Direct evidence of redox interaction between metal ion and support oxide in Ce0.98Pd0.02O2−δ by a combined electrochemical and XPS study

Abstract

A combined electrochemical method and X-ray photo electron spectroscopy (XPS) has been utilized to understand the Pd²⁺/CeO₂ interaction in Ce_1−xPd_xO_2−δ (x = 0.02). A constant positive potential (chronoamperometry) is applied to Ce_0.98Pd_0.02O_2−δ working electrode which causes Ce⁴⁺ to reduce to Ce³⁺ to the extent of ∼35%, while Pd remains in the +2 oxidation state. Electrochemically cycling this electrode between 0.0–1.2 V reverts back to the original state of the catalyst. This reversibility is attributed to the reversible reduction of Ce⁴⁺ to Ce³⁺ state. CeO₂ electrode with no metal component reduces to CeO_2−y (y ∼ 0.4) after applying 1.2 V which is not reversible and the original composition of CeO₂ cannot be brought back in any electrochemical condition. During the electro-catalytic oxygen evolution reaction at a constant 1.2 V for 1000 s, Ce_0.98Pd_0.02O_2−δ reaches a steady state composition with Pd in the +2 states and Ce⁴⁺ : Ce³⁺ in the ratio of 0.65 : 0.35. This composition can be denoted as Ce⁴⁺_0.63Ce³⁺_0.35Pd_0.02O_2−δ−y (y∼0.17). When pure CeO₂ is put under similar electrochemical condition, it never reaches the steady state composition and reduces almost to 85%. Thus, Ce_0.98Pd_0.02O_2−δ forms a stable electrode for the electro-oxidation of H₂O to O₂ unlike CeO₂ due to the metal support interaction.