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The first example where the presence of water vapour considerably enhances the rate of the reaction NO, C3H6, O2 -+ N2, COX, H20 in the entire range of reaction temperature (250-500 "C) is found when a mechanical mixture of MnzO3 and Sn-ZSM-5 is used as a catalyst.Catalytic reduction of NO by using hydrocarbons in the presence of an excess of oxygen has attracted much attention. In addition to early reports on Cu-~eolites,l-~ many other catalysts have been reported.4-6 However, the most serious problem encountered is the retarding effects caused by the presence of water vapour which is inevitably contained in combustion effluents. Two such effects are observed; rapid reversible inhibition and slow but irreversible degradation of catalytic activity. Here, we report a catalytic system which solves the former problem caused by water vapour.We have recently reported that the activity of Ce ionexchanged zeolites'-9 for the reduction of NO by propene was enhanced by the mechanical mixing of them with Mn203 or Ce02.10 This promotional effect was explained based on several experimental results by a bifunctional mechanism, in which Mn203 or Ce02 accelerates the oxidation of NO to NO2 and the subsequent steps between NO2 and propene are catalysed by Ce-ZSM-5. If one admits this mechanism, there arise much wider opportunities for the selection of catalysts; that is, the combination of two catalyst components by mechanical or chemical mixing. For example, a certain metal ion-exchanged zeolite that has previously been regarded inactive could be an efficient catalyst when mixed with Mn203 or Ce02.In this work, we examined the combination of various metal ion-exchanged ZSM-5 systems with Mn203 by mechanical mixing and its resistance to water vapour. Mn203 was chosen since there was an indication in our previous study that the activity of Mn203 for the oxidation of NO was relatively resistant to water vapour.Mn203 (6.7 m2 g-I) was mixed in a mortar for ca. 20 min with Sn-ZSM-5 which was prepared by ion-exchange of Na-ZSM-5 (Si02/Al2O3 = 23.8) with an aqueous solution of SnC12. Ce-ZSM-5 prepared as in the previous study was similarly mixed.lo The ion-exchange levels of Sn and Ce were 100 and 15%, respectively. The Mn2O3/M-ZSM-5 ratios in the mechanical mixtures were 1 : 1 in mass. The details of the preparation have been described in a previous paper.10 Hereafter the

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