Reaction Mechanism of Selective Catalytic Reduction of NO with NH3 over Fe-ZSM-5 Catalyst

Fe-exchanged ZSM-5 has been found previously to be much more active than commercial vanadia-based catalysts for selective catalytic reduction (SCR) of NO to N2 with NH3. The NO reduction mechanism is studied in this work using combined in situ FTIR (to observe surface-adsorbed species) and online mass spectrometry (to analyze reaction products). NH3 adsorbs quickly on Fe-ZSM-5 to generate NH4+ ions, and the catalyst is quite active in oxidizing NO to NO2 by O2. The temperature-programmed surface reaction (TPSR) from 100 to 400°C shows that the reactivities of NH4+ with NO and NO2 decrease in the order NO+NO2 (1 : 1 ratio, producing N2)>NO2 (producing N2+N2O)⪢NO (producing N2), with the same total NOx concentration for the three cases. This trend is also observed in their reactivities at 200°C. It is concluded that both NO and NO2 are involved in the reaction with NH4+ ions to form N2. Also, the reactivity of NH4+ ions with NO+O2 on Fe-ZSM-5 is much higher than that on H-ZSM-5, but the two catalysts show the same activity for the reaction between NH4+ and NO+NO2. This suggests that NOx reduction probably takes place on the Brønsted acid sites and that the role of Fe3+ irons is to oxidize NO to NO2 by O2. A possible reaction mechanism for NO reduction involves the reaction between one NO2 molecule and two neighboring NH4+ ions to form an active intermediate NO2(NH4+)2, which subsequently reacts with another NO to produce N2 and H2O. The intermediate has been detected by FTIR with a band near 1602 cm−1.