Clustering segregation

Questions regarding possible kinetic limitations at the low temperature range [83], the mechanism and driving forces for cluster segregation, and the possibility of clustering/ordering due to segregating Ag atoms (drawn schematically in Fig. 18) remain open. 

When two metals are combined within a nanoparticle, potentially they could mix in different patterns called as nanostructures. A few commonly nanostructures are (a) core-shell, (b) sub-cluster segregated, (c) homogeneous (ordered or random) alloy and (d) multi-shell or onion like structure. In the case of supported AuPd catalysts, homogeneous... 

P/M Tool Steels. In conventionally produced high alloy tool steels (slowly cooled cast ingots), carbide tends to segregate (48). Segregated clusters of carbide persist even after hot working, and cause undesirable effects on tool fabrication and tool performance. P/M tool steels, on the other hand, provide very fine and uniform carbides in the compact, the final bar stock, and the tools. Several tool steel suppHers consoHdate gas-atomized tool steel powder by HIP to intermediate shapes, which are then hot-worked to final mill shapes. Water-atomized tool steel powder is also available (see also T OOL materials). ... 

Ab-initio studies of surface segregation in alloys are based on the Ising-type Hamiltonian, whose parameters are the effective cluster interactions (ECI). The ECIs for alloy surfaces can be determined by various methods, e.g., by the Connolly-Williams inversion scheme , or by the generalized perturbation method (GPM) . The GPM relies on the force theorem , according to which only the band term is mapped onto the Ising Hamiltonian in the bulk case. The case of macroscopically inhomogeneous systems, like disordered surfaces is more complex. The ECIs can be determined on two levels of sophistication ... 

When the second-site revertants were segregated from the original mutations, the bci complexes carrying a single mutation in the linker region of the Rieske protein had steady-state activities of 70-100% of wild-type levels and cytochrome b reduction rates that were approximately half that of the wild type. In all these mutants, the redox potential of the Rieske cluster was increased by about 70 mV compared to the wild type (51). Since the mutations are in residues that are in the flexible linker, at least 27 A away from the cluster, it is extremely unlikely that any of the mutations would have a direct effect on the redox potential of the cluster that would be observed in the water-soluble fragments. However, the mutations in the flexible linker will affect the mobility of the Rieske protein. Therefore, the effect of the mutations described is due to the interaction between the positional state of the Rieske protein and its electrochemical properties (i.e., the redox potential of the cluster). 

Figure 4.28. STM image of a PtRh(lOO) surface. Although the bulk contains equal amounts of each element, the surface consists of 69% of platinum (dark) and 31 % of rhodium (bright), in agreement with the expected surface segregation of platinum on clean Pt-Rh alloys in ultrahigh vacuum. The black spots are due to carbon impurities. It is seen that platinum and rhodium have a tendency to cluster in small groups of the same elements.

This Study has shown that reasonably uniform platinum crystallites can be made on y-alumlna, and that platinum and palladium can be segregated and maintained In that form for the most part even after exposure to high temperature oxidation-reduction conditions. Highly dispersed clusters of palladium, nickel, cobalt, and Iron can be observed. Cluster size determination could not be accurately made because of the lack of contrast between the cluster and the support. The marginal detectability by EDS for these clusters enabled elemental Identification to be made, however, mass uniformity determinations could not be made. 

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).

Complete clustering, or segregation, of tautomers may occur during crystal growth. This can lead, for example, to crystals that consist of two tautomers in equal proportions and located at crystallographically independent sites. This is the case for isocytosine, 47 (104). Similarly, in crystalline anthranilic acid the neutral molecule and the zwitterion coexist (10S). Another example, 39, has been mentioned already (82). 

Further evidence for microphase separahon has been seen by AFM. As expected, BPSH 00, with no ionic regions, displays no significant features in its AFM image. For BPSH 20, isolated ionic clusters have dimensions of 10-25 nm. These clusters are even more readily discerned from the non-ionic matrix in BPSH 40, but the domains appear to remain relatively segregated from each other. In the case of BPSH 50 and 60, connections between domains are clearly visible, especially in the case of the latter sample. It also should be noted, however, that these samples were in a dehydrated state. Therefore, it might be expected that even in the case of the lower acid content samples, it is likely that some channel formation between ionic domains will still occur upon the uptake of water. This can be clearly seen in its linear conductivity behavior as a function of disulfonated monomer (i.e., the percolation threshold has been reached by at least 20-30% content of disulfonated monomer). 

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