INSITU INFRARED-SPECTROSCOPY OF CO ADSORBED AT ORDERED PT(110)-AQUEOUS INTERFACES

Surface infrared spectra in the terminal CO stretching region are reported for CO adsorbed on ordered Pt(110) in 0.1 M HClO4 as a function of CO coverage, θ (0.05 ≲ θ ≲ 1.0) and electrode potential, E (-0.25 V ≤ E < 0.2 V versus SCE), primarily to explor the effects of coverage-dependent surface reconstruction. Substantial differences in the coverage-dependent CO stretching frequency, νιCO, were observed for CO adlayers formed directly by dosing from suitably dilute solution concentrations (~ 2 × 10-5 M), and by partial electrooxidative removal ("stripping") from saturated irreversibly adsorbed layers. The latter procedure typically yielded only small (≲ 5 cm-1) decreases in νιCO and essentially constant bandwidth (FWHM ≈ 11 cm-1) from θ = 1.0 to 0.1, the presence of coadsorbed hydrogen (at -0.25 V) and with coadsorbed water (at 0.05 V). This indicates that large domains ("islands") of close-packed CO remain on patches of (1 × 1) substrate even down to low average coverages under these conditions. For CO adlayers formed by means of the former ("dosing") procedure at 0.05 V, however, a separate lower-frequency νCO band (at 2030-2052 cm-1) appears for θ ≲ 0.6, supplementing the higher-frequency feature (at 2060-2072 cm-1) and supplanting it for θ ≲ 0.4. The lower-frequency νCO band is attributed tentatively to CO bound within (1 × 2) reconstructed Pt(110) domains, having smaller local coverages (i.e. packing densities). Voltammetric electrooxidation of this low-frequency νCO form, as deduced from simultaneous infrared and electrochemical measurements, occurs at significantly (0.1-0.15 V) lower overpotentials than for the high-frequency form. The corresponding νιCO-θ behavior in the presence of coadsorbed hydrogen (i.e. at -0.25 V) is substantially different, a single νCO band exhibiting a continuous frequency decrease from 2072 (θ = 1.0) to 2005 cm-1 (θ ≈ 0.1) being obtained. Qualitatively similar νιCO-θ behavior was also observed in the presence of adsorbed sulfate or chloride anions. This is attributed to co-adsorption yielding a wider and more θ-dependent distribution of CO island sizes. Significant differences are noted with the corresponding infrared spectra observed for the Pt(110)/CO in UHV at 300 K, with regard to the νCO frequency-, band intensity-, and bandwidth-θ dependencies. The absolute νιCO values for the electrochemical and UHV systems also differ when they are compared at the same surface potential. These differences are attributed both to the effects of the double-layer field and to co-adsorbing components on the structure of the electrochemical CO adlayer. © 1990.