Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5

Abstract The influence of NO 2 on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO 2 in the feed drastically enhanced the NO x removal efficiency (DeNOx) up to 600 °C, whereas the promoting effect was most pronounced at the low temperature end. The maximum activity was found for NO 2 /NO x  = 50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH 3 :NO:NO 2 ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO 2 /NO x  = 100%. The addition of NO 2 to the feed gas was always accompanied by the production of N 2 O at lower and intermediate temperatures. The absence of N 2 O at the high temperature end is explained by the N 2 O decomposition and N 2 O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO 2 and water suppresses the oxidation of NO to NO 2 , which is an essential preceding step of the actual SCR reaction for NO 2 /NO x 2 is formed with NO and the side-product N 2 O by reaction with gas phase NO 2 .