Catalytic reactivity of Co3O4 with different facets in the hydrogen abstraction of phenol by persulfate

SO 4 − and OH radicals are important oxidative species that play important roles in the oxidation of organic contaminants by peroxomonosulfate (PMS)/persulfate (PS) and Co 3 O 4 under acidic conditions. However, the facet effect of Co 3 O 4 on the catalytic oxidation of phenol by PS under alkaline conditions has not been investigated yet. In this study, we synthesized three Co 3 O 4 nanoparticles with different facets [rod (110), cube (100) and octahedra (111)]. The Co 3 O 4 rods show the highest catalytic reactivity in the oxidation of phenol by PS, followed by Co 3 O 4 cubes and Co 3 O 4 octahedra. Quenching experiments and EPR analysis rule out contributions of SO 4 − , OH, O 2 − and 1 O 2 to phenol oxidation and suggest that H-abstraction from phenol to phenoxyl radicals is an important pathway in all Co 3 O 4 -PS systems. The analysis of total organic carbon (TOC) and solid characterizations reveal that more than 96.6 % phenol has been transformed into polyphenols, instead of being mineralized into smaller organic molecules or CO 2 in the rod-PS-phenol system. The DFT (density functional theory) calculations further confirm that PS is more readily activated via a non-radical pathway on the Co 3 O 4 (110) plane with appropriate Co(II)-Co(II) distances, and the activated O O bonds then abstract H atoms from phenol molecules with the production of phenoxyl radicals, as well as SO 4 − and OH radicals. The produced phenoxyl radicals then link together to form dimers, trimers, oligomers and eventually insoluble polymers. This study not only enriches our understanding on the facet effect of different Co 3 O 4 in the oxidation of phenol, but also gives new insight into turning phenolic wastes into polyphenol "treasures".