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A screening of new base metal oxides, which suppress H2S formation in Pt/Rh/CeO2 TWC catalysts, has been conducted using thermodynamic calculations. As a result, several promissing candidates were found, and most of the candidates exhibited H2S suppression effects in an engine H2S formation test. However, many of these candidates for Pt/Rh/CeO2 TWC catalyst negatively affected catalytic performance after thermal and engine agings. Among the candidates, the most prospective candidate as a non-Ni type H2S scavenger was GeO2. Test results of an advanced type Pt/Rh/CeO2 TWC catalyst with GeO2 also showed both H2S suppression effectiveness and no negative effects on catalytic performance. This catalyst is expected to be one of non-Ni low H2S Pt/Rh/CeO2 TWC catalyst candidates for near future applications.

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.

New requirements for catalyst resulting from changes in FTP modes are discussed in this study. Air conditioner(AC) operation results in dramatic changes in both engine-out and tailpipe emissions, especially NOx emission. Even though NOx emission limits will be loosened by 15% in the proposed regulations, the effect of AC operation is much greater than that. Prolonging the intermediate soak time would result in lower catalyst bed temperatures for both the close-coupled and underfloor positions. This results in a delay in light-off by 15 to 35 seconds. Tightening the A/F control under aggressive driving results in an increase in catalyst bed temperature of approximately 150°C. Also, CO and NOx emission breakthroughs are found in high space velocity(SV) operation. Addressing this situation requires improvements in catalyst performance: 1)increase in activity, 2)lower light-off, 3)higher thermal stability and 4)increase in reactivity under extreme A/F and flow rate conditions.

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 reaction mechanism in a NOx-trap type catalyst for a partial lean burn engine is discussed using a thermodynamic calculation approach. The thermodynamic calculation and catalyst characterization suggest the following reaction mechanism; During lean operation, NO2, which was formed from NO oxidation reaction by precious metals, reacted with M-Carbonate(M: NOx Trap Material such as alkali earth elements) to form the corresponding M-Nitrate on the catalyst. When the A/F switches to rich, M-Nitrate decomposed to M-oxide and NO2. Released NO2 was purified to N2. M-oxide reacted with CO2 to form M-Carbonate. Thermodynamic calculation further suggested that NOx trap performance depended on the basicity of added NOx trap material, and evaluation results of the performance in Pt/Rh type catalyst supported this tendency. Furthermore, impacts of catalyst formulations and reaction parameters on NOx trap performance were investigated for identification of the NOx trap reaction mechanism.