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.