Although the basic electrochemical response of palladium in aqueous acid media is frequently considered as being rather similar to that of platinum (apart from a region at low potentials where palladium displays unusual behavior due to the absorption/desorption of large quantities of hydrogen and its isotopes), there are significant differences between these two metals. Palladium is of higher reactivity: it is more prone to dissolution, hydrous oxide growth, formation of the higher oxide, and possibly oxygen absorption into the outer layers of the metal lattice. As a catalyst for oxygen gas reduction at the air cathode of an acid fuel cell Pd is prone to dissolution and formation of inhibiting hydrous oxide species, i.e. , its performance decays significantly with time at the potential region of interest, ca. 0.8 V (RHE). Thick hydrous oxide films may be grown on Pd in acid, at temperatures up to 40°C, by the potential cycling technique; unlike Pt, dissolution tends to predominate at higher temperatures. The reduction behavior of such films is also more complex than in the case of Pt. The presence of a curious, widely ignored but independently reported, minor reversible response just above 0.2 V (RHE) in cyclic voltammograms recorded for Pd in acid was confirmed; the significance of this feature will be considered in more detail in a forthcoming paper.
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Burke et al. (1993) studied this question.