Mechanism of the Br−/Br2 Redox Reaction on Platinum and Glassy Carbon Electrodes in Nitrobenzene by Cyclic Voltammetry

We report the determination of a complete mechanism for the Br−/Br2 reaction in nitrobenzene using simulations of cyclic voltammograms at platinum and glassy carbon macroelectrodes and platinum ultramicroelectrodes at varying scan rates (0.02-500 V/s) and concentrations of Br−, Br3−, and Br2 (2.5-20 mM). As in other nonaqueous solvents and ionic liquids, we observe two consecutive redox processes at lower and higher potentials that we assign to the Br−/Br3− and Br3−/Br2 redox reactions, respectively. A complete reaction mechanism, including the elementary steps of each process, was fit to the voltammetric data using DigiElch® simulation software. The model proposed here is different from previous models of halide reactions in nonaqueous solvents in three ways. First, it proposes the direct oxidation and reduction of Br3−, whereas previous models describe the reduction and oxidation of X3− as a CE process, with the dissociation of X3− into X− and X2 occurring first. Second, it is able to accurately match data at a wide range of scan rates and concentrations without the use of unrealistically large homogeneous rate constants. Finally, it can match data at platinum and glassy carbon electrodes by simply changing the apparent heterogeneous kinetics of the electron transfer reactions.