Ceria films on zirconia substrates: models for understanding oxygen-storage properties

The oxygen-storage properties of ceria in three-way automotive catalysts ar e promoted and stabilized by mixing with zirconia. In the present study, th is promotion was investigated using model catalysts in which ceria lilms we re vapor deposited onto alpha-Al2O3, polycrystalline ZrO2, polycrystalline Y2O3-stabilized ZrO2 (YSZ), and YSZ(1 0 0), (1 1 1), and (1 1 0) single cry stals. Following deposition of Pd, both TPD of CO and steady-state CO-oxida tion kinetics suggest that the ceria films on the zirconia-based substrates were much more easily reduced than films on alpha-Al2O3. Polycrystalline z irconia and YSZ and the YSZ single crystals were equally effective in promo ting ceria reducibility. Structural studies of ceria on YSZ(1 0 0), using b oth TEM and EDSXD (energy-dispersive, surface X-ray diffraction), demonstra te 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 CeO2, but the coherence length suggests that the overl ayer may have a high defect density. It is suggested that the structure-dir ecting properties of zirconia are responsible for the enhanced properties o f ceria-zirconia mixed oxides. (C) 1999 Elsevier Science B.V. All rights re served.