Catalyst system for treating the exhaust from a lean-burn

Abstract

A catalyst system for converting NO.sub.x, CO, and HC contained in the exhaust gas from a gasoline-fueled engine controlled to operate under lean-burn conditions (R=0.03-0.9) such that the a ratio of NO.sub.x to HC in the exhaust gas in in the range of 1/3 to 3/1 (i.e., minimum C.sub.3 H.sub.6 of 250 ppm and NO.sub.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 0.5-20% lanthana, 0.2-30% palladium, and advantageously an outer washcoat TiO.sub.2. 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.sub.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 0.5-20% lanthana and 2-30% palladium.