Mechanistic Insight into the Promotion of the Low-Temperature NH₃–SCR Activity over NiMnFeO_x LDO Catalysts: A Combined Experimental and DFT Study

Mn-based catalysts have attracted much attention in the field of the low-temperature NH₃ selective catalytic reduction (NH₃–SCR) of NO. However, their poor SO₂ resistance, low N₂ selectivity, and narrow operation window limit the industrial application of Mn-based oxide catalysts. In this work, NiMnFeO_x catalysts were prepared by the layered double hydroxide (LDH)-derived oxide method, and the optimized Ni_0.5Mn_0.5Fe_0.5O_x catalyst had the best denitration activity, excellent N₂ selectivity, a wider active temperature range (100–250 °C), higher thermal stability, and better H₂O and/or SO₂ resistance. A transient reaction revealed that Ni_0.5Mn_0.5Fe_0.5O_x inhibited the NH₃ + O₂ + NO_x pathway to generate N₂O, which may be the main reason for its improved N₂ selectivity. Combining experimental measurements and density functional theory (DFT) calculations, we elucidated at the atomic level that sulfated NiMnFeO_x (111) induces the adjustment of the acidity/basicity of up and down spins and the ligand field reconfiguration of the Mn sites, which improves the overall reactivity of NiMnFeO_x catalysts. This work provides atomic-level insights into the promotion of NH₃–SCR activity by NiMnFeO_x composite oxides, which are important for the practical design of future low-temperature SCR technologies.