Electrolytic reduction of chlorinated solvents - Kinetics, electrode selectivity, and electrode materials

The electrolytic reductions of CCl4 and other chlorinated solvents on electrodes consisting of elemental Ag, Au, Cu, Fe, Ni, Pd, and Zn were compared in terms of reaction kinetics, product distribution and rates of competitive reactions (predominantly H-2 (g) generation). Aside from the selection of cathode materials, conditions (electrode potential, bulk solution pH) in all reactors were identical. Reaction rates conformed to Butler-Volmer kinetics, modified to account for mass transfer limitation at the cathode surface. As expected, the limiting current was independent of cathode material selection. At higher (less negative) potentials that did not produce mass transfer limitation, rates of CCl4 reduction were metal-dependent with Ni > Cu > Pd > Fe > Au > Ag > Zn. The extent of dehalogenation reactions at the same series of metal cathodes was established by measuring metal-dependent CCl4 reaction products under otherwise identical reactor conditions. On that basis, Ni and Cu electrodes were superior to other metals tested. At E-c=-1.0V (vs. SHE) and pH 7.0, methane production accounted for 80 percent of the CCl4 transformed at the Ni electrode. Based on product identification of reduction of chlorinated ethanes and ethenes, dihalo-elimination and dehydrohalogenation were major reduction pathways for vicinal polychloroethanes and germinal polychloroethanes, respectively. For other chlorinated compounds, electrolytic reductions underwent hydrogenolysis to form less chlorinated or unchlorinated products. Based on selectivity for CCl4 reduction, cathode materials were ordered Zn > Cu > Fe, > Ni, suggesting that electrode selectivity was inversely related to the material-dependent exchange current density for hydrogen gas generation.