Electrocatalysis of Ethanol on a Pd Electrode in Alkaline Media: An in Situ Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy Study

In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy in conjunction with H–D isotope replacement is used to investigate the dissociation and oxidation of CH3CH2OH on a Pd electrode in 0.1 M NaOH, with a focus on identifying the chemical nature of the pivotal intermediate in the so-called dual-pathway (C1 and C2) reaction mechanism. Real-time spectroelectrochemical measurements reveal a band at ∼1625 cm–1 showing up prior to the multiply bonded COad band. CH3CD2OH and D2O are used to exclude the spectral interference with this band from interfacial acetaldehyde and H2O, respectively, confirming for the first time that the ∼1625 cm–1 band is due to the adsorbed acetyl on the Pd electrode in alkaline media. The spectral results suggest that the as-adsorbed acetyl (CH3COad) is oxidized to acetate from approximately −0.4 V as the potential moves positively to conclude the C2 pathway. Alternatively, in the C1 pathway, the CH3COad is decomposed to α-COad and β-CHx species on the Pd electrode at potentials more negative than approximately −0.1 V; the α-COad species is oxidized to CO2 at potentials more positive than approximately −0.3 V, while the β-CHx species may be first converted to COad at approximately −0.1 V and further oxidized to CO2 at more positive potentials.