Effect of Crystallinity on the Photoreduction of Cerium Oxide: A Study of CeO2 and Ce/Al2O3 Catalysts

The effect of crystallinity on the photoreduction of cerium oxide (Ce4+ to Ce3+) during X-ray photoelectron spectroscopy (XPS) analysis of CeO2 and Ce/Al2O3 catalysts has been determined. Cerium oxide samples were prepared by calcining cerium(IV) methoxyethoxide in air at different temperatures. The structure of the resulting metal oxides was determined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The oxide obtained from calcination at 200 °C consisted primarily of amorphous cerium oxide, while the material produced at 750 °C contained large CeO2 crystallites. XPS Ce 3d spectra obtained for the samples showed that the amorphous cerium oxide sample was reduced more extensively than the crystalline material during XPS analysis. Ce/Al2O3 catalysts (5.1 wt % CeO2) were prepared by incipient wetness impregnation using ammonium cerium(IV) nitrate and cerium(IV) methoxyethoxide precursors. XRD and XPS analyses of the Ce/Al2O3 catalyst derived from the nitrate precursor (designated CeN) indicated that most of the cerium was present as poorly dispersed CeO2 crystallites. The catalyst prepared using the alkoxide precursor (designated CeA) also contained poorly dispersed cerium oxide; however, XRD data suggested that very little of the cerium oxide phase was crystalline. The XPS Ce 3d spectrum obtained for the CeN catalyst was typical of Ce(IV) oxide while the spectrum collected for the CeA catalyst was similar to a Ce(III) compound. These results have been attributed to enhanced photoreduction of the amorphous cerium oxide present in the CeA catalyst.