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The transition from Euro2 to Euro3 emission standards for passenger diesel vehicles requires very good hydrocarbon control coupled with moderate, passive NOx reduction. In some cases, CO and PM conversion is also required. Passive NOx reduction, that is, selective catalytic reduction of NOx by hydrocarbons without delivery of hydrocarbon specifically for this purpose, requires good management of a scarce hydrocarbon resource. At the same time, increased requirements for HC control demand that unconverted HC be further minimized. Appropriately designed lean NOx catalysts with hydrocarbon adsorber capabilities offer very good HC control with moderate NOx reduction performance when fresh. However, certain zeolite structures appear quite unstable under high temperature aging, yielding significant declines in aged NOx performance. Instabilities can be avoided through proper choice of molecular sieve structure and composition, together with suitable washcoat structure.

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.

The pathbreaking papers by Held and Iwamoto has led to a reinvigorated search for lean NOx catalysts worldwide. Extensive effort is currently being expended to develop precious metal lean NOx catalysts, with application intended for diesel engines and low-temperature lean-burn gasoline engines. Here we concentrate on the properties of such a catalyst, referred to as LNX3. The NOx reduction selectivity of LNX3 is examined with model reducing agents. Hydrocarbons are more effective at reducing NOx than CO or H2. Next it is shown that excessively high HC/NOx ratios can degrade the performance of noble metal lean NOx catalysts. This is because the exotherm from the hydrocarbon oxidation can increase the temperature in a significant portion of the catalyst bed beyond the effective temperature window wherein the catalyst is able to reduce NOx.