CO adsorption on Ni₄ and Ni₈ clusters deposited on regular and defect sites of the MgO(0 0 1) surface

The properties of Ni₄(CO)₄ and Ni ₈(CO)₄ model clusters supported on the MgO(0 0 1) 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 Fs+ 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(0 0 1) surface, the Ni clusters show an intrinsic instability when CO is added to the cluster in linear fashion atop Ni atoms; some clusters (Ni₄) fragmentate, while others (Ni₈) 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 Ni₄ and Ni₈ 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 Fs+ centers the transfer of about one electron occurs from the surface to the metal particle.