STUDY OF THE ELECTROCHEMICAL HYDROGENATION OF NITROBENZENE IN Cu AND CuPd ELECTRODES.
Carvajal David a, Arcas Ramón a, Mesa Camilo a, Giménez Sixto a, Mas Elena a, Fabregat Francisco a
a Institute of Advanced Materials (INAM), Universitat Jaume I (UJI), Avenida de Vicent Sos Baynat s/n, Castelló de la Plana, Spain
Proceedings of International Conference on Frontiers in Electrocatalytic Transformations (INTERECT)
València, Spain, 2021 November 22nd - 23rd
Organizers: Elena Mas Marzá and Ward van der Stam
Poster, Carvajal David, 022
Publication date: 10th November 2021

The production of cheap energy from renewable sources, like solar energy, provides the opportunity to use electrochemistry for the synthesis of added-value products in a cost-effective manner. The reduction half-reaction has typically been used for the production of H2, due to its high-density energy per weight unit. Additionally, the reduction of CO2, to energy-rich chemicals (CO, formic acid, etc.) is gaining increasing attention these days. In this line, there are other alternative chemical routes, such as the synthesis of products for the chemical industry, which despite being much less developed, may present a good intrinsic economical interest.

The production of aniline by reduction of nitrobenzene is a very useful transformation, as this species are widely employed as building blocks for the production of aniline-based dyes, explosives, pesticides and drugs. This is a 3-steps mechanism, involving a 6 electron and 6 proton process. Each of these steps involve the insertion of 2e- and 2H+ species in the NO2 group of the molecule. This organic transformation requires the formation (and stabilization) of H* in the electrode surface to reach an effective reduction and hydrogenation. This requirement highlights the importance of developing materials capable of producing H* in order to succeed in this electro-transformation.

Electrodes made of Cu and Cu based compounds have efficiently been used for the electro-reduction of nitrobenzene in aqueous media due to their high energy of activation for the competing hydrogen evolution reaction (HER), thus enhancing the reactivity of the hydrogen radical in the organic reduction and increasing the coulombic efficiency for the organic transformation. Compared with copper, palladium show high activity for the hydrogenation of organic compounds, mainly due to their affinity for the adsorption and storage of H* species.

In this work, decoration of Cu foil surface with Pd by galvanic replacement technique was used to improve the catalytic properties of this material. Using bare Cu, the reduction of concentrated solutions of nitrobenzene has been tested. The introduction of Pd in the Cu surface enhanced the performance and selectivity of the electrode, achieving a complete reduction of a 30 mM nitrobenzene solution in 12 h at -0.8 V (vs Ag/AgCl), obtaining aniline with 70% yield. Finally, a detailed analysis using Impedance Spectroscopy has revealed the improvement in the catalytic performance of Cu with Pd electrodes by increasing the adsorption of hydrogen in the electrode surface, favouring the selectivity in the reduction of nitrobenzene to aniline.

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