Investigation of selective catalytic reduction of N ₂O by NH ₃ over an Fe-mordenite catalyst: Reaction mechanism and O ₂ effect

We systematically investigated the reaction mechanism and effect of O ₂ on N ₂O reduction by NH ₃ over an Fe-Mordenite (MOR) catalyst. O ₂ has no inhibitory effect on N ₂O reduction, and NH ₃ selective catalytic reduction (SCR) of N ₂O is superior to NH ₃ oxidation by O ₂. We found that the mechanism of NH ₃ SCR of N ₂O involves the redox cycle of Fe(III)-OH sites, with Fe(III)-OH reduction by NH ₃ as the first and rate-determining step. Then N ₂O is activated at the reduced Fe(II)-OH sites into NO/N or N ₂/O, reoxidizing the Fe(II)-OH into Fe(III)-OH sites. Next, the NO formed in situ reacts with adsorbed NH ₂ to form NH ₂NO, which further decomposes to N ₂ and water. In addition, some NO may join with O to form NO ₂, which reacts with NH ₄ ⁺ to produce NH ₄NO ₂ and further decomposes to N ₂ and water. It is possible that under the steady state, N-NO breaking accounts for two-thirds of N ₂O splitting. The formation of NO intermediates plays a crucial role in this reaction. The structural arrangement of MOR zeolites and the high content of Fe ions provides two proximal Fe ions, that is, Fe(III) ···Fe(III) pairs, as the active sites for this N-NO breaking, resulting in the high activity of Fe-MOR.