Coverage- and potential-dependent binding geometries of carbon monoxide at ordered low-index platinum- and rhodium-aqueous interfaces: comparisons with adsorption in corresponding metal-vacuum environments

Abstract

Infrared spectra for carbon monoxide adsorbed at ordered low-index platinum- and rhodium-aqueous interfaces as a function of electrode potential, E, and CO coverage, θ coare compared with each other and with vibrational spectra obtained at corresponding metal-ultrahigh vacuum (UHV) surfaces in order to explore the influences of the doublelayer environment on the CO adlayer structure. Earlier results for Pt(111), (100), (110), and Rh(100) electrodes in aqueous perchloric acid are supplemented by additional data in neutral and alkaline electrolytes in order to expand the accessible potential range. New results are also reported for Rh(111) and Rh(110) for each of these conditions. The form of the electrochemical infrared spectra in the C-O stretching ( v CO) region, especially the relative intensities of the characteristic terminal and twofold bridging v CO bands, tends to be most similar to spectra obtained at the metal-UHV surfaces at high θ CO and when the former are evaluated at relatively positive potentials. Altering the potential in the negative direction on most electrodes at high θ CO favors increasingly CO binding in bridging rather than terminal sites, especially on the rhodium surfaces. This is consistent with the greater extent of dπ-2π ∗ metal-CO back donation expected under these conditions. Bridging CO coordination is also favored increasingly at most electrochemical interfaces towards lower θ CO, these effects of water and/or hydrogen coadsorption are compatible with the behavior of analogous UHV systems. The v CO frequency- E slopes at constant adsorption site occupancy vary substantially with the CO binding geometry, being ∼ 30–35, 40–45, and 50–60 cm −1 V −1 for terminal, twofold and threefold bridging sites, respectively, at high θ CO. These findings, together with the observed increases in d v CO/d E towards lower θ CO. are consistent with the anticipated variations in back bonding. At least at high θ CO, the observed smaller v CO frequencies for the electrochemical relative to the corresponding UHV interfaces are largely compatible with the lower surface potentials that characterize the former, along with differences in site occupancy.