Rh-Pd-alloy catalyzed electrochemical hydrodefluorination of 4-fluorophenol in aqueous solutions

Abstract

Hydrodefluorination (HDF) is the key step in fluoroaromatic pollutant (FAP) degradation, 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 HDF catalyst (a Rh-Pd alloy) and investigated the underlying catalytic reaction mechanism, using 4-fluorophenol (4-FP) as a model FAP. Specifically, the rapid aqueous-phase HDF of 4-FP at pH 3 was achieved on a Rh-Pd alloy-modified Ni foam electrode, which was followed by ring hydrogenation (RH) of the HDF product (phenol) to afford cyclohexanone and cyclohexanol. Inductively coupled plasma optical emission spectrometer (ICP-OES) and thermodynamic analyses showed that, 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 foam support. Hydrogenation and cyclic voltammetry experiments indicated that the HDF of 4-FP probably follows an indirect mechanism (featuring adsorbed H atoms as the direct reductant) and that the strength of 4-FP adsorption onto the surface of the catalyst significantly influences its HDF activity.