(673c) Elucidating Redox Pathways for Nitrous Oxide Emissions Control over Metal-Zeolite Catalysts

Cu ions exchanged into small-pore Chabazite (CHA) zeolites facilitate the selective catalytic reduction of NO_x with NH₃ in diesel aftertreatment systems. However, they unintentionally produce nitrous oxide (N₂O); N₂O is an ozone-depleting substance and the third most deleterious greenhouse gas (GHG). Thus, new catalytic technologies are urgently needed to simultaneously mitigate NO_x and N₂O emissions. Fe-zeolites display order-of-magnitude lower N₂O emissions during NO_x-SCR, and are known to activate N₂O in stoichiometric partial hydrocarbon oxidation chemistries. Yet, the mechanisms by which Fe-zeolites decompose N₂O in SCR systems in the presence of NO and NH₃ (Figure 1a) remain incompletely understood.

Here, we synthesize model Fe-CHA zeolite materials and investigate their reactivity towards N₂O-SCR reactions involving NH₃ and/or NO as reductants, using steady-state kinetics and redox titrations. We prepared 1.1 wt% Fe-CHA (Fe/Al = 0.22) on a parent H-form CHA zeolite (Si/Al = 12), and validated the exclusive presence of Fe ions using UV-visible spectroscopy and NH₃-temperature programmed desorption (TPD). Steady-state reactivity measurements in a differential laboratory reactor with varying gas mixtures (T = 623 K) reveal disparate redox pathways for N₂O-SCR (Figure 1b), with products quantified using an infrared analyzer. Rates of N₂O-SCR in the presence of NH₃ are 5× higher than rates of N₂O decomposition without NH₃. In the presence of NH₃ alone, N₂O is reduced via Eqn. 1, implying a redox cycle with N₂O as oxidant and NH₃ as reductant. In the presence of NO and NH₃, N₂O consumption occurs via a linear combination of two competitive redox pathways, Eqn. 1 (NH₃ redundant) and Eqn. 2 (NO and NH₃ co-reductants). Redox titrations and reaction order measurements validate the proposed redox cycle and indicate that oxidation is rate-limiting. Our findings provide fundamental insights into N₂O-SCR redox pathways over metal-zeolites for GHG emissions control.