Remarkable Structural Effect on the Gold-Hydrogen Analogy in Hydrogen-Doped Gold Cluster.

In accordance with the well established gold-hydrogen analogy, a hydrogen atom mimics the properties of a gold atom in gold clusters. In a recent study it has been demonstrated that the properties of a hydrogen atom doped small gold cluster (Au7H) are not in conformity with the aforementioned analogy. In this paper we study the properties of the Au7H cluster exhaustively to re-examine the validity of the gold-hydrogen analogy in the context of adsorption of CO and O2 molecules on pristine gold and hydrogen atom doped gold clusters. For this purpose we first determine the most stable structure of the Au7H cluster by using an ab initio density functional theory based method with generalized gradient approximation (GGA) and Meta-GGA exchange-correlation functionals. We carry out geometry optimization by considering various planar and three-dimensional isomers of the Au7H cluster as initial geometries. We find that the lowest energy structure of Au7H is a planar one with C2 v symmetry, and it is very close to the structure of the Au8 cluster with D4 h symmetry. Furthermore, to examine the validity of the gold-hydrogen analogy we carry out a detailed investigation of the adsorption of CO and O2 molecules on the most stable as well as various other low energy isomers of the Au7H cluster. We find that the adsorption energies and the extent of activation of CO and O2 molecules on the most stable planar isomer of Au7H are almost the same as those on the parent Au8 cluster with D4 h symmetry proving the validity of the gold-hydrogen analogy. On the other hand, for the high energy three-dimensional isomers of the Au7H cluster obtained from the pristine Au8 cluster with T d symmetry, we find a significant enhancement in adsorption energy as well as the extent of activation of CO and O2 molecules as compared to those for the corresponding pristine cluster. Therefore, the high reactivity of the 3D isomer of the Au7H cluster may be attributed to its existence in a state which is higher in energy than its most stable planar isomer.