Electrolytic reduction of CCl4 -: effects of cathode Material and potential on kinetics, selectivity, and product stoichiometry

Carbon tetrachloride (CCl4) was reductively transformed via electrolysis at metal cathodes consisting of pure Ag, Al, Au, Cu, Fe, Ni, Pd, and Zn. Primary products included CHCl3, CH2Cl2 and CH4. Traces of CH3Cl, C2H4, C2H6, C3H8, and other unidentified products were also observed in headspace samples. CHCl3 was reduced to CH2Cl2 and CH4 under the same experimental conditions, although at a slower rate. CH2Cl2 was essentially stable during the course of these experiments (k(s) < 0.05 L/m(2) . min). CCl4 reduction was first order in the bulk liquid phase concentration. Pseudo-first-order rate constants were a function of cathode material, pH, and applied cathode potential (E-c) in the range -1.4 V less than or equal to E-c less than or equal to -0.4 V vs. SHE. At all electrodes, reaction rates conformed to Butler-Volmer kinetics, modified to account for mass transfer limitations. Under comparable conditions, the kinetics of CCl4 electrolysis at Ni and Cu electrodes were superior to those of transformation using any other metal tested. At E-c = -0.6 and -0.8 V, rate constants for CCl4 reduction at a Cu electrode decreased with increasing pH in the range 2 < pH < 6. At E-c less than or equal to -1.2 V, reduction was limited by mass transfer to the electrode surface and was independent of cathode material and pH, Electrode efficiency (ratio of dehalogenation rate to total electrolytic current) for CCl4 reduction was inversely related to the material-dependent exchange current density for hydrogen gas generation, suggesting that production of H-2 (g) is the most significant competitive reaction under the conditions of the experiments. Based on efficiency considerations, the electrode materials were ordered Zn > Cu > Fe > Ni.