Influence of temperature on the growth of surface oxides on palladium electrodes

Surface oxides on Pd electrodes were formed by anodic polarization in 0.5 M aqueous H2SO4 at various potentials, Ep, from 0.80 to 1.80 V, for polarization times, tp, up to 104 s and at 278≤T≤338 K. Application of Ep between 0.80 and 1.00 V resulted in formation of a thin oxide layer that revealed one feature in the oxide–reduction profiles, the peak OC1. Upon application of Ep>1.00 V, development of a thick surface oxide commenced and the oxide–reduction profiles showed an increase of OC1 and a voltammetric wave, VW1, at a higher potential with respect to OC1. The OC1 feature corresponded to PdO reduction and VW1 to reduction of PdO2. It was observed that the growth of the thin (inner) oxide layer (OC1 peak) always preceded the development of the thick (outer) one (VW1), thus the oxide film had a two-layer structure. The oxide growth behavior was influenced by the experimental conditions such as Ep and tp, and in general the higher Ep and the longer tp, the thicker the oxide (apart from polarization at 0.80 ≤ Ep ≤ 0.95 V for tp ≥ 102 s when the initial Pd dissolution occurred). Temperature augmentation also affected the oxide thickness (for Ep and tp constant), yet did not result in new oxide species. Theoretical data treatment indicated that Pd oxide growth followed two distinct kinetic laws each one arising from a different mechanism: (i) logarithmic growth at 0.95≤Ep≤1.40 V, thus when PdO was formed; and (ii) inverse-logarithmic growth at Ep>1.40 V, thus when PdO2 was developed on top of PdO. The logarithmic growth originated from the interfacial place exchange between Ochem and the top-most Pd atoms, whereas the inverse-logarithmic law arose from the process being limited by the escape of the metal cation from the metal into the oxide at the inner metal | oxide interface. The dipole charge, δ, assigned to the surface dipole moment of the PdO layer was of the order of 0.1–0.3 of an electron. The strength of the electric field that assisted the interfacial Pd cation escape was of the order of 108–109 V m−1.