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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.

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.