Metal copper cluster cores

The results of the EXAFS studies on osmium-copper clusters lead to conclusions similar to those derived for ruthenium-copper clusters. That is, an osmium-copper cluster Is viewed as a central core of osmium atoms with the copper present at the surface. The results of the EXAFS investigations have provided excellent support for the conclusions deduced earlier (21,23,24) from studies of the chemisorption and catalytic properties of the clusters. Although copper is immiscible with both ruthenium and osmium in the bulk, it exhibits significant interaction with either metal at an interface. 

For the copper reference catalyst, the magnitude of the main peak in the transform is halved as a result of exposure of the catalyst to oxygen. Presumably the outer layers of the copper clusters are oxidized, leaving metallic copper cores in the interior. The oxidation of the copper apparently ceases with the formation of the outer oxide layers. 

The size of the osmium-copper clusters of interest in the catalyst considered here is such that the number of metal atoms which could be present in a full surface layer is significantly higher than the number that would be located in the interior core. For a stoichiometry of one copper atom per osmium atom, there are, then, too few copper atoms to form a complete surface layer around the osmium. It should be realized that parameters derived from the EXAFS data on the osmium-copper clusters are average values, since there is very likely a distribution of cluster sizes (9) and compositions in a silica-supported osmium-copper catalyst. 

For the catalyst containing osmium alone on silica, the osmium clusters behave as if they are more electron deficient than pure metallic osmium that is, there appear to be more unfilled d states to accommodate the electron transitions from the 2pin core level of the absorbing atom. In the silica-supported osmium-copper clusters, however, the osmium atoms appear to be less electron deficient than they are in the pure osmium clusters dispersed on silica. The presence of the copper thus appears to decrease the number of unfilled d states associated with the osmium atoms. This observation is the first that we have made regarding the electronic interaction between the components of a bimetallic cluster catalyst. Further studies of such interactions are currently in progress on other bimetallic catalysts. 

Two bimetallic clusters derived from [Ru6C(CO),6]2- have been prepared, in which eight and thirteen metal atoms are present. The reaction of Cu(MeCN)JBF4 with 18 results in the addition of two copper vertices to the Ru6C core [Eq. (14)] (50). 

Since the pirn and mpim molecules are quite large, any ab initio calculation including a cluster of metal atoms will necessarily be very tedious. Therefore, it is desirable to model the ligands by a subsystem which includes only the part that is primarily involved in the interaction with the copper surface. As mentioned in the introduction, data from XPS core level spectroscopy show that the nitrogen atom is the primary reaction site in the interaction between pirn and a copper surface, and that the effect of this interaction does not extend significantly into the benzene moiety of the pirn molecule. Therefore, we chose as a model system for pirn the molecule NH(CHO)2 (di-formimide, see Figure 1). As will be shown in the next section, the electronic structure of this model molecule has an electronic structure which very much resembles that of the imide part... 

To model the copper (100) surface a two-layer cluster of C4V symmetry, with 5 copper atoms in one layer and 4 copper atoms in the other layer, has been used. In this cluster, all the 9 metal atoms were described by the LANL2DZ basis set. The LANL2DZ basis set treats the 3s 3p 3d 4s Cu valence shell with a double zeta basis set and treats all the remainder inner shell electrons with the effective core potential of Hay and Wadt [33]. The non-metallic atoms (C and H) were described by the 6-3IG basis set of double zeta quality with p polarization functions in... 

Other metallic clusters that have been demonstrated to show the QDL effect are palladium [116, 117], silver [118] and copper [119]. Palladium MFCs capped with mixed monolayers of hexanethiolate/dodecanethiolate and ferrocene thiolate ligands are prepared in a manner similar to that employed for gold MFCs. The DPV studies exhibit a quantized charging effect but the current peaks are not as well defined as those observed for Au-MPCs. Capacitance values of the order of 0.35 aF are obtained, indicating smaller core sizes or thicker monolayer dielectrics [116]. 

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