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–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–N_x–C SACs (x > 4), highlighting their practical applications in environmental remediation.

中文翻译：

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高度协调的Fe-N5位点有效促进过一硫酸盐活化以降解4-氯苯酚


具有高配位数（x>4）的M-N _x 单原子催化剂（SAC）是消除有机污染物的有效催化剂，但高活性SAC的起源仍然难以捉摸。在这项工作中，我们成功合成了具有轴向N配位的Fe-N ₅ SAC，该SAC通过过一硫酸盐（PMS）活化在宽pH范围内降解4-氯酚（4-CP）时表现出优异的催化性能(4.0–10.0)。 Fe-N ₅ 的速率常数（2.99 min ⁻¹ ）是 Fe-N ₄ 的 6.36 倍，周转频率（TOF） Fe-N ₅ 被发现比文献中报道的通过 PMS 活化降解 4-CP 的最先进的 SAC 和纳米催化剂高 4-149 倍。此外，Fe-N ₅ 并未受到共存物质（HA、HCO ₃ ⁻ 、SO ₄ ²⁻ PO ₄ ⁻ 、 NO ₃ ⁻ 和 Cl ⁻ O ₂ 在Fe-N ₅ /PMS体系中发挥着关键作用用于 4-CP 降解。密度泛函理论计算证实，Fe-3d 能带中心与费米能级之间较窄的间隙增强了 Fe-N ₅ 中的电子转移，从而促进了 PMS 激活。此外，Fe-N ₅ /PMS 系统在实际废水中显示出良好的应用潜力。 上述研究结果为配位化学设计 M–N _x –C SAC (x > 4) 的未来提供了重要启示，突出了它们在环境修复中的实际应用。