Silver And Palladium Catalyzed Selective Hydrogenative Dechlorination Of Chlorinated Organic Compounds

The reductive dechlorination reaction has important application value in the synthesis and degradation of chlorinated organic compounds?COCs?.In synthesis,it can convert readily available polychlorinated COCs into partially chlorinated COCs that are difficult to synthesize;In degradation,it can completely dechlorinate of COCs to less toxic chlorine-free organics.Silver and palladium have excellent catalytic propertiesinelectrochemicalreductiondechlorinationandchemical hydrodechlorination?HDC?,respectively.Therefore,the related research has important significance in the theoretical development and practical application.In this paper,the electrochemical dechlorination of 3,5,6-trichloropicolinic acid?TCP?on silver electrode and the mechanism of electrochemical dechlorination of chloroacetic acid?MCAA?on silver nanocrystals?Ag NCs?were studied,In addition to,Pd nanocrystals?Pd NCs?as the probe catalyst to investigate the HDC catalytic sites and the reaction mechanism,and to discuss the reason for the effects of support on HDC.The main research contents and conclusions are as follows:???Electrochemical reduction of 3,5,6-trichloropicolinic acid?3,5,6-T?at glassy carbon,Ni,Cu,and Ag cathodes in aqueous solutions at different pH values was investigated by cyclic voltammetry?CV?and electrolysis.The results suggest that the selectivity of the reduction strongly depends on the cathode materials used and the pH of the aqueous solutions.A high selectivity for the hydrodechlorination of 3,5,6-T to3,5-dichloropicolinic acid?3,5-D?has been achieved exclusively at an activated Ag cathode and at pH=3.?a selectivity of 95%at 42%conversion under potentiostatic mode and a selectivity of 89%at 88%conversion under intentiostatic mode.????The active sites and possible mechanisms of electrochemical reduction dechlorination of MCAA in aqueous solution were studied by using cubic Ag NCs as probe catalyst.The results show that the reduction peak potential and peak current change continuously during the CV process.After the reaction,Ag NCs become rounded and a large number of silver clusters are formed on the surface.These results indicate that the low-coordinated corner and edge atoms on Ag NCs are likely to be the main active sites,and the dechlorination reaction is likely to involve an oxidative addition process.???Pd nanocrystals?vary size and shape?were used as the probe catalyst to investigate the HDC catalytic sites in aqueous solution with 4-chlorophenol as the model chlorinated organic pollutants.Experiments suggest that the numbers of edge and corner atoms on cubic Pd NCs with varying sizes?6.3-18.4 nm?showed linear correlation with the HDC activity,suggesting that these defect atoms are the main catalytic sites for HDC.This conclusion can provide a reference for the design and preparation of Pd catalysts with high HDC performance.???The reaction mechanism of the HDC was studied through substituents effect,the detection of biphenol in HDC,the isotope labeling experiments,and the determination of chloride ion and Ullmann coupling product.The electron-withdrawing substituents favored the HDC of ACs.Almost no phenol-d could be produced from HDC of 4-CP in D₂O,and no Ullmann coupling product was detected during HDC.These results strongly suggested that the HDC catalyzed on Pd NCs would not follow the conventional electrophilic aromatic substitution,anion,or radical mechanism.In contrast,the production of chloride ion,the absence of Ullmann coupling product in the absence of H₂,and the positive effect of electron-withdrawing substituents on HDC demonstrated that the HDC should include an oxidative addition process,which probably follows the nucleophilic aromatic substitution?S_NAr?mechanism.???The effects of support on the the performance of HDC catalyzed by Pd NCs and the intrinsic reasons were studied by using Al₂O₃,SiO₂,TiO₂ and activated carbon?AC?as support.The results show that the HDC activity decreases as following:Pd NCs/AC>Pd NCs/Al₂O₃?Pd NCs?Pd NCs/SiO₂>Pd NCs/TiO₂.This order is consistent with the electron supply of the Pd NCs by the carrier.The result indicates that the electronic effect caused by support may be the main reason for the higher performance of Pd NCs catalyted HDC.