Abstract: The catalysts supported by Mn₇Fe₄Ce₉O_x/AlO_x and Mn₃Fe₁Ce₆O_x/AlO_x prepared by co-precipitation method were first evaluated by standard SCR mechanism reaction under 7777 h⁻¹ intake condition. The catalytic activity of the two catalysts was only about 10% at 60 ℃. Then, the concentration ratio of NO₂/NO_x in intake air was increased under the same conditions, and the concentration ratio of NO₂/NO_x in intake air was 0%, 14.3%, 28.6%, 42.8%, 57%, 71.4%, 85.7% and 100%. The results show that the denitrification efficiency of Mn₃Fe₁Ce₆O_x/AlO_x catalyst can be 64% at 60 ℃, which is about 58% higher than that of the first evaluation. Experimental and theoretical calculations show that Mn₃Fe₁Ce₆O_x/AlO_x catalyst has larger specific surface area and stronger adsorption and activation of NO₂, which improves the efficiency of NO₂ fast SCR mechanism reaction. At the same time, the in situ infrared test found that the adsorption mode of Mn₃Fe₁Ce₆O_x/AlO_x catalyst changed significantly when the concentration ratio of NO₂/NO_x increased, and the E-R and L-H mechanism mainly adsorbed by NH₃ changed to the E-R and L-H mechanism mainly adsorbed by NO₂. The change of adsorption mechanism may be the key factor to improve the performance of catalyst at ultra-low temperature. This work provides a promising strategy for exploring efficient and economical denitrification of NH₃-SCR, as well as experience for ultra-low temperature flue gas treatment and guidance for new flue gas treatment processes.