Spin-state engineering of confined iron-based catalysts for efficient heterogeneous electro-Fenton process

The confinement effect of the carbon shell highlights its potential for constructing stable heterogeneous electro-Fenton (EF) catalysts; however, conventional Fe-C composites often drive the over-reduction of adsorbed hydroxyl (*OH), compromising their oxidative efficiency. Here, we introduce sulfur into the carbon shell to regulate the spin state of Fe in carbon-encapsulated Fe₂O₃. Synchrotron-based X-ray absorption spectroscopy and temperature-dependent magnetization measurements confirmed that sulfur doping converted Fe from a medium-spin to a low-spin configuration. The resulting low-spin Fe₂O₃@SC exhibited a seven-fold enhancement in intrinsic EF activity for water purification compared with Fe₂O₃@C. Combined spectroscopic analyses and density functional theory calculations revealed that the low-spin state increased the energy gap between the singly occupied molecular orbital of *OH and the highest occupied molecular orbital of the Fe sites, thereby suppressing its over-reduction to OH^-. Moreover, the empty e_g orbitals of low spin Fe facilitated O₂ adsorption, enhancing the O₂ reduction activity in the EF process. This spin-state modulation strategy provides critical insights for constructing sustainable EF catalysts.