- 3:34 PM

Evidence for Entropy Effects in the Reduction of Ceria-Zirconia Solutions

Parag R. Shah1, Taeyoon Kim1, Gong Zhou1, Paolo Fornasiero2, and Raymond J. Gorte1. (1) Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Room 311A Towne Building, 220 South 33rd Street, Philadelphia, PA 19104, (2) Chemistry Department, INSTM Trieste RU and Centre of Excellence for Nanostructured Materials, University of Trieste, Via L. Giorgieri 1, Trieste, I-34127, Italy

An instrument for Coulometric-titration measurements was built and used to measure the thermodynamic redox properties for a 10-wt% Cu/silica catalyst at 973 K and for two ceria-zirconia solid solutions, Ce0.81Zr0.19O2 and Ce0.25Zr0.75O2, between 873 and 1073 K. For Cu/silica, the equilibrium data show two well-defined steps in the oxygen isotherms, associated with equilibrium between Cu and Cu2O and between Cu2O and CuO; and the P(O2) associated with these two steps are in close agreement with values expected for pure Cu. The oxidation enthalpies were similar for both Ce0.81Zr0.19O2 and Ce0.25Zr0.75O2, between -500 and -550 kJ/mol O2, and independent of the extent of reduction. However, there is a step change in partial entropy of reduction for Ce0.81Zr0.19O2, from ~-250 J/mol.K to less than -100 J/mol.K, after removal of approximately one oxygen for every two Zr+4. A model is presented which views the reduction of ceria-zirconia as removal of oxygen from "pyrochlore-like" clusters, with some of the changes in reducibility associated with the number of sites from which oxygen can be removed from such clusters.