Adsorbed Enolate as the Precursor for the C–C Bond Splitting during Ethanol Electrooxidation on Pt

Ethanol is a promising alternative fuel to methanol for direct alcohol fuel cells. However, the complete electrooxidation of ethanol to CO₂ involves 12 electrons and C-C bond splitting so that the detailed mechanism of ethanol decomposition/oxidation remains elusive. In this work, a spectroscopic platform, combining SEIRA spectroscopy with DEMS, and isotopic labeling were employed to study ethanol electrooxidation on Pt under well-defined electrolyte flow conditions. Time- and potential-dependent SEIRA spectra and mass spectrometric signals of volatile species were simultaneously obtained. For the first time, adsorbed enolate was identified with SEIRA spectroscopy as the precursor for C-C bond splitting during ethanol oxidation on Pt. The C-C bond rupture of adsorbed enolate led to the formation of CO and CH_x ad-species. Adsorbed enolate can also be further oxidized to adsorbed ketene at higher potentials or reduced to vinyl/vinylidene ad-species in the hydrogen region. CH_x and vinyl/vinylidene ad-species can be reductively desorbed only at potentials below 0.2 and 0.1 V, respectively, or oxidized to CO₂ only at potentials above 0.8 V, and thus they poison Pt surfaces. These new mechanistic insights will help provide design criteria for higher-performing and more durable electrocatalysts for direct ethanol fuel cells.