Spin-State Engineering of Fe Single-Atom Catalysts for ¹O₂-Mediated Degradation of Emerging Organic Pollutants

Emerging organic pollutants (EOPs) present structural diversity and complex reactivity in advanced oxidation processes (AOPs), yet the intrinsic physicochemical determinants of their degradation remain unclear. Here, Fe-CN-X catalysts with tunable Fe spin states were constructed by modulating nitrogen coordination, enabling systematic evaluation of pollutant-catalyst interactions. Magnetic susceptibility and electron paramagnetic resonance analyses confirmed that Fe-CN-900 possesses the highest spin state (µ_eff = 2.38), which favors electron occupation in e_g orbitals and increases Fe─O charge density. This electronic configuration markedly enhances PMS adsorption and elevates singlet oxygen (¹O₂) yield from 48% (Fe-CN-700) to 85% (Fe-CN-900). Kinetic studies of 7 representative pollutants revealed a strong correlation between ionization potential and degradation rates. High-spin Fe sites markedly promoted electron transfer from high-ionization-potential pollutants, while pollutant-specific degradation trends highlighted diminishing catalytic enhancement for readily degradable molecules (e.g., BPA) but substantial improvement for recalcitrant species (e.g., NB). Theoretical calculations further demonstrated that pyrrolic N coordination redistributes Fe valence electrons, facilitating ¹O₂-mediated electron transfer through vacant π* orbitals and lowering the electron escape barrier. This work establishes a direct mechanistic link between pollutant electronic properties and catalytic performance, providing theoretical guidance for rational spin-state engineering of catalysts toward efficient and selective degradation of chemically resistant EOPs.