Abstract
The oxygen-storage properties of ceria in three-way automotive catalysts are promoted and stabilized by mixing with zirconia. In the present study, this promotion was investigated using model catalysts in which ceria films were vapor deposited onto α-Al 2O 3, polycrystalline ZrO 2, polycrystalline Y 2O 3-stabilized ZrO 2 (YSZ), and YSZ(1 0 0), (1 1 1), and (1 1 0) single crystals. Following deposition of Pd, both TPD of CO and steady-state CO-oxidation kinetics suggest that the ceria films on the zirconia-based substrates were much more easily reduced than films on α-Al 2O 3. Polycrystalline zirconia and YSZ and the YSZ single crystals were equally effective in promoting ceria reducibility. Structural studies of ceria on YSZ(1 0 0), using both TEM and EDSXD (energy-dispersive, surface X-ray diffraction), demonstrate that ceria forms ordered overlayers on YSZ(1 0 0), oriented with respect to the YSZ surface. The lattice parameter for ceria is decreased by only 0.6% compared to bulk CeO 2, but the coherence length suggests that the overlayer may have a high defect density. It is suggested that the structure-directing properties of zirconia are responsible for the enhanced properties of ceria–zirconia mixed oxides.
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