Confined Co@NCNTs as highly efficient catalysts for activating peroxymonosulfate: free radical and non-radical co-catalytic mechanisms

A series of transition metal (Co, Ni, Fe) nanoparticles were confined in N-doped carbon nanotubes (NCNTs) prepared (Co@NCNTs, Ni@NCNTs, and Fe@NCNTs) by the polymerization method. The structure and composition of catalysts were well characterized. The catalytic activity of catalysts for activating peroxymonosulfate (PMS) was conducted via acid orange 7 (AO7) degradation. Among the catalysts, Co@NCNTs performed the best catalytic activity. Additionally, Co@NCNTs performed good catalytic activity in pH values of 2.39–10.98. Cl⁻ and SO₄^2- played a promoting roles in AO7 degradation. NO₃⁻ presented a weak effect on the catalytic performance of Co@NCNTs, while HCO₃⁻ and CO₃²⁻ significantly suppressed the catalytic performance of Co@NCNTs. Both non-radical (¹O₂ and electron transfer) and free-radical (·OH and SO₄^·−) pathways were detected in the Co@NCNTs/PMS system. Notably, ¹O₂ was identified to be the main active specie in this study. The catalytic activity of Co@NCNTs gradually decreased after cycle reuse of Co@NCNTs. Finally, the toxicity of the AO7 degradation solution in the study was evaluated by Chlorella pyrenoidosa.