Excellent performance of one-pot synthesized Fe-containing MCM-22 zeolites for the selective catalytic reduction of NOx with NH3

Abstract

A superior Fe-containing MCM-22 zeolite prepared by a one-pot synthesis method (OP-Fe/M22) showed excellent NH₃ selective catalytic reduction (NH₃-SCR) activity and almost 100% N₂ selectivity in a wide temperature window (200–500 °C). To investigate the effects of preparation strategies on the NH₃-SCR reaction, other three Fe/MCM-22 zeolites with similar iron content (4.4–4.8 wt%) were prepared by solid-state ion exchange (SSIE-Fe/M22), incipient wetness impregnation (IM-Fe/M22) and mechanical mixing (MM-Fe/M22) methods, respectively. Catalytic results in NH₃-SCR demonstrate that OP-Fe/M22 gives significantly higher activity in the whole temperature range (100–550 °C) than the other three post-synthesized catalysts. As revealed by XRD, UV-vis, H₂-TPR, NH₃-TPD, and Py-IR experiments, OP-Fe/M22 has the highest concentrations of isolated Fe³⁺ species and acid sites, and the lowest concentration of Fe₂O₃ nanoparticles among the four catalysts, which ensures its highest activity in the NH₃-SCR reaction. Besides, OP-Fe/M22 exhibits the higher activity both in NH₃ oxidation and NO oxidation reactions than IM-Fe/M22, indicating the better NO oxidation ability and NH₃ storage capacity of OP-Fe/M22 which also facilitates the NH₃-SCR process. In contrast with Cu-based MCM-22 zeolite obtained from the incipient wetness impregnation method at similar metal content (5% Cu/M22), OP-Fe/M22 shows comparable activity with 5% Cu/M22 in the low-temperature range (100–300 °C) while obviously higher activity at high temperatures (300–550 °C). Moreover, by removing part of undesired iron oxides in OP-Fe/M22 through mild acid post-treatments, the NH₃-SCR activity of OP-Fe/M22 in the medium and high temperature ranges (200–550 °C) can be further improved, suggesting that OP-Fe/M22 is a potential catalyst with highly-efficient NH₃-SCR activity in a wide temperature window.