| Electrocatalytic hydrogenolysis (ECH) was widely used for chlorinated organic compounds’synthesis and degradation. Because of the adsorption of the chloride atom on the silver electrode and strong ability of palladium atomic to adsorb hydrogen and produce chemisorbed H, chlorinated organic compounds showed a remarkable catalytic activity towards the taken off of the atomic chlorine. In this paper, we employed 2,4-Dichlorophenoxyacetic acid (2,4-D) as a mode compound to investigate various factors that might affect the dechlorination of 2,4-D over palladium loaded on roughened reticulated silver cathode (Pd/Ag(r)). Under the optimized conditions,92% 2,4-D could be dechlorinated and 66% current efficiency. And silver electrode has especial response in Raman spectroscopy, the Raman signal of the molecule adsorb on the roughened silver cathode surface would be enlarge. Concequently, electrochemical surface-enhanced Raman spectroscopy (SERS) method was used to study the reduction mechanism of tetrachloropicolinic acid in different pH. In present paper, following research contents were carried out:(1) Different Pd/Ag(r) were prepared by different metallic displacement temperature (load palladium temperature) and different palladium loading with roughened silver electrode as a basement. The Scanning Electron Microscope mirror (SEM), Energy Dispersive Spectrometer (EDS) were used for observing the surface of different Pd/Ag(r). The electrocatalytic activity and stability of Pd/Ag(r) cathodes were tested through the constant current electroanalysis. Finally, the optimum preparation conditions of Pd/Ag(r) were determined. The economic analysis showed that the Pd/Ag(r) cathode is an economical cathode material to dechlorinate 2,4-D compared with other palladium modified cathodes.(2) The effect of opration parameters on the rate and current efficiency of the dechlorination of 2,4-D on Pd/Ag(r) were evaluated, including dissolved oxygen, dual-reference electrodes method of IR compensation, electrolyte composition,2,4-D initial concentration, current density, electrolysis temperature.(3) The reduction mechanism of 3,4,5,6-tetrachloropicolinic acid in different pH were studied through electrochemical surface-enhanced Raman spectroscopy (SERS) method. We get the adsorption form of 3,4,5,6-tetrachloropicolinic acid molecule on silver electrode through analyzing characterisitic Raman peak at different potential.According to all research above, conclusions as following can be obtained:(1) The optimum preparation conditions of Pd/Ag(r) cathode: Roughing silver electrode at 25℃ by the following sequence in 0.5M NaCl with current density 6.061 mA-cm"2:5 min oxidize â†' 5 min ruduce â†' 5 min oxidize â†' 5 min ruduce. Loading palladium by metallic replacement in 3 mM PdCl2+0.1 M HCl for 100 ml with magnetic stirring until the yellow PdCl2 solution turned colorless at 25℃. Electroreduction in 0.5 M NaOH until generate lager amount of hydrogen on the electrode surface.(2) The effect of opration parameters on dechlorination reaction of 2,4-D on Pd/Ag(r):there are no effect from dissolved oxygen and without dual-reference electrodes method of IR compensation. Cl- is detrimental to the 2,4-D dechlorination, while OH" can drastically promotes the dechlorination of 2,4-D. Ethanol would deactivate the Pd/Ag(r) cathode, if an alkalin aqueous solution was used for the dechlorination of 2,4-D. The dechlorination efficiency increased with increasing the initial concentrations of 2,4-D (from 5 to 75 mM), which indicates that higher initial concentration (< 75 mM) is favorable for the dechlorination of 2,4-D on Pd/Ag(r) cathodes. Current density strongly influence the dechlorination efficiency of 2,4-D, moderate current density (1.39-2.78 mA·cm-2) have the most beneficial effect on the dechlorination process. Temperature is favorable for 2,4-D mass transfer and hydrogen desorption. The hydrogen desorption enhancement greater than 2,4-D mass transfer at high temperature(45～75℃), so the rate of 2,4-D dechlorination decrease. The mechanism of dechlorination of 2,4-D on a Pd/Ag(r) electrode has been revised as following:(1) the HCl formed by the catalytic hydrogenation probably is steadily adsorbed on a Pd/Ag(r) electrode; (2) the amount of adsorbed 2,4-D on a Pd/Ag(r) electrode may decrease rapidly in a sufficiently negative potential region due to the potential-driven desorption effect.(3)In different pH, the reduction mechanism of 3,4,5,6-tetrachloropicolinic acid on Ag(r) electrode were studied through electrochemical surface-enhanced Raman spectroscopy (SERS) method. There just has a different combination between 3,4,5,6-TeCP and Ag electrode at different pH. But the reduction mechanism of acid, neutral and alkaline system just the same process:3,4,5,6-TeCPâ†' 3,5,6-TrCPâ†'3,6-DCP. Through Cl" and -COO", 3,4,5,6-Tetrachloropicolinic acid was adsorbed on the cathode surface, the fourth Cl atom was taken off at the first step. The fifth Cl atom was taken off at the second step. |
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