Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammograms. Part 2: Pt(100)

As in previous work with Pt(1 1 1) electrodes, we have combined CO-stripping cyclic voltammetry, CO charge–displacement and FT-IR spectroscopy measurements to determine changes in the coverage and structure of CO adlayers on Pt(1 0 0) electrodes in 0.1 M H2SO4 as a function of potential. In CO-free solutions the maximum coverage is θCO = 0.79, which can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, Ed) is more negative than 0.25 V vs. RHE. Although the highest CO coverage of Pt(1 0 0) electrodes in contact with CO-saturated solutions could not be determined, our FT-IR results clearly show that, as in the case of Pt(1 1 1) electrodes, removing CO from the solution results in a partial, reversible desorption of the CO adlayer, and, hence, that the CO adlayer on the Pt(1 0 0) electrode is in equilibrium with the CO-containing solution. The lowest CO coverage at which hydrogen adsorption on the Pt(1 0 0) electrode is completely blocked is θCO = 0.75, which corresponds to Ed = 0.40 V vs. RHE. The results reported here provide support to the hypothesis that the process at the pre-peak in CO-stripping voltammograms (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of CO by reaction with oxygenated species nucleating at steps, the main CO-oxidation peak appearing when nucleation of oxygenated species at the terraces also occurs.