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 re-oxidation 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.

Graphical abstractCe0.9Ti0.1O2 and Ce0.8Ti0.2O2 were prepared by the citric-acid method and characterized using XRD, coulometric titration, and WGS reaction rates. The mixed oxides are shown to be significantly more reducible than bulk ceria, with enthalpies for re-oxidation being approximately −500 kJ/mol O2. However, WGS rates over 1 wt% Pd supported on ceria, Ce0.8Ti0.2O2, and Ce0.81Zr0.192O2 were nearly the same.Pictogram: Figure optionsDownload full-size imageDownload as PowerPoint slideOxygen-to-metal ratios for (○) CeO2, (□) Ce0.9Ti0.1O2, and (△)Ce0.8Ti0.2O2 as a function of P(O2) at 973 K, measured by coulometric titration, for samples were calcined at 973 K for 5 h. The closed squares (■) show oxygen-to-metal ratios for Ce0.9Ti0.1O2 at 973 K after the sample had been calcined at 1323 K. The closed triangles (▴) show the oxygen to metal ratio of Ce0.9Ti0.1O2 at 973 K measured using flow titration.