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Interest in the electrochemical reduction of organic compounds as an alternative to hydrogen electrosynthesis is driven, among other reasons, by the fact that the products obtained in this process may have more economic value than H2. Furthermore, the technique, coupled with renewable sources of electricity, may open the door for large volume green chemical synthesis. In the present article, the electrochemical reduction and hydrogenation of nitrobenzene to aniline at neutral pH is focused upon, using copper electrodes with and without palladium decoration. Both electrodes present good activity, with a decrease in the overpotential for nitrobenzene reduction and the competing hydrogen evolution reaction when Pd is deposited in the surface of the Cu electrode. It is observed that the rate and selectivity of nitrobenzene reduction to aniline improves at high concentrations, obtaining yields close to 70% for 30 × 10−3 m initial concentrations of nitrobenzene when using Pd decorated Cu electrodes. Impedance spectroscopy analysis reveals that the role of Pd in the decorated electrodes is to enhance the production of H* radicals, necessary for the nitrobenzene reduction process. This explains the increase in the selectivity toward aniline production by the incorporation of small amounts of Pd in the surface of the Cu electrode.
Hydrogenation of nitrobenzene to form aniline is a multistep process than can be eased if copper electrodes are decorated with palladium. While first step of the reaction, reduction of nitrobenzene to nitrosobenzene is very efficient on the copper surface, subsequent hydrogenation steps are more efficient on the Pd surface, that supplies these reactions with hydrogen radicals.