Pt metal-CeO₂ interaction: direct observation of redox coupling between Pt⁰/Pt²⁺/Pt⁴⁺ and Ce⁴⁺/Ce³⁺ states in Ce_0.98Pt_0.02O_2-δ catalyst by a combined electrochemical and x-ray photoelectron spectroscopy study

Abstract

Pt ions-CeO₂ interaction in Ce_1-xPt_xO_2-δ (x = 0.02) has been studied for the first time by electrochemical method combined with x-ray diffraction and x-ray photoelectron spectroscopy. Working electrodes made of CeO₂ and Ce_0.98Pt_0.02O_2-δ mixed with 30% carbon are treated electrochemically between 0.0-1.2 V in potentiostatic (chronoamperometry) and potentiodynamic (cyclic voltametry) mode with reference to saturated calomel electrode. Reversible oxidation of Pt0 to Pt²⁺ and Pt⁴⁺ state due to the applied positive potential is coupled to simultaneous reversible reduction of Ce⁴⁺ to Ce³⁺ state. CeO₂ reduces to CeO_2-y (y = 0.35) after applying 1.2 V, which is not reversible; Ce_0.98Pt_0.02O_2-δ reaches a steady state with Pt²⁺:Pt⁴⁺ in the ratio of 0.60:0.40 and Ce⁴⁺:Ce³⁺ in the ratio of 0.55:0.45 giving a composition Ce_0.98Pt_0.02O_1.74 at 1.2 V, which is reversible. Composition of Pt ion substituted compound is reversible between Ce_0.98Pt_0.02O_1.95 to Ce_0.98Pt_0.02O_1.74 within the potential range of 0.0-1.2 V. Thus, Ce_0.98Pt_0.02O_2-δ forms a stable electrode for oxidation of H₂O to O₂ unlike CeO₂. A linear relation between oxidation of Pt²⁺ to Pt⁴⁺ with simultaneous reduction in Ce₄₊ to Ce³⁺ is observed demonstrating Pt-CeO₂ metal support interaction is due to reversible Pt⁰/Pt²⁺/Pt⁴⁺ interaction with Ce⁴⁺/Ce³⁺ redox couple.