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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.