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The surface species formed in the reaction of NO and NO2 with pre-adsorbed NH3 over a FeZSM-5 catalyst (1.27 wt.％ Fe,SiO2/Al2O3 =25) at low temperature (140℃) were studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).Through using a background spectrum of NH3-saturated Fe-ZSM-5,we clearly observed the formation of common intermediates resulting from the reaction of NO2 or NO + O2 with pre-adsorbed NH3.This presents strong evidence that the oxidation of NO to form surface nitrates and nitrites is the key step for standard SCR at low temperature.In addition,the results suggest that in the SCR reaction at low temperature,the NH4+ ions absorbed on Brφnsted acid sites are less active than NH3 adsorbed on Lewis acid sites related to Fe species.