Characterization of the equilibrium redox properties for ceria-based mixed oxides

The ability of cerium to cycle between Ce+3 and Ce+4 is crucial for ceria-based materials in many catalytic applications. However, our fundamental understanding of how structure and composition affect ceria reducibility is really rather poor. The primary goal of this thesis was to provide a better understanding of the redox properties for ceria-based catalysts and also to establish a connection between the redox ability of ceria and the activity of catalytic reactions carried out by ceria-based catalysts. The energetics for oxidation of ceria-based mixed oxides were determined from the equilibrium oxygen isotherms measured at a temperature range between 873 and 1073 K using both flow-titration and coulometric-titration measurements. First, the thermodynamic properties of surface ceria were examined on ceria/La-modified-alumina (LA) samples because the surface area of pure ceria was found to be unstable under redox conditions. Oxidation enthalpies on ceria/LA were found to vary with the extent of reduction, ranging from −500 kJ/mol-O2 at low extents of reduction to near the bulk value of −760 kJ/mol O 2 at higher extents. The redox properties were also investigated for the following ceria-based mixed oxides: ceria-zirconia, ceria-titania, ceria-manganese, ceria-hafnia, and ceria-terbia. The oxidation enthalpies for each of the ceria-zirconia solid solutions were between −500 and −550 kJ/mol-O2 and essentially independent of the extent of reduction, but the oxidation entropies depended strongly on the sample composition and the extent of reduction. A model was presented which views reduction of the solid solutions in terms of the local atomic structure, with the formation of pyrochlore-like clusters. The solid solutions of ceria-titania were also significantly more reducible than bulk ceria, with enthalpies for reoxidation being approximately −500 kJ/mol-O2. Following calcination at 1323 K, the solid solutions separated into ceria and titania phases. Investigation of mixed oxides of cerium and manganese showed that oxygen isotherms exhibited steps associated with MnO-Mn3O4 and Mn3O4-Mn 2O3 equilibria, implying that manganese oxide must exist as separate phases in the solids. The P(O2) for the MnO-Mn 3O4 equilibrium is shifted to lower values in the mixed oxides, indicating that the manganese-oxide phase is affected by interactions with ceria. The redox properties of ceria-hafnia are nearly identical to that of ceria-zirconia. Finally, mixed oxides of ceria with terbia and praseodymia also formed single-phase, fluorite structures at 1323 K, but the oxidation isotherms for the solid solutions were similar to what would be expected for physical mixtures of these oxides. Both WGS and hydrocarbon-oxidation rate measurements were used to characterize the catalytic properties of the mixed oxides. The activities and activation energies of the WGS reaction for both 1wt% Pd/Ce0.8Zr0.2O 2 and 1wt% Pd/Ce0.8Ti0.2O2 were nearly identical to that of 1wt% Pd/CeO2. The ceria-manganese mixed oxides were more active for hydrocarbon-oxidation reactions than either ceria or manganese oxide individually