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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.

Sulfate generation by diesel oxidation catalysts (DOC) is still a problem although sulfur concentration in the diesel fuel will be reduced in future. Two approaches were attempted to reduce the sulfate generation without inhibiting the HC and CO oxidation performance. One was to use an optimized support material that adsorbs less SO2 and has sufficient specific surface area for HC/CO oxidation. Another approach was to apply a layer on the catalyst, which prevents SO2 adsorption. Sulfate generation was successfully reduced while maintaining high HC/CO oxidation performance.

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.