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The relative effectiveness of Pt and Pd in TWC catalysts for converting hydrocarbon (HC) species was investigated using engine dynamometer and vehicle FTP evaluations. An engine-aged Pd/Rh TWC catalyst showed higher HC conversions in Bag-1 than a comparably aged Pt/Rh TWC catalyst even though light-off activity for the Pt/Rh was approximately 40 °C lower than for the Pd/Rh. Analysis of the Bag-1 HC species by capillary gas-chromatography suggested that Pd was more effective in oxidizing both C2-C5 paraffins and aromatic HCs. In Bag-2 and -3, where Pd/Rh HC conversions were lower than those of the Pt/Rh, the Pd/Rh was superior to the Pt/Rh in converting the aromatic HCs. A Pt/Rh at the front (upstream location) and the Pd/Rh at the rear position was found effective for lowering HC emissions in comparison to the Pt/Rh-Pt/Rh converter system while maintaining CO and NOx conversion performances.

Current major concerns m auto exhaust three-way conversion (TWC) catalyst are: 1) improved thermal stability for high temperature applications, such as low emission vehicles (LEV), and 2) high O2 storage capacity for on-board diagnostic (OBD) systems to meet OBD-2 regulations. These are challenges to catalyst technologies posed by the regulations. Of the many possible approaches, stabilization of Rh and CeO2 by ZrO2 shows promise in TWC formulations. This paper summarizes our investigations of thermally stabilized Zr containing TWC catalysts, including the chemistry of CeO2 stabilization with ZrO2, and their OBD-2 characteristics.

A new Pd-Rh three-way catalyst (TWC) for close-coupled (CC) applications was developed to improve low temperature gas activity. In this study the TWC has a layered structure with Pd in the top layer and Rh in the bottom layer. The specific objectives of this study was to compare Ba and La additives to Pd in the top layer. Alumina was used for the Pd support and La or Ba were co-impregnated with Pd. The catalysts were engine aged at 950°C for 200 h and evaluated on a vehicle using the European NEDC test, for CO, HC and NOx performance. After this aging, the Pd-La catalyst showed higher gas performance than the Pd-Ba catalyst, especially in the cold start region. This improvement was correlated to the Pd particle size and the sintering suppression observed upon addition of La. Sintering suppression was also observed upon addition of Ba; however, the mechanism appears to be different from that of La addition.