Substrate oxidation enhances the electrochemical production of hydrogen peroxide

Hydrogen peroxide (H₂O₂) is electrochemically produced via oxygen (O₂) reduction on a carbon cathode surface. In order to enhance the production of H₂O₂, anodic loss pathways, which significantly reduce the overall H₂O₂ production rate, should be inhibited. In this study, we investigate the effects of organic electron donors (i.e., typical chemical contaminants) on the anodic loss pathways of H₂O₂ in a single-cell electrochemical reactor that employs an anode composed of TiO₂ over-coated on a mixed-metal oxide ohmic contact catalyst, Ir_0.7Ta_0.3O₂, deposited on a Ti-metal that is coupled with a graphite rod cathode in a sodium sulfate (Na₂SO₄) electrolyte that is saturated with oxygen (O₂). Organic electron donors are shown to enhance the electrochemical production of H₂O₂, while simultaneously undergoing oxidative degradation. The observed positive effect of organic electron donors on the electrochemical production of H₂O₂ is due in part to a preferential adsorption of organic substrates on the TiO₂ outer layer of the anode. The sorption of the organic electron donors inhibits the formation of surficial titanium hydroperoxo species ([bond, triple bond]Ti-OOH) on the anode surface. The organic sorbates also act as scavengers of surface-bound hydroxyl radical [bond, triple bond]Ti-OH. As a result, the decomposition of H₂O₂ on the anode surface is significantly reduced. The cathodic production rate of H₂O₂ at low pH is enhanced due to proton coupled electron transfer (PCET) to O₂, while the anodic decomposition of H₂O₂ is inhibited due to electrostatic interactions between negatively-charged organic substrates and a positively-charged outer surface of the anode (TiO₂ pH_zpc = 5.8) at low pH.