Mechanistic Study of the Validity of Using Hydroxyl Radical Probes To Characterize Electrochemical Advanced Oxidation Processes

The detection of hydroxyl radicals (OH^•) is typically accomplished by using reactive probe molecules, but prior studies have not thoroughly investigated the suitability of these probes for use in electrochemical advanced oxidation processes (EAOPs), due to the neglect of alternative reaction mechanisms. In this study, we investigated the suitability of four OH^• probes (coumarin, p-chlorobenzoic acid, terephthalic acid, and p-benzoquinone) for use in EAOPs. Experimental results indicated that both coumarin and p-chlorobenzoic acid are oxidized via direct electron transfer reactions, while p-benzoquinone and terephthalic acid are not. Coumarin oxidation to form the OH^• adduct product 7-hydroxycoumarin was found at anodic potentials lower than that necessary for OH^• formation. Density functional theory (DFT) simulations found a thermodynamically favorable and non-OH^• mediated pathway for 7-hydroxycoumarin formation, which is activationless at anodic potentials > 2.10 V/SHE. DFT simulations also provided estimates of E° values for a series of OH^• probe compounds, which agreed with voltammetry results. Results from this study indicated that terephthalic acid is the most appropriate OH^• probe compound for the characterization of electrochemical and catalytic systems.