Oxidation enthalpies for reduction of ceria surfaces

Abstract

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 1.6 × 10 −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.