Active iron sites associated with the reaction mechanism of N 2O conversions over steam-activated FeMFI zeolites

Abstract

The relation between the intrinsic mechanism of various N 2O conversions over FeMFI catalysts and the nature of the active iron site(s) has been analyzed. To this end, direct N 2O decomposition and N 2O reduction with CO in the absence or presence of NO were investigated using a combination of transient pulse and steady-state techniques over steam-activated FeMFI zeolites with a similar iron content (0.6–0.7 wt% Fe) and different framework compositions (Si–Al, Si–Ga, Si–Ge, and Si). The forms of iron in the catalysts were characterized by UV/vis and HRTEM. The intrinsic reaction mechanism determines the optimal iron site distribution, which can be modulated by tuning the steaming temperature during activation. Oligonuclear iron oxo clusters in the zeolite channels are essential in direct N 2O decomposition due to a faster desorption of O 2 as compared to isolated ions. Such forms of active iron can be achieved at a lower steam-activation temperature over FeAlMFI and FeGaMFI (900 K) than over FeGeMFI and FeMFI (1150 K). Contrarily, zeolites with a more uniform distribution of isolated iron species lead to higher activities in N 2O reduction with CO as compared to highly clustered catalysts. In this case, O-removal as CO 2 is strongly accelerated vis-à-vis O 2 desorption in direct N 2O decomposition. The dual role of NO as a promotor in N 2O decomposition and as an inhibitor in N 2O reduction also supports the participation of different sites in both types of conversions. NO selectively inhibits N 2O reduction over isolated iron ions, further evidencing the essential role of oligonuclear iron clusters in the NO-assisted N 2O decomposition.