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

Operando infrared (IR) spectroscopy and density functional theory (DFT) calculations were combined to investigate the selective catalytic reduction (SCR) of NOx by NH3 over H-AFX zeolites. The steady-state kinetics shows that SCR reactions involving NO2 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 NO2 with NH3 to form N2, H2O, and NH4NO3 at low temperatures (50–150 °C). In combination with DFT results, these data suggest that NO promotes the reduction of NH4NO3 to NH4NO2, which then decomposes into N2 and H2O. Therefore, the accumulation of NH4NO3 in the zeolite is inhibited by NO. Furthermore, when NO is absent, NH4NO3 decomposition into N2O and H2O 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 NO2 by NH3 and that BASs are responsible for the NH3-SCR reactions involving NO2.