N₂O Activation and NO Adsorption Control the Simultaneous Conversion of N₂O and NO Using NH₃ over Fe-ZSM-5

Fe-exchanged zeolites are heterogeneous catalysts that can potentially ensure simultaneous conversion of nitrous oxide (N₂O) and nitric oxide (NO) using ammonia (NH₃) as a selective reducing agent through their selective catalytic reduction reaction (N₂O-NO-SCR). In this study, we rationalize the origin of the beneficial effect of N₂O on the NO conversion by combining catalytic experiments with <i>ex situ</i> characterization and in <i>situ/operando</i> X-ray absorption spectroscopy (XAS) and infrared spectroscopy in diffuse reflectance mode (DRIFTS) on a series of Fe-ZSM-5 catalysts where we attempted to control Fe speciation at constant Fe content. The catalytic activity data revealed that N₂O can promote NO conversion at different temperatures and to different extents. This behavior was found to be related to the activity of the catalysts in the NO-mediated N₂O decomposition reaction, which ensures the oxidative transformation of NO and thus sustains the N₂O-NO-SCR chemistry. The oxidation activity is in turn determined by processes of N₂O activation and NO adsorption, which are a function of the Fe speciation and are likely catalyzed by a minority of isolated Fe²⁺ sites coordinated in different cationic environments. In agreement, the concentrations of the Fe species able to activate N₂O (<i>C</i>_α) and of the Fe species able to coordinate NO (<i>C</i>_Fe-NO) decrease with an increasing degree of Fe agglomeration and govern especially the promotion of the NO conversion induced by N₂O in this dual-site mechanism. Maximization of the concentration of both species is therefore essential to design Fe-exchanged zeolites with the highest activity toward the N₂O-NO-SCR reaction.