Cu-doped Ni-LDH with abundant oxygen vacancies for enhanced methyl 4-hydroxybenzoate degradation via peroxymonosulfate activation: key role of superoxide radicals

Oxygen vacancies (OVs) were introduced into Ni-based layered double hydroxides (LDHs) through Cu doping, and the catalytic performance of the resulting Ni x Cu-LDHs were investigated for peroxymonosulfate (PMS) activation and methyl 4-hydroxybenzoate (MeP) degradation. Compared with that of Ni-LDH, the catalytic performance of Ni x Cu-LDHs were significantly enhanced and increased with increasing OV content in the catalysts, indicating that Cu doping introduced OVs into Ni x Cu-LDHs and greatly improved their catalytic activity with PMS. Quenching experiments and EPR analyses confirmed that oxidation processes dominated by superoxide radicals (O 2 •− ) and singlet oxygen ( 1 O 2 ), rather than sulfate radicals (SO 4 •− ) or hydroxyl radicals (•OH) used by traditional LDH catalysts, were responsible for MeP degradation by Ni 15 Cu-LDHs. In addition, quenching experiments with different systems showed the fate of reduced SO 4 •− and •OH, and demonstrated that O 2 •− and 1 O 2 concentrations grew with increasing OV content, confirming that the presence of OVs affected the process of PMS activation. Notably, O 2 •− mainly originated from adsorbed oxygen or dissolved oxygen (DO) by acquiring electrons from OVs in Ni 15 Cu-LDHs, since OVs possess abundant localized electrons. Consequently, an OV-mediated oxidative mechanism was proposed for Ni 15 Cu-LDHs/PMS. This study provides new clues for enhancing the catalytic performance of LDH catalysts by introducing OVs via metal doping in PMS-based AOPs systems.