Briquet, L.G.V.; (2010) Modelling active sites of supported metal catalysts. Doctoral thesis , UCL (University College London).

Abstract

Many chemical processes involving heterogeneous catalysis such as automotive catalysts or the steam reforming process are vital for industry. The chemistry of nanoclustered metal particles is distinctly altered by their interaction with the support. Metal/metal oxide interactions are therefore considered to be crucial to tune the catalytic properties of many catalysts. In order to increase our understanding of the interactions between the metal oxide surface and the metal adsorbate, computational techniques, based on both Density Functional Theory and interatomic potentials, have been used. As the presence of sodium cations in alumina supports can be an issue for the catalyst industry, the segregation of sodium is investigated and compared to available experimental data. It has been found that sodium tends to segregate at a subsurface site in the (0001) α-alumina surface, while the migration of sodium defects is much more facile in γ-alumina. In this thesis, we investigate the interactions of a group of catalytically active metals (nickel, palladium, platinum and gold) with different surfaces of alumina, starting with various faces of α-Al2O3. Although subtly different, the metal/support interaction for each surface is consistent, with metal promoting a charge transfer from the surface oxygen to the top-most aluminium. As water is known to bond strongly to the alumina surfaces, the effect of surface hydroxylation on the adsorption mechanisms has also been investigated. An important stabilisation of the metal is observed with, in some cases, the migration of a proton from the surface to the metal, forming hydride species. To assess further the impact of the surface conditions on the catalytic properties of the metal particle, we also investigated the interactions between the metal and a carbon monoxide probe. All metals show different behaviour toward the carbon monoxide bond length and vibrational frequency. The hydroxylation of the alumina surface has a strong impact on the carbon monoxide/metal complex stability as the carbon monoxide probe enters into competition with the hydride specie bonded to the metal. This investigation allows us to obtain a detailed insight into the nature of the metal support interaction and the active sites of complex catalysts.

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