Electroreduction of aliphatic chlorides at silver cathodes in water

Abstract

The effect of water on the reduction of aliphatic chlorides at silver cathodes was investigated by cyclovoltammetric experiments in acetonitrile, water and their mixture and focused electrolyses in water solutions. The addition of water to acetonitrile gives rise to the formation of new peaks and/or the displacement of pre-existent peaks at dramatically less negative potentials. In a water medium, silver shows a very large electrocatalytic effect towards the reduction of chlorinated aliphatic compounds belonging to the classes of chloromethanes, chloroethanes and chloroethenes, dramatically higher than that observed in aprotic solvents such as dimethylformamide (DMF) or acetonitrile (ACN). As an example, for 1,2-dichloroethane, a peak at about -2.5 V and -0.9 V vs. SCE are observed in acetonitrile and water, respectively. It was observed that the reduction at silver cathodes of most of the investigated compounds occurs in water medium at less negative potentials with respect to the water discharge, thus allowing their direct reduction and opening interesting perspectives for the electrochemical detection and abatement of chlorinated aliphatic hydrocarbons in water. The electrochemical reduction of 1,1,2,2-tetrachloroethane and 1,2-dichloroethane was also performed by long time electrolyses with high final abatements (>90%). In the case of 1,2-dichloroethane no halogenated by-products were observed while for Cl2HC{single bond}CHCl2, cis- and trans-1,2-dichloroethylene were formed as the main halogenated intermediates and by-products that presented otherwise very low concentrations at the end of the electrolyses.