Mechanism of Ethanol Oxidation at a Platinum Electrode

Abstract

The kinetics of ethanol oxidation at a roughened Pt electrode was investigated by a potentiostatic pulse method, and the reaction mechanism was discussed on the basis of the kinetic data obtained. Both current and surface coverage during the constant-potential electrolysis of ethanol did not reach to steady states within 6 minutes from the beginning of the electrolysis. However, the current densities per unit area of the "free" electrode surface reached to the steady state after about 200 seconds. In the potential range of 0.30 to 0.45 V vs. RHE, a Tafel relation was found to hold provided that the steady current densities were adopted. The Tafel slope observed was 0.133 V per decade. The reaction order with respect to the ethanol concentration was 0.98 in the ethanol concentration range of 0.05 to 2 mol. dm-3when the surface coverage, the potential and the pH were constant. Based on these kinetic data, it was concluded that the ethanol oxidation occurred in the following sequence in the potential range of 0.30 to 0.45 V
C₂H₅OH_sol →r.d.s. C₂H₅O_ad H⁺+ e
C₂H₅O_ad→CH₂CHO_sol H+ e where "r. d. s. " denoted the rate-determining step. At the potentials above 0.5 V, the ethanol oxidation was inhibited by acetic acid, which was one of oxidation products.