Electrochemical quartz crystal microbalance studies of a palladium electrode oxidation in a basic electrolyte solution

Anodic oxidation of Pd in basic solutions (0.1 M KOH_aq and 0.1 M NaOH_aq) has been examined via cyclic voltammetry (CV) and an electrochemical quartz crystal microbalance (EQCM). Admittance tests show that Pd(II) layer behaves as a rigid one. The anodic vertex potential influences mass response during formation of the Pd(II) layer. For low anodic vertex potentials, obtained absolute mass per mole values suggest Pd(OH)₂ or PdO·H₂O to be oxidation products. At this stage of the oxidation process, contribution from adsorbed H₂O/OH⁻ in Pd(II) layer formation could explain the lower-than-expected mass gain, although the extent of H₂O/OH⁻ adsorption is unclear. The mass gain decreases with further increase in the anodic vertex potential, eventually reaching the value of ca. 8 g mol⁻¹ at about 700 mV vs. SCE. Comparing the influence of vertex potential in CV experiments on the mass and reduction potential of the Pd(II) species points to the formation of PdO at higher oxidation potentials. At this stage of the process, a fraction of the PdO species is generated during transformation of previously formed Pd(OH)₂/PdO·H₂O. A shift of the main Pd(II) reduction potential peak depends on both the anodic vertex potential and on the composition of the Pd(II) film. The order of the Pd(II) reduction process is the opposite of that observed for the oxidation process. The Pd(IV) species formed at E ≥ 500 mV vs. SCE and those reduced between 50 and 350 mV are hydrated or contain hydroxyl groups.