Core level spectroscopy of oxidized and reduced CexU1-xO2 materials

A series of CexU1-xO2 been studied by core level spectroscopy, where x = 0, 0.25, 0.5, 0.75, 0.95, and 1. These materials have shown potential for thermal generation of hydrogen from water with better activity than CeO2 alone. In addition, Ce0.5U0.5O2 was also studied by in situ XRD as a function of temperature (from 300 to 973 K). The fresh material had the fluorite structure with crystallite size of about 15 nm. The lattice spacing is typical of complete miscibility of Ce and U cations in the fluorite structure. Phase segregation, of the uranium oxide, when the temperature was raised to 500 K and above was seen for the fresh material. The phase segregated uranium oxide is tentatively attributed to alpha-U3O8 (although it can be attributed to the hexagonal alpha- phase or the orthorhombic phase of U2O5). The used Ce0.5U0.5O2, composed of larger crystallite size (about 60 nm), was more stable with negligible phase segregation. Study of the XPS U4f regions indicated the presence of three types of U cations (U6+, U5+, and U4+). U6+ cations are mainly present in the fresh material with high % independent of U concentration (XPS U4f(7/2) binding energy at 381.1 eV and associated pairs of satellites at 4.5 and 10.0 eV above each split). Upon sputtering with Ar ions, a large fraction of these U6+ cations are reduced to U5+ (XPS U4f(7/2) binding energy at 380.4 eV and satellite at 8.2 eV above each split line) then to U4+ cations (XPS U4f(7/2) at 379.9 eV and its typical satellite at 6.9 eV above). After five minutes Ar ion sputtering the CexU1-xO2 materials were mainly composed of U4+ cations. On the other hand, XPS Ce3d lines indicated the presence of some contribution of Ce3+ cation in addition to lines characteristic of Ce4+ cations in the as prepared materials. Upon reduction considerable amount of Ce4+ is reduced to Ce3+ (up to 75%) in the mixed oxides while CeO2 alone showed very mild reduction. This is interpreted as due to charge transfer from the U cations further reducing Ce4+ cations. Valence band analyses corroborated the core level results with the reduced U containing mixed oxide showing large contribution of U5f/Ce4f lines while CeO2 alone has small Ce4f contribution even upon Ar ion sputtering. (C) 2013 Elsevier B.V. All rights reserved.