Atomic-Scale View of VO_X-WO_X 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 VO_X and WO_X 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 Brønsted 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 Brønsted acid sites in multicomponent oxide catalysts.