An electrochemical study of the composition of thin, compact Pd oxide films

Abstract

An electrochemical study of the properties of thin, compact oxide films (Pd α-oxides) formed at polycrystalline Pd wire electrodes and at Pd-sputter-coated quartz crystals (to enable in situ mass measurements) in alkaline and neutral sulfate solutions has been carried out. In alkaline solutions, the mass of the film is consistent with the formation and reduction of Pd(OH)₂· H₂O, in agreement with the ca.–58 mV pH dependence observed over a narrow alkaline pH range using solutions that do not lead to competitive anion adsorption effects. This film can be transformed, without a change of oxidation state or significant thickening, by a particular continuous potential cycling regime, to one which reduces at ca. 270 mV more negatively, consistent with the development of a deprotonated, charge compensated, hydrated oxide film, i.e., Pd(OH)₂·yH₂O ·xOH^–·xNa⁺. In neutral sulfate solutions, evidence for sulfate adsorption on the reduced Pd surface and in the early stages of α-oxide formation is seen. Nevertheless, the mass of the α-oxide film could be measured, indicating it to be PdO · H₂O in this medium.