Selective reduction of NO with Fe-ZSM-5 catalysts of low Fe content: I. Relations between active site structure and catalytic performance

Fe-ZSM-5 catalysts (0.2–1.2 wt% Fe) were prepared by an exchanging of Na-ZSM-5 with Fe2+ ions formed by the dissolution of iron in acidic medium, and characterized by UV–vis, EPR, and X-ray absorption spectroscopy and by TPR and TEM. Their catalytic properties were investigated for the selective catalytic reduction (SCR) of NO by isobutane (2000 ppm NO, 2000 ppm isobutane, 3% O2, 42,000 h−1) or by NH3 (1000 ppm NO, 1000 ppm NH3, 1% O2, 750,000 h−1). The catalysts were highly active in both reactions, competing favorably with catalysts prepared by chemical vapor deposition of FeCl3 into H-ZSM-5. The spectroscopic studies showed that at Fe contents less than or equal to 0.3 wt%, ca. 95% of the iron was present in mononuclear sites of different coordination. At higher Fe contents, small oligomeric clusters coexisted with mononuclear sites, and at 1.2% Fe, large, poorly ordered Fe oxide aggregates were also detected. By correlation of the activities with the concentration of Fe sites as determined from UV–vis spectra, it was established that mononuclear Fe ions are active sites for both SCR reactions, but oligomers contribute as well. At the same time, oligomers (and aggregate surfaces) are more active in unselective oxidation of the reductant, which limits the temperature window of selective NO reduction. This unselective attack by clustered species occurs at low temperatures with isobutane; hence the best performance was found for a catalyst of low Fe content (0.3 wt%), which is at variance with previous optimization strategies. With NH3, the unselective attack occurs at a much higher temperature; hence the best catalysts for NH3-SCR are those with the highest exposure of Fe sites.