Synergistic activation of peroxymonosulfate through oxygen vacancies-rich Ni1.8Co1.2O4/halloysite catalyst for efficient environmental restoration: Performance and mechanism insights

Introducing oxygen vacancies (OVs) is a prospective method to enhance the performance of eliminating persistent organic pollutants via sulfate radical-based advanced oxidation processes. In this paper, a novel Ni_1.8Co_1.2O₄/halloysite catalyst with abundant OVs was synthesized through water bath and calcination followed by non-stoichiometric doping method. By means of optimization and control experiments, the optimized catalyst achieved a norfloxacin (NOR) removal efficiency of 90.3 % within 40 min in the presence of peroxymonosulfate (PMS). The reaction rate constant in Ni_1.8Co_1.2O₄/halloysite-30 %/PMS system was 0.34 min⁻¹, which was around 1.5 times more than that in NiCo₂O₄/halloysite-30 %/PMS system. Halloysite alleviated the agglomeration of ultrafine Ni_1.8Co_1.2O₄ and contributed to the formation of OVs. The generation of OVs furnished supplementary active sites for the reaction and expedited the electron transfer. Radical quenching experiments verified that both ¹O₂ and SO₄^•− were the principal active species during the oxidation process. The enhanced catalytic activity could be ascribed to the fact that a substantial quantity of OVs in Ni_1.8Co_1.2O₄/halloysite-30 %, promoted the generation of ¹O₂ and SO₄^•−, especially ¹O₂. Based on the analysis of HPLC-MS, possible degradation pathways of NOR and the degradation mechanisms were deduced. In brief, this study could offer a new and innovative idea for the advancement of crystal defect engineering on the surface of spinel catalysts within the environmental domain.