Structural characterization of CeO2-MO2 (M = Si4+, Ti4+, and Zr4+) mixed oxides by Raman spectroscopy, X-ray photoelectron spectroscopy, and other techniques

Microstructure evolution of ceria-based mixed oxides CeO2-MO2 (M = Si4+, Ti4+, and Zr4+) after thermal treatments in the temperature range of 773-1073 K were investigated by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and other techniques. The CeO2-SiO2 was synthesized by a deposition precipitation method, and a coprecipitation procedure was adopted to make CeO2-TiO2 and CeO2-ZrO2 binary oxides. The XRD measurements revealed the presence of crystalline cubic CeO2 on the surface Of SiO2 in CeO2-SiO2, CeO2 and TiO2 (anatase) in CeO2-TiO2, and Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 phases in CeO2-ZrO2 samples. The crystallinity of these phases increased as the calcination temperature increased. Estimations of the cell parameter a indicated an expansion of the CeO2 lattice in the case of CeO2- TiO2 samples, whereas a contraction was noted in the case of CeO2-ZrO2. Some incorporation of Si4+ ions into the CeO2 lattice was noted at higher calcination temperatures for the CeO2-SiO2 samples. Raman measurements revealed the presence of oxygen vacancies, lattice defects, and the displacement of oxide ions from their normal lattice positions in the case of the CeO2-TiO2 and CeO2-ZrO2 samples. The XPS studies revealed the presence of silica, titania, and zirconia in their highest oxidation states-Si4+, Ti4+, and Zr4+-at the surface of the materials. Cerium is present in both Ce4+ and Ce3+ oxidation states, but in different proportions, depending on the mixed-oxide system and the calcination temperature used.