Transformation of hydroquinone to benzoquinone mediated by reduced graphene oxide in aqueous solution

The mediation effect of reduced graphene oxide (rGO) on the oxidative transformation of 1,4-hydroquinone (H(2)Q) to 1,4-benzoquinone (BQ) in aqueous solution was investigated using a batch method and electron paramagnetic resonance. The results showed that the autoxidation of H(2)Q was spin-restricted and extremely slow in acidic and neutral pH range, but this process can be dramatically accelerated when rGO was added. In the presence of 33.3 mg L-1 rGO, more than 76.0% of H(2)Q was oxidized to BQ within 36 h. The enhancement effects of rGO were attributed to the combined contribution of the high chemical reactivity of graphenic edges and defects on rGO and the high electron conductivity of graphene basal surface of rGO. It is proposed that dissolved oxygen reacted with graphenic edges and defects of rGO to produce surface-bound oxygen intermediates, which capture H atoms from the phenolic hydroxyl groups of H(2)Q and facilitate the generation of semiquinone radical (SQ(center dot-)). The generated SQ(center dot-) continued to transfer an electron to molecular oxygen to yield superoxide radical (OF) and BQ. As a chain-carrying radical, OF further reacted with H(2)Q to produce SQ(center dot-) and H2O2. (C) 2015 Elsevier Ltd. All rights reserved.