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

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, Ce 0.81 Zr 0.19 O 2 and Ce 0.25 Zr 0.75 O 2, 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 Cu 2 O and between Cu 2 O and CuO; and the P(O 2 ) associated with these two steps are in close agreement with values expected for pure Cu. The oxidation enthalpies for both Ce 0.81 Zr 0.19 O 2 and Ce 0.25 Zr 0.75 O 2 were similar, between −500 and −550 kJ/mol of O 2, and independent of the extent of reduction. However, there is a step change in −Δ S of reduction for Ce 0.81 Zr 0.19 O 2, from ∼250 to <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” structures, with some of the changes in reducibility associated with the number of sites from which oxygen can be removed.