Axial oxygen coordination drives spin-regulated electron transfer in single-atom Fe catalysts for selective pollutant transformation

Precise modulation of spin states of single-atom catalysts (SACs) offers a promising route to fine-tune peroxide activation behaviors and selectivity toward different oxidation pathways. Here, we report a spin-tunable Fe SAC composed of iron phthalocyanine (FePc) axially coordinated via oxygen bridges (-O-) onto annealed nanodiamond (AND), denoted as FePc-O-AND. The axial oxygen coordination induces a spin transition from high-spin (t_2g⁵e_g³) to an intermediate-spin (t_2g⁴e_g²) state. This transition generates an unoccupied Fe 3dz² orbital that enables oriented electron transfer to peracetic acid (PAA) via hydroxyl oxygen coordination. In situ synchrotron-based Fourier-transform infrared spectroscopy (SR-FTIR) reveals a distinct PAA activation pathway involving inner-sphere complexation and a non-radical electron-transfer mechanism. As a result, the FePc-O-AND/PAA system drives a non-radical electron-transfer pathway with a high reaction rate (2.11 min^-1), selectively converting phenolic pollutants into high-molecular-weight polyphenolic products (n ≥ 5). Density functional theory (DFT) calculations reveal that axial oxygen coordination in FePc-O-AND enhances PAA adsorption energy (-0.89 eV) and induces a favorable inner-sphere interaction with the hydroxyl oxygen, thereby facilitating effective PAA activation. The FePc-O-AND/PAA system exhibits strong resistance to water matrix interferences and maintains high performance over 130 h of continuous-flow operation. These findings establish axial coordination-mediated spin-state regulation as a powerful strategy for engineering SACs for sustainable water purification and recycling of micropollutants.