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

M-N-x 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-N-5 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-N-5 (2.99 min(-1)) was 6.36 times higher than that of Fe-N-4, and the turnover frequency (TOF) of Fe-N-5 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-N-5 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 O-1(2) played a key role in the Fe-N-5/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-N-5, which resulted in promoted PMS activation. In addition, the Fe-N-5/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-N-x-C SACs (x > 4), highlighting their practical applications in environmental remediation.