Study On Electrocatalytic Hydrodechlorination Of P-chlorophenol On Palladium-doped Multi-walled Carbon Nanotube Particles

With the acceleration of the modernization and the rapid development of the economy and society,the pollution caused by human activities has surpassed the self-regulation ability of the environment and destroyed the balance of the ecosystem.Among them,chlorophenols have a wide distribution on a global scale because they are difficult to metabolize and transformand persist in the environment.Therefore,it is essential to develop a cost-effective,eco-friendly treatment method that can eliminate the toxicity of chlorophenols to maintain environmental stability as well as human safety.In this study,a three-electrode system was used to reduce and dechlorinate 4-chlorophenol（4-CP）with particle electrodes.The specific research contents are as follows:A series of palladium loaded multi-walled carbon nanotubes（Pd/MWCNTs）were synthesized by impregnation method,and in situ chemical reduction of samples was carried out with NaBH₄,ethanol and H₂ as reducing agents,referred as Pd/MWCNTs-B,Pd/MWCNTs-E and Pd/MWCNTs-H,respectively.X-ray diffraction（XRD）and transmission electron microscopy（TEM）analysis showed that Pd nanoparticles were successfully supported on the surface of MWCNTs and ha d a typical face-centered cubic（fcc）structure.According to the Scherrer formula,the particle sizes of Pd nanoparticles supported on Pd/MWCNTs-B,Pd/MWCNTs-E and Pd/MWCNTs-H were 6.4 nm,9.5 nm and 13.1 nm,respectively.X-ray photoelectron spectroscopy（XPS）results showed that the Pd loading of Pd-MWCNTs-B,Pd/MWCNTs-E and Pd/MWCNTs-H catalysts was 0.51 wt%,0.47 wt%and 0.49 wt%,respectively.The electrocatalytic hydrodechlorination performance of 4-CP was studied in a three-dimensional（3D）electrochemical reactor using Pd/MWCNTs as particle electrodes.The complete removal of 4-CP by Pd/MWCNTs-B was achieved after 30min,at this time,only 60%and 29%of 4-CP was dechlorinated by Pd/MWCNTs-E and Pd/MWCNTs-H,respectively.Moreover,the processes exactly followed a pseudo-first-order model.The effects of different current densities,initial pH and dissolved oxygen（DO）concentration on 4-CP reduction were investigated.The experimental results showed that the removal and reaction rate of 4-CP increased with the increase of current density.There was no significant difference in 4-CPremoval（100%）for different initial pH after reaction 30 min,indicating that the electrocatalytic dechlorination of4-CP could be accomplished by Pd/MWCNTs-B under a broad pH range.DO had an adverse effect on the reduction reaction of 4-CP in the reaction system,and the DO concentration was negatively correlated with the removal of 4-CP.The mechanism of indirect electrochemical 4-CP reduction by Pd/MWCNTs particle electrodes was divided into three steps.Firstly,the titanium cathode generates a large amount of H₂ as a hydrogen source,which is dispersed in solution b y magnetic stirring.Secondly,the H₂ bubbles are captured on the surface of Pd NPs and then split into two active atomic H*,which served as the reducing agents for the hydrodechlorination of 4-CP.Finally,the Cl atoms in 4-CP are substituted by the active atomic H*and concurrently 4-CP is dechlorinated to phenol.The Pd NPs played the catalytic active sites for dissociating H₂ to generate H*,which was confirmed to be responsible for the 4-CP dechlorination.Pd/MWCNTs-B exhibited excellent stability and there was not significant deactivation and metal loss after 5 recycles.This research indicated that electrocatalytic dechlorination in a 3D electrochemical reactor with Pd/MWCNTs as theparticle electrodes was a promising method for the 4-CP removal.