Rationally Tailored Redox Properties of a Mesoporous Mn–Fe Spinel Nanostructure for Boosting Low-Temperature Selective Catalytic Reduction of NO_x with NH₃

Mn–Fe spinel oxides are considered as promising catalysts for low-temperature selective catalytic reduction of NO_x with NH₃ (NH₃-SCR), but the operation temperature window severely suffers from their excessive redox properties. Here, a novel mesoporous nanostructured Mn_0.5Fe_2.5O₄ spinel catalyst (Mn_0.5Fe_2.5O₄-S) with tailored redox properties was synthesized by a facile self-assembly method and applied for NH₃-SCR. The morphological structure and physicochemical properties of the as-prepared catalysts were affirmed through comprehensive characterization methods. Compared with the conventional Mn_0.5Fe_2.5O₄ nanoparticle catalyst (Mn_0.5Fe_2.5O₄-P), the Mn_0.5Fe_2.5O₄-S sample exhibited excellent low-temperature De-NO_x performance, a wider operation temperature window, lower apparent activation energy, and higher N₂ selectivity. The superior catalytic activity of the Mn_0.5Fe_2.5O₄-S catalyst was mainly attributed to its moderate redox properties derived from the unique mesoporous nanostructure with regular dispersed active sites. In situ DRIFTS results indicated that a large amount of −NH₂ species were formed on the Mn_0.5Fe_2.5O₄-S due to the appropriate redox properties. Meanwhile, the optimized redox properties could suppress the unwanted NH₃ oxidation and thus broaden the temperature window in the middle temperature region. DFT calculation results proved that the Mn_0.5Fe_2.5O₄-S catalyst with the preferentially exposed (220) crystal plane exhibited a lower energy barrier for the activation of NH₃ to −NH₂. This should be the key factor for intermediate formation and activity enhancement.