Density Functional Theory Study of Ag-Cluster/CO Interactions

摘要

The interactions between carbon monoxide and small clusters of silver atoms are examined. Optimal geometries of the cluster-molecules complexes, i.e. silver cluster - carbon monoxide molecule, are obtained for different sizes of silver clusters and different numbers of carbon monoxide molecules. This analysis is performed in terms of different binding energy of these complexes and analysis of the frontier orbitals of the complex compared to those of its constituents. The silver atom and the dimer (Ag_2) bond up to three carbon monoxide molecules per Ag atom, while the larger clusters appear to saturate at two CO's per Ag atom. Analysis of the binding energy of each CO molecule to the cluster reveals that the general trend is a decrease with the number of CO molecules, with the exception of Ag where the second CO molecule is the strongest bound. A careful analysis of the frontier orbitals shows that the bent structures of AgCO and Ag_2CO are a result from the interaction of the highest occupied orbital of Ag (5s) and Ag_2 (σ) with the lowest unoccupied orbital of CO (π~*). The same bent structure also appears in the bonding of CO to some of the atoms in the larger clusters. Another general trend is that the CO molecules have a tendency to bond atop of an atom rather than on bridge or face sites. These results can help us elucidate the catalytic properties of small silver clusters at the atomic level.