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Catalytic reduction of NOx from heavy duty diesel engines via addition of reductant to the exhaust is accompanied by a substantial exotherm in the catalyst bed which does not occur, for example, in a diesel oxidation catalyst. Engine tests show that thermal management in the aftertreatment system is required for optimum reductant use and maximum NOx conversion by the low-temperature (200-300°C) catalyst NSP-5, but of less importance with the high temperature (> 350°C) Catalyst A. Understanding thermal effects is also important for reconciling test results in the near-adiabatic environment of a full-sized catalyst on an engine with the near-isothermal one of a test piece in a laboratory reactor. The effects of reductant type and concentration on NOx conversion on NSP-5 were shown to result in part from non-steady state behavior of the catalyst during steady state engine operation.

The desire for improved fuel economy, and lower emissions of green house gases, such as CO2, is projected to increase the demand for diesel and lean-burn gasoline engines throughout the world. Several commercial diesel oxidation catalysts (DOCs) were developed in the last 3-4 years to reduce hydrocarbon, CO, and particulates emitted from the exhaust of diesel passenger cars and trucks. To meet future U.S. and European NOx standards, it is essential to develop catalyst technology that will allow NOx reduction in addition to the other three pollutants. Two materials that attracted great attention as lean NOx catalysts are the Cu/ZSM-5 and Pt based. Cu containing ZSM-5 are active for lean-NOx reduction at temperatures above 350°C, provided sufficient hydrocarbons are present as reductants.

The technical issues related to NOx abatement for diesel applications are summarized. Data on improved catalysts and a novel approach which involves temporarily trapping of NOx before reduction are presented. New high temperature lean NOx materials have been identified which have better hydrothermal stability than the state of the art Cu/ZSM-5. One of these materials, Catalyst A, was shown to reduce the NOx emitted from a 2.5 L diesel engine at temperatures ≥ 350°C using injected diesel fuel as a reductant. Catalyst A also showed reasonably good durability after aging for 500 h at ca. 500°C on a 14 L diesel truck engine. Pt/Al2O3, a low temperature lean NOx reduction catalyst (200-300°C), demonstrated fairly good performance after 125 h of aging on a 4 L diesel truck engine, however sulfate make and N2O formation are high on this material. New low temperature NOx traps show promise for transient removal of NOx below 200-400°C.