Revisiting the Acetaldehyde Oxidation Reaction on a Pt Electrode by High-Sensitivity and Wide-Frequency Infrared Spectroscopy

High-sensitivity and wide-frequency attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is highly demanded in unraveling electrocatalytic processes at the molecular level. In this work, an in situ ATR-SEIRAS technique incorporating a micromachined Si wafer window, <i>p</i>-polarized infrared radiation, and isotope labeling is extended to revisit the acetaldehyde oxidation reaction (AOR) on a Pt electrode in an acidic medium. New spectral features in the fingerprint region are detected, including ω­(C–H) at 1078 cm^–1 and ν_as(C–C–O) at 919 cm^–1 for adsorbed acetaldehyde and δ­(O–C–O) at 689 cm^–1 for adsorbed acetate, besides the other enhanced and clearly discriminated spectral signals at higher frequencies. Time-evolved and potential-dependent ATR-SEIRAS measurements together with advanced density functional theory calculations considering the coadsorption of CO and C₂ species enable clarification of the structures and roles of surface C₂ intermediates (η¹(C)-acetyl and η¹(H)-acetaldehyde), as reflected by the two bands at 1630 and 1663 cm^–1, respectively, leading to updated pathways for the AOR on a Pt electrode.