A Thermodynamic Investigation of the Redox Properties of Ceria-Titania Mixed Oxides

Ceria-titania solutions with compositions of Ce0.9Ti0.1O2 and Ce0.8Ti0.2O2 were prepared by the citric-acid (Pechini) method and characterized using x-ray diffraction (XRD) for structure, coulometric titration for redox thermodynamics, and water-gas-shift (WGS) reaction rates. Following calcination at 973 K, XRD suggests that the mixed oxides exist as single-phase, fluorite structures, although there was no significant change in the lattice parameter compared to pure ceria. The mixed oxides are shown to be significantly more reducible than bulk ceria, with enthalpies for reoxidation being approximately -500 kJ/mol O2, compared to -760 kJ/mol O2 for bulk ceria. However, WGS rates over 1-wt% Pd supported on ceria, Ce0.8Ti0.2O2, and Ce0.8Zr0.2O2 were nearly the same. For calcination at 1323 K, the mixed oxides separated into ceria and titania phases, as indicated by both the XRD and thermodynamic results.