Insights into Electroreductive Dehalogenation Mechanisms of Chlorinated Environmental Pollutants

Abstract

The electroreductive cleavage of carbon‐chloro bonds has attracted increasing attention from both environmental and mechanistic points of view over the past decades, taking into consideration a large variety of chlorinated organic compounds. Important discoveries have been made on the underlying electron/hydrogen transfer mechanisms and the enhancement of dechlorination performance for electrocatalysis purposes over the last years. However, there is still missing a combined knowledge from individual researches to specially elucidate the electroreductive dehalogenation mechanisms of organic chlorides. Herein, we summarize the diverse electrochemical strategies applied to reductive dehalogenation of chlorinated environmental pollutants, including direct electrochemical reduction, electrocatalytic hydro‐dechlorination and electrochemically mediated reduction methods, and, especially, highlight the differences in reductive dehalogenation mechanisms. Specifically, direct electron transfer is the most common dechlorination mechanism involved in the direct electrochemical reduction of organic chlorides, in which the electron transfer to C−Cl bonds can be classified as concerted or stepwise dissociative electron transfer mechanism. Indirect atomic hydrogen reduction is the primary dechlorination mechanism in the electrocatalytic hydro‐dechlorination process, which often competes with the direct electrochemical reduction route. Electrochemically mediated reduction, as another indirect route, uses a redox‐active mediator to indirectly transfer electrons from cathode to organic chlorides, resulting in reductive dehalogenation reaction. Moreover, the effects of critical factors on the dehalogenation reactivity and mechanisms are also discussed to better demonstrate the electroreductive dehalogenation of organic chlorides. Lastly, this contribution highlights recent relevant advances in electroreductive dehalogenation issues, as well as the challenges and potentials of this process.

Electrochemically activated carbon–halogen bond cleavage and C–C coupling monitored by in situ shell-isolated nanoparticle-enhanced Raman spectroscopy

“…44 Typically, there are two electrochemical reduction mechanisms for organic halides. 2,9 One is the direct electron transfer from the cathode to the molecules through two possible ways: RX + e − ⇋ RX •− → R • + X − (stepwise manner) and RX + e − → R • + X − (concerted manner). 22,45 Then, the R • could be further reduced by the cathode and combined with the H + in the solution and turned into an RH product (R • + e − + H + ⇋ RH).…”

“…It has been proved that the hydrodehalogenation process on the metal surface in the presence of a suitable proton donor has high activity towards breaking the C-X bonds. 9,48 In our experiments, the potential-dependent conversion ratios of CTP and BTP are estimated by intensity variation of ν(C-Cl) and ν(C-Br). Fig.…”

“…5, firstly, BTP is reduced by direct electron transport or active atomic hydrogen to form a benzene free radical (Ar • ), which is a key intermediate that has been widely reported in electroreductive dehalogenation. 6,9,22,40 Then two adjacent Ar • could undertake an Ullmann-type self-coupling reaction to form BPDT. Secondly, the Ar • could further be reduced by the cathode through the 1e − process Ar • + e − ⇋ Ar − .…”

“…3,4 Meanwhile, various techniques, such as cyclic voltammograms (CVs), 5,6 gas chromatography-mass spectrometry (GC-MS) 7 and electrochemical nuclear magnetic resonance (NMR), 8 have been employed to investigate the mechanistic features of the electroreductive dehalogenation processes associated with electron transfer coefficients and reaction intermediates. 9 However, the above techniques lack in situ characterization of transient chemical structures at the electrode/liquid interfaces, which limits the understanding of the electrocatalytic process and the mechanism of C-X bond cleavage.…”

Metallic nanoparticles for electrocatalytic reduction of halogenated organic compounds: A review

“…In the next sections, we focus on the Ag, Pd, and Cu electrode materials, which have been extensively studied for dehalogenation reaction. Since the mechanisms involved in reductive dehalogenation process have been recently reviewed [137,138], we will more focus this part on the performances of each electrode material for the reduction of halogenated POPs.…”