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

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 NOx with NH3 (NH3-SCR). For both fresh (without hydrothermal aging treatments) and aged (with hydrothermal aging treatments) zeolites, the NOx conversions of OP-Fe-FER-x zeolites are much higher than that of IMFe/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 NOx conversions of all four aged zeolites decrease in the low-temperature range (100-300 degrees C), while they increase at high temperatures (450-550 degrees C) to different extents. Based on XRD, N-2 physisorption, NH3-TPD, UV-vis, XPS and H-2-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 degrees 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.