Electrochemical infrared spectroscopy of carbon monoxide on ordered rhodium (111): Comparisons with vibrational spectra on Pt(111) and in related surface–vacuum environments

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Abstract
Surface infrared spectra are reported for carbon monoxide adsorbed on a well-ordered Rh(111) crystal in 0.1 M HClO4 , and in neutral and alkaline electrolytes as a function of CO coverage θ as well as electrode potential. The results are compared in detail with corresponding vibrational spectra obtained recently for CO at the Pt(111)–aqueous interface, and on Rh(111) and Pt(111) in ultrahigh vacuum (uhv) in the absence and presence of coadsorbed water, in order to explore the possible influences of the aqueous electrochemical environment upon the CO surface coordination geometry. The potential-dependent surface infrared spectra for high CO coverages in 0.1 M HClO4 exhibited a major C–O stretching band (νCO ), due to terminal (i.e., linearly bound) CO at 2025–2042 cm−1 over the potential range −0.25 to 0.25 V vs SCE; a weaker νCO band, attributed to CO bound to twofold bridging sites, was observed at 1792–1815 cm−1 under these conditions. The latter form undergoes electro-oxidation preferentially, as deduced from spectral sequences obtained during slow voltammetric sweeps. At CO coverages close to saturation, θ≊0.75, the linear/bridged site occupancy ratio is estimated to be roughly 2. At lower CO coverages within the hydrogen adsorption region, a band at about 1870–1875 cm−1 appears at the expense of the terminal νCO feature. Corresponding spectra obtained in neutral 0.1 M NaClO4 and alkaline electrolytes also yielded similar terminal and bridging νCO bands, downshifted in frequency from those in 0.1 M HClO4 to an extent (≲50 cm−1 ) commensurate with the more negative electrode potentials involved. Comparison of the electrochemical infrared spectra with vibrational spectra obtained for the analogous Rh(111)/CO system in uhv indicates that the presence of the aqueous environment yields only small (≲20 cm−1 ) downshifts in the νCO band frequencies. Essentially the same conclusion is obtained from a corresponding comparison between vibrational spectra obtained at the electrochemical and uhv Pt(111)/CO interfaces. Cognizance is taken in these analyses of the known (or anticipated) differences in the potentials of zero charge and work functions in the electrochemical and uhv environments, respectively. These findings are also compared with recent vibrational spectra obtained for CO coadsorbed with water on Rh(111) and Pt(111) in uhv at low temperatures.

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