Evolution of Co _n Al clusters and chemisorption of hydrogen on Co _n Al clusters

The growth behavior of Co _n Al (n = 1–15) and the chemisorptions of hydrogen on the ground state geometries have been studied using the density functional theory (DFT) within the generalized gradient approximation (GGA). The growth pattern for Co _n Al is Al-substituted Co _n+1 clusters, and it keeps the similar frameworks of the most stable Co _n+1 clusters except for n = 2, 3, and 6. The Al atom substitutes the surface atom of the Co _n+1 clusters for n ≤ 13. Starting from n = 14, the Al atom completely falls into the center of the Co-frame. The dissociation energy, the second-order energy differences, and the HOMO–LUMO gaps indicate that the magic numbers of the calculated Co _n Al clusters are 7, 9, and 13, corresponding to the high symmetrical structures. To my knowledge, this is the first time that a systematic study of chemisorption of hydrogen on cobalt aluminum clusters. The twofold bridge site is identified to be the most favorable chemisorptions site for one hydrogen adsorption on Co _n Al (n = 1–6, 8, 10), and two hydrogen adsorption on Co _n Al (n = 1–7), while threefold hollow site is preferred for one hydrogen adsorption on Co _n Al (n = 7, 9, 11–15) and two hydrogen adsorption on Co _n Al (n = 8–10, 12–15) clusters. The ground state structure of two hydrogen adsorption on Co₁₁Al is exceptional. In general, the binding energy of both H and 2H of Co _n Al (n = 1–12) is found to increase with the cluster size. And the result shows that large binding energies of the hydrogen atoms and large fragmentation energies for Co₁₁AlH and Co₁₂AlH make these species behaving like magic clusters.