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, C2H6, 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, CH4) 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 (H2-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.