Study On Pd-Fe/Graphene Multifunctional Catalytic Cathode And Degradation Of 4-Chlorophenol By Multi-electrodes System

The chlorophenols pollutants have high toxicity, carcinogenic properties, and persistence in the environment. They are difficult to be effectively degraded with conventional treatment methods. In the chlorophenol treatments, the typical electrochemical method and apparatus are not sufficient, and likely to cause secondary pollution. In this paper, the Pd-Fe/graphene multifunctional catalytic cathodes and Ti/IrO2/RuO2 anode were used to construct a three-electrode electrolysis system and the electrocatalytic degradation mechanism of 4-chlorophenol was proposed.The different amounts and proportions metal/graphene catalysts were prepared by improved Hummer oxidation method and photoreduction method and then were characterized by X-ray diffraction> scanning electron microscopy、transmission electron microscopy、X-ray photoelectron spectroscopy and atomic force microscope. Metal Pd and Fe loaded on the graphene shown their characteristic peaks at 39.3°and 44.6°, respectively. The layer thickness of 1 wt% Pd/graphene,1 wt% Fe/graphene, Pd-Fe(0.5 wt%-0.5 wt%)/graphene catalysts were about 1.2 nm. And the metal particle sizes were less than 7 nm and were uniform loaded on graphene surface. In electrochemical performance tests (cyclic voltammetry curve, differential pulse voltammetry curve, current-time curve), the oxygen reduction peak was closed to -0.35 V. The decay rate, stable current, and peak shape were significant in the oxygen saturated and alkaline conditions. The hydrogen adsorption reduction peak was closed to -0.036 V. The electrochemical oxidation and reduction properties of Pd-Fe(0.5 wt%-0.5 wt%)/graphene catalyst was superior than other catalysts.The three electrodes system was used to degrade 4-chlorophenol with Ti/IrO2/RuO2 anode and Pd-Fe/graphene multifunctional catalytic cathodes. The current density, electrolyte concentration, initial pH, ventilation mode and reaction time were optimized by the single factor method and the response surface methodology. When the initial solution pH was 7, the electrolyte concentration was 0.03 mol/L, the current density was 25 mA/cm2, the hydrogen gas prior to 60 min and then continuous 60 min air, the degradation effect of 4-chlorophenol was the best. The total electro-catalytic kinetic equation was k=e-6.503 C0.877 M-0.762 with the first-order reaction kinetics. When the initial pH was 7, the degradation percentage of 4-chlorophenol could be improved by increase the current density or decrease the electrolyte concentration.Under the optimal degradation conditions, the degradation mechanism and generated intermediates of 4-chlorophenol were analyzed using ultraviolet spectral scanning, high performance liquid chromatography, ion chromatography and compared with the two-electrode system. The results showed that after 120 min electrochemical degradation, the degradation percentage of 4-chlorophenol reached 95.8% (cathode 1),95.7% (cathode 2) and 96.5% (anode), respectively. And TOC removal percentage reached 93.1% (cathode 1),92.4% (cathode 2) and 90.4% (anode) respectively. The dechlorination degree reached 93.7% in the anode and cathode compartments. Compared with the single cathode system, the degradation effect had improved. The degradation mechanism was proposed as following:the dechlorination reaction was firstly occurred to generate phenol which was then oxidized to hydroquinone and p-benzoquinone. At the following step, the aromatic ring was opened to generate fumaric acid and other short chain organic acids. The continued oxidation generated formic acid, acetic acid and other small molecules. Finally,4-chlorophenol was completely degraded to carbon dioxide and water finally.