Conventional V₂O₅-WO₃/TiO₂ catalysts for NH₃-SCR suffer from poor low temperature activity, limited thermal stability, and toxicity associated with vanadium, which restrict their application under flexible operating conditions. In this work, a series of Fe-Ce-Mn ternary oxide catalysts were synthesized by a sol-gel route and evaluated for the NH₃-SCR of NO in the temperature range of 150 to 400 ℃. The optimized composition Fe_0.1Ce_0.04Mn_0.02 delivers more than 90% NO conversion between 150 and 350 ℃, while maintaining N₂ selectivity above 80% even at elevated space velocity and under varied O₂ concentrations and NH₃/NO mol ratio feed ratios, indicating excellent tolerance to operating fluctuations. Structural and surface characterizations reveal uniformly distributed Fe, Ce and Mn species, a high specific surface area of about 160 m²/g, and a mesoporous network dominated by 4 to 5 nm pores, together with enriched surface adsorbed oxygen (O_α) and highly oxidized Fe³⁺/Ce⁴⁺/Mn⁴⁺ species. It synergizes with acidic sites to provide the surface chemistry foundation for NH₃ adsorption activation and NO_x surface conversion. In situ DRIFTS coupled with NH₃-TPD, NO-TPD and DFT calculations reveal that NH₃ preferentially adsorbs onto Lewis acid sites and undergoes deprotonation to form surface *NH₂ species, while NO is converted into monodentate and bridging nitrate species via migratory surface adsorbed oxygen species. These nitrogen containing intermediates coexist isothermally within the 150–350 ℃ range and undergo in situ coupling via the key NH₂NO intermediate, ultimately yielding N₂ and H₂O. Mn incorporation increases the density of electronic states near the Fermi level and significantly lowers the energy barriers for NH₃ dehydrogenation and NH₂ nitrate coupling, Gibbs free energy analysis indicates that the key step in this primary pathway is thermodynamically more favorable (with a lower ΔG), while the electronic structure (PDOS) reveals that charge transfer at the interface occurs more readily. Together, these findings collectively support this dominant reaction pathway. This study provides a rational design strategy for vanadium free NH₃-SCR catalysts with broad temperature window activity.