A Strategic NH3-Dosing Approach for the Minimization of N2O Production During NH3-SCR Reactions over Cu-SSZ-13 Catalysts

The undesired production of N 2 O during NH 3 -SCR reactions is investigated over a reference commercial Cu-CHA catalyst. Steady-state experiments performed in the 150–500 °C temperature range exhibit a bimodal trend in the N 2 O formation profile, confirming the existence of two different reaction mechanisms occurring at low and high temperatures. Focusing on a low-to-medium T -range, N 2 O production, usually ascribed to NH 4 NO 3 formation and decomposition, increases with the NO 2 /NO x ratio. However, an excess of NO 2 leads to a decrease in the N 2 O release due to ammonium nitrate deposition and catalyst clogging phenomena. Steady-state and dynamics experiments show the promoting effect of both NH 3 feed concentration and NH 3 storage on N 2 O production at T > 200 °C. Surprisingly, N 2 O decreases with increasing NH 3 /NO x ratio at lower temperature. A novel approach based on the strategic injection of NH 3 is also applied to mitigate the N 2 O formation while maintaining high deNO x activity. Remarkably, complete NO x conversion and ~ 11% N 2 O saving are achieved (with inlet NO 2 /NO x = 0–0.5) at temperatures exceeding 200 °C; in addition, a peculiar behavior is observed in the N 2 O profile, which increases and decreases when adding and removing NH 3 from the feed, respectively. Notably, the opposite trend is observed in the N 2 O profile at 200 °C. When under Standard SCR conditions, this so far unreported observation challenges the NH 4 NO 3 formation route for N 2 O and suggests the existence of different controlling phenomena at different temperature regimes: i) the Cu/redox chemistry at T ≤ 200 °C and ii) the NH 3 storage at higher temperature, ideally up to 300 °C.