Title

Investigation of the paired synthesis of ozone and hydrogen peroxide in a proton exchange membrane reactor

Date of Completion

January 1993

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

The simultaneous use of ozone and hydrogen peroxide (an Advanced Oxidation Process) has been shown to successfully destroy low concentrations of hazardous organic compounds present in wastewater streams. This study explored the paired synthesis of ozone and hydrogen peroxide at the two electrodes in a proton exchange membrane (PEM) reactor. An electrochemical reactor and a flow system were designed and constructed for this purpose. The oxidants were generated using pure water and oxygen as the reactants with the proton exchange membrane serving as the electrolyte as well as the separator between the cathodic and the anodic sections of the reactor.^ The research methodology was split up into four reaction systems: (1) oxygen evolution at the anode and hydrogen evolution at the cathode (water electrolysis), (2) ozone + oxygen evolution at the anode and hydrogen evolution at the cathode, (3) oxygen reduction at the cathode (leading to hydrogen peroxide synthesis) and oxygen evolution at the anode, and (4) simultaneous synthesis of ozone and hydrogen peroxide. The optimum conditions for generating the oxidants were determined as a function of the applied voltage, electrode materials catalyst loadings, reactant flowrates, and pressure. Measured and calculated quantities included the cell current, aqueous and liquid phase oxidant concentrations, oxidant production rates, and current efficiencies for oxidant evolution.^ Ozone was synthesized at the anode at room temperature and pressure using lead dioxide powders bonded to a proton exchange membrane (Nafion$\sp{\rm R}$ 117). Maximum ozone concentrations of 3.1 mg/l in the aqueous phase and 50 mg/l in the gas phase were obtained using a lead dioxide loading of 7.2 mg/cm$\sp2$ and an aqueous flowrate of 27.5 l/hr yielding a current efficiency of 5.5%. The cathodic reaction was hydrogen evolution.^ Hydrogen peroxide was synthesized at the cathode at room temperature and 15 psig with an oxygen flowrate of 100 cc/min. The electrocatalysts investigated were different loadings of gold, carbon and graphite powders which were either bonded to the membrane or on to a carbon fiber paper pressed up against the membrane. Peroxide evolution was observed from all the catalysts that were studied with the graphite powders yielding the highest concentrations (25 mg/l). The corresponding anodic reaction was oxygen evolution.^ Finally, ozone and hydrogen peroxide were synthesized simultaneously hence demonstrating the feasibility of the process. However, the high power consumption (345 kwh/kg O$\sb3$) required restricts its use until higher oxidant current efficiencies are obtained at lower applied potentials. ^

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