives from coal, over a mercury chloride catalyst but the availability of inexpensive ethene from oil led to a change of manufacturing route despite the lower yields, lower product purity and higher costs for waste treatment. Now, dwindling oil reserves and increasing environmental awareness is dictating a search for to the alternatives to the oxychlorination route. The safety and environmental issues posed by HgCl2 prevent a return to the former catalyst but Hutchings and co-workers have proven that gold supported on carbon can be a viable alternative. Our project will inform the development of such catalysts but will also provide an important new window on gold catalysis.Whilst catalysis by gold nanoparticles is an established phenomenon particularly in relation to selective oxidation processes, the key points of the active site and the reaction mechanism remain very controversial despite extensive work. Different researchers have separately emphasised the importance of different aspects of gold catalysts for example: the size of the gold particles; the interface between the gold and the support and the oxidation state of the gold. Fierro-Gonzalez & Gates ascribe this controversy to the fact that attention has largely focussed on mechanistic studies of CO oxidation and suggest that this experimentally appealing reaction is not as straight forward as it appears. They recommend that more diverse systems be investigated at a fundamental level if further progress is to be made in understanding the nature of the active site in gold catalysis and thus provide direction to the development of better gold catalysts. Au/C catalysts have received remarkably little attention from the surface science community (in contrast to the gold/oxide catalysts) and a study of the fundamental chemistry underlying this system therefore offers the prospect not only of advancing the development of an important future catalyst but also of shedding new light on some of the fundamental principles of gold catalysis. A powerful combination of surface spectroscopy, surface microscopy and surface kinetic methods applied in conjunction with computational chemistry will be focussed on identifying and characterising the active states of gold in ideal models of the hydrochlorination catalyst. However, in practice, the Au/C hydrochlorination catalyst consists of a high area activated carbon exhibiting a variety of functional groups at the surface which may influence the deposition and stability of the adsorbed gold halides. To address this issue the project involves three separate parallel strands with studies of gold deposited from aqueous solutions on modified HOPG and activated carbon under ambient conditions providing the bridge between model surface science/computational studies of gold halides on HOPG and the practical catalysis.