INFRARED-SPECTROSCOPY AS A PROBE OF CO ADSORPTION AT PT(335) UNDER AQUEOUS ELECTROCHEMICAL CONDITIONS

Experiments measure the infrared spectral features for carbon monoxide adsorbed at Pt(335) (Pt(S)-[4(111) X (100)]) under aqueous electrochemical conditions. The coverage and potential dependence of the C-O stretching vibrational mode is measured for CO adlayers formed by dosing under potential control from electrolyte solutions containing dilute levels (ca.4x10(-5) M) of dissolved CO. Infrared bands assignable to CO chemisorbed at sites on the step edge and sites on the terrace in both on-top and bridging coordination environments are observed. At low coverages, spectral features appear that are attributable to CO at on-top and bridging sites on the step edge. Plots of on-top CO peak energy versus electrode potential are unexpectedly nonlinear over the range of low coverages, with slopes that approach zero at potentials in the classical hydrogen adsorption region and increase to large values at potentials in the double layer region. At intermediate coverages, spectral features appear that are assignable to on-top and bridging CO at both terrace and edge sites. The effects of dipole-coupling between terrace and edge CO adsorbates become apparent at intermediate coverages, as the intensity of the higher energy feature for on-top CO at terrace sites is enhanced at the expense of the lower energy feature for on-top CO at edge sites. At the highest coverages, dipole-coupling effects become a major factor in determining the appearance of the infrared spectral features; the on-top CO vibrational bands are identical to features observed for CO at Pt(111) under analogous electrochemical conditions, while the spectral features of bridging CO coincide with those observed for CO at Pt(100) in the high-coverage limit. Potential-and coverage-dependent structural transformations of the adsorbed CO adlayer are discussed within the context of models derived in earlier studies which probed CO adsorption at stepped platinum surfaces through the use of imaging and vibrational spectroscopic techniques.