Zirconium-cerium mixed oxides were prepared by two methods: (i) sol-gel hydrolysis of alcoholic solutions of zirconium alkoxides (n-propoxide and n-butoxide) in the presence of aqueous solutions of cerium nitrate (method SG) or (ii) coprecipitation of aqueous solutions of zirconyl and cerium nitrates by ammonia (method NP). Dried or calcined solids were characterized by XRD, DTA, BET and oxygen storage capacity (OSC ) measurements, O 2 adsorption was monitored by infrared spectroscopy. Regardless of the zirconium precursor, SG samples calcined at 900 °C have a fluorite-type structure with BET surface areas varying between 35 and 45 m2 g−1. Cerium-rich samples (0.85∏x∏0.90), with excellent OSC properties and high thermal stability can be prepared by the sol-gel method. FTIR studies of O 2 adsorption reveal the presence of stable superoxide species whose concentration strongly depends on the cerium content and the method of preparation. A good correlation between the relative amount of O 2 − species and OSC values was observed. Superoxide ions may be formed at the same anionic vacancies as those involved in oxygen storage.Sol-gel method. Two Zr precursors were used: zirconium n-

Ce-Pr-O mixed oxides prepared by coprecipitation and calcined at 1173 K reveal a non stoichiometric structure with oxygen vacancies in the bulk. In situ XRD and Raman spectroscopy were used to investigate these properties simultaneously and identified a thermal expansion coefficient twice as high as that of pure ceria at room temperature and the presence of a Raman band at 560 cm 21 , characteristic of the oxygen vacancies. These results confirm that praseodymium atoms are able to induce oxygen mobility in the bulk at 298 K. This is due to a structure containing a high number of anionic vacancies pre-existing at ambient temperature.

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