The low-temperature synthesis of cation-ordered Ce–Zr-based oxide via an intermediate phase between Ce and Fe

Abstract

Ce_0.5Zr_0.5O₂ is a well-known oxygen storage material used in automobiles to maintain the air–fuel ratio during operation. It has been reported that the cation ordering of Ce and Zr ions and doping with transition metals improves the oxygen storage capacity (OSC) of the Ce–Zr-based oxides. In this study, Ce–Zr-based oxides were co-doped with Co and Fe to determine their effect on the oxygen storage capacity at low temperatures. Using the Pechini method, samples of (CeZrO₄)_100−x–(CoFe₂O₄)_x (xCFO; x = 1 to 5) are prepared and their solubility limit, oxygen storage capacity, and stability are clarified. The solubility limit of xCFO was found to be 5CFO. A high OSC of 440.0 μmol O₂ g⁻¹ at 400 °C was found for 5CFO, which is 13.5 times higher than the 32.5 μmol O₂ g⁻¹ found for Ce_0.5Zr_0.5O₂ without doping. This was attributed to the low formation enthalpy of oxygen vacancy and faster surface exchange kinetics. This solid solution is stable at 400 °C and decomposes at 900 °C. The results indicate that Fe induces phase transition to a cation-ordered phase, a pyrochlore-type Ce₂Zr₂O₇, and lowers its transition temperature to 800 °C. The acceleration of the reduction of Ce and cation diffusion due to the formation of the intermediate phase of CeFeO₃ resulted in these changes. From the results of this study, it can be concluded that Fe is a key element in improving the OSC properties of the Ce–Zr-based oxide and expands the possibilities for a cation-ordered κ-Ce₂Zr₂O₈ catalyst.