Computer Simulation Studies of Oxide and Oxyhalide Catalysts.

This thesis describes the role of computer simulation techniques in modelling the structural and defect properties of solid oxidation catalysts for the conversion of methane. In particular, attention is focused on oxide and oxyhalide materials, namely La2O3, LaOCl and BiOCl, which are active catalysts for the oxidative coupling of methane reaction. The interatomic potentials correctly reproduce the crystal structures of these materials. Simulations of La2O3 find anion Frenkel disorder to be the predominant intrinsic defect with negligible deviation from ideal stoichiometry, in accord with the known properties of the pure oxide. The highest solubility, for a range of dopants, is calculated for Sr which is consistent with experimental studies of the promoted catalyst. A low activation energy for oxygen vacancy migration suggests fast diffusion through the bulk and to the surface. We suggest that the catalytic activity is linked to an oxidation reaction in which oxygen holes are created from the annihilation of anion vacancies by molecular oxygen. Surface simulations have been performed and include the important effect of surface relaxation. The results predict that the {001} and {011} surfaces would dominate the equilibrium crystal morphology, which is consistent with the available electron microscopy data. The formation of O- and O2 2- peroxide species, which are believed to be responsible for methane activation and surface segregation effects, are examined in some detail. Defect calculations on LaOCl and BiOCl find Schottky-like disorder to be the predominant intrinsic defect, with ion transport governed by Cl-vacancy migration. The {001} surface is predicted to dominate the crystal morphology of LaOCl and BiOCl, leading to flat, 'platelike' crystals, in accord with observation. The formation of both oxide and chloride species, as possible active sites, is investigated. The examination of the Bi0. 5La0.5OCl solid solution predicts Schottky-like disorder to be less favourable than in the pure materials; this result, and the lower Cl- mobility, may be critical factors for the increased stability but lower catalytic performance of the mixed system