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 O 2 titration of samples equilibrated in flowing mixtures of H 2 and H 2 O. Samples having the following compositions were studied after calcinations at 973 and 1323 K: CeO 2 , Ce 0.92 Zr 0.08O2 , Ce 0.81 Zr 0.19O2 , Ce 0.59 Zr 0.41 O 2 , Ce 0.50 Zr 0.50O2 , Ce 0.25 Zr 0.75 O 2 , Ce 0.14 Zr 0.86 O 2 , and ZrO 2. While the oxidation enthalpy for CeO 2 was between −750 and −800 kJ/mol O 2 , the oxidation enthalpies for each of the solid solutions were between −500 and −550 kJ/mol O 2 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 CeO 2 , Ce 0.92 Zr 0.08 O 2 , and Ce 0.14 Zr 0.86 O 2 , the samples remained single-phase after calcination at 1323 K and the thermodynamic redox properties were unaffected. By contrast, Ce 0.59 Zr 0.41 O 2 formed two phases following calcination at 1323 K, Ce 0.78 Zr 0.22 O 2 (71 wt.%) and Ce 0.13 Zr 0.87 O 2 (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.

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 for both Ce0.81Zr0.19O2 and Ce0.25Zr0.75O2 were similar, between −500 and −550 kJ/mol of O2, and independent of the extent of reduction. However, there is a step change in −ΔS of reduction for Ce0.81Zr0.19O2, from ∼250 to <100 J/mol·K, after removal of approximately one oxygen for every two Zr4+. 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.

The thermodynamic properties of surface ceria were investigated through equilibrium isotherms determined by flow-titration and coulometric-titration measurements on high-surface-area ceria and ceria supported on La-modified alumina (LA). While the surface area of pure ceria was found to be unstable under redox conditions, the extent of reduction at 873 K and a P(O 2 ) of 1.6x10 -26 atm increased with surface area. Because ceria/LA samples were stable, equilibrium isotherms were determined between 873 and 973 K on a 30-wt% ceria sample. Oxidation enthalpies on ceria/LA were found to vary with the extent of reduction, ranging from -500 kJ/mol O 2 at low extents of reduction to near the bulk value of -760 kJ/mol O 2 at higher extents. To determine whether +3 dopants could affect the oxidation enthalpies for ceria, isotherms were measured for Sm +3 -doped ceria (SDC) and Y +3 -doped ceria. These dopants were found to remove the phase transition observed in pure ceria below 973 K but appeared to have minimal effect on the oxidation enthalpies. Implications of these results for catalytic applications of ceria are discussed. AbstractThe thermodynamic properties of surface ceria were investigated through equilibrium isotherms determined by flow-titration and coulometric-titration measurements on high-surfacearea ceria and ceria supported on La-modified alumina (LA). While the surface area of pure ceria was found to be unstable under redox conditions, the extent of reduction at 873 K and a P(O 2 ) of 1.6x10 -26 atm increased with surface area. Because ceria/LA samples were stable, equilibrium isotherms were determined between 873 and 973 K on a 30-wt% ceria sample. Oxidation enthalpies on ceria/LA were found to vary with the extent of reduction, ranging from -500 kJ/mol O 2 at low extents of reduction to near the bulk value of -760 kJ/mol O 2 at higher extents.To determine whether +3 dopants could affect the oxidation enthalpies for ceria, isotherms were measured for Sm +3 -doped ceria (SDC) and Y +3 -doped ceria. These dopants were found to remove the phase transition observed in pure ceria below 973 K but appeared to have minimal effect on the oxidation enthalpies. Implications of these results for catalytic applications of ceria are discussed.

Cerium-manganese mixed oxides with compositions of Ce 0.5 Mn 0.5 O 1.75 and Ce 0.8 Mn 0.2 O 1.9 were prepared by the citric-acid (Pechini) method and their catalytic properties were compared to CeO 2 and Mn 2 O 3 . The mixed oxides exhibited higher specific rates than either CeO 2 or Mn 2 O 3 for oxidation of both methane and n-butane. While XRD measurements of the mixed oxides suggested that the materials had primarily the fluorite structure, oxygen isotherms, measured by coulometric titration at 973 K, exhibited steps associated with MnO-Mn 3 O 4 and Mn 3 O 4 -Mn 2 O 3 equilibria, implying that manganese oxide must exist as separate phases in the solids. The P(O 2 ) for the MnO-Mn 3 O 4 equilibrium is shifted to lower values in the mixed oxides, indicating that the manganese-oxide phase is affected by interactions with ceria.

The thermodynamics of oxidation and reduction were investigated for a series of vanadium-containing oxides, including CeVO 4 , LaVO 4 , ZrV 2 O 7 , Mg 3 (VO 4 ) 2 , CrVO 4 , AlVO 4 , and V 2 O 5 , in the temperature range between 873 and 973 K, using coulometric titration to measure oxidation isotherms. For each of the compounds, the oxygen uptakes corresponded to oxidation from V +3 to V +5 as oxygen fugacities increased from 10 -25 to 10 -2 atm. With V 2 O 5 and ZrV 2 O 7 , the only materials in this group that contain V-O-V linkages, there was evidence for two oxidation processes, corresponding to V +3 -V +4 and V +4 -V +5 equilibria, whereas oxidation occurred in a single step for the other materials. CrVO 4 and AlVO 4 were unstable to deep reduction, but oxidation enthalpies and entropies were reported for CeVO 4 , ZrV 2 O 7 , Mg 3 (VO 4 ) 2 , and V 2 O 5 .