In Situ/Operando Spectroscopic Studies on the NH3–SCR Mechanism over Fe–Zeolites

Reduction/oxidation half-cycles of the selective catalytic reduction of NO with NH3 (NH3–SCR) over Fe-exchanged mordenite (MOR) zeolites at 300 °C were investigated by in situ/operando spectroscopy (infrared, UV–vis, and Fe K-edge X-ray absorption near edge structure) and density functional theory (DFT) calculation. The reduction of Fe3+ into Fe2+ and the simultaneous formation of N2 and H2O in the reduction half-cycle (under NO + NH3) were demonstrated by different spectroscopic results. In the subsequent oxidation half-cycle (under O2 or NO + O2), Fe2+ was reoxidized into Fe3+. The reduction half-cycle comprises several elementary steps. Reduction of Fe3+–OH by NO producing Fe2+ and NO+ species was observed at low temperatures (<100 °C), while N2 formation due to the reduction of NO+ was observed under subsequent NH3 exposure at 100 °C. Under transient conditions, NH3 on Brønsted acid sites (B–NH3) reacted with NO to generate N2 when the coverage of B–NH3 was low, indicating that B–NH3 is not a spectator but a reservoir of NH3. Transition state calculation theoretically suggested that the formation of nitrous acid (HONO) intermediates from [Fe3+(OH–)2]+ at a Al site and gaseous NO was a facile process (Ea = 29.2 kJ/mol). Combining the experimental observation and DFT calculation, the mechanism of the reduction half-cycle over Fe–zeolites was proposed; [Fe3+(OH–)2]+ is reduced by NO to produce a HONO intermediate, which then reacts with NH3 on Brønsted acid sites to yield H2O and N2 via NO+ species. Based on the mechanistic insights above, Fe–zeolites (MOR and β) with different Fe loadings and Si/Al ratios were tested for NH3–SCR reaction. Consequently, 2.7 wt % Fe-loaded zeolites with a relatively large number of Brønsted acid sites (Al-rich β with a Si/Al ratio of 5) showed the highest NOx conversion in a low-temperature region.