Atomic-Scale View of VOX–WOX Coreduction on the α-Al2O3 (0001) Surface

The catalytic activity of oxide-supported vanadium oxide is improved by the presence of tungsten oxide for the selective catalytic reduction of nitric oxides. We propose a mechanism for V–W synergy through studies of the reduction–oxidation behavior of near-monolayer VOX and WOX species grown by atomic layer deposition on the α-Al2O3 (0001) single crystal surface. In situ X-ray standing wave measurements reveal an overlayer of W6+ species that is correlated with the substrate lattice as well as a redox-reversible shift from uncorrelated V5+ to correlated V4+. X-ray photoelectron spectroscopy and electronic structure calculations show a partial reduction of W6+ in the presence of V4+, improving the Bronsted acidity in mixed V–W catalyst systems. This mechanism of V–W synergy suggests that control of W d-states might be used as a design parameter for Bronsted acid sites in multicomponent oxide catalysts.