Mechanism of perchlorate formation on boron-doped diamond film anodes

This research investigated the mechanism of perchlorate (ClO ₄^-) formation from chlorate (ClO₃^-) on boron-doped diamond (BDD) film anodes by use of a rotating disk electrode reactor. Rates of ClO₄^- formation were determined as functions of the electrode potential (2.29-2.70 V/standard hydrogen electrode, SHE) and temperature (10-40 °C). At all applied potentials and a ClO ₃^- concentration of 1 mM, ClO₄^- production rates were zeroth-order with respect to ClO₄^- concentration. Experimental and density functional theory (DFT) results indicate that ClO₃^- oxidation proceeds via a combination of direct electron transfer and hydroxyl radical oxidation with a measured apparent activation energy of 6.9 ± 1.8 kJ·mol^-1 at a potential of 2.60 V/SHE. DFT simulations indicate that the ClO₄^- formation mechanism involves direct oxidation of ClO₃^- at the BDD surface to form ClO₃^•, which becomes activationless at potentials > 0.76 V/SHE. Perchloric acid is then formed via the activationless homogeneous reaction between ClO₃ ^• and OH^• in the diffuse layer next to the BDD surface. DFT simulations also indicate that the reduction of ClO ₃^• can occur at radical sites on the BDD surface to form ClO₃^- and ClO₂, which limits the overall rate of ClO₄^- formation.