Abstract
The electrooxidation pathways of ethanol and acetaldehyde in 0.1 M HClO 4 on ordered Pt (111) surfaces in comparison with disordered Pt (111) and polycrystalline Pt were examined by means of FTIR spectra obtained in real time during slow (2–10 mV s −1) thin-layer voltametric sweeps. This “single potential alteration infrared” (SPAIR) procedure enables the potential-dependent quantity of the various reaction products, acetic acid, acetaldehyde, and CO 2 to be determined quantitatively, and the role of CO and other CO 2-producing adsorbates in the reaction kinetics to be assessed. For ethanol electrooxidation on ordered Pt (111), acetic acid and acetaldehyde are formed, along with smaller amounts of CO 2 beyond ca. 0.3 V (vs. SCE) where oxidation of adsorbed CO occurs; acetic acid is the major product during the reverse potential sweep. Only small CO coverages (θ ≈ 0.1) are obtained for ethanol concentrations of 0.05-0.5 M. Substantially higher CO coverages are obtained for disordered Pt (111) and especially polycrystalline Pt. The product distributions are not altered greatly from ordered Pt (111). Markedly different behavior, however, was observed for acetaldehyde electrooxidation on ordered Pt (111) in that little acetic acid is formed until near the end of the positive-going potential sweep, 0.8 V. A greater quantity of adsorbed CO (θ ≈ 0.15) is present, but this undergoes complete electrooxidation by 0.45 V. This marked reaction inhibition is attributed instead to the presence of another (or other) adsorbed species, which is observed to undergo oxidation to CO 2 only between 0.5–0.7 V. The inhibition of acetic acid formation is much less marked on disordered Pt (111), and essentially absent on polycrystalline Pt. Comparisons are also made between the electrooxidation kinetics for some related unifunctional reactants, including formic acid and isopropanol, on ordered and disordered Pt (111) surfaces. Markedly smaller amounts of adsorbed CO are generally formed from organic species on the former surface; some possible electrocatalytic consequences are discussed.
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