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More stringent emission regulations have forced an overall systems approach to meeting standards in the most effective manner. An important part of the emissions after-treatment system is the substrate. In recent years, thin wall high cell density substrates have become available. These substrates reportedly offer lower thermal mass and better heat transfer properties for faster light-off and better mass transfer properties for increased performance under stabilized conditions. This paper examines the behavior of high cell density substrate systems under a series of test conditions. A comparison of 400/6, 600/4, 600/3 and 900/2 was carried out both on a bench engine for stabilized and light-off conversion, and also under the ECE/EUDC test procedure. Bench engine results showed significant benefits for HC, CO and NOx emissions when using the higher cell density thinwall substrates.

The current method for on-board monitoring of catalyst performance involves detecting the degradation in the oxygen storage capacity as the catalyst ages. Inherent in this method is the need to correlate the deactivation in HC perform-ance with oxygen storage capacity. However, as HC standards become more stringent, light off becomes the key factor impacting HC emission levels, and it is increasingly difficult to detect failures in HC performance based on OSC deactivation. A possible approach to address this challenge is to include catalyst formulation as a variable in performing OBD-II calibrations. This study explores the potential for tailoring the OBD-II performance of a catalyst by customizing the PGM/OSC component to give the desired degree of thermal stability. The effects of sulfur and aging conditions are also investigated. The potential for independent adjustment of OSC and HC performance is discussed.

The FTP HC, CO, and especially NOx emissions of standard Pd Only three way catalysts have been demonstrated to be more sensitive to fuel sulfur content than Pt/Rh, Pd/Rh, and Pt/Pd/Rh catalysts. To understand the interaction between Pd Only three way catalysts and sulfur, a laboratory study was conducted to examine the relationships between washcoat composition, washcoat structure, and catalyst activity under various evaluation sulfur conditions. First, Pd promoters were examined for their ability to increase activity under both low and high sulfur test conditions. Second, an examination of catalyst structure demonstrated that control of the interaction between Pd and the stabilized ceria washcoat component is critical to achieving high catalyst activity under both low and high sulfur test conditions.

With the advent of stricter vehicle emission standards, the improvement of three way catalyst performance and durability remains a pressing issue. A critical consideration in catalyst design is the potential for variations in fuel sulfur levels to have a significant impact on the ability to reach TLEV, LEV, and ULEV emission levels. As a result, a better understanding of the role of PGM composition in the interplay between thermal durability and sulfur tolerance is required. Three way catalysts representative of standard Pd-only, Pd/Rh and Pt/Rh formulations were studied over a variety of aging and evaluation conditions. The parameters investigated included aging temperature, air fuel ratio and sulfur level. Evaluations were performed on a 1994 TLEV vehicle using different sulfur level fuels. The effect of PGM loading was also included within the study.