INSITU INFRARED-SPECTROSCOPY OF CARBON-MONOXIDE ADSORBED AT IRIDIUM(111)-AQUEOUS INTERFACES - DOUBLE-LAYER EFFECTS ON THE ADLAYER STRUCTURE

Surface infrared spectra in the C-O stretching (nu-CO) frequency region are reported for CO adsorbed on ordered Ir(111) in aqueous media, primarily 0.1 M HClO4, as a function of CO coverage, theta-CO, and electrode potential, E. In contrast to other platinum-group (111) surfaces examined hitherto, the adsorption sites were found to be exclusively atop (or near-atop), even at potentials down to -0.8 V vs SCE (in 0.1 M NaClO4 + 1 mM KOH), as discerned from the appearance of a single nu-CO feature at ca. 1960-2060 cm-1. Large (up to ca. 100 cm-1) frequency upshifts are observed at increasing dosed coverages, especially for adsorption in the "hydrogen" potential region (ca. -0.25 to -0.05 V vs SCE), where fractional CO coverages of up to 0.6 could be obtained. Significantly smaller saturated theta-CO values, ca. 0.45, were attained for adsorption in the "double-layer" region, 0-0.2 V. Adlayers formed in these two potential regions also exhibit different spectral and electrochemical properties. For example, CO adlayers prepared within the hydrogen region on Ir(111) exhibit extensive island formation during electrooxidative removal, as discerned from the relatively invariant nu-CO(t) values, in a similar fashion to adlayers on low-index Pt and Rh surfaces. For adlayers formed in the double-layer region on Ir(111), however, CO island formation is rather more limited under these conditions, as seen from a markedly larger nu-CO(t)-theta-CO dependence which matches more closely that obtained for partial coverages formed by direct dosing with dilute CO solutions.