Electrocatalytic Properties Of Transition Metals Towards Reduction Of Chlorinated Volatile Organic Compounds (Cl-VOCs)

Chlorinated volatile organic compounds (Cl-VOCs), including polychloromethanes(PCMs), polychloroethanes (PCAs) and polychloroethylenes (PCEs), are among the mostubiquitous pollutants in environment. Some of them have been listed as the priority pollutantsby China, U.S.EPA and European Union, and are strictly controlled in drinking water.Therefore, searching for innovative, high efficient remediation technologies for the abatementof Cl-VOCs waste and for the removal of Cl-VOCs from contaminated water and soil sitesare significantly important. In this work, electrochemical reductive cleavages of Cl-VOCswere carried out in dimethylformamide (DMF)+0.1M tetrapropylammoniumtetrafluoroborate (TPABF4) at different cathodic materials under different solvent conditions,to investigate the dechlorination mechanisms of Cl-VOCs, evaluate the electrocatalyticactivities of different transition metal electrodes, and further examine the feasibility of thismethod on the abatement of Cl-VOCs. Finally, an innovative electrochemical hydrogenationtechnology on the removal of acetylene, which is the main electrolysis intermediate ofpolychloroethanes, was explored.Electrochemical reduction of PCAs were investigated in DMF+0.1M TPABF4at Ag,Au, Pd, Pt, Cu, Fe, Ni, Pb, Zn and glassy carbon (GC), the last used as a reference systemwith negligible catalytic properties. Results indicate electrocatalytic activities of transitionmetals are strongly dependent on the nature of electrode and chemical structure of PCAs. Inany case, Ag, Au and Cu behave the most remarkable catalytic effects toward the reductivecleavage of PCAs. Based on the positive shift values of catalytic electrodes with respect toGC for PCAs reduction, the electrochemical reactivity of all the investigatedpolychloroethanes follow the order:1,1-DCA <1,1,1-TCA <1,2-DCA <HCA <1,1,2,2-TeCA <1,1,2-TCA <1,1,1,2-TeCA. Electron transfer (ET) coefficients indicate thatthe first ET to PCAs undergoes concerted dissociative electron transfer (DET) mechanism,regardless of the nature of electrodes and chemical structure of PCAs, where the electrontransfer and reductive cleavage of C-Cl bond take place simultaneously. Theactivation-driving force relationships of the dechlorination reactions for all investigated PCAsare found to conform to "Sticky DET" model, in which a sizable interaction between the caged fragments is survived.Based on the voltammetric investigations of PCAs and the electrolysis results of1,1,1-trichloroethane (1,1,1-TCA) and1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) at GC and Agelectrode in different solvent conditions, respectively, the dechlorination mechanism is foundstrongly dependent on the structure of PCAs. Geminal PCAs undergo a sequentialhydrodechlorination mechanism, in which one chlorine atom is lost with successivehydrogenation occurrence at one reduction step until completely dechlorination productethane are formed. In contrast, vicinal PCAs follow reductive α,β-elimination mechanism byexpelling two chlorine ions at one single step, leading to the corresponding (chloro-)olefinformation, which could be further reduced at more negative potentials. Reductions of PCAsare found very sensitive to the proton availability in the solvent. For geminal PCAs,regardless of the type of electrode, in dry aprotic DMF conditions, apart from sequentialhydrodechlorination mechanism, dehydrochlorination (base promoted α,β-elimination) andcarbene route play significant roles towards the dechlorination of PCAs process; whereas,sequential hydrodechlorination becomes the primary reduction mechanism in the presence ofstrong proton donor (acetic acid). In contrast, reductive α,β-elimination mechanism for vicinalPCAs couldn't be suppressed by the presence of proton donor, concomitant with(chloro-)olefin formation.Electrocatalytic activity of Cu was investigated towards the reductive dechlorination ofgeminal PCAs and polychloromethanes. It shows that Cu exhibits an excellent catalyticactivity for the reduction of these two types of compounds with evident positive shift ofreduction potentials with respect to GC. Results indicate the addition of proton donor (HAc,H2O) has significant effects on the reductive dechlorination process, which could activate thecatalytic activity of Cu toward the abatement of C-Cl bond. Besides, the presence of protondonor may also affect the reduction mechanisms of these two types of chlorinatedhydrocarbons by suppressing side reactions favoring the sequential hydrodechlorination route.Comparison of two different potential applied manners (sequential and single steppotentiostatic electrolysis), indicate essentially the same electrolysis results are obtainedtoward the reductive cleavage of chlorinated aliphatic hydrocarbons, with the only significantdifference that electrolysis intermediates are repulsive to accumulate in the case of single step electrolysis, since the potential applied locate at which all the intermediates are reducible.However, for the stepwise electrolysis, by choosing appropriate working conditions, it cankeep the reaction retain at a determined stage, which are very significant in electrosynthesisindustry.Eventually, electrochemical hydrogenation of acetylene was carried out extensively atprecious and non-precious electrodes in DMF+0.1M TPABF4in different solvent conditions.It shows the electrochemistry behaviors of acetylene are strongly dependent on the nature ofelectrode and water availability. Among them, Au, Ag and Pd display remarkableelectrocatalytic activity toward the reduction of acetylene. Addition of water could enhancethe peak current intensity of acetylene and promote the reduction peak shift to more positivedirection. Electrolytic results of acetylene further indicate the product distributions depend onthe cathodic materials and solvent conditions. In dry aprotic DMF solvent, high ethyleneselectivity was obtained, namely58.75%for Cu and51.81%for Ag, whereas, more than35%C4hydrocarbons were obtained in the both cases. For Pt electrode, considerable C4hydrocarbons were produced (74.68%) with78.33%conversion of acetylene. In the presenceof water, more than75%and68%ethylene were obtained at Cu and Ag electrode,respectively, concomitant with quite few C4hydrocarbons formation (3.14%for Cu and0.27%for Ag), indicating that radical coupling reaction could be effectively suppressed. However,low acetylene (34.14%) was converted at Pt electrode, concomitant with high yield of C4hydrocarbons (27.18%) due to the discharge of water. It should be noted that trace of ethanewere generated in all different solvent conditions, it appears the overhydrogenation ofacetylene could be successfully inhibited by using this method.In general, electrochemical reductive dechlorination technology as a promising approachhas been successfully applied on the abatement of Cl-VOCs in this study. Cu is found toexhibit excellent electrocatalytic activities toward the reductive cleavage of C-Cl bond,sometimes it's even comparable with Ag, the latter is believed to be the most remarkablecatalytic electrode for dechlorination of chlorinated compounds in organic solvent, but withmuch cheaper price. This work provides a significant technical support for the abatement ofCl-VOCs and the electrosynthesis process of (chloro-) hydrocarbons. A novel technology foracetylene removal has been proposed and explored in this work, we hope it can open a gate for the development of electrochemical acetylene hydrogenation technology through ourresearch, and provide an alternative technology for acetylene removal in the polymersynthetic industry.