Selective Catalytic Reduction of NO with Ammonia over Fe3+-Exchanged Mordenite (Fe–MOR): Catalytic Performance, Characterization, and Mechanistic Study

Abstract

Fe-exchanged zeolites, i.e., mordenite (MOR), clinoptilolite (HEU), beta, ferrierite (FER), and chabazite (CHA), are studied as catalysts for selective catalytic reduction (SCR) of NO with NH3. It is found that SCR activity decreases in the following order: Fe–MOR>Fe–HEU>Fe–FER>Fe-beta>Fe–CHA. Fe–MOR and Fe–HEU are much more active than the commercial vanadia catalyst. For Fe–MOR, SCR activity increases with a decreasing Si/Al ratio. Moreover, SO2 and SO2+H2O improve SCR activity. On Fe–MOR, nearly 100% NO conversion is obtained at a high space velocity (GHSV=4.6×105 h−1) in the presence of SO2. The Fe–MOR catalysts are also characterized by H2-TPR (temperature-programmed reduction) and Fourier transform infrared (FT-IR) spectroscopy. TPR profiles indicate that iron cations in the catalysts are present as approximately 73% Fe3+ and 27% Fe2+. FT-IR spectra show that NO can be oxidized by O2 to N2O3, NO2, and nitrate adsorbed species, and that they are bonded to the iron cations. NH3 molecules adsorb on Brønsted acid sites of the zeolite to form NH4+ ions. NO+O2 is very active in reacting with NH4+ ions on Fe–MOR at 300°C, but it is less active with those on H–MOR. This is in good agreement with their SCR activities and is probably related to the fast formation of NO2 on Fe–MOR. A possible reaction scheme for SCR reduction is proposed. NO reduction involves the reaction between NO2 and a pair of NH4+ ions to generate an active intermediate, which then reacts with NO to produce N2 and H2O. NO oxidation to NO2 is the rate-determining step.