Oxidation entropies and enthalpies of ceria–zirconia solid solutions

The thermodynamic redox properties for a series of ceria–zirconia solid solutions have been measured by determining their oxidation isotherms between 873 and 1073 K. Isotherms were obtained using Coulometric titration and using O₂ titration of samples equilibrated in flowing mixtures of H₂ and H₂O. Samples having the following compositions were studied after calcinations at 973 and 1323 K: CeO₂, Ce_0.92Zr_0.08O₂, Ce_0.81Zr_0.19O₂, Ce_0.59Zr_0.41O₂, Ce_0.50Zr_0.50O₂, Ce_0.25Zr_0.75O₂, Ce_0.14Zr_0.86O₂, and ZrO₂. While the oxidation enthalpy for CeO₂ was between ?750 and ?800 kJ/mol O₂, the oxidation enthalpies for each of the solid solutions were between ?500 and ?550 kJ/mol O₂ and essentially independent of the extent of reduction. The shapes of the isotherms for the solid solutions were affected by the oxidation entropies, which depended strongly on the sample composition and the extent of reduction. With CeO₂, Ce_0.92Zr_0.08O₂, and Ce_0.14Zr_0.86O₂, the samples remained single-phase after calcination at 1323 K and the thermodynamic redox properties were unaffected. By contrast, Ce_0.59Zr_0.41O₂ formed two phases following calcination at 1323 K, Ce_0.78Zr_0.22O₂ (71 wt.%) and Ce_0.13Zr_0.87O₂ (29 wt.%); the isotherm changed to that which would be expected for a physical mixture of the two phases. A model is presented which views reduction of the solid solutions in terms of the local atomic structure, with the formation of “pyrochlore-like” clusters causing the increased reducibility of the solid solutions. Some of the changes in reducibility are associated with the number of sites from which oxygen can be removed in order to form pyrochlore-like clusters.