Enhancement by water vapour of catalytic reduction of NO by propene over mechanically mixed Mn2O3 and Sn-ZSM-5

Abstract

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

Sulfur Tolerance of Selective Partial Oxidation of NO to NO2 in a Plasma

“…On catalysts that are not very effective in catalyzing the equilibration of NO+O 2 and NO 2 , the rate of N 2 formation is substantially higher when the input NO x is NO 2 instead of NO. This has been observed on γ-Al 2 O 3 [1], H-ZSM-5 [1], Na-ZSM-5 [4], Ce-ZSM-5 [4], ZrO 2 [5], Ga 2 O 3 [5], Sn-ZSM-5 [6], InAl 2 O 3 [7], In-MFI [8] and Zn-MFI [9]. It has also been observed that Group II metal oxides in general are much more effective in the SCR of NO 2 compared to NO [10].…”

“…The difficulty in explaining the results for mixture (6) does not invalidate the results given for mixtures (1) to (5). It will be shown that the SO 2 oxidation in mixture (6) has been suppressed by the hydrocarbon. The chemical kinetics calculations are presented in the next section.…”

“…For mixture (6), the level of SO 2 apparently increases after the addition of plasma power. One possible explanation is the interference due to the water vapor produced by the hydrocarbon oxidation.…”

“…For mixture (6), the relative increase in absorbance in the 1136 cm -1 integrated peak may be too much to be accounted for by water from hydrocarbon oxidation. Another possible reason for the absorbance increase in mixture (6) after addition of plasma power is the formation of formaldehyde.…”

“…This value is close to the experimentally observed SO 2 oxidation in Figure 7. Figure 18 shows the concentrations of species produced when a plasma is applied to mixture (6). The O and OH radicals are consumed in reactions with C 3 H 6 .…”