In situ infrared spectroscopy of well-defined single-crystal electrodes: adsorption and electrooxidation of carbon monoxide on platinum(111)

Abstract

Surface infrared spectra have been obtained for carbon monoxide adsorbed on an ordered Pt(111) crystal in 0.1 M HClO/sub 4/ and other acidic electrolytes as a function of CO coverage as well as electrode potential. Variations in the CO coverage, theta, were made by partial electrooxidation; theta was evaluated from the quantity of CO/sub 2/ product produced as measured from the 2343-cm/sup /minus/-1/ infrared band absorbance combined with the anodic voltammetric charge. The potential-dependent surface infrared spectra exhibited a major ..nu../sub CO/ band at 2050-2075 cm/sup /minus/1/ due to linearly bound CO, the peak frequency increasing approximately linearly with both increasing coverage and potential. An additional band at 1830-1850 cm/sup /minus/1/, due to bridge-bound CO, was also observed. The relative intensities of the bridged and linear ..nu../sub CO/ forms, I/sub b/, I/sub l/, depend somewhat on theta, the largest intensity ratios, I/sub b//I/sub l/ /approximately/ 0.2-0.3, being observed for 0.4 /approx lt/ theta /approx lt/ 0.5. In contrast to the behavior of the analogous gas-phase system, the integrated intensity of the linear ..nu../sub CO/ band is proportional to theta throughout the accessible coverage range (theta less than or equal to 0.62). The disordered Pt(111) surface formed by repeated voltammetric cyclingmore » into the anodic oxide region yielded a linear ..nu../sub CO/ band upshifted by ca. 10 cm/sup /minus/1/. While the changes in the ..nu../sub CO/ frequency during potentiostatic CO electrooxidation on ordered Pt(111) are independent of the oxidation rates, the disordered surface (as well as polycrystalline Pt) exhibited ..nu../sub CO/ frequency decreases under these conditions that were substantially more pronounced for longer oxidation time scales (/approx gt/ 20 s).« less