Atomistic Simulation Studies of the Defect and Surface Properties of Perovskite-Based Oxide Catalysts.

This thesis describes the application of advanced computational techniques to the detailed study of the defect chemistry and ion transport properties of mixed metal oxides. In particular, attention is focused on two perovskite-based oxides, La2NiO4 and LaCoO3, which have important environmentally benign applications as effective heterogeneous catalysts. First, defect simulations of La2NiO4 indicate that the oxidation reaction involving interstitial oxygen is predicted to be exothermic in accord with thermochemical studies. Furthermore, the energies of solution are examined for a range of alkaline-earth metals on the La site and various transition metals on the Ni site. These simulations are extended to examine the effect of Sr doping in La2NiO4-based solid solutions where the Ni has been partially replaced by other iso-valent transition metal ions (Mn,Fe,Co,Cu). The defect chemistry is subsequently studied at the low-index surfaces of La2NiO4 where the energetically most favourable method of charge compensation remains that of Ni hole formation. This is consistent with models in which Ni3+ species are correlated to the observed catalytic activity for hypochlorite decomposition. In general, the simulations allow the rationalisation, based on quantitative results, of why certain dopants enhance the catalytic performance. The calculated surface energies are used in the prediction of the equilibrium crystal morphology. Similar simulation studies are conducted on the LaCoO3 material to investigate key redox processes, oxygen migration mechanisms and surface structures. The Sr doping reaction involving loss of oxygen is predicted to be the most favourable process, which is consistent with the belief that oxygen vacancies are key sites with regard to the decomposition of H2O2. LaCoO3 studies are extended by the employment of molecular dynamics methods (incorporating the shell model) to study oxygen ion diffusion in the Sr doped material. The results support models in which diffusion is mediated by conventional hopping of oxygen vacancies, with the calculated diffusion coefficients and migration energies in agreement with available experimental values