CHARACTERIZATION OF CERIA FILMS ON ALPHA-AL2O3(0001) AND POLYCRYSTALLINE ZIRCONIA USING O-2 TPD WITH LABELED O-18(2)

The nature of weakly bound oxygen on ceria films was studied using tem perature-programmed desorption with labeled O-18(2). For alpha-Al2O3(0 001)-supported ceria, a desorption feature between 800 and 1300 K is s hown to result from partial reduction of ceria. However, this oxygen a ccounts for only a small fraction of the total oxygen in the ceria fil m and isotopic labeling studies suggest that this oxygen does not exch ange freely with the remaining oxygen in the film. In contrast, result s for zirconia-supported ceria demonstrate that much more oxygen desor bs in the low temperature regime below 1300 K, and that there is signi ficant isotope exchange throughout the ceria film and with the zirconi a substrate. Finally, exposure of reduced, zirconia-supported ceria to water at 670 K resulted in reoxidation of the ceria film. Oxygen from ceria was then shown to react with CO adsorbed on supported Rh partic les, completing the catalytic cycle for the water-gas-shift reaction.