Coverage-dependent dipole coupling for carbon monoxide adsorbed at ordered platinum(111)-aqueous interfaces: Structural and electrochemical implications

Surface infrared spectra in the terminal C–O stretching region are reported for various 12CO/13CO mixtures adsorbed on an ordered Pt(111) electrode in 0.1 M HClO4 as a function of CO coverage θ (0.05≲θ≲0.6) and electrode potential E. Contributions to the coverage-induced shift in the C–O stretching frequency νtCO arising from dynamic dipole–dipole coupling ΔνD, and from static dipole and ‘‘chemical’’ effects ΔνC, are obtained from the mixed isotope spectra. Similar to the corresponding ultrahigh vacuum (UHV) system, the former provides the largest contribution, ΔνD being 30–35 cm−1 for saturated CO layers (θ≊0.6). Substantial differences in the νtCO -θ (and ΔνD -θ) dependencies were observed for CO adlayers formed by dosing from suitably low solution concentrations (∼2×10−5 M), and from saturated irreversibly adsorbed layers by means of partial electrooxidative stripping. Using the latter ‘‘stripping’’ procedure, only small (≲5 cm−1) decreases in νtCO and ΔνD were observed as θ is decreased over the range 0.1≲θ0.6, whereas the former ‘‘dosing’’ procedure yielded substantially larger νtCO -θ and ΔνD -θ dependencies, especially in the presence of coadsorbed hydrogen (at −0.25 V vs SCE) rather than coadsorbed water (at 0.1 V). These behavioral differences are interpreted in terms of coverage-dependent CO island formation. For electrooxidative stripping conditions, extensive island formation survives to low CO coverages, providing direct evidence that CO electrooxidation occurs via nucleation and growth at the island edges. For dosing conditions, smaller and more coverage-dependent CO patches are apparently obtained in the presence of coadsorbed hydrogen. These undergo slightly more facile electrooxidation than the CO islands formed at intermediate coverages by prior stripping. The coverage-dependent frequencies and intensities of the bridging CO form (νbCO ≊1800–1850 cm−1) vs the terminal CO form (νtCO ≊2020–2075 cm−1) are also examined in comparison with corresponding data for related UHV systems. In contrast to Pt(111)/CO in UHV, the bridging site is occupied even at low dosed coverages (θ≳0.05): this is also suggestive of the presence of CO islands having high local coverages. Similarities are noted with vibrational spectra obtained for Pt(111)/CO with coadsorbed water and hydrogen in UHV.