Structural characterization of CeO2-ZrO2/TiO2 and V2O5/CeO2-ZrO2/TiO2 mixed oxide catalysts by XRD, Raman spectroscopy, HREM, and other techniques

摘要

Structural characteristics of CeO2-ZrO2/TiO2 (CZ/T) and V2O5/CeO2-ZrO2/TiO2 (V/CZ/T) mixed oxide catalysts have been investigated using X-ray diffraction (XRD), BET surface area, Raman spectroscopy (RS), and high-resolution transmission electron microscopy (HREM) techniques. The CeO2-ZrO2 (1:1 mole ratio) solid solution was deposited over a finely powdered TiO2 support by a deposition precipitation method. A nominal 5 Wt % V2O5 was impregnated over the calcined (773 K) CZ/T mixed oxide carrier by a wet impregnation technique. The obtained CZ/T and V/CZ/T samples were further subjected to thermal treatments from 773 to 1073 K to understand the dispersion and temperature stability of these materials. In the case of CZ/T samples, the XRD results suggest the formation of different cubic and tetragonal Ce-Zr-oxide phases, Ce0.75Zr0.25O2, Ce0.6Zr0.4O2, Ce0.5Zr0.5O2, and Ce0.16Zr0.84O2 in varying proportions depending on the treatment temperature. With increasing calcination temperature from 773 to 1073 K, the intensity of the lines pertaining to cubic Ce0.6Zr0.4O2 and Ce0.5Zr0.5O2 phases increased at the expense of cubic Ce0.75Zr0.25O2, indicating more incorporation of zirconia into the ceria lattice. The TiO2 was mainly in the anatase form whose crystallite size also increased with increasing treatment temperature. A better crystallization and more incorporation of zirconia into the ceria lattice was noted when CZ/T was impregnated with V2O5. However, no crystalline V2O5 could be seen from both XRD and RS measurements. In particular, a preferential formation Of CeVO4 compound and an intense tetragonal Ce0.16Zr0.84O2 phase were noted beyond 873 K. The HREM results indicate, in the case of CZ/T samples, a well-dispersed Ce-Zr-oxide of the size similar to5 nm over the bigger crystals (similar to40 nm) of TiO2 when treated at 873 K. The exact structural features of these crystals as determined by digital diffraction analysis of experimental images reveal that the Ce-Zr-oxides are mainly in the cubic fluorite geometry and the TiO2 is in anatase form. A better crystallization of Ce-Zr-oxides (similar to8 nm) over the surface of bigger crystals of TiO2 was noted at 1073 K. A further enhancernent in the crystallite size and zirconiarich tetragonal phase was noted in the case of V/CZ/T samples. Further, the structure of CeVO4 formed was also clearly identified in conformity with XRD and RS results.