Electrochemical Reduction of Cyanides at Metallic Cathodes: A Comparison with Biological HCN Reduction

Abstract

The electrochemical reduction of cyanides has been studied at a number of cathodes both in near-neutral and in alkaline solutions. Nickel appears as the most effective cathode material, promoting cyanide reduction with current efficiencies close to 70%, even in moderately alkaline solutions. In all cases, the eletroreduction of cyanides leads to a mixture of 4e Ϫ ͑methylamine͒ and 6e Ϫ products ͑methane and ammonia͒. The use of Nafion films loaded with Ni microparticles enabled us to markedly increase the effective current densities for cyanide reduction. The electrochemical reduction of HCN/CN Ϫ is shown to present interesting similarities and differences with the biological cyanide reduction process.Until recently, cyanides ͑i.e., CN Ϫ ions and HCN͒ were considered to be unreducible electrochemically in aqueous solution. In fact, the cathodic wave recorded at a platinum electrode immersed in an HCN ͑aq͒ solution was traditionally assigned to hydrogen evolution. 1 Although this conclusion remains essentially correct with regard to the Pt cathode, it cannot be extended to all electrode materials. In fact, we have recently reported 2 that the electroreduction of an almost neutral HCN solution, conducted using a nickel or a copper cathode, results in the formation of substantial amounts of methylamine, methane, and ammonia according to Reactions 1 and 2In contrast with the results of electrolyses conducted with Ni and Cu cathodes, only traces of the cyanide conversion products were formed at a Pt electrode. It is to be noted, however, that nitriles, including HCN, undergo chemisorption on Pt, both in the doublelayer and hydrogen-adsorption regions, and can be partially reduced and then reoxidized in the adsorbed state without formation of solution-soluble products. 3,4 In this paper, the behavior of a number of electrode materials is examined in an attempt to explain the large differences in their electrocatalytic properties toward cyanide reduction. It is shown that some metals and, in particular, nickel, promote also quite efficiently the cyanide reduction in alkaline solutions. Significant faradaic yields are maintained when operating at high ͑apparent͒ current densities using dispersed Ni cathodes. ExperimentalAll experiments were performed in two-compartment cells made either of glass or Teflon. A platinum counter electrode ͑large-area Pt grid͒ was separated from a working electrode either by a Nafion membrane or a ceramic frit. A cross section of metal ͑Ag, Au, Cu, Ni͒ rods, 3 mm diam ͑Specpure 99.999%, Johnson Matthey͒ or larger metal sheets of Pt and Pd, ͑from 0.25 to 1.0 cm 2 ͒ served in various experiments as working electrodes. The electrode surface was prepared by polishing the metals mechanically with fine emery papers and then with suspensions of 1.0 and 0.3 m alumina powders. Alumina was removed from the electrode surface via sonication in an ultrasonic bath and washing with copious amount of distilled water. Before the measurements, the electrode surface was cleaned by cycling the potential into the...

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“…Up until the year 2000, cyanide functional group was considered to be inert to ECH in aqueous solutions. This was later disproved by Milan Fedurco and co‐workers in 2001 by reducing the functional group with metals like Au, Ag, Cu, Ni, Pd and Pt, in the presence of a platinum electrode [131,186] . The reduction of cyanide group lead to formation of the respective amine, which is the 4‐electron product, or methane and ammonia which are the 6‐electron products respectively.…”

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