Microscopic Spectral Characteristics and Redox Properties of Nanosized Ce1-4x(FeAlCoLa)xO2-δ Solid Solutions
SUN Shi-long1, ZHANG Guo-fang1*, SHU Jun1, GUO Rui-hua1, LI Yi-ming1, LIU Zhuo-cheng1, XU Jian-yi1, GE Qi-lu2
1. School of Material and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, China
2. Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, China
Abstract：Hydrothermal method was used to synthesize nanosized Fe3+, Al3+, Co2+, and La3+ co-doped Ce1-4x(FeAlCoLa)xO2-δ(x=0.00~0.05) solid solutions. The solid solutions' microstructure, morphology, spectral characteristics, and redox activities were systematically characterized and analyzed by XRD, TEM, SEM, UV, PL, Raman, and temperature-programmed reduction (TPR) with H2. XRD results showed the Ce1-4x(FeAlCoLa)xO2-δ solid solutionsexhibited the CeO2 cubic fluorite structure. A tiny diffraction peak corresponding to the Co3O4 impurity phase at 36.6° was observed when the doped content reached x=0.04, indicating that x=0.04 was the solid solubility of doped ions in the CeO2 lattice. The positions of the (111) diffraction peaks were shifted towards a higher angle, which proved the doped ionsinduced the distortion of the lattice. The TEM and SEM images showed the samples were spherical with high crystallinity, and doping caused lattice contraction. The UV absorption spectra revealed that the doped samples' absorption edges were gradually red-shifted compared to pure CeO2. Extra absorption peaks corresponding to the doped ions were found in the region of 560~780 nm. The band gap energies decreased from 2.84 eV (pure CeO2) to 2.1 eV (x=0.05). The reasoncould be that the doped ions formed new impurity energy levels between the valence and conduction bands, which allowed the electrons to transition from the valence band to the lower impurity energy levels and then lowered the band gap energies. In addition, the distortion of the lattice and increased concentration of oxygen vacancies prevented the electrons from transferring to higher energies, which can also result in the reduction of band gap energies. PL spectra showed that doping significantly reduced the emission peak intensities. Raman spectra demonstrated that the dopingresulted in the shift of the F2g peak, the decrease of peak intensities, and the widening of peaks. Meanwhile, the relative intensities of the peak corresponding to the oxygen vacancies were also observed to be enhanced. Thus, both the PL and Raman spectra proved that doping increased the degree of lattice distortion and the concentration of oxygen vacancies. The H2-TPR test results showed that doping can effectively reduce the redox reaction temperatures and improve the redox activities. The sample doped with x=0.03 possess the lowest surface reduction temperature and the largest peak areas, which meansthis sample exhibited the best redox activities. It can be concluded that the redox performances of the samples were closely related to the grain sizes, lattice defects, and oxygen vacancy concentrations. This study showed that the four ions co-doped with CeO2 could effectively modify the microstructure and improve the samples' catalytic activities at a low doping concentration.
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