Isolated Molybdenum(VI) and Tungsten(VI) Oxide Species on Partly Dehydroxylated Silica: A Computational Perspective

Although silica-supported molybdenum and tungsten oxide systems are widely used in catalysis, the nature of the surface metal oxide species is still not fully recognized. In this work, comprehensive periodic and cluster density functional theory (DFT) studies of the isolated Mo(VI) and W(VI) oxide species on dehydrated amorphous silica have been performed to give insight into their heterogeneity. It is shown that the relative stabilities of the metal oxide species strongly depend on their location that influences their geometry and the strain of the dehydrated silica surface. The favorable located monooxo W(VI) species are clearly more stable than the dioxo W(VI) species, whereas no strong thermodynamic preference is predicted in the case of the Mo(VI) species. The relative stability of the monoxo species increases in the order: Cr < Mo < W. However, due to geometrical constraints on the silica surface, formation of the tetragrafted monooxo species is hampered, compared to the digrafted dioxo species and the latter are expected to be in majority. The monografted hydroxy dioxo Mo(VI) species are unstable. Based on the vibrational frequency analysis, some details regarding the structure of the experimentally observed dioxo species in the MoOx/SiO2 catalyst are proposed, while the results for the WOx/SiO2 system are somewhat ambiguous.