Structural characterization of nanosized CeO2-SiO2, CeO2-TiO2, and CeO2-ZrO2 catalysts by XRD, raman, and HREM techniques

Structural characteristics of nanosized ceria-silica, ceria-titania, and ceria-zirconia mixed oxide catalysts have been investigated using X-ray diffraction (XRD), Raman spectroscopy, BET surface area, thermo-gravimetry, and high-resolution transmission electron microscopy (HREM). The effect of support oxides on the crystal modification of ceria cubic lattice was mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahighly dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO2-SiO2 sample primarily consists of nanocrystalline CeO2 on the amorphous SiO2 surface. Both crystalline CeO2 and TiO2 anatase phases were noted in the case of CeO2- TiO2 sample. Formation of cubic Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 (at 1073 K) were observed in the case of the CeO2-ZrO2 sample. Raman measurements disclose the fluorite structure of ceria and the presence of oxygen vacancies/Ce3+. The HREM results reveal well-dispersed CeO2 nanocrystals over the amorphous SiO2 matrix in the cases of CeO2-SiO2, isolated CeO2, and TiO2 (anatase) nanocrystals, some overlapping regions in the case of CeO2-TiO2, and nanosized CeO2 and Ce-Zr oxides in the case of CeO2-ZrO2 sample. The exact structural features of these crystals as determined by digital diffraction analysis of HREM experimental images reveal that the CeO2 is mainly in cubic fluorite geometry. The oxygen storage capacity (OSC) as determined by thermogravimetry reveals that the OSC of the mixed oxide systems is more than that of pure CeO2 and is system dependent.