Performance, selectivity, and mechanism in Cu-ZSM-5 lean-burn catalysts

Abstract

Performance of Cu-ZSM-5 materials as three-way control catalysts of emissions from advanced lean-burn automotive engines has been studied by laboratory methods. Control of NO x under thexe oxidizing conditions is poorly precedented and particularly difficult. Fresh Cu-ZSM-5 catalysts exhibit moderate NO x reduction performance over a broad temperature range. The nitric oxide is reduced by hydrocarbon species present in the simulated exhaust, not by carbon monoxide or hydrogen. Propene is more effective than propane. Catalyst compositions of maximum effectiveness for NO x reduction are different when propene or propane is used as the reducing agent. We suggest copper-alkyl species are formed by an oxidative activation and that these intermediates add nitric oxide to form N-nitroso- N-alkylhydroxylamate species bound to cupric centers. Such a mechanism would account for the known requirements of: (1) variability in catalytic effectiveness as the reducing agent varies; (2) the inability of carbon monoxide or hydrogen to engage in selective NO x reduction; and (3) the requirement of oxygen for selective NO x reduction.