Reactions of acetone on the surfaces of the oxides of uranium

Abstract

The reactions of acetone were investigated on α U3O8 (stoichiometric and H2 reduced) by temperature programmed desorption. The surface and bulk characteristics of U oxides were investigated by x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD). The comparison between the XPS U 4f7/2 binding energy of β UO3, α U3O8, and UO2 indicated that the U cations in U3O8 are composed of +5/+6 (2/1) or +4/+6 (1/2) oxidation states. XPS of H2 reduced α U3O8 (at 800 K) indicated a shift of the U 4f7/2 binding energy from 381.6 (unreduced) to 380.5 eV (reduced) as well as the appearance of two satellites at 387.0 and 397.4 eV. These new XPS lines’ positions are characteristic of U+4 cations of UO2. Similar results were observed upon Ar-ion sputtering of α U3O8. Ar-ion sputtering of β UO3 also results in the reduction of U+6 cations to U+4 cations. XRD of H2 reduced α U3O8 indicated that all of the α U3O8 phase was transformed to the UO2 phase. A considerable difference between the reactivity of the surfaces of U3O8 and UO2 towards acetone was observed. A carbon–carbon bond formation reaction giving isobutene was observed only on the α U3O8 surface indicating its structure-sensitive nature and/or its sensitivity towards changing the oxidation state of U cations. This reaction has not been observed on the surfaces of actinide oxides before. On the other hand, acetone reacted on UO2 to give mainly propene. This latter reaction (which is a C–O bond dissociation) is most likely due to the capacity of UO2 to accommodate large numbers of atomic oxygen in interstitial positions while maintaining its structure intact.