Study of Water Interaction with UO2, U2O5, and UO3: Tracking the Unexpected Reduction of Uranium Cations and Characterization of Surface-Bound Hydroxyls

The interaction of water with the surfaces of metal oxidesis importantto many fields of research, extending from nuclear science to catalysisto energy and biomedical materials. One intriguing phenomenon is theobservation that, for a few oxides, water seems to reduce (not oxidize)the oxide substrate. In this work, ultraviolet photoelectron spectroscopy(UPS) has been used to study the reactions of H2O withprototype oxide nuclear fuels: UO2, U2O5, and UO3. On UO2, water adsorbs largelyin a molecular state. On U2O5, water partiallydissociates at -60 & DEG;C, thus forming surface -OHgroups, and a fraction of the uranium cations are reduced from U5+ to U4+. On UO3, a similar reductionprocess is seen (reduction of a fraction of uranium cations from U6+ to U5+), albeit less pronounced. The chemisorbedH(2)O and -OH states via their molecular orbitals(MOs), 1b(2), 3a(1), and 1b(1) for H2O and 1 & sigma; and 1 & pi; for -OH, were further analyzed.The 3a(1)-1b(1) binding energy difference(& UDelta;E) was taken as a measure of the bond strength.It was found to be larger on UO2 and U2O5 (2.9-3.0 eV) than on UO3 (2.2 eV). Thecharge state of the surface hydroxyl was found to be related to the1 & pi; /1 & sigma; intensity ratio, from which, and in conjunctionwith the created U 5f states, electron transfer to the conductionband under UPS collection was facilitated by the hole trapping capacityof surface -OH groups, at least in the case of UO3. An energy band diagram is constructed that may explain the redoxprocess observed on UO3 under UV photon excitation.