N<sub>2</sub>O Activation and NO Adsorption Control the Simultaneous Conversion of N<sub>2</sub>O and NO Using NH<sub>3</sub> over Fe-ZSM-5
Filippo Buttignol, Alberto Garbujo, Pierdomenico Biasi, Oliver Kröcher, Davide Ferri
Abstract
High Resolution Image Download MS PowerPoint Slide Fe-exchanged zeolites are heterogeneous catalysts that can potentially ensure simultaneous conversion of nitrous oxide (N 2 O) and nitric oxide (NO) using ammonia (NH 3 ) as a selective reducing agent through their selective catalytic reduction reaction (N 2 O-NO-SCR). In this study, we rationalize the origin of the beneficial effect of N 2 O on the NO conversion by combining catalytic experiments with ex situ characterization and in situ/operando 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 N 2 O 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 N 2 O decomposition reaction, which ensures the oxidative transformation of NO and thus sustains the N 2 O-NO-SCR chemistry. The oxidation activity is in turn determined by processes of N 2 O activation and NO adsorption, which are a function of the Fe speciation and are likely catalyzed by a minority of isolated Fe 2+ sites coordinated in different cationic environments. In agreement, the concentrations of the Fe species able to activate N 2 O ( C α ) and of the Fe species able to coordinate NO ( C Fe-NO ) decrease with an increasing degree of Fe agglomeration and govern especially the promotion of the NO conversion induced by N 2 O 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 N 2 O-NO-SCR reaction.