Study on the catalytic activity and hydrothermal stability of one-pot synthesized Fe-based FER zeolites for NH3-SCR†

Abstract

Three OP-Fe-FER-x zeolites (with an FER structure) synthesized by a one-pot method with different Si/Al ratios (x = Si/Al =10–30) and similar iron contents (5.4–5.6 wt%) were compared with commercial IM-Fe/SSZ-20 zeolite (with a CHA structure) prepared by an incipient wetness impregnation method to investigate their catalytic activity and hydrothermal stability for the selective catalytic reduction reaction of NO_x with NH₃ (NH₃-SCR). For both fresh (without hydrothermal aging treatments) and aged (with hydrothermal aging treatments) zeolites, the NO_x conversions of OP-Fe-FER-x zeolites are much higher than that of IM-Fe/SSZ-20 zeolite, suggesting that the OP-Fe-FER-x zeolites are better catalysts for NH₃-SCR than the commercial IM-Fe/SSZ-20 zeolite. Compared with the fresh zeolites, the NO_x conversions of all four aged zeolites decrease in the low-temperature range (100–300 °C), while they increase at high temperatures (450–550 °C) to different extents. Based on XRD, N₂ physisorption, NH₃-TPD, UV-vis, XPS and H₂-TPR experiments, the hydrothermal aging process of the OP-Fe-FER-x zeolites is depicted, which consists of partial collapse of the zeolite framework, removal of framework aluminum and iron atoms, and transformation and aggregation of iron species. Detailed characterization reveals that the OP-Fe-FER-x zeolites are more stable than the IM-Fe/SSZ-20 zeolite during the hydrothermal aging treatments. The obvious decline of NH₃-SCR catalytic activity for the aged zeolites in the low-temperature range is mainly attributed to the transformation and aggregation behaviors of active isolated Fe³⁺ species. As for the increase of NH₃-SCR activity for the aged zeolites at the high temperatures (450–550 °C), it is mainly attributed to the suppression of the NH₃ oxidation side reaction due to the deteriorated NH₃ storage capacity because of the loss of massive acid sites during the hydrothermal aging treatments.