| abstract |
A catalyst system for converting NO x , CO, and HC contained in the exhaust gas from a gasoline-fueled engine controlled to operate under lean-burn conditions (R = .03-.9) such that the a ratio of NO x to HC in the exhaust gas is in the range of 1/3 to 3/1 (i.e., minimum C₃H₆ of 250 ppm and NO x of 200-400 ppm). The system comprises a first stage catalyst containing a transition metal-exchanged zeolite (i.e., Cu-ZSM5), and a three-way catalyst for treating the effluent from said first stage catalyst. The second stage catalyst consists of a high surface area alumina support impregnated with an intimate mixture of .5-20% lanthana, .2-30% palladium, and advantageously an outer washcoat TiO₂. n A method of treating the exhaust gas from a fossil-fueled engine comprising operating the engine under lean-burn conditions while controlling the emission ratio of NO x /hydrocarbon to be in the range of 1/3 to 3/1; exposing the exhaust gas to a first stage catalyst comprising a copper ion-exchanged ZSM5 zeolite having at least 3% by weight copper; and exposing the effluent from said first stage catalyst to a second stage catalyst comprising an alumina support impregnated with an intimate mixture of .5-20% lanthana and 2-30% palladium. |