Effect of preparation method and redox treatment on the reducibility and structure of supported ceria-zirconia mixed oxide

An Al₂O₃-supported Ce_0.45Zr_0.55O₂ was prepared with a modified sol-gel procedure designed to maximize the interaction between ceria and zirconia and promote thermal stability of alumina. Its properties, characterized in detail with XRD, TPR, and laser Raman spectroscopy, were compared with those of more conventional preparations. This sample consisted of finely dispersed Ce_xZr_1-xO₂ and had a high fraction of easily reducible Ce⁴⁺. Complete insertion of Zr into the ceria fluorite structure was observed after 900°C oxidation. After reduction at 980°C and reoxidation at 900°C, the mixed oxide particles remained small and furthermore still remained single phase and homogeneous by XRD characterization. Redox treatment enhanced the reducibility of Ce⁴⁺ in this sample. One feature of the enhancement, similar to that in bulk mixed oxide of similar composition, was an increase in the ratio of Ce⁴⁺ that was reducible at ∼491°C relative to the ones at ∼577°C. A new low-temperature reduction feature appeared at 320°C. This was attributed to the segregation of a small fraction of Ce³⁺ from the mixed oxide upon reduction and the subsequent formation of a finely dispersed ceria-rich phase on the surface of mixed oxide crystallites upon reoxidation. This phenomenon was particular to the supported samples because Al₂O₃ was able to stabilize the particle size of the mixed oxide during severe redox and thermal treatments.