Redox-driven atomic-scale changes in mixed catalysts: VOX/WOX/α-TiO2 (110)†
X-ray studies of vanadium–tungsten mixed-monolayer-oxide catalysts grown on the rutile α-TiO2 (110) single crystal surface show redox behavior not observed for lone supported vanadium or tungsten oxides. Two cases are presented: sub-monolayer (sub-ML) vanadium oxide (vanadia) grown on ML tungsten oxide and ML vanadia grown on sub-ML tungsten oxide. The X-ray standing wave (XSW) and X-ray photoelectron spectroscopy (XPS) observations for both cases show coverage-dependent reversible redox-induced atomic-scale structural and chemical state changes. Atomic force microscopy shows that the mixed VOX/WOX overlayers have a conformal film-like structure in the as-deposited state. XSW analysis in light of XPS reveals that the V and W cations that are uncorrelated with the substrate lattice play an important role in catalytic redox reactions. Distinct differences in the redox-induced changes for these two mixed catalysts result from tuning the ratio of V to W, and relationships are drawn between the catalyst composition, structure, and chemistry. Comparison of these V–W mixed cases and the corresponding unmixed cases reveals a synergistic effect in which the reduction of W can be significantly enhanced by the addition of V.