Quantum Theory of Advanced Materials

Dissociative chemisorption of H-2 on the Al surface is a crucial step in the regeneration of promising hydrogen-storage materials such as alane and alanates. We show from first-principles calculations that transition metals such as V and Nb can act as effective catalysts for H-2 interaction with Al(100). When located at subsurface sites, V and Nb can reduce the activation barrier for H-2 dissociation by significantly larger values than the well-studied catalyst Ti. In addition, the binding energy of a H atom on the surface can be enhanced by as much as 0.4 eV when V or Nb is introduced in the sublayers of Al(100). The diffusion barrier for the adsorbed hydrogen is reduced by similar to 0.1 eV, showing an increased hydrogen mobility. The mechanism of promoting the metal surface reactivity by subsurface alloying with transition metals proposed in this work may serve as a new possible scheme for catalytic reactions on the metal surface.

Hydrogenation-induced metal-semiconductor transitions usually occur in simple systems based on rare earths and/or magnesium, accompanied by major reconstructions of the metal host (atom shifts >2 Angstrom). We report on the first such transition in a quaternary system based on a transition element. Metallic LaMg2Ni absorbs hydrogen near ambient conditions, forming the nonmetallic hydride LaMg2NiH7 which has a nearly unchanged metal host structure (atom shifts <0.7 Angstrom). The transition is induced by a charge transfer of conduction electrons into tetrahedral [NiH4](4-) complexes having closed-shell electron configurations.