Boosting Fe Atom Density and Forming Fe(IV) via Fe–N₅ Generation in the Catalyst for H₂O₂-Based Fenton Reaction

Fe-based single-atom catalysts display extraordinary activity in the Fenton-like reaction, but low Fe atom densities limit their overall catalytic performance. In addition, high-valent iron (as Fe^IV═O) plays a pivotal role due to the selective degradation of contaminants in water. However, it is rarely reported for Fe^IV═O generation in the H₂O₂-based Fenton-like process. This study confirmed that the Fe atom (Fe²⁺ and Fe³⁺) and the Fe cluster coexisted in the as-synthesized catalyst, in which the penta-coordinated Fe atom (Fe-N₅) was found instead of the normal Fe-N₄. The Fe content reaches 13.7 wt % in the catalyst, which is significantly higher than the normal Fe atom density (less than 5 wt %). The ratio of the Fe atom to the Fe cluster is 89.8% at the surface of the catalyst. Furthermore, Fe^IV═O was generated during the H₂O₂-based Fenton-like reaction. The density functional theory and experimental results confirm that the synergistic effect between FeN₅ and the Fe cluster enhanced the catalytic activity and Fe^IV═O formation. The catalyst showed selective degradation for electron-rich contaminants such as tetracycline hydrochloride via the Fenton-like reaction, in which the nonradical pathway by Fe^IV═O dominated the degradation process.