Rh-Pd Catalyzed Electrochemical Hydrodefluorination Of 4-Fluorophenol

The use of fluoroorganic compounds?FOCs?has become increasingly widespread,however,many FOCs are toxic and persistent in the natural environment.Hydrodefluorination?HDF?is the key step in the artificial degradation of fluoroorganic pollutant?FOPs?,therefore,making the search for efficient HDF catalysts and the determination of the corresponding mechanism a matter of great importance.In this study,we developed an efficient Rh-Pd alloy electocatalyst by screening catalyst loading and elemental ratios,using HDF of 4-fluorophenol?4-FP?as the model reaction.We also investigated the reaction pathway and the underlying reaction mechanism using various characterization means and electrochemical methods.The specific content and conclusion of this paper is as follows:Pd-and Rh-modified Ni foam electrodes with different noble metal loadings,Rh/Pd and Pd/Rh composites-modified Ni foam electrodes,and Rh-Pd alloy-modified Ni foam electrodes were fabricated by electrodepositions.At fitst,the HDF activity of these electrodes was compared in an aqueous solution with optimized pH value.Secondly,the surface morphology and composition of the Rh-Pd alloy-modified Ni foam?Rh-Pd/Ni?electrodes were analyzed by XRD and SEM assembled with EDS.Next,we also compared the degradation pathways of 4-FP on Rh-Pd/Ni electrode and Rh/Ni electrode.Finally,the mechanism of 4-FP degradation on Rh-Pd/Ni electrode was investigated using inductively coupled plasma optical emission spectrometer?ICP-OES?analysis,thermodynamic analysis,hydrogenation with H₂ as a reducing agent,and cyclic voltammetry?CV?.The experimental results show that:The Rh-Pd alloy catalyst achieved more rapid HDF of 4-FP and the subsequent ring hydrogenation?RH?of the HDF product?phenol?to afford cyclohexanone?CYC.one?and cyclohexanol?CYC.ol?than that of Rh catalyst;The fastest HDF of 4-FP was achieved at pH 3 on the Rh-Pd/Ni foam electrode,and both the conversion of 4-FP and the yield of F^-could be over 97%;In addition to those supplied by the applied current,the electrons involved in the HDF of 4-FP and the RH of phenol were largely provided by the oxidative dissolution of the Ni support;The HDF of 4-FP probably follows an indirect mechanism?featuring adsorbed H atoms as the direct reductant?and that the adsorption strength of the catalyst with 4-FP significantly influences its HDF activity.A more stable carbon paper was used as the substrate to prepare Rh/C and Rh-Pd/C electrodes.Galvanostatic electrolysis and CV experiment was utilized to study the HDF process of 4-FP on the Rh-Pd/C electrode.First,we studied the effects of carbon paper types and Pd and Rh loadings on the HDF activity of the electrodes.Then,the electrochemical characteristics of 4-FP on the Rh-Pd/C electrode were tested.Moreover,process parameters,such as current density,pH of catholyte,reaction temperature,were optimized.Finally,we investigated the degradation pathways of 4-FP.The experimental results show that:On the Rh-Pd/C electrode,rhodium and palladium exist as Rh-Pd alloys;The adsorption capacity of the 4-FP and its reduction products on Rh-Pd/C electrodes decrease in the following order:4-FP>PhOH>CYC.one>CYC.ol;Under the optimised conditions(catholyte pH=3,the current density=1.6 mA cm^-2,and the reaction temperature=303 K),the HDF of4-FP could be achieved with 96%conversion of 4-FP,and 94%yield of F^-;The main the degradation pathway of 4-FP on Rh-Pd/C electrode and Rh-Pd/Ni electrode is same,that is 4-FP?PhOH?CYC.one?CYC.ol.The high HDF activity of the Rh-Pd alloy and its tendency to afford less toxic end products make this catalyst potentially suitable for the remediation of FOPs-contaminated water.