Selective catalytic reduction of NO with ammonia over FE³⁺-exchanged mordenite (FE-MOR): Catalytic performance, characterization, and mechanistic study

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.