Effect of Ce-modified Fe/ZSM-5 zeolite for selective catalytic reduction of NO_x by ammonia

Key Laboratory of Beijing on Regional Air Pollution Control, Department of Environmental Science, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China

^a Ye Qing: yeqing@bjut.edu.cn

Abstract

A series of xCe-Fe/ZSM-5 (x = 0, 0.25, 0.5 wt%) samples were prepared by the impregnation method, and the catalytic activity was evaluated by the selective catalytic reduction of NO_x with ammonia (NH₃-SCR). The physicochemical properties of prepared samples were characterized by various techniques such as X-ray diffraction (XRD), Brunner-Emmet-Teller (BET) measurement, hydrogen temperatureprogrammed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS), ammonia temperatureprogrammed desorption (NH₃-TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). XRD and BET results demonstrated that Ce and Fe species were uniform dispersed on the surface of the ZSM-5 zeolite and the micropore structure of ZSM-5 was still maintained. H₂-TPR analysis indicated that the doping of Ce created more isolated Ce⁴⁺ and Fe³⁺ on the surface of catalysts, and the abundant Ce⁴⁺ and Fe³⁺ could enhance the reduction ability of catalysts. XPS analysis suggested that the doping of Ce could generate more oxygen vacancies, thereby increasing the number of chemisorption oxygen. According to the in-situ DRIFTS and NH₃-TPD results, Ce species provided more acidic sites, which is beneficial to the NH₃ adsorption ability of ZSM-5 zeolite. Additionally, the abundant chemisorption oxygen, medium and strong Brønsted acid sites, excellent NH₃ adsorption ability and outstanding reduction property are beneficial to the NH₃-SCR reaction. Among all prepared samples, the 0.25Ce-Fe/ZSM-5 sample possessed the widest reaction temperature window and the best catalytic performance (NO conversion over 98% at 350-450 °C), which was associated with the abundant acid sites and remarkable adsorption ability of NH₃, outstanding redox ability and abundant chemisorption oxygen after the doping of Ce.