The oxidation capacity of Mn3O4 nanoparticles is significantly enhanced by anchoring them onto reduced graphene oxide to facilitate regeneration of surface-associated Mn(III).

Metal oxides are often anchored to graphene materials to achieve greater contaminant removal efficiency. To date, the enhanced performance has mainly been attributed to the role of graphene materials as a conductor for electron transfer. Herein, we report a new mechanism via which graphene materials enhance oxidation of organic contaminants by metal oxides. Specifically, Mn3O4-rGO nanocomposites (Mn3O4 nanoparticles anchored to reduced graphene oxide (rGO) nanosheets) enhanced oxidation of 1-naphthylamine (used here as a reaction probe) compared to bare Mn3O4. Spectroscopic analyses (X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy) show that the rGO component of Mn3O4-rGO was further reduced during the oxidation of 1-naphthylamine, although rGO reduction was not the result of direct interaction with 1-naphthylamine. We postulate that rGO improved the oxidation efficiency of anchored Mn3O4 by re-oxidizing Mn(II) formed from the reaction between Mn3O4 and 1-naphthylamine, thereby regenerating the surface-associated oxidant Mn(III). The proposed role of rGO was verified by separate experiments demonstrating its ability to oxidize dissolved Mn(II) to Mn(III), which subsequently can oxidize 1-naphthylamine. The role of dissolved oxygen in re-oxidizing Mn(II) was ruled out by anoxic (N2-purged) control experiments showing similar results as O2-sparged tests. Opposite pH effects on the oxidation efficiency of Mn3O4-rGO versus bare Mn3O4 were also observed, corroborating the proposed mechanism because higher pH facilitates oxidation of surface-associated Mn(II) even though it lowers the oxidation potential of Mn3O4. Overall, these findings may guide the development of novel metal oxide-graphene nanocomposites for contaminant removal.