Structural Characterization of CeO₂−MO₂ (M = Si⁴⁺, Ti⁴⁺, and Zr⁴⁺) Mixed Oxides by Raman Spectroscopy, X-ray Photoelectron Spectroscopy, and Other Techniques

Abstract

Microstructure evolution of ceria-based mixed oxides CeO 2 −MO 2 (M = Si 4+, Ti 4+, and Zr 4+ ) 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 CeO 2 −SiO 2 was synthesized by a deposition precipitation method, and a coprecipitation procedure was adopted to make CeO 2 −TiO 2 and CeO 2 −ZrO 2 binary oxides. The XRD measurements revealed the presence of crystalline cubic CeO 2 on the surface of SiO 2 in CeO 2 −SiO 2, CeO 2 and TiO 2 (anatase) in CeO 2 −TiO 2, and Ce 0.75 Zr 0.25 O 2 and Ce 0.6 Zr 0.4 O 2 phases in CeO 2 −ZrO 2 samples. The crystallinity of these phases increased as the calcination temperature increased. Estimations of the cell parameter a indicated an expansion of the CeO 2 lattice in the case of CeO 2 −TiO 2 samples, whereas a contraction was noted in the case of CeO 2 −ZrO 2 . Some incorporation of Si 4+ ions into the CeO 2 lattice was noted at higher calcination temperatures for the CeO 2 −SiO 2 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 CeO 2 −TiO 2 and CeO 2 −ZrO 2 samples. The XPS studies revealed the presence of silica, titania, and zirconia in their highest oxidation states Si 4+, Ti 4+, and Zr 4+ at the surface of the materials. Cerium is present in both Ce 4+ and Ce 3+ oxidation states, but in different proportions, depending on the mixed-oxide system and the calcination temperature used.