Electrolytic Reduction of CCl4—Effects of Cathode Material and Potential on Kinetics, Selectivity, and Product Stoichiometry

Abstract

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/m² · 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 ≤ E_c ≤ −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 ≤ −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 H₂ (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. Key words: Reductive dehalogenation; carbon tetrachloride; groundwater remediation; electrolysis