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Phase-transfer mediated electroreduction of oxygen to hydrogen peroxide in acid and alkaline electrolytes Gyenge, Elod Lajos

Abstract

Two novel methods for the electroreductioh of O₂ to H₂O₂ have been developed and investigated in a variety of acidic and alkaline solutions. The first technique, unreported before in the literature, uses surfactants and exploits the associated interfacial effects, in order to enhance the direct, 2e⁻ reduction of O₂.The second method described in the present study is based on emulsion mediation of the electroreduction of O₂. The latter procedure employs a three-phase, L/L/G system, composed of an organic media, an aqueous electrolyte and oxygen gas. The organic phase is composed of a redox mediator (i.e. 2-ethyl-9,10-anthraquinone, EtAQ), supporting electrolyte (i.e., tetrabutylammonium perchlorate, TBAP) and cationic surfactant (i.e., tricaprylmethylammonium chloride, Aliquat® 336) dissolved in a solvent (e.g., tributyl phosphate, TBP). Hydrogen peroxide is produced in-situ, by the mediating cycle: EtAQ electroreduction - homogeneous oxidation of the anthrahydroquinone. An in-depth study and synergistic analysis was performed for several phenomena of importance for emulsion mediation, such as physico-chemical and electrochemical properties of the organic media, emulsification characteristics, partition of H₂O₂, mechanism of EtAQ - O₂ electroreduction and multiphase flow dynamics in three-dimensional electrodes. Also, an economic analysis is provided for in-situ H₂O₂ production by emulsion mediation in an acidic electrolyte. The employed experimental techniques involved: cyclic voltammetry at µA current levels, batch electrolysis using currents up to 1 A and electrosynthesis in fixed-bed 'flow-by' electrochemical cells operated at currents up to 60 A . For the electrosynthesis experiments the cathode materials were either graphite felt or reticulated vitreous carbons with 10 to 100 pores per inch. For the surfactant modified, 'direct' O₂ reduction, it was found that a triple-C₈ chain cationic surfactant, Aliquat 336® (i.e. [CH₃(CH₂)₇] ₃CH₃N⁺C1⁻) increased the standard rate constant for O₂ electroreduction to H₂O₂ on vitreous carbon, minimized the H₂O₂ electroreduction and improved the peroxide concentration and current efficiency during electrosynthesis in various acid and alkaline electrolytes. These effects were attributed to an increase of the surface pH induced by the Aliquat 336® surface structures. The emulsion mediation was most effective for acidic peroxide electrosynthesis, where at current densities above 500 A m⁻² the mediated system yielded higher peroxide concentrations and current efficiencies than the surfactant modified 'direct' O₂ electroreduction.

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