Atomic-scale study of ambient-pressure redox-induced changes for an oxide-supported submonolayer catalyst: VO _x/α-TiO ₂(110)

The activity of supported catalysts is strongly linked to the structure of the surface species. However, direct in situ structural measurements of submonolayer catalysts are lacking. Using in situ X-ray standing wave analysis combined with ex situ X-ray photoelectron spectroscopy and atomic force microscopy, we observed reversible redox-induced surface cation dynamics for three-fourths of a monolayer of vanadia grown by atomic-layer deposition on an α-TiO ₂(110) surface. On the basis of first-principles density functional theory calculations, we propose vanadia submonolayer structures that agree with and suggest explanations for the observed redox-induced structural changes. Under oxidation conditions, the correlated submonolayer structure is a 2D trimer-decorated polymeric chain along the rutile [-1 1 0] direction; under reducing conditions, the more stable polymeric chain remains unreduced, whereas the less stable decorating trimers become reduced and uncorrelated with the support lattice. These results show that a large fraction of V sites retains redox activity at submonolayer V coverages.