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Because of their relatively low particulate make, lean burn natural gas vehicles (NGV's) are a viable approach to meeting the ULEV particulate standards in urban environments where NGV's are substituted for diesel powered buses and other fleet vehicles. Our experience with oxidation catalyst technology for natural gas vehicle emissions abatement has been consistent: that palladium based catalysts maintain excellent NMHC activity and particulate reduction, but methane activity, while initially very high, decreases within the first 50 hours of operation. This paper will show that sulfur oxides at sub-ppm concentrations diminish catalyst methane activity, and that inorganic ash components from the lubricating oil (P, Zn, Ca) do not significantly contribute to the initial catalyst deactivation. Using laboratory simulations, we explore systems approaches to increasing catalyst life.

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.