Electrocatalytic dechlorination of polychloroethylenes at silver cathode

In this study, silver was considered as a catalytic electrodic material for the reduction of polychloroethylenes for possible application in environmental remediation and/or as an auxiliary process in the industrial chlorination of ethylene for production of vinyl chloride monomer. The electroreduction of tetrachloroethylene (PCE) and trichloroethylene (TCE), which are the most hazardous chlorinated ethylenes, was investigated at a Ag electrode in DMF under conditions of controlled proton availability. For the sake of comparison also 1,1-dichloroethylene (1,1-DCE) and 1,2-dichloroethylene were considered. Voltammetric investigations point out that Ag possesses good catalytic activities for the reduction of PCE, TCE, and 1,1-DCE as attested by the remarkable positive shifts (up to 0.46 V) of the reduction potentials with respect to GC, considered to be a non-catalytic material. In contrast, Ag shows no appreciable catalytic effect for 1,1-DCE. Apart from the case of 1,1-DCE, the presence of acetic acid (HAc) exalts the catalytic activity of silver, increasing the anodic shifts of the reduction potentials to about 0.57-0.70 V with respect to GC. Controlled-potential electrolyses have shown that both PCE and TCE are mainly reduced to acetylene and ethylene with overall yields exceeding 95 % after 100 % conversion. The distribution between these two products as well as formation of intermediates, which however remained always quite low, was found to be affected by the presence of proton donors such as H2O and HAc. A common reduction mechanism observed for both PCE and TCE involves the alpha,beta-elimination of two Cl- ions triggered by the electron transfer. This leads to the formation of dichloroacetylene or chloroacetylene, which are further reduced at the electrode according to a sequential hydrodehalogenation leading to acetylene. The latter can be further reduced to ethylene.