The role of Brønsted acidity in the SCR of NO over Fe-MFI catalysts

Abstract The selective catalytic reduction (SCR) of NO with isobutane and with NH3 was studied over Fe-MFI catalysts which differ strongly in Bronsted acidity but are similar in Fe content and structure of Fe sites, having shown similar activity in N2O decomposition in related work. The catalysts were prepared by exchange of Na-ZSM-5 (Si/Al ca. 14) with Fe2+ ions formed in situ by acidic dissolution of Fe powder and by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 (Si/Al ca. 30). The characterization of acidic properties by ammonia TPD and by IR of adsorbed pyridine at different temperatures revealed marked differences in acidity between exchanged and steam-activated samples, the latter being (almost) void of strong Bronsted sites. The structural similarity of the iron sites was confirmed by UV–Vis and EPR spectroscopic results. The weakly acidic samples were inferior both in isobutane-SCR and in ammonia-SCR. With isobutane, dramatic differences over the whole range of parameters studied imply a vital role of Bronsted acidity in the reaction mechanism (e.g. in isobutane activation). In NH3-SCR, large reaction rates were achieved with non-acidic catalysts as well, but a promoting effect of acidity was noted for catalysts that contain the iron in the most favorable site structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH4NO2) may be rate-limiting at low temperatures.