Copper atomic core

When the ruthenium EXAFS for the ruthenium-copper catalyst is compared with the EXAFS for a ruthenium reference catalyst containing no copper, it is found that they are not very different. This indicates that the environment about a ruthenium atom in the bimetallic catalyst is on the average not very different from that in the reference catalyst. This result is consistent with the view that a ruthenium-copper cluster consists of a central core of ruthenium atoms with the copper atoms present at the surface. 

Let us first consider what EXAFS might tell us in the case of bimetallic particles that are not too small - say a few nanometer in diameter. For a truly homogeneous alloy with a 50 50 composition, EXAFS should see a coordination shell of nearest neighbors with 50% Cu and 50% Ru around both ruthenium and copper atoms. If, on the other hand, the particle consists of an Ru core surrounded by a Cu shell of monatomic thickness, we expect that the Ru EXAFS shows Ru as the dominant neighbor, because only Ru atoms in the layer directly below the surface are in contact with Cu. The Cu EXAFS should see both Cu neighbors in the surface and Ru neighbors from the layer underneath, with a total coordination number smaller than that of the Ru atoms. The latter situation is indeed observed in Ru-Cu/Si02 catalysts, as we shall see below. 

A metal can be considered as a fixed lattice of positive ions permeated by a gas of free electrons. Positive ions are the atomic cores the electrons are the valence electrons. For example, copper has a configuration (electronic structure)... 

Following the addition of one or two protons, the Cug cube does not collapse. Preliminary results on the crystal structure of the [Cu8(r-Bu2-DED)6H]3 anion (Bu4N+ salt) show that Cu-Cu distances in the Cu8 core are shorter than those in the Cu8 core of the unprotonated cluster. If this difference is maintained in the final stages of refinement, it probably can be attributed to a decrease in the S-S repulsions in the S 2 periphery expected to occur on protonation of two of the ligands. It appears that the Cu-Cu distances in the Cu8 cubes reflect a delicate balance of (1) the coordination geometry requirements of the copper atoms, (2) the Cu-Cu attractive interactions, and (3) the S-S repulsions in the St 2 periphery. 

The properties of the [CujI S - cluster are quite remarkable. This intense blue diamagnetic cluster (Amax 654 nm, e 46,100) can be described formally as a mixed-valence Cu(I), Cu(III) cluster (164). The crystal structure of this molecule (Fig. 59) supports this formalism and reveals a Cu5 rectangular pyramidal core (166). The basal copper atoms are three-coordinate and nearly planar and can be considered as Cu(I) units. The axial copper is four-coordinate and strictly planar, a coordination geometry appropriate for ad8 Cu(III) center. 

Fig. 23. (CH3CN)2Cu2Ru6C(CO),6, 20 (50). The distorted octahedral Ru6C core is capped by two directly bonded copper atoms [Cu-Cu = 2.693(1) A], one on an Ru3 face, the second on the CuRu2 face so formed. The Ru-Ru distances range from 2.798(1) to 3.072(1) A (mean 2.89 A). Ru-Q.rtjrte distances range from 2.031(4) to 2.073(4) A (mean 2.05 A). There are thirteen terminal carbonyls, and three asymmetrically bridging Ru-Ru edges. No Cu-CO contacts are short enough to imply bonding interactions.

Fig. 30. (CHjCN)jCu2Rh C(CO), (61). The irigonal prismatic Rh6C core of 24 (Fig. 29) is essentially unperturbed by the additional copper atoms that cap the two trigonal faces of the prism. Rh-Rh distances average 2.765(1) A (basal) and 2.810( 1) A (interbasal). The Cu-Rh bonds average 2.660( I) A. The mean Rh -C, distance is 2.13 A.

A metal can be considered as a fixed lattice of positive ions permeated by a gas of free electrons. Positive ions are the atomic cores, while the electrons are the valence electrons. For example, copper has a configuration (electronic structure) ls22s22p63s23p63dl04sl (superscripts designate number of electrons in the orbit) with one valence electron (4s). The atomic core of Cu+ is the configuration given above, less the one valence electron 4s1. The free electrons form an electron gas in the metal and move nearly freely through the volume of the metal. Each metal atom contributes its valence electrons to the electron gas in the metal. Interactions between the free electrons and the metal ions makes a large contribution to the metallic bond. 

The reactions catalyzed by laccases proceed by the monoelectronic oxidation of a suitable substrate molecule (phenols and aromatic or aliphatic amines) to the corresponding reactive radical (Riva, 2006). The redox process takes place with the assistance of a cluster of four copper atoms that form the catalytic core of the enzyme they also confer the typical blue color to these enzymes because of the intense electronic absorption of the Cu-Cu linkages (Piontek et al., 2002). The overall outcome of the catalytic cycle is the reduction of one molecule of oxygen to two molecules of water and the concomitant oxidation of four substrate molecules to produce four... 

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... 

In the potassium row, the unoccupied 3d level begins to be filled its energy has dropped more slowly than that of the 3s and 3p levels, but it becomes filled before the 4p level begins to fill then in the ground state of scandium the 3d level becomes occupied with one electron. Elements in which some d states are occupied arc called transition metals. The 3d states have become completely filled when copper, atomic number 29, is reached. The 3d states become part of the atomic core as Z increases further, and the series Cu, Zn, Ga,..., gains electrons in an order similar to that of the series Na, Mg, Al,. ... 

Table 4. Atomic Core Levels of Various Copper Complexes for Ground State...

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. 

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