Abstract
CeO 2 supported V 2O 5 catalysts were prepared by the wetness impregnation technique and their surface structures were characterized by O 2 chemisorption, X-ray diffraction (XRD) and Raman spectroscopy (LRS). The surface acidity and basicity were measured by using microcalorimetry and infrared spectroscopy (FTIR) for the adsorption of NH 3 and CO 2. Temperature programmed reduction (TPR) was employed for the redox properties. In particular, isopropanol probe reactions with and without the presence of O 2 were employed to provide the additional information about the surface acidity and redox properties of the catalysts. Variation of loading of V 2O 5 and calcination temperature brought about the changes of surface structures of dispersed vanadium species, and hence the surface acidic and redox properties. Structural characterizations indicated that V 2O 5 can be well dispersed on the surface of CeO 2. The monolayer dispersion capacity was found to be about 8 V/nm 2, corresponding to about 10% V 2O 5 by weight in a V 2O 5/CeO 2 sample with the surface area of 80 m 2/g. Vanadium species in the catalysts (673 K calcined) with loading lower than 10% were highly dispersed and exhibited strong surface acidity and redox ability, while higher loading resulted in the formation of significant amount of surface crystalline V 2O 5, which showed fairly strong surface acidity and significantly weakened redox ability. Calcination of a 10% V 2O 5/CeO 2 at 873 K resulted in the formation of mainly CeVO 4 on the surface, which showed low surface acidity and redox ability. The probe reaction seemed to suggest that the calcination at higher temperature might cause the decrease of surface acidity more than redox ability. Thus, the 10% V 2O 5/CeO 2 catalyst calcined at 873 K exhibited much higher selectivity to benzaldehyde as compared to other V 2O 5/CeO 2 catalysts studied in this work, although its activity for the conversion of toluene was relatively low.
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