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Stringent new emission regulations pose challenges for the development of diesel catalyst technology. Future 4-way catalyst technology must provide for the simultaneous control of CO, HC, NOx and particulate matter (PM). Since NOx abatement is a reduction process while control of CO, HC and PM emissions requires catalytic oxidation, it is difficult to simultaneously control all four regulated pollutants under the oxidizing conditions found in diesel exhaust. New catalysts for lean NOx control have been developed for systems that operate with injection of reductant (active systems) and without injection of reductant (passive systems). In this study, two experimental catalysts which have potential as lean NOx catalysts, Pt/ZSM-5 and an Ir catalyst, were evaluated under both passive and active conditions. Conditions required for simultaneous PM and NOx abatement are discussed for a lean NOx + a conventional diesel oxidation catalyst (DOC) system.

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.

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.