Hydrothermal Deactivation of Fe-ZSM-5 Prepared by Different Methods for the Selective Catalytic Reduction of NOx with NH3

A series of Fe/ZSM-5 catalysts were prepared by liquid ion exchange, incipient wetness impregnation, and solid-state ion exchange to investigate the selective catalytic reduction (SCR) of NO by NH3 (NH3-SCR). The effect of hydrothermal deactivation of Fe-ZSM-5 catalysts prepared by different methods as a function of Fe loading was investigated. Freshly made and hydrothermal aged Fe-ZSM-5 catalysts were studied through NH3-SCR activity test and characterized using X-ray diffraction, UV-Vis diffuse reflectance spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy. Iron species on the surface of Fe-ZSM-5 catalysts were assigned to isolated Fe3+ species, oligomeric FexOy clusters, and Fe2O3 particles based on the UV-Vis spectra. The iron species distributions in the Fe-ZSM-5 catalysts prepared by these methods were quite different, which resulted in difference in SCR activity. The NH3-SCR activity of different Fe-ZSM-5 catalysts became very similar after aging, and the activity of NH3 oxidation and NO oxidation decreased with the aged catalysts. Characterization results indicated that the activity change of the aged Fe-ZSM-5 catalysts was due to the change of iron species distribution in Fe-ZSM-5 catalysts after hydrothermal aging. The relative concentration of isolated Fe3+ species was decreased, whereas the relative concentration of oligomeric FexOy clusters and Fe2O3 particles was increased in the aged catalysts. A considerable decrease in the Bronsted acidity of catalysts was observed for the aged Fe-ZSM-5 catalysts.