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Reducing NO_<i>x</i> emission in the exhaust gas under the extremely low temperature of engine cold start was important for the ecological environment and human health.MnO₂ has high catalytic activity for NO_<i>x</i> in the low temperature range, which is a promising nanocatalyst.The intrinsic relationship between the surface structural characteristics and denitrification activity of MnO₂-Fe₂O₃-CeO₂/Al₂O₃ nanocatalysts produced by ultrasonic assisted coprecipitation method with different manganese contents was discussed. The results show that when the manganese content is 15%(mass fraction), the nanocatalyst has the largest specific surface area and the smallest grain size. At the same time, the distribution of active substances is the most uniform. Meanwhile, the crystallinity of metal oxides is suppressed, and Mn, Fe and Ce are in the best coexistence state, which can promote the enrichment of chemisorption oxygen. The catalytic oxidation of NO to NO₂ improves the reaction rate of fast SCR, and the NO₂ conversion reaches 88.5% at 50 ℃. Optimizing manganese content is an extremely effective method to improve the structure-activity relationship of catalytic materials and promote the performance of NO preoxidation and fast SCR reaction.