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 0.81Zr 0.19O 2 and Ce 0.25Zr 0.75O 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 2O and between Cu 2O 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 0.81Zr 0.19O 2 and Ce 0.25Zr 0.75O 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 -ΔS of reduction for Ce 0.81Zr 0.19O 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.