Vanadia catalysts on anatase, rutile, and TiO 2(B) for the ammoxidation of toluene: An ESR and high-resolution electron microscopy characterization

Abstract

Three titania polymorphs, anatase, rutile, and TiO 2(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 NH 3(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 TiO 2(B). The monolayer on TiO 2(B) consists of V 4+ 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 V 4+ and V 4+ species. The ESR spectrum indicated a high degree of V 4+-V 4+ interaction. From the characterizations of rutile-supported samples it seems that the support interface is a solid solution of V 4+. On all supports, vanadium on top of the monolayer is present as V 5+. High-resolution micrographs of the samples with the highest loading revealed the formation of amorphous vanadia on both anatase and rutile. On TiO 2(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 TiO 2(B) show a good performance for both reactions.