| The electrochemical reductive dehalogenation of organic halides is one of the most important reactions in organic electrochemisty,it possesses important application value both for organic synthesis and for environmental protection.Ag exhibits extraordinary electrocatalytic activity and is one of the most promising cathode materials towards the reduction process.So,the research of electrochemical reductive dehalogenation on Ag cathode has very important scientific significance and application value both in the theoretical development of organic electrochemisty and the practical application of organic synthesis and environmental protection.In order to perfect the theory of electrocatalytic dehalogenation reaction on Ag cathode and develop a practical technology for the electrocatalytic dehalogenation of halogenated organic contaminants,following research contents were carried out:(1)The electrocatalytic dechlorination reaction behaviors and pathway of benzenyltrichloride were studied by normal cyclic voltammetry(CV)and potentiostatic electrolysis,and electron transfer coefficient(a)was determined in the interest of definition of the dechlorination reaction;the anode reaction between Ag electrode and leaving chlorine ion,which was produced by the reduction of benzenyltrichloride,was evaluated by wide anode region CV;program electrochemical method(controlled-current electrolysis→ potentiostatic electrolysis→ linear sweep voltammetry)was devised in order to find the potential region,in which the leaving chlorine ion could adsorbed on the silver electrode.(2)A roughened silver cathode(Ag(r)cathode)was prepared by an oxidation-reduction cycle process(ORC process).Surface structure and roughness factor of the Ag(r)cathode,and its catalytic activity and selectivity towards the dechlorination of chloroactic acids(CAAs)in aqueous solutions of different pH were studied by SEM,underpotential deposition(UPD)method,CV and controlled-current electrolysis,respectively.In addition,the effect of pH on the adsorption of leaving chlorine ion at the Ag(r)cathode was evaluated by the wide anode region CV.(3)The feasibility of complete dehalogenation of four typical halogenated phenol in basic aqueous solutions by the catalysis of Ag(r)cathode was evaluated,and the selectivity,pathway and dynamics characteristic of the dehalogenation of 2,4,6-tribromophenol was studied by CV and potentiostatic electrolysis.(4)The electrocatalytic dechlorination of 3,4,5,6-tetrachloropicolinic acid(TeCP)and 3,5,6-trichloropicolinic acid(TCP)at the Ag(r)cathode in 0.5 M NaCl04 aqueous solutions were investigated by an in situ electrochemical surface-enhanced Raman spectroscopy(SERS)coupled with density functional theory(DFT)calculations,and the surface geometry change of TeCP and its reductive product adsorbed on the Ag(r)cathode depended on the electrode potential were specially focused.(5)A roughened silver-palladium(Pd/Ag(r))cathode was fabricated by a convenient metallic replacement reaction,and its electrocatalytic property towards reductive dechlorination of 2,4-Dichlorophenoxyacetic acid(2,4-D)in basic aqueous solution have been evaluated.Moreover,The effect of operating parameters on dechloriantion efficiency of 2,4-D at the Pd/Ag(r)cathode were estimated.Based on the research contents above,conclusions as following can be obtained:(1)The reaction related to the first reduction peak,which was divided from the reduction peak of benzenyltrichloride at-1.19 V(vs.Ag/Ag+)when the scan rate is<50 mV/s,in CH3CN at Ag cathode was an adsorption controlled process and its electron transfer occurred in a concerted way,and the electron transfer coefficient(a)is approximate 0.25;the electrocatalytic dechlorination of benzyl chloride on the silver cathode can be promoted in present of acetic acid;the three reduction peak(Ep=-1.19,-1.50,-2.24 V)of benzenyltrichloride on the silver cathode are attributed to(1)→(2)→(3)→ and(1)→(2)→(3)→(4),and(2)→(6)→(7)→(8)→(9)→(10)and(2)→(6)→(7)→(8)→(4)→(11),and(10)→ toluene,respectively(Re.Fig.2-7);the potential region,in which the leaving chlorine ion could adsorbed on the Ag electrode,confirmed by the convenient program electrochemical method,is-1.05～-1.85 V(vs.Ag/Ag+).