Pd4 clusters on MgO OOl

We consider here the deposition of Ni4, Cu4, and Pd4 clusters at various sites of the MgO(OOl) surface. The cluster models used to simulate the MgO(OOl) surface are Mg909 and 09Mg9 (Fig. 21). These clusters represent the adsorption sites on an oxygen anion or a magnesium cation, respectively. The cluster models are embedded in an array of 17 x 17 x 6 point charges. Further details can be found else-where. ° To model the metal/oxide interface we have considered square planar (D4h) clusters which can be viewed as precursors of an epitaxially growing metal 

The structure of the adsorbed clusters has been partially optimized. In two of the adsorption geometries, denoted A and B (Fig. 22), the interaction is primarily between the cluster metal atoms and the surface oxygen atoms. In geometry B each metal atom is placed on-top of the surface oxygen centers this corre- 

Things are somewhat different for Pd. The calculations on Ni and Cu clusters have clearly shown a preference for the interaction at the oxide anions of the surface while experimental data on small Pd particles supported on MgO have been interpreted in terms of adsorption of the Pd atoms on top of the Mg + ions. Furthermore, Pd has a larger atomic radius than Ni and Cu and it is interesting to analyze the expansion of a Pd cluster adsorbed on MgO. Relativistic GGA calculations have been performed for Pd4/MgO. They show as an important first result that for Pd also the preferred adsorption sites are the oxide anions of the surface. Much weaker bonding is found when the Pd4 cluster interacts with the 

The supported Pd4 cluster has Pd-Pd distances which are elongated compared with those of the compound in the gas-phase. The optimum geometry of Pd4 adsorbed on the oxide anions of MgO has metal-metal distances of 2.93 A, somewhat shorter than the MgO lattice parameter. The potential energy for the Pd-Pd stretch is, however, very flat with very little energy it is possible to expand the cluster to a pseudomorphic structure on the MgO surface. Because the clusters experimentally deposited on MgO are usually much larger than the species considered here, it is conceivable that a thin Pd metal film will grow epitaxially on MgO, at variance with Ni and Cu. 

The calculations for Ni clusters supported on AI2O3 have been performed at the LDA level,f ° differently from the studies of metal deposition on MgO previously 

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