W. Chun

Coarsening of Pt particles in a model NOx trap

The effects of temperature and atmosphere on the coarsening of Pt particles in a model NOx trap (Pt/BaO/Al2O3) were examined by XRD, TEM, and CO chemisorption. The main finding, that the most significant particle growth occurs at elevated temperatures under oxidizing conditions, is relevant to NOx trap durability.

Encapsulation of Pd particles by ceria-zirconia mixed oxides

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.

Development of PGM-free catalysts for automotive applications

The activity of Ag-based catalysts in soot oxidation using NO2 and oxygen as oxidants has been characterized in laboratory tests (TGA) and under real conditions on an engine dynamometer. Under low-temperature NO2-assisted and high-temperature O2-assisted soot oxidation conditions, the activity of Ag-based catalysts was found to be comparable or higher than that of commercial Pt-catalysts. In addition, Ag-based compositions also revealed noticeable NOx storage, some passive NOx reduction ability, and activity in NO oxidation. Ag-catalysts characterized in the present paper may be promising for the retrofit applications and high-temperature periodical regenerations with air for diesel passenger cars.

Thermally stable, high-surface-area, PrOy-CeO2-based mixed oxides for use in automotive-exhaust catalysts

Abstract High-surface-area praseodymia-ceria-based mixed oxides were prepared by an organic- templating method and evaluated as support materials for palladium in model automotive- exhaust catalysts. Results demonstrate that these materials can provide much more oxygen storage capacity than ceria-zirconia at low temperature (≤ 350°C). Addition of zirconium, yttrium, or calcium to praseodymia-ceria increased the surface area and its thermal stability but decreased the low-temperature oxygen storage capacity. Overall, these appear to be viable materials for use in practical catalysts.