Clock reconstruction

Figure 5.8. Left STM picture of the so-called clock reconstruction that occurs when 0.5 ML of carbon is adsorbed on Ni(lOO). Right Hard sphere model of the reconstruction. Dotted rings indicate the unreconstructed positions of...

It has been observed that cobalt may undergo large-scale reconstruction under a synthesis gas environment.27 Reconstruction is a thermodynamically driven process that results in the stabilization of less reactive surfaces. Recent molecular modeling calculations have shown that atomic carbon can induce the clock reconstruction of an fee cobalt (100) surface.28 It has also been postulated and shown with in situ x-ray adsorption spectroscopy (XAS) on cobalt supported on carbon nanofibers that small particles (<6 nm) undergo a reconstruction during FTS that can result in decreased activity.29... 

Molecular modeling work performed by Sasol researchers on fee cobalt (100) shows that increased coverage of 50% atomic carbon will induce a clock type reconstruction (Figure 4.3) similar to that observed for the classic case of Ni (100).28 The adsorption energy of the carbon is stabilized by 15 kJ/mol compared to the unreconstructed surface, resulting in a more stable surface.28 The reconstruction results in a shorter distance between the carbon and cobalt but also an increase in coordination of the cobalt atoms and, thus, fewer broken bonds. The barrier for the carbon-induced clock reconstruction was found to be very small (1 kJ/mol), which suggested that the process is not kinetically hindered. The... 

FIGURE 4.3 Left The unreconstructed surface of 50% C/fccCo (100). Right The clock reconstructed surface of 50% C/fcc Co (100). The darker spheres represent cobalt atoms and the lighter ones (in the fourfold hollow sites) represent carbon atoms. (Reprinted from Ciobica. I. M.. van Santen, R. A., van Berge, P. J., and van de Loosdrecht, J., Adsorbate Induced Reconstruction of Cobalt Surfaces, Surface Science, 602, 17-28. Copyright 2008, with permission from Elsevier.)... 

Klink C, Olesen L, Besenbacher F, Stensgaard I, Laegsgaard E, Lang ND (1993) Interaction of C with Ni(lOO) - atom-resolved studies of the clock reconstruction. Phys Rev Lett 71 4350... 

Klink C, Stensgaard I, Besenbacher F, Laegsgaard E (1995) An STM study of carbon-induced structures on Ni(lll) - evidence for a carbidic-phase clock reconstruction. Surf Sci 342 250... 

Kirsch and Harris [176] calculated Ni(OOl) surface reconstructions induced by C, N, and O adsorption and suggested that C and N atoms prefer the nearly coplanar sites with the top Ni surface, which induce the clock reconstruction of the surface. However, O atoms prefer sites slightly above the Ni(lOO) surface plane and have little effect on the overall surface structure. The local environments of the C, N, and O atoms on these surfaces are similar to their environments in a series of late transition metal carbonyl clusters, suggesting that some of the same electronic factors may play a role in favouring different structures. Results of the calculations suggest that adsorbates occupy coplanar sites on Ni(lOO) surface disrupt the Ni-Ni bonding within the surface layer and bonds between the surface and second layers. 

On the C- and N-covered surfaces, the disruption is overcompensated by the formation of strong adsorbate-Ni bonds and by new Ni-Ni surface bonds resulting from the clock reconstruction. When O is forced into a coplanar site, however, both the higher electron count and increased electronegativity of the O atoms lead to severe disruption of the surface bonding and the formation of weak Ni-0 bonds. When O atoms sit above the surface, they form more polar Ni-0 bonds, contribute less electron density to the Ni surface bands, and cause less disruption to the Ni-Ni surface bonds. These results suggest that similar to the organometallic clusters, the... 

Fig. 3.25 STM images (reprinted with permission from [173, 174]) and models (insertions) for (a, b) the Ni(001)-4,y2(l x 1)R45°-16(C ", N ) clock reconstruction. Indicated are individual atomic valences. Although they appeared the same to that of the Rh(001)-4.y2(l x 1)R45°-160 phase, the surface atomic valences and the driving forces are different (reprinted with permission from [1])...

The 3B premise provides an alternative view on the reconstruction suggesting that O, N, and C tend to find their own environment to form the tetrahedron with bond and nonbond creation. Tetrahedron bond formation drives the radial and the subsequent clock reconstruction on the O-Rh(OOl) surface. Meanwhile, the 0 transits into an 0 with sp-orbital hybridization. The ip-orbital hybridization also holds for the C and N when they react with the Ni(OOl) surface. The electrostatic... 

C. Klink, L. Olesen, F. Besenbacher, I. Stensgaard, E. Laegsgaard, N.D. Lang, Interaction of c with Ni(lOO)—atom-resolved studies of the clock reconstruction. Phys. Rev. Lett. 71(26), 4350 353 (1993)... 

The net gain in energy provides forces that drive reconstruction, though the orbit hybridization and the antibonding formation consume some amount of energy. Analysis of the Rh(001)-O clock reconstruction indicates that the driving force for the atomic dislocation also comes from other sources such as the alternative electrostatic repulsion and attraction along the (11) rhombi chain, and a response of bond tension that stabilizes the clock rotation. 

A well-known example of adsorbate induced surface reconstruction is that of carbon on the (100) surface of nickel. Even though this surface already offers a four-fold coordination to the carbon atom in unreconstructed form, additional energy is gained by the so-called clock-anti-dock reconstruction shown in Fig. 5.8. Of course, it costs energy to rearrange the surface nickel atoms, but this investment is more than compensated by... 

Many such exist in rocks and minerals, and they enable geologists to reconstruct the history of our planet long before the first humans emerged. Uranium isotopes are some of the most useful geochemical clocks. Uranium-238 decays with a half-life of about 4.5 billion years, almost the same as the age of the Earth. A sequence of decay steps converts U to thorium-230. 

Feng D-F, Cho G, Doolittle RF (1997) Determining divergence times with a protein clock update and reevaluation. Proc Natl Acad Sci USA 94 13028-13033 Gabaldon T, Huynen MA (2003) Reconstruction of the protomitochondrial metabolism. Science 301 609... 

The maximum number of walkers that can be used scales exponentially with the dimensionality of the free energy that has to be reconstructed. In practical applications, the number of walkers can be used for reconstructing a free energy in two, three and for dimensions is of the order of 10, 100 and 1000, respectively. This feature of the algorithm allows to reconstruct free energies as a function of several variables in a clock time that depends only on the maximum number of available processors. 

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