Structure of Monomeric Chromium(VI) Oxide Species Supported on Silica: Periodic and Cluster DFT Studies

Abstract

Silica-supported chromium oxide systems are efficient catalysts for many important chemical processes. Despite many years of investigations, the structure of the surface Cr species is not unambiguously determined. In this work, comprehensive DFT investigations of the monomeric Cr(VI) oxide species on silica under dehydrated conditions are performed. A large number of advanced periodic and cluster models of the SiO2 surface, based on the β-cristobalite structure and different amorphous structures, have been applied. The calculated relative energies of the monooxo and dioxo Cr(VI) species depend on their location on the surface and on the structure of the model. It is concluded that the dioxo Cr(VI) species are thermodynamically preferred, but the presence of the monoxo Cr(VI) species, being in minority, cannot be excluded. According to the vibrational frequency analysis, the asymmetric O═Cr═O stretching mode for the dioxo species and the Cr═O stretching mode for the monooxo species can overlap.