Oxygen Adsorption and Oxide Formation on V(100) Surfaces

Abstract

The initial stages of the uptake of molecular oxygen on V(100) single-crystal surfaces were characterized by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and ion scattering spectroscopy (ISS). As in previous reports on this system, a small amount of oxygen contamination was always detected on the surface, but that could be minimized by avoiding severe annealing after sputtering of the surface and appears to not affect the chemistry of the surface in any significant way, at least in terms of methanol conversion. Oxygen adsorption is dissociative even at liquid nitrogen temperatures. At low temperatures, saturation is reached at a coverage of approximately 1.0 ML, but above 170 K further uptake can occur, leading to the formation of a thin V2O3 + VO2 mixed oxide film. Thick oxide films, with thicknesses of the order of nanometers, could only be generated above 320 K and by using high oxygen doses. In general, the temperature at which the adsorption is carried out proved to play an important role in determining the surface stoichiometry and film thickness of the resulting surface oxides. Diffusion of oxygen into the bulk starts at approximately 500 K and displays an estimated activation energy of 132 kJ/mol.