Electrocatalytic Reductive Dechlorination Of Chlorinated Phenols At Carbon Electrode With Microstructure

The electroreduction process has caused world-wide interest because of its high dechlorination efficicency, simple operation, and environmental friendly property. Therefore, the electroreduction process with the different palladium-loaded carbon materials (activated carbon fiber (ACF), graphite and multiwalled carbon nanotubes (MWCNTs)) as working electrode is employed in this study for electrocatalytic dechlorination of chlorophenols (e.g. pentachlorophenol (PCP)).The palladium-loaded activated carbon fiber (Pd/ACF) electrode, which is prepared by electrochemical deposition, is used to investigate the adsorption and electrocatalytic dechlorination of PCP in solution. The scanning electron microscopy (SEM) indicates that Pd dispersed uniformly on the surface of ACF. The adsorption rate and capacity of PCP on ACF are still high even after its surface is loaded Pd (4.2 wt.%). The Pd/ACF electrode is used to investigate the electrocatalytic dechlorination of PCP under galvanostatic conditions. The Pd/ACF electrode shows high degradation and current efficiency, but low stability. The mechanism of the electrocatalytic dechlorination for PCP is mainly electrocatalytic hydrodehalogenation (HDH) along with direct electroreduction. PCP is gradually dechlorinated, and the final product is phenol.The multiwalled carbon nanotubes (MWCNTs) on graphite are prepared by chemical vapor deposition (CVD). The Pd/MWCNTs/graphite electrode is prepared by electrochemical deposition, and characterized by SEM, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The results indicate that the diameters and surface area of MWCNTs are 40-60 nm and 54 m~2/g, respectively. Pd with the particle size of about 13 nm also disperses uniformly on the surface of MWCNTs. The electrocatalytic reduction performance of Pd/MWCNTs/graphite electrode is investigated using PCP as the target compound. The Pd/MWCNTs/graphite electrode at potentiostatic bias shows high degradation and current efficiency for PCP in solution with low concentration. The stability of Pd/MWCNTs/graphite electrode is investigated, and the results suggest that the electrode has high stability.The degradation of PCP on Pd/MWCNTs/graphite is in good agreement with the first-order adsorption kinetics equation in all the experimental cases, and shows a dependence on temperature, initial concentration, the amount of catalyzer, anodic electrolyte concentration, and potential bias. The corresponding equation is developed. The experimental results indicate that the equation shows well prediction for the degradation of PCP. The results obtained by linear sweep voltammetry and GC/MS analysis indicate that the mechanism ofdegradation of PCP is electrocatalytic HDH. PCP is gradually dechlorinated to phenol, and the final product is cyclohexanone.The degradation rate constants of PCP on Pd/ACF, Pd/graphite and Pd/MWCNTs/graphite electrode are 0.0141, 0.0092 and 0.0153 min'1, respectively. The results indicate that, compared with Pd/ACF and Pd/graphite, the Pd/MWCNTs/graphite electrode has higher electrocatalytic activity and stability.The degradation kinetics of three chlorophenols on Pd/ACF, Pd/graphite and Pd/MWCNTs/graphite electrode are also investigated in the paper. The results suggest that the degradation rate constants vary in the order: 4-chlorophenol > 2,4-dichlorophenol > PCP.In a word, the degradation properties and mechanism of electrocatalytic reduction for nondegradable chlorophenols on different carbon materials are revealed in this paper. The carbon materials exhibit high degradation and current efficiency during the electrocatalytic reduction process, which provide the theoretical and experimental basis for the application of electrocatalytic reduction using carbon electrodes in treatment of wastewater containing chlorinated organic compounds.