CO adsorption and kinetics on well-characterized Pd films on Pt(111) in alkaline solutions

The electrochemistry of CO on a bare Pt(111) electrode as well as a Pt(111) electrode modified with pseudomorphic thin palladium films has been studied in alkaline solution by means of Fourier transform infrared (FTIR) spectroscopy. First Pd films were prepared and well characterized in UHV and subsequently transferred into the electrochemical cell for the registration of the voltammetric profiles. The charge corresponding to the formation of underpotentially deposited hydrogen (Hupd) on these Pt(111)-xPd surfaces was established in sulfuric acid solution as a function of x (0 ≤ x ≤ 1 Pd monolayer (ML)). All subsequent measurements were then performed on electrochemically deposited palladium films using the above Hupd-charge vs. Pd coverage relationship to evaluate the amount of electrochemically deposited palladium. FTIR spectra for CO adsorbed on one monolayer and a submonolayer coverage are compared to those of the unmodified Pt(111) surface, all surfaces having identical 2D lattice structures. Infrared absorption bands of CO bound on either Pt(111) or Pt(111)-1ML Pd are clearly 1 distinguished. Spectra of CO adsorbed on Pd submonolayers show characteristic features of both CO bound to Pt and to Pd, indicating that on Pt(111)-xPd surfaces there is no coupling between Pt-COad and Pd-COad molecules. The kinetics of CO oxidation on these surfaces is determined either by rotating disk electrode (RDE) measurements or by FTIR spectroscopy, monitoring the CO3 production. The oxidation of COad on Pt(111) and on Pd modified platinum surfaces starts at the same potential, ca. at 0.2 V. The oxidation rate is, however, considerably lower on the Pt(111)-xPd surfaces than on the Pt(111) surface. The kinetics of CO oxidation appears to be determined by the nature of adsorbed hydroxyl anions (OHad), which are more strongly (less active) adsorbed on the highly oxophilic Pd atoms.