Evaluation of a Cu/Zeolite Catalyst to Remove NOXfrom Lean Exhaust

A Cu/ZSM-5 catalyst has been tested in both the laboratory and in a gasoline-fueled, lean-burn car. This catalyst is generally believed to be the current “state of the art” for removing NO_x from lean engine exhaust. The laboratory tests showed that the HC species was the exhaust component which reduced the NO_x. This catalyst actually produced CO, but the CO had no impact on the NO_x reduction. The NO_x conversion was inhibited by oxygen at oxygen levels below 4%, but was also modestly inhibited by oxygen levels above 4%. The NO_x, conversion of these catalysts was less affected by SO₂ than were the HC and CO conversions. These catalysts showed very poor thermal durability, losing much of their activity after aging under conditions milder than those typically used for three-way catalysts.

The vehicle tests of the fresh Cu/ZSM-5 catalyst showed between 30 and 40% average NO_x conversion. Hence, the car on which this catalyst was tested would have to have engine-out NO_x emissions of less than 0.65 g/mi in order to meet a 0.4 g NO_x/mi tailpipe standard, the most lenient of the 1996 Federal emission standards. Since this catalyst had very poor HC performance, it is likely that it would be difficult to meet the future HC standards if this catalyst were used alone. The durability of the lean NO_x catalyst must also be improved.