Structure and reactivity of vanadium oxide catalysts supported on anatase TiO2

A series of TiO2 (anatase) supported vanadia catalysts with V2O5 loadings ranging from 2 to 12% were characterized by temperature-programmed reduction (TPR), X-ray diffraction, electron spin resonance (ESR), and oxygen chemisorption studies. TPR profiles indicate the presence of a single peak for the reduction and suggest that the reducibility of vanadia depends on its loading on TiO2. Dispersion of vanadia was determined by the static oxygen chemisorption method at 640 K on the samples prereduced at the same temperature. At low vanadia loadings, i.e., below 6%, vanadium oxide is found to be present in a highly dispersed state. A comparison of dispersion of vanadia determined by low-temperature oxygen chemisorption (samples reduced at 773 K for 6 h and subsequent oxygen chemisorption at 195 K) with that determined at 640 K shows that the latter method provides a better measure of dispersion of vanadia. ESR spectra obtained under ambient conditions for the catalysts reduced at 640 K show the presence of V4+ ions in axial symmetry. The results are discussed with the possible surface vanadia species present on TiO2. The catalytic properties were evaluated for the vapor-phase ammoxidation of 3-picoline to nicotinonitrile and related to the oxygen chemisorption sites.