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. Cu/ZSM-5 based catalysts, unfortunately, have three serious limitations which are likely to prevent its use in mobile applications, namely poor hydrothermal stability, low hydrocarbon selectivity, and sensitivity to sulfur poisoning. The Pt based catalysts are active in a narrow temperature range of 200-300°C, considerably narrower than the target temperature range of 150-350°C required by most diesel engine manufacturers.
In this paper, the performance of some new selective low temperature lean NOx, catalysts will be described. These catalysts showed broader temperatures ranges for NOx, conversion than Pt/Al2O3 with interesting selectivity characteristics. This new lean NOx, technology has also been integrated with diesel oxidation catalysts resulting in 4-Way catalysis. Some engine results describing the performance of the 4-Way catalyst will also be described.