Structure and Reactivity of Vanadium Oxide Catalysts Supported on Anatase TiO2

A series of TiO 2 (anatase) supported vanadia catalysts with V 2 O 5 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 TiO 2 . 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 V 4+ ions in axial symmetry. The results are discussed with the possible surface vanadia species present on TiO 2 . The catalytic properties were evaluated for the vapor-phase ammoxidation of 3-picoline to nicotinonitrile and related to the oxygen chemisorption sites.