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 degrees 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 degrees C), while N2 formation due to the reduction of NO+ was observed under subsequent NH3 exposure at 100 degrees C. Under transient conditions, NH3 on Bronsted 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 Bronsted acid sites to yield H2O and N2 via NO+ species. Based on the mechanistic insights above, Fe-zeolites (MOR and beta) 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 Bronsted acid sites (Al-rich beta with a Si/Al ratio of 5) showed the highest NOx conversion in a low-temperature region.

Automated summary

The reduction/oxidation half-cycles of Fe-exchanged mordenite zeolites for NH3-SCR of NO were investigated using in situ/operando spectroscopy and density functional theory calculations. The mechanism involves the reduction of Fe3+ to Fe2+ under NO and NH3 exposure, followed by the reoxidation of Fe2+ under O2 or NO + O2. Fe-zeolites with a higher number of Bronsted acid sites showed the highest NOx conversion.