Molecular hydrogen adsorption and dissociation on the plutonium (111) surface

Abstract

Hydrogen molecule adsorption and the reaction barrier for dissociation on the plutonium (111) surface have been studied in detail using the generalized gradient approximation to density functional theory. All calculations have been performed at both non-spin-polarized and spin-polarized levels of the theory. Weak molecular adsorptions with a layer by layer alternate spin arrangement of the plutonium atoms were observed. Horizontal approaches on the top site both without and with spin polarization were found to be the most favorable molecular adsorption sites. For dissociative adsorption it was found that the most favorable dissociation channel needs activation energies of 0.216 and $0.305\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at the non-spin-polarized and spin-polarized levels, respectively, with considerably higher adsorption energies than those of the molecular cases. Compared to dissociation on the (100) surface, hydrogen molecule dissociation on the (111) surface is easier because the activation energies are much lower in the later case. In general the adsorption of hydrogen molecule pushes the Pu $5f$ band away from the Fermi level, except for spin-polarized dissociative adsorption, where $5f$ orbitals come closer to the Fermi level. Charge transfer to the hydrogen atoms for dissociative adsorption is larger than that of molecular adsorption due to the reduced distances of hydrogen atoms to the plutonium surface. In that case, the ionic part of $\mathrm{H}\ensuremath{-}\mathrm{Pu}$ bonding contributes along with the covalent part due to Pu $5f--\mathrm{H}$ $1s$ hybridization.