Electrochemical and infrared spectroscopic quantitative determination of the platinum-catalyzed ethylene glycol oxidation mechanism at CO adsorption potentials

The electrochemical mechanism for ethylene glycol oxidation by polycrystalline platinum at 0.30 V/SCE in 0.10 M HClO4 is investigated by reflection infrared spectroscopic, coulometric, and voltammetric measurements of 0.10 M ethylene glycol, glycolaldehyde, glyoxal, glycolic acid, glyoxylic acid, and oxalic acid. CO2, glycolic acid, and adsorbed CO are identified as reaction products for ethylene glycol and glycolaldehyde oxidation. A two-path mechanism is proposed for 0.30 V oxidation of ethylene glycol and glycolaldehyde: either the reacting molecule undergoes direct oxidation to desorbing glycolic acid or it undergoes direct dissociation of the carbon-carbon bond to form various amounts of aqueous CO;! and adsorbed CO. Calculations are performed, assuming the quantities of CO and CO2 depend statistically upon the identities of the two functional groups comprising the two-carbon reactant molecule and upon oxidation conditions. Calculation results for dissociation at 0.30 V show that nearly 100% of the carboxyl functional groups are oxidized to CO2, whereas 20% and 50-67%, respectively, of the alcohol and aldehyde groups are partially oxidized to adsorbed CO. About 20% of the ethylene glycol molecules undergo bond dissociation, whereas 25-40% of the glycolaldehyde molecules dissociate. In 70 s of electrochemical oxidation, about three times as many ethylene glycol molecules react as glycolaldehyde molecules.