Effect Of Electrode Material And Position Of Halogen Atoms On Electrochemical Dehalogenation Reactions

Halogenatedaromaticcompoundsareusedasimportant intermediates for the synthesis of a variety of wood preservatives,dyes,pesticides and so on.Which can lead to serious environmental contamination and it is very urgent to treat these contaminants.As one of the most attractive methods for degradation of halogenated aromatic compounds,electrochemical degradation method has aroused increasing interest in recent years.The position of halogen atom,the electronegativity of substituent group and the electrode materials always affect the electrochemical dehalogenation mechanism.In this paper,in situ FTIR is used as the main technique,theoretical calculation is used to supply theoretical guidance for electrochemical dehalogenation reaction.The main studies are as followed:（1）Dendritic Ag/Cu electrode was synthesized by Galvanic replacement of Ag⁺and Cu,5-bromo-2-chlorobenzoic acid（BCBA）and mono-chlorobenzoic acids（mono-CBA（2-CBA,3-CBA and 4-CBA））were used as the template molecules,the effect of the position of halogen atom on the electroreductive reaction mechanism was studied.Ag/Cu electrode exhibited high electrocatalytic activity for the dehalogenation reaction of BCBA and mono-CBA.The mechanism of the electroreductive dehalogenation reaction was represented as a sequence of electron additions and halogen expulsions.BCBA took a flat orientation on the electrode surface via an extendedπbond.With the C-Br bond breakage,BCBA received electrons and hydrogen to form intermediate 2-CBA.Then C-Cl bond broke in the same way,and the end product BA was obtained,which was connected with interaction of the COO with the electrode surface vertically.Both 3-CBA and 4-CBA bonded to the surface through the COO group and oriented perpendicularly,while 2-CBA adsorbed on the electrode surface in the perpendicular stance not bonded though COO.The end product of electroreductive reaction of mono-CBA was BA,too.（2）Pd/Ag/Cu electrode was successfully prepared on Ag dendrites with Cu substrate（Ag/Cu）by two-step Galvanic replacement reactions.The effect of the position of chlorine atom on the mechanism of the electroreductive reaction of dichlorophenols（DCP（2,3-DCP,2,4-DCP and 2,5-DCP））on Pd/Ag/Cu electrode was investigated.Compared with Pd/Cu,Ag/Cu and Cu electrodes,Pd/Ag/Cu electrode exhibited the highest electrocatalytic activity for electroreductive reaction of DCP.The electroreduction process of DCP was caused by the electrocatalytic hydrogenolysis of C-Cl bond.Chemisorbed hydrogen(H_（ads）Pd)was generated by reduction of water and C-Cl bonds broke by the reaction of adsorbed DCP with H_（ads）Pd.Because of the greater bond dissociation energy value,C2-Cl bond was more difficult to release from DCP,so2-CP was the main intermediate,and the final product was phenolate.（3）Mono-chlorophenols（mono-CP（2-CP,3-CP and 4-CP））and DCPs were chosen as the template molecules,the electrooxidation mechanism on Pt electrode were studied.Density functional theory（DFT）was used to calculate the Fukui function for mono-CP and DCP and gave us the information on possible reactive sites during electrochemical oxidation.Pt electrode exhibited a high electrochemical activity for the electrochemical oxidation reaction of mono-CP and DCP.The electrooxidation reactions were mainly caused by the attack of HO·.There were several features of the electrochemical oxidation reaction of mono-CP and DCP:（1）Pt electrode was poisoned with polymer,and lost its electrochemical activity gradually.（2）C4 was easier to be attacked by HO·than other sites due to the effect of phenolic hydroxyl;（3）CO₂ and H₂O were the final product.