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 CHx 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. CHx 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.