(690b) 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_0.19O₂ and
Ce_0.25Zr_0.75O₂, 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 Ce_0.81Zr_0.19O₂
and Ce_0.25Zr_0.75O₂, 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 Ce_0.81Zr_0.19O₂,
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.