Vanadium Oxides Supported on a Thin Silica Film Grown on Mo(112): Insights from Density Functional Theory

Abstract

Low-coverage vanadium oxide species (monomers and dimers) as well as larger VmOn clusters (m = 4 and 6) supported on an ultrathin SiO2/Mo(112) film are investigated by density functional theory in combination with statistical thermodynamics. At low vanadium chemical potentials, monomeric species are stable, and with increasing vanadium or oxygen chemical potentials, dimeric VOx species become stable. These monomeric and dimeric species are created by replacing Si atoms in the two-dimensional SiO2 film by vanadyl (V═O) groups. At high vanadium chemical potentials (high vanadia loading), the largest vanadia clusters considered (V6O15) are stable. The frequencies calculated for the V4O10 cluster anchored to the silica surface by two V−O(2)−Si interface bonds account for all vibrational frequencies observed for the vanadia/SiO2/Mo(112) model system. Its calculated infrared spectrum shows an intense band at 1048 cm−1 corresponding to a stretching of the V═O groups and a broad band in the 630−730 cm−1 range attributed to V−O−V vibrations, well-known for the V2O3 bulk. The characteristic vibrations of the ultrathin SiO2/Mo(112) support are infrared-inactive, because of the metal surface selection rule, which is in full agreement with the experimental observations.