Spontaneous Fe(III)/Fe(II) redox cycling in single-atom catalysts: Conjugation effect and electron delocalization

The mechanism of spontaneous Fe(III)/Fe(II) redox cycling in iron-centered single-atom catalysts (I-SACs) is often overlooked. Consequently, pathways for continuous SO(4)(·-)/HO⋅ generation during peroxymonosulfate (PMS) activation by I-SACs remain unclear. Herein, the evolution of the iron center and ligand in I-SACs was comprehensively investigated. I-SACs could be considered as a coordination complex created by iron and a heteroatom N-doped carbonaceous ligand. The ligand-field theory could well explain the electronic behavior of the complex, whereby electrons delocalized by the conjugation effect of the ligand were confirmed to be responsible for the Fe(III)/Fe(II) redox cycle. The possible pyridinic ligand in I-SACs was demonstrably weaker than the pyrrolic ligand in Fe(III) reduction due to its shielding effect on delocalized π orbitals by local lone-pair electrons. The results of this study significantly advance our understanding of the mechanism of spontaneous Fe(III)/Fe(II) redox cycling and radical generation pathways in the I-SACs/PMS process.