1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Electro, Physical & Theoretical Chemistry Carbon Nanotube/Graphite Mixtures Used as Cavity Cell Macroelectrodes and their Unexpected Behavior Towards Benzyl Bromide Electroreduction Luciana Santos de Oliveira, [a] Marcelo Navarro, [a] Roge ´rio Tavares Ribeiro, [b] and Eduardo Henrique Lago Falca ˜o* [a] Powder macroelectrodes were prepared by mixing carbon nanotubes (CNT) and graphite in different ratios, and evaluated as electron transfer medium for the benzyl bromide electro- reduction. 1,2-Diphenylethane (DPE) (1-electron process) was favored, instead of toluene (2-electron process), for all CNT:graphite mass ratios, while pure graphite or CNT favored toluene as major product. The 3:7 CNT:graphite mass ratio gave the best result, furnishing 70 % yield of DPE. Cyclic voltammetry showed a cathodic shift for the benzyl bromide reduction potential by using CNT:graphite electrode composition, favor- ing the 1-electron process products during the electrolyses, and corroborating the results observed. Thanks to their outstanding electronic properties, carbon nanotubes (CNTs) have become promising candidates for applications in rechargeable batteries, [1] artificial muscles, [2] electrodes, [3] and many others, either as pure materials or as composites. Electrosynthetic and electroanalytical applications have gained prominence in recent years. One such example is the electrocatalytical homocoupling of 2-halopyridines, carried out in a cavity cell using CNT:graphite mixtures as electrode. [3] This cavity cell setup is versatile, efficient, and environmentally- friendly, and has been used for a wide variety of reactions. [3,4] The setup allows the use of small or large area electrodes. The graphite powder microelectrode (~ 50 mm surface diameter x 20 mm depth) has been used in a large number of electro- analytical studies, [5,6] while the powder macroelectrode (PME, ~ 10 mm surface diameter x 1 mm depth) has been applied in electrosynthetic experiments. These and many other examples demonstrate the importance of combining CNTs with different materials to improve electrosynthetic performance and to better understand their action mechanisms. The electrochemical reduction of benzyl bromide is funda- mentally important because of its mechanistic versatility, and will be used as standard reaction in this work. Depending on whether it proceeds via a 1-electron or a 2-electron pathway (Scheme 1), this reaction can yield 1,2-diphenylethane (DPE) or toluene, respectively. [7–9] The electroreduction of benzyl halides have been studied in different electrode materials and reaction media. For instance, benzyl bromide was electro-reduced in droplets/aqueous [10] and hexane/butanol/water microemulsion [11] electrolytes, indi- cating the formation of the benzyl radical as intermediate. In other studies, the electrochemical reduction of benzyl halides was carried out in electrochemical cavity cell in the absence and presence of solvent (N  N-dimethyformamide), and differ- ent electrode materials: graphite and silver doped graphite, or silver/graphite composite powders. [8,12] In this work, PMEs were prepared by mixing CNTs and commercial powder graphites in different ratios. [7,8] The graph- ites are henceforth denoted as SA or F, depending on the graphite commercial source – see Experimental section. The mixtures (CNT:SA or CNT:F) were used as PME in an electro- chemical cavity cell. The development of sustainable synthetic methodologies for decreasing or eliminating the use of organic solvents and organometallic by-products (green chemistry), and the better understanding of the reaction mechanisms on the electrode interface have motivated these investigations. [8–14] Figure 1 shows SEM images of CNT, SA graphite and a CNT:SA mixture. The majority of the CNT powder is in the form [a] Dr. L. Santos de Oliveira, Prof. M. Navarro, Prof. E. H. Lago Falc¼o Departamento de Química Fundamental Universidade Federal de Pernambuco Av. Prof. Moraes Rego, S/N, Recife, 50670-901, Brazil E-mail: eduardo.lfalcao@ufpe.br efalcao.ufpe@gmail.com [b] Dr. R. Tavares Ribeiro Centro de Tecnologias EstratØgicas do Nordeste Av. Prof. Luis Freire 01, Recife, 50740-545, Brazil Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201700959 Scheme 1. Possible mechanistic pathways for the carbon-halogen direct electrochemical reduction (R = benzyl, X =I, Br or Cl). [7–9] Communications DOI: 10.1002/slct.201700959 7771 ChemistrySelect 2017, 2, 7771 – 7775  2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim