First-principles Study Of Al Mixed-Metal Clusters

Started from the four types 13-atom high-symmetric（Ih, Oh, D5 h, D3h） close-packed structures and by replacing a 3d transition metal atom in the nonequivalent position. There are two parts in this paper.（1） The geometrical and electronic properties of the doped Al₁₂X（X=Sc-Zn） clusters are systematically studied by using the density-functional theory. Close-packed（icosahedral-like） structures are found to be favorable for the ground state geometries and the degenerate isomers of Al₁₂X（X = Sc, Ti, V, Ni, Cu） clusters. The magnetic moments of the doped Al₁₂X（X = Cr, Mn and Fe） are substantially increased as compared with that of the pure Al13, which are mainly derived from the strong spin splitting of the d electrons of the doped atoms. For the absorption of H, O and N on the close-packed Al₁₂ X clusters, it is found that H atom tend to occupy the top or bridge site instead of the hollow site, but the adsorption sites of O and N atom are more complex. O and N are always adsorbed around the doped atom of the doped cluster with the doped atom on the surface and the adsorption energies of O and N on the doped clusters are all enhanced as compared with that on pure Al13, but it is quite different for the adsorption of H, which implies that the influences of the d electrons of the doped atoms on O and N are stronger than that on H. All doped clusters exhibit the same selective sequence of adsorption: O > N >H.（2） Based on the icosahedral Al₁₂X（X= Ni, Cu） clusters with the doped atoms in the center, the adsorption of H atom and H2 molecule on the surface of these clusters are systematically investigated by using the density-functional theory calculations, and the obtained results are compared with that of the adsorption of H and H2 on the pure Al13 cluster. Our results show that: Compared to the bridge site of the H atom on pure Al13, H adsorbs on the top site of each of the Al₁₂X（X=Ni、Cu） clusters; The geometric structure of Al₁₂ Ni is greatly distorted in the adsorption of either a hydrogen atom or a H2 molecule. For the comparison of the dissociative adsorption of H2 on the surface of pure Al13 cluster, the reaction energies all increase and the potential barriers all decrease in the dissociation reaction processes of H2 on the surfaces of the doped Al₁₂X（X= Ni, Cu） clusters, which implies that the doped Al₁₂X（X= Ni, Cu） clusters are more favorable for the occurrence of the dissociative adsorption of H2 than the pure Al13 cluster.