Adsorption of molecular hydrogen and hydrogen sulfide on Au clusters.

The authors present theoretical results describing the adsorption of H2 and H2S molecules on small neutral and cationic gold clusters (Au(n)((0/+1)), n=1-8) using density functional theory with the generalized gradient approximation. Lowest energy structures of the gold clusters along with their isomers are considered in the optimization process for molecular adsorption. The adsorption energies of H2S molecule on the cationic clusters are generally greater than those on the corresponding neutral clusters. These are also greater than the H2 adsorption energies on the corresponding cationic and neutral clusters. The adsorption energies for cationic clusters decrease with increasing cluster size. This fact is reflected in the elongations of the Au-S and Au-H bonds indicating weak adsorption as the cluster grows. In most cases, the geometry of the lowest energy gold cluster remains planar even after the adsorption. In addition, the adsorbed molecule gets adjusted such that its center of mass lies on the plane of the gold cluster. Study of the orbital charge density of the gold adsorbed H2S molecule reveals that conduction is possible through molecular orbitals other than the lowest unoccupied molecular orbital level. The dissociation of the cationic Au(n)SH2+ cluster into Au(n)S+ and H2 is preferred over the dissociation into Au(m)SH2+ and Au(n-m), where n=2-8 and m=1-(n-1). H2S adsorbed clusters with odd number of gold atoms are more stable than neighboring even n clusters.