Vanadium oxide particles were prepared by physical vapor deposition of vanadium in oxygen ambient onto ice-precovered well-ordered silica thin films. Morphology, electronic structure, and vibrational properties of the vanadia deposits were studied by scanning tunneling microscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, and infrared reflection absorption spectroscopy. The results show that the ice behaves as an oxidative agent, that favors vanadium oxidation up to V4+, and as a buffer layer that precludes strong interaction of the V adatoms with the silica film. At room temperature, upon desorption of the unreacted water, nanosized particles of vanadia hydroxide-like gel, containing V-OH and, to a lesser extent, vanadyl (VO) species, are formed. Vacuum annealing at 550 K leads to the total dehydration and partial reduction of the particles to V2O3, which expose the V-terminated surface. Subsequent reoxidation in 10(-6) mbar of O-2 irreversibly transforms the surface to the VO terminated, that is, the same as for the reactive deposition onto the clean silica surface. The results suggest that the structure of silica-supported vanadia catalysts is determined by a calcination step and should be considered under low oxygen pressure conditions as vanadium sesquioxide nanoparticles with the VO terminated surface.
2007. Vol. 111, no 14, 5337-5344 p.