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Increasing interest in lean NOx catalysis at temperatures between about 300-550°C has led to development of catalytic materials with thermal durability considerably improved over academic benchmark catalysts such as Cu-ZSM-5. The breaching of thermal durability barriers brings new obstacles into focus. Practical implementation of high temperature HC-based lean NOx catalysis entails delivery of hydrocarbons to the catalyst inlet at high temperatures. We have found initially unexpected, but scientifically precedented, phenomena regarding gas-phase kinetic instability of hydrocarbons in diesel exhaust atmospheres above 300°C. Around 300°C, homogeneous hydrocarbon oxidation can begin to occur. Rates of oxidation decrease between about 350-450°C and then increase again at higher temperatures. Some apparent NOx disappearance that does not correspond to chemical reduction of NOx can also occur homogeneously throughout this temperature range.

Hydrocarbon conversion across emission control catalysts is a strong function of inlet temperature. The bulk of unconverted hydrocarbon emissions arises in Bag 1 of the FTP-75 cycle before the emission control system goes closed-loop. A general strategy for improving converter hydrocarbon efficiency is to heat up the catalyst early in Bag 1. One strategy for doing this is to place a small catalytic converter near the engine manifold. This approach to hydrocarbon control is well established and represents a production feasible method. This paper explores the use of close-coupled catalysts in conjunction with conventional underfloor converters for achieving the California low emission vehicle standards. The paper identifies catalytic formulations for both converters that optimize emission system performance. The benefits of double wall exhaust pipe connecting the two converters and thin walled substrate for the light-off catalyst were also studied.