A High-Resolution Electron Microscopy Investigation of TiO2(B)-Supported Vanadium Oxide Catalysts

TiO2(B) crystals were found to be isomorphic with those of its precursor, K2Ti4O9. The former crystals had a large number of facetted voids, 3–30 nm, formed as a result of the removal of potassium and water in preceding hydrolysis and calcination steps, respectively. TiO2(B)-supported vanadium oxide catalysts with loadings in the range 0.25−10 theoretical layers were prepared by impregnation of the support with an oxalic acid solution of NH4VO3 followed by calcination in air. HREM micrographs of catalysts with a low vanadium loading, recorded using a low electron-dose imaging technique, showed that the surfaces, in the initial stage, were without any anomalous surface structure. This observation may be due to a similar structure of the support and the deposited vanadia phase. At high vanadium loadings, both amorphous and crystalline particles were seen, in agreement with the features revealed by the use of IR spectroscopy. For catalysts with low vanadium loadings, the IR difference spectra showed the presence of tetrahedrally coordinated V4+ and V5+ species. In a fully converged electron beam, reduction of the support and the vanadia phases occured, resulting in the formation of small crystallites. The catalysts were used for the oxidation of toluene to benzaldehyde. However, contrary to what has been observed for the ammoxidation producing benzonitrile, no enhanced catalytic properties, in comparison with those of crystalline V2O5, were obtained using TiO2(B) as support