VANADIUM-OXIDE MONOLAYER CATALYSTS - PREPARATION, CHARACTERIZATION AND CATALYTIC ACTIVITY

The purpose of this review is to provide a guide to the recent literature on the preparation, structure and catalytic properties of vanadium oxide monolayer catalysts. Monolayers of vanadium oxide, represented as VO(x), may be formed on the surface of a supporting oxide by impregnation with a suitable vanadium source, followed by calcination, by grafting methods using VOCl3 or a vanadyl alkoxide, by heating mechanical mixtures or by coprecipitation. Information on the chemical structures present when there is less than a monolayer of VO(x) can be obtained from vibrational and electron spectroscopies (Raman, FT-IR, XPS). Temperature-programmed reduction (TRP), XRD, ESR and V-51 NMR provide additional characterisation of the materials. VO(x) monolayers are particularly stable on TiO2 and on Al2O3, but are not readily formed on SiO2, where aggregation to V2O5 occurs. At low vanadium contents, isolated VO4 tetrahedra have been detected, and polymeric octahedral VO6 species at higher loadings: various disordered and 'paracrystalline' V2O5 phases have been seen when more than the monolayer loading is present. Species in the monolayer are effective catalysts for oxidation of o-xylene to phthalic anhydride, for ammoxidation of aromatics and for selective reduction of nitric oxide by ammonia, but not for oxidation of butadiene to maleic anhydride.