Highly coordinated Fe–N5 sites effectively promoted peroxymonosulfate activation for degradation of 4-chlorophenol

M–Nx single-atom catalysts (SACs) with a high coordination number (x > 4) are effective catalysts for eliminating organic pollutants, while the origin of SACs with high activity still remains elusive. In this work, we successfully synthesized an Fe–N5 SAC with axial N coordination, which exhibited exceptional catalytic performance by peroxymonosulfate (PMS) activation for degrading 4-chlorophenol (4-CP) in a wide pH range (4.0–10.0). The rate constant of Fe–N5 (2.99 min−1) was 6.36 times higher than that of Fe–N4, and the turnover frequency (TOF) of Fe–N5 was found to be 4–149 times higher than those of state-of-the-art SACs and nanocatalysts reported in the literature for 4-CP degradation by PMS activation. Moreover, Fe–N5 was not significantly affected by coexisting substances (HA, HCO3−, SO42−, H2PO4−, NO3−, and Cl−) and had satisfactory degradation efficiency for various chlorophenols. Electron paramagnetic resonance (EPR), quenching experiments, and radical probe experiments demonstrated that 1O2 played a key role in the Fe–N5/PMS system for 4-CP degradation. Density functional theory calculations confirmed that a narrower gap between the Fe-3d band center and Fermi level enhanced the electron transfer in Fe–N5, which resulted in promoted PMS activation. In addition, the Fe–N5/PMS system showed good potential for application in real wastewater. The above findings offer important implications for the future of coordination chemistry in designing M–Nx–C SACs (x > 4), highlighting their practical applications in environmental remediation.