Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over vanadium oxides supported on titania

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Abstract
Catalysts containing 0.1–10 wt% V have been prepared by impregnating TiO2(Degussa P25) by aqueous NH4VO3 and calcining at 773 K. The catalytic performance in the oxidation of toluene was investigated and correlated with the features shown by X.r.d., ESCA, u.v.–visible spectroscopy and i.r. spectroscopy of adsorbed CO. Both the activity in the oxidation of toluene and the initial selectivity for benzaldehyde increase rapidly with increasing vanadium loading up to 2 wt% V. The changes between 2 and 10 wt% V are small. Surface areas, pore structure, primary particle size and anatase/rutile ratio change with the vanadium content. Up to 2 wt% V, a small increase in the anatase/rutile ratio is obtained. In these catalysts, surface-bonded vanadium species are present, and it is suggested that they induce a reconstruction in the titania surface. Larger and opposite effects are observed for 10 wt% V, where crystalline V2O5 facilitates the anatase to rutile phase transition. Sintering of the primary particles and a large reduction in the specific surface area are also obtained. U.v.–visible spectroscopy of reduced catalysts indicated the predominance of VIV species in tetrahedral coordination. VIII species could not be observed by i.r. spectroscopy of adsorbed CO. I.r. spectra indicate the presence of V—OH groups. Breakpoints in the spectral data were obtained at ca. 1.1 wt% V corresponding to the consumption of two support OH groups per vanadium species. It is suggested that the isolated vanadium species are oxohydroxyvanadium bonded to titania through two oxygen bridges. These constitute the active site at low loadings. Two adjacent vanadium species are thought to form the reduced tetrahedral structure. New active sites present at higher loadings may be formed by the reaction between V—OH groups and vanadium precursors. The new sites possibly represent a VV/VIV redox couple: It is suggested that its presence is an important factor governing the catalytic activity.

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