Exhaust treating system for lean-burn cng engine

Abstract

A three-way catalyst system for efficiently converting the exhaust gas from a CNG-fueled engine when operated under lean conditions at a redox ratio of 0.02-0.9, the system comprising a first stage catalyst comprising a transition metal-containing zeolite; means for injecting a hydrocarbon into said exhaust gas prior to entry of said exhaust gas into said first stage catalyst, said hydrocarbon having a greater affinity than CH.sub.4 in its ability to react with NO; and a second stage catalyst for treating the effluent from said first stage catalyst and comprising a high surface area alumina impregnated with discontinuous La.sub.2 O.sub.3 and palladium. The invention also comprehends a method of treating exhaust gases from a lean-burn CNG-fueled engine, operating at a redox ratio of 0.02-0.9; exposing such exhaust gases to a first stage catalyst consisting of copper-ZSM5 zeolite having at least 3% by weight ion-exchange copper; injecting a fast-acting hydrocarbon into said exhaust gas prior to entry of the exhaust gas into said first stage catalyst, the hydrocarbon having a greater affinity than CH.sub.4 in its ability to react with NO; and exposing the effluent from the first stage catalyst to a second stage catalyst comprising palladium supported on alumina containing discontinuous La.sub.2 O.sub.3.