High-efficient M-NC single-atom catalysts for catalytic ozonation in water purification: Performance and mechanisms

Heterogeneous catalytic ozonation (HCO) holds promise in water purification but suffers from limited accessible metal sites, metal leaching, and unclear structure-activity relationships. This work reported M-NC (M=Co, Ni, Fe, and Mn) single-atom catalysts (SACs) with high atomic efficiency and minimal metal release. The new HCO systems, especially the Co-based system, exhibited impressive performance in various refractory contaminant removal, involving various reactive species generation, such as •OH_ads, •OH_free, *O, and ¹O₂. For sulfamethoxazole removal, the normalized k_obs for Co-NC, Ni-NC, Fe-NC, and Mn-NC were determined as 13.53, 3.94, 3.55, and 4.13 min^-1·mM_metal^-1·g·m^-2 correspondingly, attributed to the abundant acid sites, faster electron transfer, and lower energy required for O₃ decomposition and conversion. The metal atoms and hydroxyl groups, individually serving as Lewis and Bronsted acid sites (LAS and BAS), were the primary centers for •OH generation and O₃ adsorption. The relationships between active sites and both O₃ utilization and •OH generation were found. LAS and BAS were responsible for O₃ adsorption, while strong LAS facilitated O₃ conversion into •OH. Theoretical calculations revealed the catalytic mechanisms involved O₃→ *O→ *OO→ O₃•^-→ •OH. This work highlights the significance of SAC design for HCO and advances the understanding of atomic-level HCO behavior.