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) ≫3NO (producing N₂), with the same total NO_x 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 NO_x 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.