Spin-State Engineering of Tetrahedral Fe Sites Directs Selective NO Oxidation to Overcome the Activity-Selectivity Trade-off in NH3-SCR

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FeMn-based oxides are widely recognized as one of the most promising catalyst systems for low-temperature selective catalytic reduction (SCR) of NOx with NH3; however, their practical application is constrained by the intrinsic trade-off between activity and selectivity. Herein, Cr was introduced into FeMnOx to regulate the oxidation behavior and reaction pathway through spin-state engineering of Fe sites. Cr doping induces a phase transformation from α-Fe2O3 to γ-Fe2O3 and generates high-spin tetrahedrally coordinated Fe (FeTd) sites. Consequently, FeMnCrOx maintains NOx conversions above 90% over a broad temperature window of 120–255 °C, while increasing N2 selectivity at 175 °C from below 40% to above 90%, with satisfactory resistance toward H2O and SO2. Characterizations and density functional theory calculations reveal that the spin-polarized FeTd sites selectively channel activated oxygen toward NO oxidation rather than NH3 over-oxidation. FeTd serves as the preferential adsorption and activation site for NO, and the enhanced orbital-selective π-backdonation between FeTd and adsorbed reactants (dx2-y2 → NO π*, dxy → O2 π*), enables cooperative NOδ−/O2δ− activation, lowering the NO oxidation barrier from 2.25 to 1.41 eV. The enhanced NO2 formation enables a Lewis-acid-mediated fast SCR pathway. This work establishes a strategy for directing oxidation capability toward desired reaction pathways through spin-state engineering, providing mechanistic insights into the design for overcoming activity-selectivity trade-offs in NH3-SCR catalysis.