水热老化对不同方法制备的Fe-ZSM-5用于NH_3选择性催化还原NO_x的影响

Hydrothermal Deactivation of Fe-ZSM-5 Prepared by Different Methods for the Selective Catalytic Reduction of NO_x with NH_3

采用液体离子交换、等体积浸渍和固相离子交换制备了一系列Fe-ZSM-5催化剂,并将其用于NH3选择性催化还原NOx(NH3-SCR)反应.运用X射线衍射、紫外-可见漫反射吸收光谱和原位漫反射傅里叶变换红外光谱对催化剂进行了表征.结果表明,Fe-ZSM-5催化剂表面Fe物种可分为孤立Fe3+物种、低聚Fe氧化物团簇和Fe2O3,各催化剂上NH3-SCR反应活性不同的根本原因是其表面Fe物种分布不同.水热老化后,Fe-ZSM-5催化剂上400oC以下反应时NOx转化率降低,而高温活性略有提高,各催化剂活性差异减小;同时NH3氧化活性和NO氧化活性均明显降低.这可归因于水热老化使Fe-ZSM-5催化剂表面的孤立Fe3+物种相对浓度降低,低聚Fe氧化物团簇和Fe2O3相对浓度增加,以及催化剂表面Br?nsted酸性位显著减少所致.

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 NOx 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 Brφnsted acidity of catalysts was observed for the aged Fe-ZSM-5 catalysts.