Methods for the formation of M-Nx-C active sites on single-atom catalysts and their role in persulfate activation by non-radical paths

• M-Nx-C active sites, particularly unconventional UM-Nx-C (x≠4), are crucial for non-radical persulfate activation in single-atom catalysts. • Direct and indirect synthesis methods for M-Nx-C sites are systematically reviewed, with emphasis on UM-Nx-C formation. • Catalyst supports such as g-C3N4, MOFs, COFs, and other carbon materials significantly influence the formation and performance of M-Nx-C sites. • The review highlights the role of M-Nx-C in non-radical pathways (e.g., high-valence metals, singlet oxygen, electron transfer) for organic contaminant degradation.