Kink sites, blocked

A block model of defects on a single-crystal surface is depicted in Figure 2.4.17 The surface itself in reality is a two-dimensional defect of the bulk material. In addition, one-dimensional defects in the form of steps which have zero-dimensional defects in the form of kink sites. Terraces, which are also shown in the figure, have a variety of surface sites and may also exhibit vacancies, adatoms, and point defects. Surface boundaries may be formed as a result of surface reconstruction of several equivalent orientations on terraces. 

The discovery that kink sites in steps are effective in breaking C C bonds in addition to C-H and H H bonds, thereby initiating hydrogenolysis reactions, may also explain the effect of trace impurities or second component metals that introduce selectivity. Since these kink sites have fewer nearest neighbors than step or terrace sites, they are likely to bind impurities or other metal atoms with stronger chemical bonds. Thus, these sites are readily blocked by impurities. As a result selective poisoning of hydrogenolysis may be obtained by minute concentrations o veil-chosen impurities or another metal component. 

If, however, for any reason, the electrode potential deviates from the equilibrium potential and the balancing equilibrium with kink atoms is blocked, as, e.g., in the absence of steps and kink sites (cf. also Section 2.3), the adatom concentration may change until a dynamic equilibrium between deposition and dissolution is reached diss,free ads dep.free > free Th adatom concentration, ads, or Cads, becomes a function of electrode potential, E, or overpotential rj = E- E y z+, so that... 

Naturally occurring oxoanions like SO/ and H2P04 at concentrations representative of those encountered in natural waters can inhibit dissolution and weathering reactions. A very low concentration of inhibitors can often be effective, because it may suffice to block the functional groups of solution-active sites (such as the kink sites). The effect of specifically adsorbable cations on the reduction of dissolution (weathering) rates of minerals is important. A case was documented by Grandstaff (32), who showed that thorium, Pb(II),... 

Figure 8.3a displays a simplified block model of a metal oxide surface showing one-dimensional defects in the form of steps. These steps in turn give rise to point defects in the form of kink sites. Terraces can also possess a variety of different surface sites such as adatoms, vacancies, and substitutional and interstitial point defects. Figure 8.3b shows that the... 

Additives can control the growth rate by blocking surface processes, e.g., surface diffusion or the transfer in and out of kink site positions. Therefore, interaction of additives with the different crystallographic structures is a dominating factor in the development of crystallographic shape and structure. 

Fig. 3. Kossel s model of molecular attachment kinetics (layer-by-layer growth). Building blocks from a liquid nutrient phase are attached (adsorbed) by a growing crystal surface at a site (A) and migrate on it to join the most beneficial attachment places with the largest number of free bonds to be saturated by such a linking (kink sites K). (S) and (F) correspond to the stepped and closely packed (flat) faces, respectively.

Impurities usually cause a reduction in the growth rates of crystalline materials due to blocking of kink sites, thereby leading to smaller crystals than required. This is a common problem, which is often related to contamination of the feed solution. In some other cases, however, impurities can enhance growth rates, thought to be due to a reduction in interfacial tension and hence increase in surface nucleation rates. 

Undoubtedly, the most important contribution of Kossel and Stranski to the theory of nucleation and crystal growth is the definition of the concept half-crystal position. In an infinitely large crystal, the atom in this, often called kink site position, (Figure 1.19b), is bonded with a semi-infinite crystal block, with a semi-infinite crystal lattice plane and with a semiinfinite crystal row. Attaching or detaching one atom to and from the halfcrystal position a new half-crystal posihon is created and this is what makes... 

Upon increasing tin content the number of accessible Pt sites decreases The H/Pt ratio decreases much faster than the CO/Pt ration. This is an indication that tin strongly blocks kink and corner sites involved in hydrogen activation ... 

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