SELECTIVE CATALYTIC REDUCTION OF NITRIC-OXIDE WITH ETHANE AND METHANEON SOME METAL EXCHANGED ZSM-5 ZEOLITES

The selective reduction of nitric oxide by methane or ethane, in the p resence and in the absence of a large excess of oxygen, has been inves tigated on Cu/ZSM-5, Co/ZSM-5, Rh/ZSM-5 and Pt/ZSM-5 catalysts over a wide range of temperatures. It has been found that the maximum nitric oxide conversion is higher with ethane than with methane and the tempe rature of this maximum is lower with ethane. In the absence of oxygen the order of activity is Rh/ZSM-5 > Pt/ZSM-5 > Co/ZSM-5 much greater t han Cu/ZSM-5 with the Cu/ZSM-5 being essentially inactive, while in th e presence of oxygen the order is: Rh/ZSM-5 > Co/ZSM-5 > Cu/ZSM-5 much greater than Pt/ZSM-5 when ethane is used as reductant and: Rh/ZSM-5 > Co/ZSM-5 much greater than Cu/ZSM-5 > Pt/ZSM-5 when methane is used. The effect of the oxygen content has been investigated for the Co/ZSM -5 catalyst. It has been found that with a small quantity of oxygen th e catalytic activity decreases markedly; with higher oxygen content th e activity of the catalyst rises again. It appears that two different reaction schemes may be operative, one in the absence of oxygen the ot her in the presence of oxygen. It is concluded that neither carbonaceo us deposits, nor nitrogen dioxide formation in the gas phase are impor tant in the reaction mechanism on metal-containing zeolites. It is pro posed that the reaction is essentially a redox process in which decomp osition of nitric oxide occurs on reduced metallic or metal ion sites (the relative activity of each of these depending on the choice of met al), leading to the formation of gaseous nitrogen and adsorbed oxygen, followed by the removal of the adsorbed oxygen by the hydrocarbon, th us recreating the active centres.