Substrate Oxidation Enhances the Electrochemical Production of Hydrogen Peroxide

Abstract

Hydrogen peroxide (H_2O_2) is electrochemically produced via oxygen (O_2) reduction on a carbon cathode surface. In order to enhance the production of H_2O_2, anodic loss pathways, which significantly reduce the overall H_2O_2 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 H_2O_2 in a single-cell electrochemical reactor that employs an anode composed of TiO_2 over-coated on a mixed-metal oxide ohmic contact catalyst, Ir_(0.7)Ta_(0.3)O_2, deposited on a Ti-metal that is coupled with a graphite rod cathode in a sodium sulfate (Na_2SO_4) electrolyte that is saturated with oxygen (O_2). Organic electron donors are shown to enhance the electrochemical production of H_2O_2, while simultaneously undergoing oxidative degradation. The observed positive effect of organic electron donors on the electrochemical production of H_2O_2 is due in part to a preferential adsorption of organic substrates on the TiO_2 outer layer of the anode. The sorption of the organic electron donors inhibits the formation of surficial titanium hydroperoxo species ( Ti-OOH) on the anode surface. The organic sorbates also act as scavengers of surface-bound hydroxyl radical Ti-OH. As a result, the decomposition of H_2O_2 on the anode surface is significantly reduced. The cathodic production rate of H_2O_2 at low pH is enhanced due to proton coupled electron transfer (PCET) to O_2, while the anodic decomposition of H_2O_2 is inhibited due to electrostatic interactions between negatively-charged organic substrates and a positively-charged outer surface of the anode (TiO_2 pH_(zpc) = 5.8) at low pH.