Effect of Preparation Method on Catalytic Performance of Fe-Cu-ZSM-5 for Selective Catalytic Reduction of NO_x by NH₃

Nitrogen oxides is one of the major pollutants in diesel exhaust, which has caused serious environmental pollution. The selective catalytic reduction (SCR) is the most efficient way to control NO_x emission, and the key is the catalysts with high activity. Recently, zeolites-based catalysts have been attracted more attention because of their special pore structure and catalytic performance. In this paper, impregnation, liquid-state ion exchange, and liquid-state ion exchange followed by impregnation methods were employed to prepare Fe-Cu-ZSM-5 catalyst. Over 80% of NO conversion can be achieved at the temperature region of 188-530℃, 200-520℃, and 240-530℃ for catalyst Fe_0.7-Cu_0.97-ZSM-5(IWI), Fe_0.59-Cu_1.0-ZSM-5(IE) and Fe_0.65(IWI)-Cu_0.95(IE)-ZSM-5, respectively. It was found that the catalyst Fe_0.7-Cu_0.97-ZSM-5 prepared with impregnation method exhibited the best catalytic performance for SCR reaction. However, the Fe_0.65-Cu_0.95-ZSM-5 catalyst prepared with liquid-state ion exchange followed by impregnation exhibited the lowest catalytic performance. The catalysts were characterized by XRD, UV-vis, H₂-TPR, XPS, NH₃-TPD, and NO oxidation. It was revealed that the catalyst Fe_0.7-Cu_0.97-ZSM-5(IWI) had more isolated Cu²⁺ and Fe³⁺ species at the exchange site, which are the active species for NH₃-SCR reaction at the low-temperature region. In addition, the catalyst Fe_0.7-Cu_0.97-ZSM-5(IWI) had the stronger redox ability and NO oxidation ability, as well as more acid sites on catalyst surface compared with catalyst Fe_0.59-Cu_1.0-ZSM-5(IE) and Fe_{0.65 (IWI)}-Cu_{0.95 (IE)}-ZSM-5. Above all, the states of active Cu and Fe species, reducibility of the catalyst and acidic properties on the catalyst surface are the critical factors for the catalytic performance of the Fe-Cu-ZSM-5 catalysts.