A detailed investigation of electrochemical hydrodefluorination of 4-fluorophenol: Rh-doped cathode preparation, reaction conditions optimizations, reactive species identification, and in situ characterization

The discharge of widely used fluorinated aromatics (FAs) leads to their notoriously high persistence and toxicity in natural environments. Defluorination is the key step for FAs pollutants detoxification. Due to its easy operation and mild reaction conditions, hydrodefluorination (HDF), especially electrochemical HDF, is a promising approach for FAs detoxification. A comprehensive and detailed investigation of the effective cathode and mechanism insights are contributable for the development of effective FAs defluorination techniques. In this study, 4-fluorophenol (4-FP) was selected as a model compound to optimize the electrochemical HDF process and understand the reaction mechanism. Cathode coating material investigation revealed that Rh outperformed Pd due to its higher reactivity towards CF bond cleavage. The Rh coating was primarily elemental and in Rh(1 1 1) crystalline form which helped store the generated atomic hydrogen (H*). Reaction conditions optimizations showed 4-FP degradation favored acidic > alkaline > neutral environment and a cell potential higher than −3 V was required for efficient 4-FP degradation. Dissolved oxygen (DO) negatively affected 4-FP degradation and the rapid 4-FP decay only occurred after DO depleted in the solution. Based on quenching experiments and ESR analysis, H* was determined as the main reactive species. In situ Raman characterization suggested that electrochemical 4-FP degradation was initiated by 4-FP adsorption through R···F···Rh bridging and H* generation/adsorption on Rh coated cathode, resulting in phenol generation. The generated phenol further experienced hydrogenation to yield cyclohexanone and cyclohexanol with lower toxicity and better biodegradation potential, demonstrating that electrochemical treatment of 4-FP simultaneously achieved defluorination along with detoxification. The outcomes of this work provide support for electrocatalyst technology development towards efficient FAs treatment while providing in-depth observation of the electrochemical HDF process.