Influences of Surface Substitutional Ti Atom on Hydrogen Adsorption, Dissociation, and Diffusion Behaviors on the α‑U(001) Surface

The hydrogen adsorption, dissociation,
and diffusion behaviors on both clean and Ti-doped α-U(001)
surfaces are systematically studied with density functional theory
method. Through detailed potential energy surface calculations, we
find that the dissociation at the bridge sites is energetically more
favorable, where the H2 molecule dissociates without any
energy barrier and the dissociated hydrogen atoms move into two neighboring
3-fold sites. Once a substitutional Ti atom exists on the α-U(001)
surface, the hydrogen molecule similarly dissociates without any energy
barriers. However, the diffusion of the dissociated hydrogen atoms
is dramatically changed after introduction of a surface substitutional
Ti atom. The into-bulk penetration of a hydrogen atom through a defect-free
surface is endothermic and needs to overcome an energy barrier of
0.8–0.9 eV. In contrast, the penetration to the subsurface
sites near the doped Ti atom is exothermic, and the activation barrier
decreases by 0.3–0.4 eV. Our results indicate that surface
doped titanium atoms in the outermost layer may behave like hydrogen
trapping sites for α-U.