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

Mn-Fe spinel oxides are considered as promising catalysts for low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), but the operation temperature window severely suffers from their excessive redox properties. Here, a novel mesoporous nanostructured Mn0.5Fe2.5O4 spinel catalyst Mn0.5Fe2.5O4-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 asprepared catalysts were affirmed through comprehensive characterization methods. Compared with the conventional Mn0.5Fe2.5O4 nanoparticle catalyst (Mn0.5Fe2.5O4-P), the Mn0.5Fe2.5O4-S sample exhibited excellent low-temperature De-NOx performance, a wider operation temperature window, lower apparent activation energy, and higher N-2 selectivity. The superior catalytic activity of the Mn0.5Fe2.5O4-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 Mn0.5Fe2.5O4-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 Mn0.5Fe2.5O4-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.