In situ infrared reflection absorption spectroscopic studies of coadsorption of CO with underpotential-deposited lead on Pt(111) in an aqueous acidicsolution

The effects of underpotential-deposited lead on the adsorption of CO on Pt( 111) surfaces have been investigated in 0.1 M HClO4 by in situ infrared ref lection absorption spectroscopy (IRAS). Lead coverages of about 0.4 on Pt(1 11) electrodes polarized at 0.1 V vs RHE prevent CO from adsorbing on multi ply bonded sites, reducing the overall coverage to about a third of that ob served in the absence of coadsorbed Pb under otherwise identical conditions , yielding a single, sharply-defined feature in the infrared reflection abs orption spectra characteristic of atop-bonded CO. However, the integrated a bsorption band of atop CO at 0.1 V in the presence of coadsorbed Pb was fou nd to be about 3 times higher than that predicted from the corresponding sp ectral features observed for Pb-free surfaces at that specific coverage cor rected for dipole-dipole coupling interactions. Although much of this effec t may be due to a Pb-induced decrease in the dielectric screening within th e CO adlayer, the gains in intensity are too large to be explained solely o n this basis, pointing to chemical factors, such as Pt-Pb charge polarizati on, as an important factor.