paper

The different roles of Pu-oxide overlayers in the hydrogenation of Pu-metal: An ab initio molecular dynamics study based on vdW-DFT+U

arXiv:1403.1087 · doi:10.1063/1.4873418

Abstract

Based on the van der Waals density functional theory (vdW-DFT)+U scheme, we carry out the ab initio molecular dynamics (AIMD) study of the interaction dynamics for H $_{2}$ impingement against the stoichiometric PuO $_{2}$ (111), the reduced PuO $_{2}$ (111), and the stoichiometric $I^\pm$ -Pu $_{2}$ O $_{3}$ (111) surfaces. The hydrogen molecular physisorption states, which can not be captured by pure DFT+\textit{U} method, are obtained by employing the vdW-DFT+\textit{U} scheme. We show that except for the weak physisorption, PuO $_{2}$ (111) surfaces are so difficult of access that almost all of the H $_{2}$ molecules will bounce back to the vacuum when their initial kinetic energies are not sufficient. Although the dissociative adsorption of H $_{2}$ on PuO $_{2}$ (111) surfaces is found to be very exothermic, the collision-induced dissociation barriers of H $_{2}$ are calculated to be as high as $3.2$ eV and $2.0$ eV for stoichiometric and reduced PuO $_{2}$ surfaces, respectively. Unlike PuO $_{2}$ , our AIMD study directly reveals that the hydrogen molecules can penetrate into $I^\pm$ -Pu $_{2}$ O $_{3}$ (111) surface and diffuse easily due to the $25$ \ native O vacancies located along the $⟨$ 111 $⟩$ diagonals of $I^\pm$ -Pu $_{2}$ O $_{3}$ matrix. By examining the temperature effect and the internal vibrational excitations of H $_{2}$ , we provide a detailed insight into the interaction dynamics of H $_{2}$ in $I^\pm$ -Pu $_{2}$ O $_{3}$ . The optimum pathways for hydrogen penetration and diffusion, the corresponding energy barriers ( $1.0$ eV and $0.53$ eV, respectively) and rate constants are systematically calculated. Overall, our study fairly reveals the different interaction mechanisms between H $_{2}$ and Pu-oxide surfaces, which have strong implications to the interpretation of experimental observations.

29 pages, 8 figures