ENCAPSULATION OF PD PARTICLES BY CERIA-ZIRCONIA MIXED OXIDES

Redox aging above 1000 degrees C of initially high-surface-area (of or der 100 m(2)/g) cerium-rich ceria-zirconia mixed oxides containing a f ew wt% Pd results in the loss of most of the surface area and concomit ant encapsulation of a substantial fraction of the Pd. The encapsulate d Pd exists in the form of metallic particles on the order of 10 nm in diameter held under a large (as much as 3.6 GPa) compressive stress b y the mixed oxide. This stress, evidently arising from changes in latt ice parameter of the mixed oxide, induced by aging, produces a volume contraction of Pd, which appears as a shift of Pd-peak positions in XR D measurements. Subsequent reduction and reoxidation of the mixed oxid e in turn relieves and restores the stress on the Pd particles, which remain metallic throughout, demonstrating that this portion of the Pd is effectively lost for catalytic purposes.