El Jamal, G. / Gouder, T. / Eloirdi, R. / Idriss, H. / Jonsson, M. (2023)

J. Phys. Chem. C 2023, 127, 29, 14222–14231

Abstract

The interaction of water with the surfaces of metal oxides is important to many fields of research, extending from nuclear science to catalysis to energy and biomedical materials. One intriguing phenomenon is the observation 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 with prototype oxide nuclear fuels: UO2, U2O5, and UO3. On UO2, water adsorbs largely in a molecular state. On U2O5, water partially dissociates at −60 °C, thus forming surface −OH groups, and a fraction of the uranium cations are reduced from U5+ to U4+. On UO3, a similar reduction process is seen (reduction of a fraction of uranium cations from U6+ to U5+), albeit less pronounced. The chemisorbed H2O and −OH states via their molecular orbitals (MOs), 1b2, 3a1, and 1b1 for H2O and 1σ and 1π for −OH, were further analyzed. The 3a1–1b1 binding energy difference (ΔE) 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). The charge state of the surface hydroxyl was found to be related to the 1π /1σ intensity ratio, from which, and in conjunction with the created U 5f states, electron transfer to the conduction band under UPS collection was facilitated by the hole trapping capacity of surface −OH groups, at least in the case of UO3. An energy band diagram is constructed that may explain the redox process observed on UO3 under UV photon excitation.

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