Formation and Reactions of NH4NO3 during Transient and Steady-State NH3-SCR of NOx over H-AFX Zeolites: Spectroscopic and Theoretical Studies

This is the authors' abstract. We don't add key points for this paper.

Operando infrared (IR) spectroscopy and density functional theory (DFT) calculations were combined to investigate the selective catalytic reduction (SCR) of NO x by NH 3 over H-AFX zeolites. The steady-state kinetics shows that SCR reactions involving NO 2 proceed much more rapidly than those of NO. Data from in situ IR combined with online mass spectrometry under transient conditions demonstrate that Brønsted acid sites (BASs) promote the reaction of NO 2 with NH 3 to form N 2, H 2 O, and NH 4 NO 3 at low temperatures (50–150 °C). In combination with DFT results, these data suggest that NO promotes the reduction of NH 4 NO 3 to NH 4 NO 2, which then decomposes into N 2 and H 2 O. Therefore, the accumulation of NH 4 NO 3 in the zeolite is inhibited by NO. Furthermore, when NO is absent, NH 4 NO 3 decomposition into N 2 O and H 2 O occurs only at high temperatures (>200 °C). A comparison of H-AFX and Cu-AFX implies that Cu sites are not active for the reduction of NO 2 by NH 3 and that BASs are responsible for the NH 3 -SCR reactions involving NO 2 .