Structural characterization of CeO2-TiO2 and V2O5/CeO2-TiO2 catalysts by Raman and XPS techniques

Structural characteristics of ceria-titania and vanadia/ceria-titania mixed oxides have been investigated using X-ray powder diffraction (XRD), Raman spectroscopy (RS), and X-ray pbotoelectron spectroscopy (XPS) techniques. The (]:I mole ratio) mixed oxide was obtained by a coprecipitation method, and a nominal 5 wt % V2O5 was deposited over its surface by a wet impregnation technique. Both of the materials were then subjected to thermal treatments from 773 to 1073 K and were characterized by the above-mentioned techniques. The XRD results suggest that the CeO2-TiO2 mixed oxide calcined at 773 K primarily consists of poorly crystalline CeO2 and TiO2-anatase phases and that a better crystallization of these oxides occurs with increasing calcination temperature. The "a" cell-parameter values suggest some incorporation of titanium into the ceria lattice. Impregnation of vanadia on ceria-titania enhances the crystallization of CeO2 and TiO2 oxides. However, no crystalline V2O5 could be observed from XRD and RS measurements. Furthermore, the dispersed molecular vanadium oxide (polyvanadate), evidenced by Raman measurements, interacts preferentially with the CeO2 portion of the mixed oxides and forms the CeVO4 compound at higher calcination temperatures. The XRD and RS results provide direct evidence about the formation of CeVO4. The XPS electron-binding energies indicate that ceria, titania, and vanadia are mainly in their highest oxidation states, Ce(IV), Ti(IV), and V(V). The formation of Ce(III) has also been noticed in both CeO2-TiO2 and V2O5/CeO2-TiO2 samples at all temperatures.