Surface characterization of CeO2/SiO2 and V2O5/CeO2/SiO2 catalysts by Raman, XPS, and other techniques

X-ray diffraction, Raman, and X-ray photoelectron spectroscopy were utilized to characterize CeO2/SiO2 supports and V2O5/CeO2/SiO2 catalysts calcined at different temperatures from 773 to 1073 K. The CeO2/SiO2 support was obtained by an aqueous deposition precipitation method, and vanadium oxide was applied to the calcined support (773 K) by using a wet impregnation technique. The XRD and Raman results suggest that the CeO2/SiO2 carrier is thermally quite stable up to 1073 K calcination temperature and primarily consists of a CeO2 overlayer on the SiO2 substrate. The CeO2/SiO2 support also accommodates a layer of vanadia in a highly dispersed state on its surface. In particular, no crystalline V2O5 was observed. The O 1s, Si 2p, Ce 3d, and V 2p core level photoelectron peaks of CeO2/SiO2 and V2O5/CeO2/SiO2 are sensitive to the calcination temperature. The XPS line shapes and the corresponding binding energies indicate that the dispersed vanadium oxide interacts selectively with the ceria portion of the CeO2/SiO2 carrier and readily forms CeVO4. The XRD and Raman techniques, in particular, provide direct evidence about the formation of CeVO4. The V/Ce and V/Si atomic ratios, as determined by XPS measurements, reveal that vanadia is mainly confined to ceria at all calcination temperatures. No significant changes in the oxidation states of V(V) and Si(IV) are noted with increasing calcination temperature. However, in the case of cerium oxide stabilization of Ce(III) was observed at higher calcination temperatures.