Structure sensitivity of ion-exchanged Fe-MFI in the catalytic reduction of nitrous oxide by methane under an excess oxygen atmosphere

We investigated the effect of the loading amount of Fe over ion-exchanged Fe-MFI catalysts (Fe-MFI) on the catalytic performance in N2O reduction with CH4 under an excess oxygen atmosphere. It is found that the turnover frequency (TOF) was very low on Fe/Al<0.1; in contrast, the TOF increased drastically with increasing Fe/Al in the range of Fe/Al⩾0.15. In temperature-programmed reduction (TPR) with hydrogen, the reduction started at a lower temperature over higher loading catalysts. In the profiles of temperature-programmed desorption (TPD) of oxygen, two peaks were observed. The lower temperature desorption peak (673–873 K), which was not observed on Fe-MFI (Fe/Al<0.15), appeared on Fe-MFI (Fe/Al⩾0.15). The amount of O2 desorption in the lower temperature range is closely related to the TOF of N2O reduction with CH4 under an excess oxygen atmosphere. This suggests that the active site is Fe species which can give a lower temperature O2-TPD peak, and it is thought that this is a binuclear Fe ion species judging from the loading amount dependence of the properties. Furthermore, according to the pulse CH4 + N2O reaction over Fe-MFI (Fe/Al=0.10 and 0.40), Fe2+ ions can promote a CH4 + N2O reaction over both catalysts, and Fe3+ ions can also promote it over Fe-MFI (Fe/Al=0.40), but not over Fe-MFI (Fe/Al=0.10). This indicates that the reduction and oxidation processes were balanced at a much higher level on Fe-MFI (Fe/Al=0.40) than on Fe-MFI (Fe/Al=0.10). This can be due to binuclear Fe ion species present on Fe-MFI (Fe/Al=0.40), and high catalytic activity can be caused by this property.