NO2 FORMATION OVER CU-ZSM-5 AND THE SELECTIVE CATALYTIC REDUCTION OF NO

The extent of the selective catalytic reduction (SCR) of nitric Oxide to dinitrogen in the presence of excess oxygen is enhanced by the oxyg en on several zeolite-based catalysts and using different reductants. When the catalyst is Cu-ZSM-5 and the reductant is a hydrocarbon, an N O2 intermediate has been suggested by several investigators. This work shows that at short residence times, with excess reductant and in the absence of oxygen, the N02 itself is reduced only back to NO. Thus, f or the selective reduction of NO2 to N2 (N-pairing) strongly oxidizing conditions are required, same as for the complete reduction of NO. In the presence of excess oxygen the activity of Cu-ZSM-5 in the NO + O2 reaction to form NO2 parallels the SCR in every respect. It is higher over Cu-ZSM-5 than on Cu/Al2O3 or on H-ZSM-5. The copper-containing z eolite is also active in the decomposition of NO2 back to NO and O2 wh ile the other catalysts are much less active. The inhibiting effect of water on the NO + O2 catalytic reaction is also parallel to the effec t on SCR. This evidence strengthens the notion of an N02 inter mediate .