(2)A roughened silver cathode(Ag(r)cathode),having a porous surface with a roughness factor of approx.63.9 cm2/cm2,can be prepared by the ORC process.The Ag(r)cathode is an excellent electrocatalytic material for direct electrodechlorination of three CAAs,not only in comparison with such inert electrodes as GC,but also in comparison with more electrocatalytic material such as Cu and Cu/Hg.The complete dechlorination of TCAA to safe AA with excellent current efficiency(near 95%)can be successfully achieved by using this peculiar metal without resorting to the use of organic solvents.The reductive dechlorination of TCAA is probably stepwise following the pathway of TCAA → DCAA→ MCAA → AA,either in strong acidic and alkaline solutions or in weak acidic solutions.The reduction activities of those substances,usually co-existing in chlorinated acetate-contaminated drinking water,probably follow the decreasing order:TCAA>DCAA>MCAA>H+>H2O,by the catalysis of Ag.The Cl-produced by the reduction of CAAs probably strongly adsorbed on the silver cathode in the form of HCl in strong acidic solutions.(3)The electrocatalytic activities of three cathodes,toward the reductive dehalogenation of halogenated phenol,follow the decreasing order:Ag(r)>Ag(p)>GC;and the reduction activities of four halogenated phenol substances follow the decreasing order:PBP>TBP>PCP>TCP;complete dehalogenation of PBP and TBP to phenol could achieved with high current efficiency,while PCP can only be reduced to 2,3,5,6-tetrachlorophenol in basic aqueous solutions by the catalysis of Ag(r)cathode;the reduction of 2,4,6-tribromophenol was determined to be a stepwise dehalogenation as follows:2,4,6-tribromophenol → 2,4-dibromophenol →2-bromophenol → phenol.In addition,the activity energy needed for the dissociative reduction of the ortho carbon-halogen bond was similar to the para carbon-halogen bond for 2,4,6-tribromophenol and a higher energy was probably required for 2,4-dibromophenol to cleave the ortho carbon-halogen bond compared to the para carbon-halogen bond.(4)The dechlorination of 3,4,5,6-tetrachloropicolinic acid(TeCP)in neutral aqueous solutions on the Ag(r)cathode was stepwise,following the pathway of TeCP→TCP → 3,6-D(3,6-dichloropicolinic acid);a large number of molecule level information concerned with the dechlorination mechanism of TeCP can be obtained by the in situ electrochemical SERS coupled DFT calculations:when the potential is-0.5 V(vs.SCE),TeCP adsorbed with the carboxyl group and 3-position Cl toward the Ag(r)cathode;when the potential reaches-0.55 V,the 4-position Cl of TeCP started to be reduced;where the potential is-0.75 V,most of TeCP on the surface of Ag(r)cathode had been converted to TCP,which also adsorbed onto Ag(r)cathode surface through carboxyl but the pyridine ring plane rotated 90 degrees;where the potential is-1.0 V,TCP produced at-0.75 V were reduced to 3,6-D further and most of which left Ag(r)cathode surface as quickly as it was produced.(5)Pd/Ag(r)appears to be an excellent electrocatalytic material for electrodechlorination of 2,4-D even in comparison with more electrocatalytic materials such as Pd/GC and Pd/Ti;The maim reductive dechlorination pathway of 2,4-D on Pd/Ag(r)cathode is following:2,4-D→2-ClPAA→PAA;and under the optimized conditions(Pd load = 240 μg/cm2,Temperature-30℃,Applied current density = 0.76 mA/cm2 and Concentration of electrolyte = 0.5 M NaOH),25 mM of 2,4-D could be wholly dechlorinated to PAA with a conversion of 2,4-D(95%),yield of PAA(84%)and current efficiency(61%)after 4 h electrolysis. |
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