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The electrochemical behaviour of palladium-modiÐed Pt(111) electrodes in perchloric acid solutions has been studied by means of cyclic voltammetry and CO charge displacement. From the results of the charge displacement experiments, values of the potentials of zero total charge (pztc) have been determined and their dependence on the palladium coverage has been studied in solutions without speciÐcally adsorbing anions. These pztc values are compared to those obtained in sulfuric acid solutions. Whilst in the presence of bisulfate anions the pztc decreases signiÐcantly as a consequence of the deposited palladium, in the case of perchloric acid solutions the pztc remains constant within experimental error. This constancy has been explained by means of a compensation between the displacement of the potential of zero free charge toward less positive potentials and the increase of hydrogen coverage on the palladium-covered electrode surfaces. The inÑuence of the concentration of speciÐcally adsorbed (bi)sulfate anions at constant pH has been pointed out. The oxidation of the CO adlayer resulting from the charge displacement has also been studied. Despite the apparently easier OH adsorption, the oxidation of adsorbed CO takes place at higher potentials on the modiÐed surfaces. CO coverage values, calculated from knowledge of the pztc value and the CO stripping charge, diminish as the amount of palladium increases.

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A combined in situ spectroelectrochemical and theoretical study of CO adsorbed on clean and palladium-covered Pt(111) single-crystal electrodes is presented. The in situ spectra of CO adsorbed on the Pt(111) and Pd supported on Pt(111) single-crystal electrodes show different bands corresponding to the C−O stretching mode that are assigned with the help of density functional cluster model calculations. For the palladium-covered Pt(111) electrode with a palladium coverage close to the monolayer, the resulting spectra can be interpreted by using a pure Pd cluster model only. A shift to higher vibrational frequencies is observed which increases with increasing CO coverage. This shift can be interpreted as a change in the adsorption site because of coverage effects in agreement with UHV results at 300 K. The present study shows that the combined use of spectroelectrochemical and first principles calculations is a promising and powerful tool for the interpretation of complex electrochemical interfaces.