Modeling of supported metal clusters: a density functional study of CO chemisorption on Ni clusters deposited on alumina

The nature of the bonding at the interface between a Ni cluster and an alumina surface has been investigated theoretically by means of linear combination of Gaussian-type orbital local density functional (LCGTO-LDF) cluster model calculations. One- and two-layer Ni₆ clusters have been deposited on a substrate A1₆O₁₂H₆ cluster model of an oxygen-terminated alumina surface. CO has been used as a probe molecule to monitor the electronic changes induced in the supported cluster by the interaction with the substrate. To this end, the CO chemisorption properties, and in particular the CO vibrational frequency, have been determined for the free and the supported Ni₆ clusters. We found that a one-layer Ni₆ cluster is strongly bound to the oxide substrate; this bonding is associated with a charge transfer from the Ni atoms to the oxygen atoms. The extent of this charge transfer is discussed in terms of a population analysis, the Ni(2p) core level shifts, and, for a single adsorbed Ni atom, the dipole moment curve. The partial oxidation of the Ni atoms is essentially restricted to the first layer of the cluster resulting in a blue-shift of the vibrational frequency of a chemisorbed CO molecule. When a two-layer cluster is considered, no significant changes of this observable are found. The results are discussed in view of the existing experimental data and of the proposed models of the metal-oxide interface.