As promising approaches in wastewater treatment, advanced oxidation processes (AOPs) can efficiently degrade and mineralize the organic wastes and contaminants by generating reactive radicals like ·OH, ·SO₄^-, and ·O₂^-. However, due to the stronger oxidability, these reactive radicals exhibit lower selectivity of degradation. Conversely, ¹O₂, which has high selectivity for electron-rich substances degradation by removing electrons, is a weak oxidant with long life and wide pH tolerance. Currently, the commonly used method of producing ¹O₂ is the disproportionation of O₂^-. But the competitive reaction, the Haber-Weiss reaction (·O₂^- + H₂O₂ → ·OH + OH^- + O₂), significantly reduces the yield of ¹O₂. Thus, to activate PMS or PDS by effective catalysts for highly selective and efficient generation of ¹O₂ is necessary. Owing to the atomic dispersion of active sites and higher selectivity, single-atom catalysts (SACs) have great potential to approach that goal. Particularly, the N-doped carbon supported SACs can efficiently activate PMS with markedly improved ¹O₂ generation selectivity. However, the sparse and random distribution of N atoms which act as coordination sites for metal atoms causes the low metal loading and inhomogeneous local coordination environments, and further retards the high-efficient generation of ¹O₂.

Graphitic carbon nitride (g-C₃N₄), which has abundant and uniform N sites as single metal atomic fixation sites, has been considered as ideal support to obtain SACs with uniform and densely loaded atomic dispersion of metal active sites. Nevertheless, the current g-C₃N₄ supported SACs are generally accompanied by metal clusters or even nanoparticles inevitably due to the migration of metal ions during the pyrolysis process, especially in the case of high metal content. Designing an appropriate strategy to prepare high-loading g-C₃N₄ supported SACs with uniform single metal atom sites is an effective approach to facilitate the efficient activation of PMS to generate ¹O₂ with high selectivity.^[1] Recently, writing in Angewandte Chemie International Edition, Zou and colleagues reported that carbon nitride supported high-loading Fe SAC (Fe₁/CN) can efficiently activate PMS to generate ¹O₂ with 100% selectivity. 

The Fe₁/CN catalyst with a loading up to 11.2 wt% is prepared via the supermolecule method.^[2] The Fe₁/CN catalyst is proved to have atomic dispersion of Fe sites with highly uniform Fe-N₄ centers. As a result, compared to CN supported Fe nanoparticles (Fe_NP/CN) catalysts, Fe₁/CN can efficiently activate PMS to generate ¹O₂ with 100% selectivity (Figure 1a). The sole generation of ¹O₂ was proved by radical quenching experiments and electron paramagnetic resonance (EPR) tests (Figure 1b-d). The high efficiency and selective generation of ¹O₂ from Fe₁/CN activated PMS can rapidly degrade and mineralize p-chlorophenol (4-CP) with higher apparent rate constants (K_obs) even more than the homogeneous Fe²⁺ in the PMS system did. Due to the sole generation of ¹O₂, the Fe₁/CN activated PMS system displays stronger resistance to obstruction from natural water, and has wider pH tolerance and better cycle stability. The density functional theory (DFT) calculation results demonstrate that the Fe sites in Fe₁/CN tend to adsorb the terminal O of PMS, which can facilitate the oxidization of PMS to form ·SO₅^-, and thereafter efficiently generate ¹O₂ with 100% selectivity.