Performance of Fe-ZSM-5 for selective catalytic reduction of NOx with NH3: Effect of the atmosphere during the preparation of catalysts

Abstract

Fe-ZSM-5 catalysts for selective catalytic reduction (SCR) of NOx with ammonia were prepared by ion exchange under different atmospheres and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Temperature-programmed desorption of NH3 and NO (NH3-TPD and NO-TPD), Ultraviolet–visible (UV–vis) spectroscopy and in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFT). The results show that the atmosphere during ion exchange and calcination significantly affects the cation exchange capacity and the nature of the Fe species in Fe-ZSM-5 catalysts. An air atmosphere was conducive to improving the ability of ZSM-5 zeolites for Fe ionic exchange. However, isolated Fe3+ sites were dominant in Fe-ZSM-5 catalyst (Fe-Z(N2)) prepared under a N2 atmosphere, and small oligomeric FexOy clusters and FexOy nano-particles were markedly increased in Fe-ZSM-5 catalyst prepared in air, resulting in the higher SCR activity and wide temperature window of Fe-Z(N2). Moreover, Fe-Z(N2) catalyst possessed more Brønsted acid sites and could adsorb more NH3 species, promoting its SCR activity. In situ DRIFT studies showed that NH3 can be adsorbed on both Brønsted and Lewis acid sites on Fe-ZSM-5 catalyst, and this adsorbed NH3 can react with gaseous NOx. Additionally, NOx can be adsorbed on Fe-ZSM-5 catalyst, mainly as a bidentate nitrate, but these nitrate species cannot be confirmed to react with gaseous NH3 because of the overlap with the strong absorption band of adsorbed NH3 species. Based on the IR results and weak adsorption of NOx over Fe-ZSM-5 catalysts, it was concluded that NH3-SCR over this Fe-ZSM-5 catalyst predominantly follows the Eley-Rideal reaction mechanism.