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

SO4 center dot- and (OH)-O-center dot radicals are important oxidative species that play important roles in the oxidation of organic contaminants by peroxomonosulfate (PMS)/persulfate (PS) and Co3O4 under acidic conditions. However, the facet effect of Co3O4 on the catalytic oxidation of phenol by PS under alkaline conditions has not been investigated yet. In this study, we synthesized three Co3O4 nanoparticles with different facets [rod (110), cube (100) and octahedra (111)]. The Co3O4 rods show the highest catalytic reactivity in the oxidation of phenol by PS, followed by Co3O4 cubes and Co3O4 octahedra. Quenching experiments and EPR analysis rule out contributions of SO4 center dot-, (OH)-O-center dot, O-2(center dot-) and O-1(2) to phenol oxidation and suggest that H-abstraction from phenol to phenoxyl radicals is an important pathway in all Co3O4-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 CO2 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 Co3O4(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 SO4 center dot- and (OH)-O-center dot 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 Co3O4 in the oxidation of phenol, but also gives new insight into turning phenolic wastes into polyphenol "treasures".