Elucidation of site-specific red-ox kinetics in the CO-assisted N2O decomposition over Fe–ferrierite by combining modulation excitation with operando EPR spectroscopy†

Abstract

The catalytic conversion of N₂O over Fe-exchanged zeolites is an essential process for controlling its anthropogenic emissions and detrimental environmental impact. In the present study, we monitored an industrial Fe–ferrierite catalyst under conditions of CO-assisted decomposition of N₂O using operando electron paramagnetic resonance (EPR) spectroscopy within the modulated excitation (ME) paradigm. Exploiting this approach, we demonstrated that N₂O decomposition occurs via reversible Fe^II/Fe^III transitions localized exclusively on isolated Fe^II centers located in the β-cationic position, successfully distinguished among various spectator species. The temporal evolution of the reversible β-Fe^II/Fe^III transitions under oxidizing and reducing atmospheres was determined with multivariate curve resolution (MCR) and via double integration of their EPR signal, allowing us to calculate the apparent activation energies for the oxidation and reduction half-cycles. Despite the reaction is controlled by the reduction half-cycle, i.e. N₂O promotes full oxidation of the active β-Fe^II centres irrespective of temperature, the kinetic results indicate that temperature enhances the rate of this oxidation reaction more than the rate of reduction in CO-rich conditions. This study shows that quantitative and qualitative reaction monitoring at sub-minute temporal resolution via operando EPR spectroscopy is possible and sufficient signal-to-noise can be obtained if the experiments are performed according to the ME approach and if phase-sensitive detection (PSD) is employed. Furthermore, our results also indicate that analytical methods, such as MCR, can produce reliable results in the framework of time-resolved EPR spectroscopy.