Oxygen vacancy model in strong metal-support interaction

Abstract

A model is proposed to account generically for strong metal-support interactions, a term which is used in a broad sense, and for SMSI, an acronym which is used in a specific and narrow sense. The generalized model is operative through the interaction of metal atoms with oxygen ion lattice vacancies in oxide supports. The occupancy of vacancies by metal atoms is equally applicable to different oxide systems, but the resulting effects may vary profoundly depending on the nature of the support. When applied to fluorite-type oxide supports, the model accounts for experimental observations of high metal dispersion, high sorption, and high catalytic activity, and unusual stability of these against sintering, up to temperatures ca. 1400 K under strong oxidation conditions. In contrast, when applied to rutile the model accounts for SMSI. The marked differences in behavior of metals on the two types of supports is explained on the basis of the uniformly compact or localized open cation sublattices. On the first type, interactions are mainly limited to metal occupancy of surface vacancies due to the diffusion barrier created by the support cations. In the second type, metal diffusion into support bulk vacancies and metal burial are possible, under hydrogen reduction, because of the open nature of the cation sublattice. The opposite phenomenon, dislodging the metal back to the support surface through oxidation, is also presented. As such, the model accounts for the typical observations in SMSI systems. The model has predictive value.