| As a common organic chloride,chlorophenols（CPs）are extensively used in industrial and agricultural production,like as the role of raw materials for the production of agrochemicals,dyes,germicides and other chemicals.These compounds are easily released into the environment during their production and use processes,which can cause severe environment pollution issues and even serious human health problems due to their high toxicity,carcinogenicity,mutagenicity,and low biodegradability.In this context,CPs have been classified as priority pollutants in many countries.To dispose these contaminants and abate their negative impacts on the environment,some effective technologies have been developed and successfully applied for the removal of CPs.Among them,electroreductive dechlorination has attracted extensive interest owing to its remarkable properties,such as high reactivity,easy operation,absence of secondary pollution,and mild experimental conditions.The electroreductive dechlorination can generally be divided into three pathways,including direct electrochemical reduction,electrocatalytic hydro-dechlorination and electro-chemically mediated reduction.The electroreductive dichlorination of 4-CP has not been reported yet.Moreover,the reduction peak associated with direct electrochemical reduction of 4-CP is seldom observed under aqueous conditions,primarily due to their high dechlorination overpotentials and the deficiency of vigorous catalysts.Here we report a facile method for the synthesis of monodisperse palladium nanoparticles（PdNPs（DMF））by wet-chemical reduction with DMF as both solvent and metal-dispersant,Pd（OAC）2as the Pdprecursor and sodium borohydride as the reductive agent.The electrocatalytic dechlorination activity of PdNPs（DMF）and its stability was determined though voltammetric studies and bulk electrolyses.Meanwhile,the mechanisms for the direct electrochemical reduction and electrocatalytic hydro-dechlorination of 4-CP during the process of electrocatalytic dechlorination over the PdNPs（DMF）electrocatalyst was investigated.In addition,The catalytic activity and dechlorination mechanisms for electrochemically mediated reduction of 4-CP and other chlorinated pollutants over DCA were studied.Characterization results of TEM,HRTEM and XRD displayed that the as-obtained PdNPs（DMF）were uniformly dispersed with an average size of 4.0 nm.The normalized ECSA of PdNPs（DMF）was 40.8 m2·（g Pd）-1,which was much higher than that of commercial Pd/C catalyst（28.42）and PdNPs（H2O）（17.89）,showing the extraordinary electrochemical property of this PdNPs（DMF）catalyst.Monodisperse PdNPs（DMF）with remarkable electrocatalytic activity could expose more active sites,which largely enhanced the generation and storage capacity of H*.PdNPs（DMF）electrocatalyst showed a distinctively electrocatalytic activity towards electrocatalytic dechlorination of 4-CP.Moreover,the voltammetric reduction of 4-CP over PdNPs（DMF）went through both the direct and indirect reduction routes in the electrocatalytic dechlorination process.The k value was much smaller than 0.5,indicating that cleavage of C-Cl bond of 4-CP over PdNPs（DMF）electrocatalyst conformed to the concerted DET mechanism.Both H*abs and H*ads were active in the electrocatalytic hydro-dechlorination of 4-CP,and H2 could be used as a reducing agent for the dechlorination of 4-CP.In the process of electrolysis,93%of 4-CP was removed after 5 h,and the apparent rate constant(kapp)was0.0088/min,validating the outstanding dechlorination performance of this catalyst.In addition,Chronoamperometric image showed that the PdNPs（DMF）catalyst exhibited good stability.DCA was added to the solution to transfer electrons from cathode to 4-CP,leading to the cleavage of C-Cl bond.Compared to the direct electrochemical reduction of 4-CP,the reduction potential of the mediator was more positive,which contributed to the higher energy efficiency.Furthermore,DCA displayed excellent catalytic activity to the electrochemically mediated reduction of other chlorinated pollutants in the environment.In summary,the three dechlorination mechanisms for the electroreduction of 4-CP were explored systematically in this study.The superior electrocatalytic property of the PdNPs（DMF）implies that the reduction process may involve the surface interactions of 4-CP with the catalyst and/or the promoted electron transfer on the catalyst surface,resulting in the significantly lower of intrinsic barrier of the dechlorination reaction,thereby greatly boosting the direct electrochemical reduction of 4-CP.cyclic voltammetry was performed to distinguish the direct and indirect reduction pathways in electrocatalytic dichlorination,and their quantitative relationship to dechlorination performance are successfully established.This can be considered as guidelines for the future electrocatalytic dechlorination works.Besides,DCA-mediated electrochemical system with high energy efficiency exhibits a good stability and is expected to have a vast potential in the remediation of chlorinated environmental pollutants. |
