The interaction of V 2O 5 with Ti0 2(anatase): Catalyst evolution with calcination temperature and O-xylene oxidation

Abstract

The interaction of V 2O 5 with the surface of TiO 2(anatase) was studied over the temperature range 110–750 °C. The V 2O 5 TiO 2 (anatase) system was characterized with laser Raman spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, X-ray diffraction, thermal gravimetric analysis, BET, and catalytic performance for o-xylene oxidation to phthalic anhydride. The state of V 2O 5 TiO 2 (anatase) possessing high loadings of vanadia is strongly dependent on calcination temperature. In the presence of vanadia the TiO 2(anatase) support exhibits a simultaneous loss in surface area and structural transformation to rutile at elevated calcination temperatures. The morphology of the supported vanadia phase also depends on calcination temperature. At low calcination temperatures, 110–200 °C, the vanadia exists as vanadyl oxalate, the starting vanadia salt. At intermediate calcination temperatures, 350–575 °C, vanadia is present as a complete monolayer of surface vanadia species coordinated to the titania support and V 2O 5 crystallites. At calcination temperature of 575 °C and above, the supported vanadia phase reacts with the TiO 2(anatase) support to yield V x Ti 1 − x O 2(rutile). These structural changes have a pronounced effect on the catalytic performance of V 2O 5 TiO 2 (anatase) catalysts for the oxidation of o-xylene. The optimum catalytic performance is observed for prolonged calcination at intermediate temperatures, 350–575 °C, where a complete monolayer of surface vanadia exists on the TiO 2(anatase) support. The complete monolayer of surface vanadia and crystalline vanadia phases remain intact during the o-xylene oxidation reaction and become partially reduced by the reaction environment.