SELECTIVE REDUCTION OF NITROGEN-OXIDES UNDER OXIDIZING EXHAUST-GAS CONDITIONS

Temperature-programmed reaction traces for Cu/ZSM-5 heated under an oxidising exhaust-gas are not consistent with the hydrocarbon acting as a direct reductant of NO. Steady-state kinetic studies eliminate another possible mechanism (catalytic decomposition of NO), but reveal a near first order dependence of NO conversion on propene partial pressure. The results are reconciled by a surface mechanism in which the hydrocarbon generates a reactive intermediate capable of reducing NO, but which itself can be depleted by oxidation. This intermediate may be a partial oxidation product of the hydrocarbon, or deposited coke; it does not appear to be CO.