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

<i>In situ</i> attenuated total reflection surface-enhanced infrared absorption spectroscopy in conjunction with H–D isotope replacement is used to investigate the dissociation and oxidation of CH<sub>3</sub>CH<sub>2</sub>OH 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<sup>–1</sup> showing up prior to the multiply bonded CO<sub>ad</sub> band. CH<sub>3</sub>CD<sub>2</sub>OH and D<sub>2</sub>O are used to exclude the spectral interference with this band from interfacial acetaldehyde and H<sub>2</sub>O, respectively, confirming for the first time that the ∼1625 cm<sup>–1</sup> band is due to the adsorbed acetyl on the Pd electrode in alkaline media. The spectral results suggest that the as-adsorbed acetyl (CH<sub>3</sub>CO<sub>ad</sub>) 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 CH<sub>3</sub>CO<sub>ad</sub> is decomposed to α-CO<sub>ad</sub> and β-CH<sub><i>x</i></sub> species on the Pd electrode at potentials more negative than approximately −0.1 V; the α-CO<sub>ad</sub> species is oxidized to CO<sub>2</sub> at potentials more positive than approximately −0.3 V, while the β-CH<sub><i>x</i></sub> species may be first converted to CO<sub>ad</sub> at approximately −0.1 V and further oxidized to CO<sub>2</sub> at more positive potentials.