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 CO_ad 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 (CH₃CO_ad) 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 CH₃CO_ad is decomposed to α-CO_ad and β-CH_x species on the Pd electrode at potentials more negative than approximately −0.1 V; the α-CO_ad species is oxidized to CO₂ at potentials more positive than approximately −0.3 V, while the β-CH_x species may be first converted to CO_ad at approximately −0.1 V and further oxidized to CO₂ at more positive potentials.