Encapsulation of Pd particles by ceria-zirconia mixed oxides

Abstract

Redox aging above 1000°C of initially high-surface-area (of order 100 m²/g) cerium-rich ceria-zirconia mixed oxides containing a few wt% Pd results in the loss of most of the surface area and concomitant encapsulation of a substantial fraction of the Pd. The encapsulated 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 by the mixed oxide. This stress, evidently arising from changes in lattice parameter of the mixed oxide, induced by aging, produces a volume contraction of Pd, which appears as a shift of Pd-peak positions in XRD measurements. Subsequent reduction and reoxidation of the mixed oxide 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.