CO adsorption on Ni-4 and Ni-8 clusters deposited on regular and defect sites of the MgO(001) surface

The properties of Ni-4(CO)(4) and Ni-8(CO)(4) model clusters supported on the MgO(001) surface have been studied by means of density functional all-electron calculations. Two- and three-dimensional Ni clusters have been adsorbed on regular terrace sites and on neutral and charged oxygen vacancies (F-s and F-s(+) centers) of the MgO surface; four CO molecules have been added in various sites of the supported clusters and their properties have been investigated. On the MgO(001) surface, the Ni clusters show an intrinsic instability when CO is added to the cluster in linear fashion atop Ni atoms; some clusters (Ni-4) fragmentate, while others (Ni-8) undergo a strong distortion which transforms the cluster from three-dimensional to a nearly planar configuration. This is the result of the weakening of the metal-metal bond induced by CO and of the preference for metal-oxide bonds under CO exposure. The effect is not observed when the Ni clusters are anchored at a point defect like an oxygen vacancy. In this case the electrons trapped in the vacancy are partially transferred to the metal cluster and reinforce the metal-metal bonds. Addition of CO to Ni-4 and Ni-8 on F centers does not alter the cluster topology, but results in measurable shifts of the CO frequency with respect to the unsupported complex. A comparison with the unsupported {[Ni-8(CO)(4)](-) cluster anion suggests that on F-s and F-s(+) centers the transfer of about one electron occurs from the surface to the metal particle. (C) 2004 Elsevier B.V. All rights reserved.}