(512b) Intrinsic kinetics of disparate redox pathways for N2O selective catalytic reduction by NO and NH3  over Fe-zeolites

Nitrous oxide (N₂O) is a potent greenhouse gas and ozone-depleting substance emitted from industrial and automotive processes such as NH₃ combustion, threatening to erase environmental benefits of using NH₃ as a decarbonized fuel. Thus, new catalytic technologies are needed to mitigate N₂O. Fe-zeolites are active for NO_x selective catalytic reduction (SCR) with NH₃^[1] and have been shown to activate N₂O for partial methane oxidation,^[2] suggesting Fe-zeolites are promising candidates for N₂O-SCR. However, the kinetics and mechanisms of N₂O-SCR by Fe-zeolites remain incompletely understood.

Here, we prepared a 1 wt.% Fe-chabazite (Si/Al = 12) to investigate the redox pathways of N₂O-SCR by NH₃ and NO. Temperature programmed titration measurements confirm Fe is present entirely as extraframework cations. Steady-state N₂O reduction rates were measured in a differential plug flow reactor. At 623 K, N₂O reduction occurs via two competitive, parallel pathways, involving either NH₃-only (Eqn. 2) or NO+NH₃ (Eqn. 1) as reductants. We varied reaction gas compositions (in either N₂O/NO/NH₃, or N₂O/NH₃) and temperature to gain insights of N₂O-SCR rates and reaction pathway selectivities. N₂O-SCR rates in both gas mixtures are 0 order in NO (Fig. 1a), 1^st order in N₂O (Fig. 1b), and display similar apparent activation energies (~175 kJ mol^-1), consistent with both redox cycles having the same rate-limiting oxidation step. However, selectivity towards Eqn. 1 increases with NO pressure (Fig. 1c), because reduction by NO+NH₃ has an intrinsically 1^st order dependence on NO. Selectivity toward Eqn. 1 decreases with temperature, suggesting the NO+NH₃ pathway has a lower apparent activation energy than the NH₃-only pathway. This work elucidates the kinetics of disparate reduction pathways of N₂O-SCR, aiding design of catalysts for N₂O abatement in applications including NH₃ engines.

1) Yasumura, et al. ACS Catal. 2022, 9983–9993. 2) Bols, et al. J. Am. Chem. Soc., 2018, 140.