Effect of reductants in N₂O reduction over Fe-MFI catalysts

We investigated the effect of reductants over ion-exchanged Fe-MFI catalysts (Fe-MFI) based on the catalytic performance in N₂O reduction in the presence and absence of an oxygen atmosphere. In the case of N₂O reduction with hydrocarbons (CH₄, C₂H ₆, and C₃H₆) in the presence of excess oxygen, the order of N₂O contribution was as follows: CH₄ > C₂H₆ > C₃H₆. This indicates that CH₄ is a more efficient reductant than C₂H₆ and C₃H₆. The TOFs of N₂O decomposition and the N₂O reduction by various reductants (H₂, CO, CH ₄) in the absence of oxygen increased with increasing Fe/Al ratio (Fe/Al≥0.15), wheras the TOFs were lower and constant in the range of Fe/Al≤0.10. Temperature-programmed reduction with hydrogen (H ₂-TPR) showed that the catalysts with a higher Fe/Al ratio were reduced more easily than those with a lower Fe/Al ratio. Temperature-programmed desorption of O₂ (O₂-TPD) showed that oxygen was desorbed at lower temperatures over the catalysts with a higher Fe/Al ratio. As the result of extended X-ray absorption fine structure (EXAFS) analysis, only mononuclear Fe species were observed over Fe(0.10)-MFI after treatment with N₂O or O₂. On the other hand, binuclear Fe species and mononuclear Fe species were observed over Fe(0.40)-MFI after treatment with N₂O or H₂. More reducible Fe species, which gave lower-temperature O₂ desorption, can be due to Fe binuclear species. Since the N₂O reduction with reductants proceeds via a redox mechanism, the reducible binuclear Fe species can exhibit higher activity. Furthermore, CH₄ can be oxidized by N₂O more easily than can H₂ and CO, although it is generally known that the reactivity of methane is very low.