Water reactions over stoichiometric and reduced UO2(1 1 1) single crystal surfaces

The reaction of water has been investigated on the surfaces of stoichiometric and oxygen defected UO2(1 1 1) single crystal. On the stoichiometric surface most of the adsorption of H2O molecules at 300 K is reversible (>80%). The remaining water on the surface desorbs in two TPD peaks at 400 and 530 K with no evidence for H2 formation. On the contrary on Ar+-sputtered surfaces, water is strongly adsorbed. The Ar+-sputtered surfaces contained large amounts of U metal or U cations in lower oxidation state than +4 (a shoulder at BE ≈ 377 eV). Upon H2O adsorption at room temperature most of the shoulder at 377 eV has disappeared suggesting a dissociative adsorption of water molecules. Further evidence of water dissociation was given by H2 desorption during TPD. The amount of desorbed H2 tracked the extent of surface reduction. A complex set of H2 desorption (three peaks) was seen during TPD. These peaks were both sensitive to prior surface reduction and surface coverage.