Pt metal-CeO2 interaction: Direct observation of redox coupling between Pt/Pt2+/Pt4+ and Ce4+/Ce3+ states in Ce0.98Pt0.02O2−δ catalyst by a combined electrochemical and x-ray photoelectron spectroscopy study

Pt ions-${\text{CeO}}_2$ interaction in ${\text{Ce}}_{1-x}{\text{Pt}}_x{\text{O}}_{2-\delta }$ $\left(x=0.02\right)$ has been studied for the first time by electrochemical method combined with x-ray diffraction and x-ray photoelectron spectroscopy. Working electrodes made of ${\text{CeO}}_2$ and ${\text{Ce}}_{0.98}{\text{Pt}}_{0.02}{\text{O}}_{2-\delta }$ 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 ${\text{Pt}}^0$ to ${\text{Pt}}^{2+}$ and ${\text{Pt}}^{4+}$ state due to the applied positive potential is coupled to simultaneous reversible reduction of ${\text{Ce}}^{4+}$ to ${\text{Ce}}^{3+}$ state. ${\text{CeO}}_2$ reduces to ${\text{CeO}}_{2-y}$ $\left(y=0.35\right)$ after applying 1.2 V, which is not reversible; ${\text{Ce}}_{0.98}{\text{Pt}}_{0.02}{\text{O}}_{2-\delta }$ reaches a steady state with ${\text{Pt}}^{2+}:{\text{Pt}}^{4+}$ in the ratio of 0.60:0.40 and ${\text{Ce}}^{4+}:{\text{Ce}}^{3+}$ in the ratio of 0.55:0.45 giving a composition ${\text{Ce}}_{0.98}{\text{Pt}}_{0.02}{\text{O}}_{1.74}$ at 1.2 V, which is reversible. Composition of Pt ion substituted compound is reversible between ${\text{Ce}}_{0.98}{\text{Pt}}_{0.02}{\text{O}}_{1.95}$ to ${\text{Ce}}_{0.98}{\text{Pt}}_{0.02}{\text{O}}_{1.74}$ within the potential range of 0.0–1.2 V. Thus, ${\text{Ce}}_{0.98}{\text{Pt}}_{0.02}{\text{O}}_{2-\delta }$ forms a stable electrode for oxidation of ${\text{H}}_2\text{O}$ to ${\text{O}}_2$ unlike ${\text{CeO}}_2$ . A linear relation between oxidation of ${\text{Pt}}^{2+}$ to ${\text{Pt}}^{4+}$ with simultaneous reduction in ${\text{Ce}}^{4+}$ to ${\text{Ce}}^{3+}$ is observed demonstrating $\text{Pt}–{\text{CeO}}_2$ metal support interaction is due to reversible ${\text{Pt}}^0/{\text{Pt}}^{2+}/{\text{Pt}}^{4+}$ interaction with ${\text{Ce}}^{4+}/{\text{Ce}}^{3+}$ redox couple.