Electrocatalytic properties of transition metals toward reductive dechlorination of polychloroethanes

Abstract

Chlorinated volatile organic compounds (VOCs) such as polychloromethanes and polychloroethanes (PCEs) are among the most ubiquitous pollutants in the environment. Reductive electrochemical dechlorination at catalytic cathodes is one of most promising methods of abatement of these pollutants. In this study, the electrochemical reduction of several PCEs has been investigated in DMF + 0.1 M (C₃H₇)₄NBF₄ at Ag, Au, Pd, Pt, Cu, Fe, Ni, Pb, Zn and glassy carbon (GC), the last used as a reference system with negligible catalytic properties. The reduction potentials of all investigated PCEs are remarkably influenced by the nature of the electrode material and the chemical structure of the compound. In any case, Ag, Cu, and Au show powerful electrocatalytic activities. Based on voltammetric investigations on all PCEs and electrolyses of 1,1,1-trichloroethane and 1,1,1,2-tetrachloroethane in different conditions, the reduction mechanism is shown to be strongly dependent on the nature of the PCE. Geminal PCEs undergo a sequential hydrodechlorination mechanism in which one chlorine atom is lost in each reduction step until a completely dechlorinated ethane is obtained. In contrast, reduction of vicinal PCEs involves removal of two chlorine atoms in an overall 2e⁻ process resulting in the formation of the corresponding (chloro)ethylene, which may be further reduced at more negative potentials.

Introduction

Chlorinated volatile organic compounds (VOCs) are among the most ubiquitous environmental pollutants owing to their widespread use as solvents and raw materials in industrial applications, and their poor biodegradability [1]. These compounds are also among the most toxic pollutants, some of them being suspected carcinogens, and are recalcitrant to the common remediation technologies. Therefore, some of them are included in the US Environmental Protection Agency (EPA) priority list of pollutants [2] and in the European Commission list of priority substances [3]. Several abatement techniques based on physical, chemical, as well as biological methods have been developed for these pollutants over the last decades. These include adsorption by activated carbon, air stripping, bio-degradation under aerobic [4], [5] or anaerobic conditions [6], [7], photocatalytic degradation [8], chemical reduction by zero valent metals or metal alloys [9], [10], [11], electrochemical oxidation [12], [13], electrochemical reduction [14] and combined electrochemical oxidation-reduction processes [15].

Recently, electrocatalytic reduction of chlorinated VOCs has been widely investigated at various electrode materials in different media, including water, aprotic solvents and aqueous solvent mixtures [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28] not only for reasons of pollution remediation but also from the standpoints of reduction mechanism [20], [21], [22], [23] and electrosynthesis [23], [24], [25], [26]. These studies have revealed that the nature of the cathode material plays a crucial role in dechlorination efficiency and overall reaction mechanism [21], [22], [28], [29]. In particular, electrode materials with remarkable catalytic activities are required in order to reduce the high overpotential associated with the dissociative electron transfer to CCl bonds and thus avoid H₂ evolution in H₂O. Indeed, much attention has already been devoted to this important issue, investigating a number of metals and composite materials modified with metal catalysts [17], [30], [31], [32]. Among the various metals investigated, Ag, Cu and Pd have been found to exhibit splendid electrocatalytic activities toward the reduction of organic halides, with remarkable positive shifts of reduction potentials and high current efficiencies [21], [28], [30], [31].

The reduction mechanism is also affected by the solvent and the proton availability of the reaction medium. According to our own research [22], [23], electrochemical dechlorination of polychloromethanes takes place through two different reaction routes: sequential hydrodechlorination leading to methane through successive removal of Cl atoms, and a carbene route leading to the same final product. The first is favored by the presence of proton donors, whereas the second dominates in dry aprotic solvents such as DMF. Another reaction route involving reduction by atomic hydrogen, produced at the electrode by H₂O electrolysis, has been proposed by Li and Farrell [10], [16] for the dechlorination of trichloroethylene (TCE) at an Fe electrode.

Although various electrode materials have been investigated for the activation of carbon–halogen bonds, a systematic study on the electrocatalytic properties of metal electrodes on the reductive dehalogenation of polychloroethanes (C₂H_6 − xCl_x; x = 1–6) is still missing. It is the aim of the present paper to provide a comparison of the catalytic properties of several transition metals for the reductive cleavage of polychloroethanes (PCEs). The study was performed in DMF to make easy evaluation of the electrocatalytic activity, which is often represented by the magnitude of positive shift of the reduction peak potential of RX on a given electrode with respect to glassy carbon, taken as the best approximation to a non-catalytic system. The role of proton availability, which was found in previous studies [22], [23] to significantly affect the electrocatalytic activity of Ag, was also examined.