Synergistic Effect of Co3O4 Nanoparticles and Graphene as Catalysts for Peroxymonosulfate-Based Orange II Degradation with High Oxidant Utilization Efficiency

Cobalt oxide and graphene nanocomposites (Co3O4/graphene) are fabricated as heterogeneous catalysts to accelerate sulfate radical generation in Orange II degradation. The Co3O4/graphene catalyst is characterized through X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy. Results show that the Co3O4/graphene catalysts are prepared successfully. Co3O4 or graphene solely exhibits slight catalytic activity, but their hybrid (Co3O4/graphene) efficiently degrades and removes Orange II from an aqueous solution in the presence of peroxymonosulfate (PMS). Orange II is completely removed or degraded (100%) within 7 min by using the composite catalysts; by contrast, Orange II is partially removed when Co3O4 or graphene is used alone under the same conditions. These phenomena suggest a synergistic catalytic activity of Co3O4 and graphene in the hybrid. To investigate the causes of the synergistic interactions of the Co3O4/graphene composites, we summarize previous studies and propose an electron transfer pathway between Co3O4 and graphene. We then perform density functional theory calculations to describe the specific features of the composite structures. The hybrid structure is more conductive than the individual semiconductor cobalt oxide clusters because of the hybridization between Co-4d orbital and graphene-p orbital. Fukui indices of electrophilic attack indicate that Co2+, not Co3+, is the active site. Therefore, the PMS activation processes and Orange II degradation pathways are involved in an electrochemical process. Graphene functions as a wire because of its excellent electrical conductivity during oxidation.