Unraveling the Fundamentals of Axial Coordination FeN Sites Regulating the Peroxymonosulfate Activation for Fenton-Like Activity.

Precise modulation of the axial coordination microenvironment in single-atom catalysts (SACs) to enhance peroxymonosulfate (PMS) activation represents a promising yet underexplored approach. This study introduces a pyrolysis-free strategy to fabricate SACs with well-defined axial-FeN coordination structures. By incorporating additional out-of-plane axial nitrogen into well-defined FeN active sites within a planar, fully conjugated polyphthalocyanine framework, FeN configurations are developed that significantly enhance PMS activation. The axial-FeN catalyst excelled in activating PMS, with a high bisphenol A (BPA) degradation rate of 2.256 min, surpassing planar-FeN/PMS systems by 6.8 times. Theoretical calculations revealed that the axial coordination between N and the Fe sites forms an optimized axial FeN structure, disrupting the electron distribution symmetry of Fe and optimizing the electron distribution of the Fe 3d orbital (increasing the d-band center from -1.231 to -0.432 eV). Consequently, this led to an enhanced perpendicular adsorption energy of PMS from -1.79 to -1.82 eV and reduced energy barriers for the formation of the key reaction intermediate (O*) that generates O. This study provides new insights into PMS activation through the axial coordinated engineering of well-defined SACs in water purification processes.