Abstract
•Rate of standard SCR per Fe atom in ZSM-5 exhibits maximum with growing Fe content.•Rate of NO oxidation per Fe atom monotonically decreasing with Fe content.•Rate of fast SCR per Fe atom decreasing with Fe content.•Standard SCR proceeds on different Fe3+ sites (isolated, oligomers).•NO oxidation and fast SCR proceed on minority Fe sites.
Fe-ZSM-5 catalysts (Si/Al in ZSM-5–14) were prepared by different routes and used for an investigation of relations between normalized reaction rates (rates per Fe atom present) and Fe content in reactions relevant for NOx abatement by selective catalytic reduction (SCR) with NH3: standard SCR, fast SCR, and NO oxidation. Samples were characterized by XRD, ex situ UV–vis and EPR spectroscopy. In standard SCR, normalized reaction rates increased with Fe content to a maximum around 0.5wt.% (Fe/Al≈0.07) and decreased at further growing Fe loading. This suggests the existence of Fe sites of different activity, which we identify with isolated Fe3+ sites (on α and/or β cation positions, lower activity) and Fe3+ in small oligomeric structures (higher activity) on the basis of our characterization data and recent results from operando-EPR experiments (Pérez Vélez et al., 2014). The decay with further increasing Fe loading is assigned to Fe being included in more aggregated structures. Normalized reaction rates of NO oxidation to NO2 decrease from very small Fe contents on, which indicates that NO oxidation is catalyzed by a small minority of Fe sites, most likely by a variety of isolated sites, in strong contrast to standard SCR. Normalized reaction rates for fast SCR are difficult to establish due to complexity and high rate of the reaction. The results suggest, however, that fast SCR proceeds on a small minority of Fe sites as well, in agreement with our recent operando EPR and Moessbauer studies (Pérez Vélez et al., 2014). Fast SCR proceeds also in absence of Fe, but rates are inferior and easily exceeded even by contributions arising from Fe impurities usually present in technical zeolites.