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

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Abstract
A combined electrochemical method and X-ray photo electron spectroscopy (XPS) has been utilized to understand the Pd2+ /CeO2 interaction in Ce1−xPdxO2−δ (x = 0.02). A constant positive potential (chronoamperometry) is applied to Ce0.98Pd0.02O2−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 CeO2−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, Ce0.98Pd0.02O2−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+ 0.35Pd0.02O2−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, Ce0.98Pd0.02O2−δ forms a stable electrode for the electro-oxidation of H2O to O2 unlike CeO2 due to the metal support interaction.

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