Vacancy Generation and Oxygen Uptake in Cu-Doped Pr-CeO₂ Materials using Neutron and in Situ X-ray Diffraction

The oxygen uptake ability of Pr-CeO₂-based oxygen carriers, catalysts, and solid oxide fuel cells can be attributed to 3+ cation generation and the presence of vacant oxygen sites. Oxygen occupancies of CeO₂, Pr-CeO₂, and 5% Cu-doped Pr-CeO₂ were investigated using neutron diffraction and related to the oxygen uptake as determined using thermogravimetric analysis (TGA). The presence of vacant tetrahedral oxygen sites at room temperature did not correspond to low-temperature oxygen uptake. The materials did not uptake oxygen at 420 °C, but oxygen uptake was observed at 600 °C, which indicated that a minimum temperature needs to be met to generate sufficient vacancies/3+ cations. Variations in the lattice parameter as a function of temperature were revealed using in situ X-ray diffraction (XRD). With increasing temperature the lattice parameter increased linearly due to thermal expansion and was followed by an exponential increase at ∼300-400 °C as cations were reduced. Despite segregation of Cu into CuO at high dopant concentration, at 600 °C a higher O₂ uptake was obtained for Ce_0.65Pr_0.20Cu_0.15O_2-δ (120 μmol g^-1), in comparison to Ce_0.75Pr_0.2Cu_0.05O_2-δ (92 μmol g^-1), and was higher than that for Ce_0.8Pr_0.2O_2-δ (55 μmol g^-1). Both Pr and Cu introduce vacancies and promote the O₂ uptake of CeO₂.