Matching molecular scale with active site spacing induces distinct mechanisms in single-atom catalysts for persulfate activation

Extensive research has been devoted to single-atom activation of persulfates in recent years. However, mechanistic understanding of the distinct interactions between different persulfates ( i.e. , peroxymonosulfate (PMS) and peroxydisulfate (PDS)) and the coordination environments of single-atom catalysts (SACs) remains critical for advancing their practical applications. Herein, we developed a Fe-N 4 SAC exhibiting dual activation capabilities for both PMS and PDS. Intriguingly, experimental results revealed divergent activation mechanisms: PMS activation was predominantly mediated via single-site reactions generating singlet oxygen ( 1 O 2 ), whereas PDS activation proceeded through both dual-site and single-site pathways involving concurrent 1 O 2 generation and electron transfer processes. Density functional theory calculations further demonstrated that the geometric alignment between the inter-site distances of Fe-N 4 centers and the molecular dimensions of PDS serves as the key determinant for enabling the electron transfer pathway. This fundamental structure-reactivity correlation suggests that the intrinsic molecular-scale differences between PMS and PDS govern their distinct interaction mechanisms with Fe-N 4 SACs. Finally, the scale-up experiments realized nearly complete sulfamethoxazole degradation during 120 h continuous operation without obvious decline in both the Fe-N 4 /PMS and Fe-N 4 /PDS systems. This work provides fundamental insights into molecular-scale effects on persulfate activation mechanisms, establishing new design principles for SACs optimization in advanced oxidation processes.