Electronic interactions in the vanadium/TiO 2(110) and vanadia/TiO 2(110) model catalyst systems

Abstract

We have studied model catalysts comprised of vanadium or vanadia deposited onto both stoichiometric and reduced single-crystal TiO 2(110) surfaces by using XPS, UPS, Auger, LEED and work-function measurements. Changes in the electronic and geometric structure of TiO 2 supports are monitored during overlayer formation in the submonolayer and monolayer regimes. Deposition of vanadium in an O 2 ambient onto the stoichiometric TiO 2 surface results in the formation of lower oxides of vanadium that interact only weakly with the support. However, the stoichiometric TiO 2 surface is attacked violently by vanadium metal atoms deposited in UHV; in the submonolayer range, V-O bonding results in electron transfer to the substrate due to the strong affinity between V and O. This substrate reduction is confirmed by the appearance of Ti 3+ features in XPS spectra. For greater than monolayer coverage, a metallic overlayer is formed. In both the vanadia/TiO 2 and V/TiO 2 systems, the initially sharp (1 × 1) LEED patterns disappear for small coverages, indicating disordering of the surfaces. Although defects on TiO 2 are generally active chemisorption sites, the reduced TiO 2 surface interacts only weakly with metallic vanadium.