INSITU INFRARED-SPECTROSCOPY OF CO ADSORBED AT ORDERED PT(100)-AQUEOUS INTERFACES - DOUBLE-LAYER EFFECTS UPON THE ADSORBATE BINDING GEOMETRY

Surface infrared spectra for carbon monoxide adsorbed on ordered Pt(100) in 0.1 M HClO4 are reported as a function of CO coverage, 0.1 ≲ θ ≲ 0.85, and electrode potential, -0.25 V ≤ E ≲ 0.25 V vs SCE. Vibrational C-O stretching bands, vCO, corresponding to both terminal and 2-fold bridging geometries are observed, with intensities that depend markedly upon E and θ. For coverages formed by dosing with dilute (ca. 2 × 10-5 M) CO solutions for varying times (≲15 min), alterations in θ at positive potentials (E ≳ 0.1 V) yield dramatic changes in the site occupancy in that bridging and terminal features are dominant for θ ≲ 0.3 and θ 0.7, respectively. Qualitatively similar, yet less marked, θ-induced alterations in the CO binding geometry are observed at negative potentials (E < 0 V vs SCE), where hydrogen rather than water constitutes the major coadsorbate. The potential-induced changes in site occupancy at near-saturation CO coverages (θ ≈ 0.8-0.85), increasingly favoring terminal versus bridging CO at more positive potentials, are ascribed to the diminishing influence of dπ-2π* back-bonding under these conditions. The markedly different potential-dependent site occupancies observed at lower coverages are attributed to the influence of coadsorbed hydrogen and water. As for the corresponding Pt(111) and Pt(110) electrochemical surfaces, coverage-dependent spectra for CO layers formed by electrooxidative stripping exhibit much smaller decreases in band frequency than for coverages attained directly by solution CO dosing. Combined with coverage-dependent spectral measurements of dipole-dipole coupling by utilizing 12CO/13CO mixtures, these results indicate that large CO islands are formed during adsorbate electrooxidation. Dissipation of these domains into adsorbate structures similar to those obtained by solution CO dosing occurs typically within ca. 15 min at -0.25 V. Significant differences are also observed in the voltammetric electrooxidation of subsaturated CO layers formed under "stripping" and "dosing" conditions, the latter but not the former exhibiting increasingly facile kinetics for lower initial coverages. © 1990 American Chemical Society.