Evolution of small Ti clusters and the dissociative chemisorption of H$_{2}$

The addition of small Ti clusters in certain complex metal hydrides has been found to improve significantly the kinetics of H$_{2}$ adsorption and desorption processes. The catalytic activity of doped Ti in these hydrogen storage materials is not fully understood. Here we report a systematic study of the sequential growth of small Ti clusters from $n$ = 2 - 15 atoms and the dissociative chemisorption of H$_{2}$ on the minimum energy clusters using density functional theory under the generalized gradient approximation. It has been found that the low energy clusters follow a pentagonal growth pattern. The clusters Ti$_{7 }$and Ti$_{13}$ show higher stability with a configuration of pentagonal bipyramid and icosahedron structures, respectively. The second difference of binding energy plot indicates that these two clusters are highly stable which agrees with the experimental collision-induced dissociation studies. Subsequently, a systematic study of the chemical reactivity of small Ti$_{n}$ clusters with $n$ = 2 - 15 towards dissociative chemisorption of H$_{2}$ has been performed. It is found that the chemisorption occurs preferentially at the two adjacent edges of any Ti atom. The chemisorption energy as a function of cluster size shows considerable structural changes in the Ti$_{n}$ clusters due to H$_{2}$ adsorption and dissociation.