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

Abstract

We investigated the effect of the loading amount of Fe over ion-exchanged Fe-MFI catalysts (Fe-MFI) on the catalytic performance in N 2O reduction with CH 4 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 O 2 desorption in the lower temperature range is closely related to the TOF of N 2O reduction with CH 4 under an excess oxygen atmosphere. This suggests that the active site is Fe species which can give a lower temperature O 2-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 CH 4 + N 2O reaction over Fe-MFI (Fe/Al=0.10 and 0.40), Fe 2+ ions can promote a CH 4 + N 2O reaction over both catalysts, and Fe 3+ 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.