Surface-Enhanced Raman Spectroscopy Studies on the Adsorption and Electrooxidation of Carbon Monoxide at the Platinum−Formic Acid Interface

Abstract

The vibrational spectrum of carbon monoxide, exerted by dissociation of formic aid, has been investigated at the platinum electrode as a function of applied potential by using the surface-enhanced Raman spectroscopy (SERS) technique. The electrolyte is 0.1 M LiClO_4. Two typical SERS features observed at 475−490 and 2055−2080 cm^(-1) are attributed to the platinum−CO (ν_(Pt-C)) and intramolecular C−O (ν_(C-O)) stretching vibration, respectively, indicating linearly adsorbed CO on platinum. Comparisons of the present data with previous studies in aqueous solutions show that solution components, particularly the dielectric in the inner double layer, may significantly influence the interaction of CO with platinum, especially the CO intramolecular mode. Electrooxidation of CO was observed to occur at potentials more positive than 0.6 V, being slightly negative relative to previous studies for CO on smooth platinum, suggestive of a higher electrocatalytic activity for the present highly roughened platinum surface. At positive and moderately negative potentials (−0.2 to 0.6 V), the Pt−C and CO intramolecular bands exhibit opposite frequency changes with decreasing potential, with Stark tuning rate being −6 and 24 cm^(-1)/V, respectively. At more negative potentials, both ν_(Pt-C) and ν_(CO) exhibit nonmonotonic potential dependences. The gradually decreasing slope for ν_(Pt-C) can be explained in terms of the nearly offsetting contributions from the π-back-donation and σ-bonding, along with increasing steric repulsion from negatively charged surfaces. The nearly potential-invariant frequencies for ν_(CO) were observed at potentials more negative than −1.0 V, indicating the key role of increasing concentration of H^+ in the inner double layer, possibly weakening the electron density back-donated from platinum to CO 2π^* orbitals.

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