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Electrochemical reduction of nitroaromatic compounds by single sheet iron oxide coated electrodes

Huang, Li-Zhi, Hansen, Hans Christian B., Bjerrum, Morten Jannik
Journal of hazardous materials 2016 v.306 pp. 175-183
aqueous solutions, coatings, drinking water, electrochemistry, electrodes, hydroxides, indium tin oxide, iron, iron oxides, mass transfer, models, nitroaromatic compounds, pH
Nitroaromatic compounds are substantial hazard to the environment and to the supply of clean drinking water. We report here the successful reduction of nitroaromatic compounds by use of iron oxide coated electrodes, and demonstrate that single sheet iron oxides formed from layered iron(II)-iron(III) hydroxides have unusual electrocatalytic reactivity. Electrodes were produced by coating of single sheet iron oxides on indium tin oxide electrodes. A reduction current density of 10 to 30μAcm⁻² was observed in stirred aqueous solution at pH 7 with concentrations of 25 to 400μM of the nitroaromatic compound at a potential of −0.7V vs. SHE. Fast mass transfer favors the initial reduction of the nitroaromatic compound which is well explained by a diffusion layer model. Reduction was found to comprise two consecutive reactions: a fast four-electron first-order reduction of the nitro-group to the hydroxylamine-intermediate (rate constant=0.28h⁻¹) followed by a slower two-electron zero-order reduction resulting in the final amino product (rate constant=6.9μM h⁻¹). The zero-order of the latter reduction was attributed to saturation of the electrode surface with hydroxylamine-intermediates which have a more negative half-wave potential than the parent compound. For reduction of nitroaromatic compounds, the SSI electrode is found superior to metal electrodes due to low cost and high stability, and superior to carbon-based electrodes in terms of high coulombic efficiency and low over potential.