Cluster layer-segregated

More recently, Adams and coworkers have provided a very interesting case of heteronuclear clusters that are very active for the hydrogenation of alkynes [4, 54, 55]. The high-nuclearity layer-segregated Pt-Ru complex [Pt3Ru6(CO)21(jU3-H)(/ -H)j], consisting of three stacked triangular layers of metal atoms with an... 

Scheme 8.8 Mechanism for the hydrogenation of diphenyl-acetylene catalyzed by layer-segregated Pt3Ru6 clusters (CO ligands omitted for clarity).

These results indicate that alumina acts on CrgOg phase to prevent its clustering and segregation with high coordinate Cr + ions. This dispersive effect of the support provides a suitable environment for the formation on the surface of low coordinate chromium ions (ref. 2). However this effect of alumina tends to depress when the content of chromium exceeds 30 % at 410°C. This result seems to Indicate a saturation of the surface sites of the support which interact with the chromium (surface of alumina covered with a layer of chromium oxide). Then an excess of Cr O deposited leads to its clustering and crystallization. Consequently the coordination of chromium ions changes from tetrahedral (low coordination) to octahedral (high coordination). 

When the ratio of surface atoms to total atoms approaches unity, the notion of complete or nearly complete segregation of the platinum in a surface layer and of iridium in a central core cannot be accommodated if the clusters are spherically symmetrical. The notion can, however, be accommodated without difficulty if the clusters have a two-dimensional, "raftlike" shape rather than a spherical shape (48). One could then visualize a central iridium or iridium-rich raft with platinum atoms around the perimeter. In very highly dispersed catalysts of this type, the effect of the platinum on the catalytic... 

A phenomenon that sometimes occurs in bulk alloys is the preferential segregation of one of the components at the surface, that is, the enrichment of the outermost surface layers(s) in one of the components. The slight outward shift of the Na atoms relative to the Li atoms can be interpreted as a manifestation of this tendency. The difference between the Wigner-Seitz radii of Na and Li is JR -s(Na-Li) = 0.72 a.u. This difference increases for K-Na and Cs-Na, namely dRH s(K-Na) = 0.87 a.u. and dRirs(Cs-Na) = 1.81 a.u. Since the properties of alkali metals and alkali atoms vary smoothly with atomic number, we expect that the effects observed in Li-Na clusters will be enhanced for K-Na and Cs-Na clusters. 

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