CO Adsorption on One-, Two-, and Three-Dimensional Au Clusters Supported on MgO/Ag(001) Ultrathin Films

The adsorption properties of CO molecules adsorbed on free and MgO supported Au clusters have been investigated by means of a first principles density functional theory (DFT) approach. We have focused our attention on the vibrational properties of CO adsorbed on gas-phase Au3-7, Au-12, and Au-22 cluster anions, and on Au3-5, Au-12, and Au-22 clusters deposited on MgO/Ag(001) films or on the bare MgO(001) surface. We also considered a full gold monolayer deposited on MgO/Ag(00 1). CO does not bind to the high-coordinated Au atoms of one- or two-dimensional (1D or 2D) gold clusters or islands but only to the low-coordinated atoms at the periphery of these structures. A red-shift of about 50-60 cm(-1) is found in omega(CO) for gold clusters deposited on MgO/Ag(001) thin films compared to the same clusters deposited on bare MgO(001) or to the neutral gas-phase counterparts. This shift, due to the occurrence of a charge transfer from the metal support to the deposited gold cluster, is found only for small 1D or 2D clusters or, for 3D clusters, when the CO molecule is bound at the MgO/Au interface. On 3D gold clusters, CO adsorbs on the top layers with binding energies and vibrational frequencies typical of neutral supported gold particles. This shows that the charging effect observed on ultrathin MgO films is largely restricted to the gold layer at the interface with the oxide support.