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Performance improvements resulting from the adoption of a new type CeO2-ZrO2-based material were found using a two-brick three way catalyst (TWC) system. Compared to a conventional CeO2-ZrO2-based material, this new type CeO2-ZrO2 has both a larger oxygen storage capacity and a slower oxygen release rate. Such characteristics were confirmed by fundamental studies. Vehicle evaluations showed this material was most effective for hot NOx control when used in the rear catalyst, especially after fuel cuts. The improvement in NOx was thought to be caused by the increased oxygen storage capacity (OSC), which effectively stored excess oxygen during fuel cuts. As a result, the air-to-fuel ratio (A/F) surrounding the active sites could be kept at stoichiometry even if the engine control shifted to a lean setting.

Beyond the ULEV regulation, a stringent NOx emission regulation like LEV-2 is enacted not only to control smog, but also to reduce environmental pollution like an acidic rain. LEV-2 is a typical example. Rhodium (Rh) has been known for its superior NOx reduction activity but was restricted to limited usage because of its scarcity and high cost. Therefore, it is essential to maximize the Rh function the three-way catalyst (TWC) so that the usage of Rh can be minimized. FTP results indicate that most NOx is emitted during high exhaust-gas flow rate, such as hard acceleration and high-speed cruising. The emission worsened in many cases during deceleration, especially after a fuel cut operation. Under this circumstance, (1) a quick restoration of the active Rh metallic species from an oxide form and (2) having sufficient Rh accessibility to exhaust gases, is important.

In order to improve the NOx performance and thermal stability of Pd-based catalyst, the effect of promoters was studied. The TWC performance, especially CO/NOx, and light-off were dramatically improved by the electronegative promoters. A ranking of the performance agreed with the electronegativity of the promoters. It was found that the PdO decomposition temperature was shifted higher by the promoter and the shift was proportional to the electronegativity of the promoter. Therefore, it was suggested that a Pd sintering would be suppressed by the PdO stabilization. An advanced-2 catalyst design based on the above results showed an excellent performance. The catalysts with this technology have been used for Japanese market and US LEV regulation since 1998. Furthermore, the technology has a potential for enhanced performance that can meet tighter regulation with lower Pd and Rh usage.

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.

A close coupled plus underfloor catalyst system was developed for achieving the ULEV performance targets. Catalyst technologies of various precious metal combinations, as well as substrates of different cell densities and wall thicknesses were investigated. Catalyst systems with relatively high precious metal loadings were required to achieved the ULEV targets after simulated engine aging. High Pd-based three way catalysts (TWC), including trimetal and Pd/Pt catalysts together with substrates of low thermal mass, were demonstrated as the appropriate selections for the close coupled position. Pt/Rh and trimetal catalysts combined with substrates of high cell density, were shown to be the promising candidates for the underfloor position.

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.

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.

A study was made into thermal deactivation of Pt/Rh/Ce three way conversion (TWC) catalyst by the use of aging engines, as well as the changes in catalytic properties developed by thermal deactivation. As a result, crystallization of Pt and CeO2 and the accompanied decrease in O2 storage capacity were found as factors resulting from the thermal deactivation. To minimize the thermal deactivation of the catalyst, CeO2 became a focus of the study and a method to inhibit CeO2 crystallization was pursued. The results of engine high temperature durability testing on improved catalysts employing the new technique confirmed inhibition effects of thermal deactivation. The study successfully resulted in a new TWC catalyst minimizing thermal deactivation.