A synchrotron XPS study of the vanadia-titania system as a model for monolayer oxide catalysts

The deposition of vanadium metal onto stoichiometric TiO2(110) has been stu died with photoelectron spectroscopy from low to high coverages of vanadium . A synchrotron source was employed in XPS experiments for the study of sub monolayer coverages of vanadium in order to determine the oxidation state o f the vanadia species formed at submonolayer coverages. The exposure of the titania surface to vanadium metal results in charge-transfer from vanadium to titania at the interface, At low doses of the metal vigorous interactio n between the metal and titania surface yields reduction of the Ti4+ specie s to Ti3+ at the interface, as evidenced by both changes in the lineshape o f the Ti 2p XPS spectra and Ti LIII-edge spectra. Concurrent with this surf ace reduction vanadium metal is oxidized. At higher vanadium doses the vana dium 2p binding energy indicates the formation of metallic vanadium. When m etallic vanadium deposition is followed by exposure of the surface to oxyge n, only one vanadium species remains on the surface, the binding energy of which corresponds to that of the oxide present initially at low doses of va nadium metal. By comparison of the V 2p binding energies to those of bulk o xides, it appears that the oxidation state of the vanadium in the oxide spe cies is + 3, suggesting the formation of V2O3 on the surface. Vanadium LIII -edge data also suggest that V2O5 is not formed by the oxidation of predose d vanadium metal. (C) 1999 Elsevier Science B.V. All rights reserved.