Potential of zero total charge of palladium modified Pt(111) electrodes inperchloric acid solutions

The electrochemical behaviour of palladium-modified Pt(111) electrodes in p erchloric acid solutions has been studied by means of cyclic voltammetry an d CO charge displacement. From the results of the charge displacement exper iments, values of the potentials of zero total charge (pztc) have been dete rmined and their dependence on the palladium coverage has been studied in s olutions without specifically adsorbing anions. These pztc values are compa red to those obtained in sulfuric acid solutions. Whilst in the presence of bisulfate anions the pztc decreases significantly as a consequence of the deposited palladium, in the case of perchloric acid solutions the pztc rema ins constant within experimental error. This constancy has been explained b y means of a compensation between the displacement of the potential of zero free charge toward less positive potentials and the increase of hydrogen c overage on the palladium-covered electrode surfaces. The influence of the c oncentration of specifically adsorbed (bi)sulfate anions at constant pH has been pointed out. The oxidation of the CO adlayer resulting from the charg e displacement has also been studied. Despite the apparently easier OH adso rption, the oxidation of adsorbed CO takes place at higher potentials on th e modified surfaces. CO coverage values, calculated from knowledge of the p ztc value and the CO stripping charge, diminish as the amount of palladium increases.