Oxygen Activation and Dissociation on Transition Metal Free Perovskite Surfaces

Density functional theory and low energy ion scattering spectroscopy were applied to study the mechanism of oxygen dissociation on the SrO-terminated surfaces of strontium titanate (SrTiO₃) and iron-doped strontium titanate (SrTi_1-xFe_xO_3Δ). Our study reveals that while O₂ dissociation is not favored on the SrO-terminated perovskite surface, oxygen vacancies can act as active sites and catalyze the O-O bond cleavage. Electron transfer from lattice oxygen atoms to the O₂ molecule, mediated by the subsurface transition metal cations, plays an important role in the resulting formation of surface superoxo species. The O₂ molecule dissociates to produce oxygen ions, which are incorporated into the perovskite lattice, and highly active oxygen radicals on the perovskite surface, which further recombine to O₂ molecules. Our focus on the SrO-terminated surface, rather than the TiO₂ layer, which is presumed to be more catalytically active, was driven by experimental observation using low energy ion scattering spectroscopy, which reveals that the surface of SrTiO₃ after high temperature heat treatment is SrO-terminated, and hence this is the surface that is technologically relevant for devices such as solid oxide fuel cells (SOFCs). Our study demonstrates that although the more active BO₂-perovskite layer is not exposed at the gas-solid interface, the SrO-terminated surfaces also actively participate in oxygen exchange reaction.