Dual-site reaction mechanism for the simultaneous reduction of nitrous and nitric oxides

In this work, we unveiled the reaction mechanism sustaining the simultaneous Selective Catalytic Reduction of N₂O and NO by NH₃ (N₂O-NO-SCR) using a combination of steady-state and transient catalytic experiments as well as a suite of complementary operando spectroscopies in transient mode, including X-ray absorption spectroscopy, electron paramagnetic resonance and diffuse reflectance infrared spectroscopy.

Our approach allowed to track three concerted reaction cycles occurring on a minority of isolated Fe sites in beta cationic exchange position (Fe_b) and in gamma cationic exchange position (Fe_g) as well as on Brønsted acid sites of a commercial Fe-exchanged Ferrierite (Fe-FER) zeolite. Square-planar Fe_b²⁺ sites are responsible for N₂O activation via transition towards highly oxidizing square-pyramidal Fe_b³⁺ hydroxyls. Owing to the close vicinity of Fe_b and Fe_g sites in the FER cavity, the Fe_b³⁺ hydroxyl oxidizes NO adsorbed on Fe_g, thus promoting the formation of an activated complex, possibly adsorbed HONO, which ultimately triggers the consumption of NH₃ adsorbed on neighboring Brønsted acid sites.

These molecular insights underscore the essential role of the combination of results obtained from several dynamic operando spectroscopic experiments and that N₂O-NO-SCR is a promising conversion strategy thanks to the mutual cooperation between the individual red-ox processes. N₂O aids the NO-SCR reaction by supplying reactive oxidizing sites and favoring NO oxidation, while NO-SCR provides an efficient reduction half-cycle driving enhanced N₂O decomposition.

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