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.