Role of structural defects and oxygen ion migration in the catalytic activity of La2O3

Abstract

Atomistic computer simulation techniques are used to investigate the role of key defect and transport properties of La₂O₃ that may be involved in the catalytic activity. The theoretical techniques are based upon efficient energy minimization procedures and Mott–Littleton methodology for accurate defect modelling. The interatomic potentials for La₂O₃ correctly reproduce the hexagonal structure. Anion Frenkel disorder is found to be the predominant intrinsic defect with negligible deviation from ideal stoichiometry, in accord with the known properties of the pure material. The highest solubility, for a range of dopants, is calculated for Sr which would increase the oxygen vacancy concentration; this is consistent with experimental studies of the promoted catalyst. A low activation for O^2– 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-hole centres are created from the annihilation of oxygen vacancies by reactant gas-phase oxygen. Furthermore, calculated binding energies indicate the possible formation of stable (dopant–O^–) centres.