Monomeric Fe in Six-Membered Rings at Work: Operando Spectroscopy of Nitrous Oxide Activation and Decomposition over Fe-Exchanged Zeolites

The heterogeneous speciation of ion-exchanged zeolites hinders a rational understanding of their reactivity. This is particularly the case for Fe-containing zeolites that are widely used in both the activation and decomposition of N₂O. Herein, we explored the reactive structure of Fe ions in the small pore zeolite SSZ-13 utilizing a combination of operando techniques, including diffuse reflectance UV–vis (DRUV) and infrared (DRIFTS) spectroscopy, as well as electron paramagnetic resonance spectroscopy (EPR), showcasing complementary sensitivity to different structural properties of the catalyst. Coupling these techniques with modulated excitation (ME) and phase-sensitive detection (PSD) enabled site-selective tracking of the behavior of Fe ions and demonstrated (i) the redox reaction of specific Fe ions in six-membered rings (Fe_6MR²⁺↔ Fe_6MR³⁺–O^–) during oxidation (OHC) and reduction (RHC) half-cycles and (ii) the rate-determining nature of the RHC. The PSD response in these spectroscopic experiments emerged as a suitable descriptor of the Fe-SSZ-13 activity toward N₂O decomposition. Changes in Fe speciation obtained by synthesis influenced the capability of the Fe ions to undergo this site-selective redox-mediated mechanism. The direct correlation between the redox kinetics of monomeric Fe sites in the six-membered rings (Fe_6MR, 107 ± 11 kJ mol^–1) and the reaction rate of N₂O decomposition (113.6 ± 16 kJ mol^–1) allowed us to unambiguously assign the global reactivity of Fe-SSZ-13 to these specific Fe ions.