A Theoretical Study of H2 Reacting on Ti/Al(100) Surfaces

Using density functional theory, we study H-2 dissociation on Ti/Al(100) alloy surfaces, modeled by different Ti coverages varying from 1/4 to 1 monolayer. The minimum barrier height for H2 dissociation depends on the specific Ti coverage on the Al surface. In view of experimental information on Ti in hydrogenated and dehydrogenated NaAlH4, the most realistic model promoting H-2 dissociation is found to be the (100) surface of Al covered by one monolayer of Ti in a c(2 x 2) structure, with Ti atoms in the first and third layers. This model has a late dissociation barrier with a height of only 0.23 eV, preceded by a deep molecular chemisorption well with a well depth of 0.45 eV. The projected density of states analysis provides a molecular orbital view in which the barrierless approach to the molecular chemisorption well is mainly explained by an occupied-virtual attraction between the H-2 sigma(g) and Ti 3d(z)2 orbitals. The barrier separating the molecular chemisorption well and the dissociated state can be understood as resulting from a competition between increasing overlap of the H-2 sigma(u) and Ti 3d(xz) orbitals and decreasing overlap of the H2 sigma(g) and Ti 3d(z)2 orbitals.