STRUCTURE AND REACTIVITY OF VANADIUM-OXIDE CATALYSTS SUPPORTED ON ANATASE TIO2

A series of TiO2 (anatase) supported vanadia catalysts with V2O5 loadi ngs ranging from 2 to 12% were characterized by temperature-programmed reduction (TPR), X-ray diffraction, electron spin resonance (ESR), an d oxygen chemisorption studies. TPR profiles indicate the presence of a single peak for the reduction and suggest that the reducibility of v anadia depends on its loading on TiO2. Dispersion of vanadia was deter mined by the static oxygen chemisorption method at 640 K on the sample s prereduced at the same temperature. At low vanadia loadings, i.e., b elow 6%, vanadium oxide is found to be present in a highly dispersed s tate. A comparison of dispersion of vanadia determined by low-temperat ure oxygen chemisorption (samples reduced at 773 K for 6 h and subsequ ent oxygen chemisorption at 195 K) with that determined at 640 K shows that the latter method provides a better measure of dispersion of van adia. 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 presen t on TiO2. The catalytic properties were evaluated for the vapor-phase ammoxidation of 3-picoline to nicotinonitrile and related to the oxyg en chemisorption sites.