Investigation of surface structures of dispersed V2O5 on CeO2-SiO2, CeO2-TiO2, and CeO2-ZrO2 mixed oxides by XRD, Raman, and XPS techniques

The techniques of X-ray diffraction (XRD), Raman spectroscopy (RS), and X-ray photoelectron spectroscopy (XPS) were employed to investigate dispersion and structural characteristics of V2O5/CeO2-MO2 (M = Si4+, Ti4+, and Zr4+) catalysts calcined at different temperatures from 773 to 1073 K. The CeO2-MO2 (1:1 mole ratio) mixed oxides were obtained by soft chemical methods with ultrahigh-dilution solutions, and a nominal monolayer equivalent of V2O5 was deposited over the calcined (773 K) supports. The XRD and RS results suggest that the CeO2-MO2 carrier calcined at 773 K exhibits the presence of nanocrystalline cubic CeO2 on the surface of SiO2 in CeO2-SiO2, CeO2 and TiO2 (anatase) in CeO2-TiO2, and Ce0.75Zr0.25O2 in CeO2-ZrO2 samples. The impregnation of vanadia over CeO2-MO2 and their subsequent calcination at higher temperatures leads to various modifications. The deposited V2O5 is in a highly dispersed state when calcined at 773 K. In particular, no crystalline V2O5 is Observed at all calcination temperatures. The dispersed vanadia on the CeO2-MO2 carrier induces better crystallization of various phases and a preferential formation of CeVO4 in all samples. Incorporation of more zirconia into the Ce0.75Zr0.25O2 cubic lattice leading to the manifestation of a Ce0.16Zr0.84O2 tetragonal phase at higher calcination temperatures is noted in the case of CeO2-ZrO2 samples. The RS measurements disclose the presence of oxygen vacancies/Ce3+ in all samples in different proportions. The XPS results reveal that O 1s, Ce 3d, and V 2p core-level photoelectron peaks are sensitive to the calcination temperature and the nature of mixed-oxide system. The formation of CeVO4 at various temperatures is established from all characterization results.