Vanadia Catalysts on Anatase, Rutile, and TiO2(B) for the Ammoxidation of Toluene: An ESR and High-Resolution Electron Microscopy Characterization

Three titania polymorphs, anatase, rutile, and TiO2(B), were used as supports for vanadia. Catalysts with nominal loadings corresponding to 1.5 and 5 theoretical layers were prepared. True monolayer samples were obtained by NH3(aq)-treatment of the samples with 5 nominal vanadia layers. Prepared catalysts were characterized by chemical analysis, ESR, and high-resolution electron microscopy. It was observed that an almost complete monolayer of vanadia can be formed on anatase and TiO2(B). The monolayer on TiO2(B) consists of V4+ species. A magnetic species attributable to tetrahedral states of vanadium was found to dominate the ESR resonance. Chemical analysis of the monolayer on anatase showed the presence of both V4+ and V4+ species. The ESR spectrum indicated a high degree of V4+-V4+ interaction. From the characterizations of rutile-supported samples it seems that the support interface is a solid solution of V4+. On all supports, vanadium on top of the monolayer is present as V5+. High-resolution micrographs of the samples with the highest loading revealed the formation of amorphous vanadia on both anatase and rutile. On TiO2(B), vanadia was found to grow coherently at the titania interface. The catalysts were used in the oxidation and ammoxidation of toluene to benzaldehyde and benzonitrile, respectively. Considering both activity and selectivity, multilayers of vanadia supported on TiO2(B) show a good performance for both reactions.