Effect of reductants in N 2O reduction over Fe-MFI catalysts

Abstract

We investigated the effect of reductants over ion-exchanged Fe-MFI catalysts (Fe-MFI) based on the catalytic performance in N 2O reduction in the presence and absence of an oxygen atmosphere. In the case of N 2O reduction with hydrocarbons (CH 4, C 2H 6, and C 3H 6) in the presence of excess oxygen, the order of N 2O contribution was as follows: CH 4 > C 2H 6 > C 3H 6. This indicates that CH 4 is a more efficient reductant than C 2H 6 and C 3H 6. The TOFs of N 2O decomposition and the N 2O reduction by various reductants (H 2, 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 O 2 (O 2-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 N 2O or O 2. On the other hand, binuclear Fe species and mononuclear Fe species were observed over Fe(0.40)-MFI after treatment with N 2O or H 2. More reducible Fe species, which gave lower-temperature O 2 desorption, can be due to Fe binuclear species. Since the N 2O reduction with reductants proceeds via a redox mechanism, the reducible binuclear Fe species can exhibit higher activity. Furthermore, CH 4 can be oxidized by N 2O more easily than can H 2 and CO, although it is generally known that the reactivity of methane is very low.