Kink, site

Fig. 9. Schematic of a two-dimensional cross section of an AgBr emulsion grain showing the surface and formation of various point defects A, processes forming negative kink sites and interstitial silver ions B, positive kink site and C, process forming a silver ion vacancy at a lattice position and positive kink...

Figure 5.4 Attachment of a growth unit into a kink site...

Impurities usually eause a reduetion in the growth rates of erystalline materials due to bloeking of kink sites, thereby leading to smaller erystals than required. This is a eommon problem, whieh is often related to eontamination of the feed solution. In some other eases, however, impurities ean enhanee growth rates, thought to be due to a reduetion in interfaeial tension and lienee inerease in surfaee nueleation rates. 

It has been often stressed that low eoordinated atoms (defeets, steps, and kink sites) play an important role in surfaee ehemistry. The existenee of dangling bonds makes steps and kinks espeeially reaetive, favoring the adsorption of intermediate species on these sites. Moreover, smdies of single-crystal surfaces with a eomplex geometry have been demonstrated very valuable to link the gap between fundamental studies of the basal planes [Pt( 111), Pt( 100), and Pt(l 10)] and applied studies of nanoparticle eatalysts and polycrystalline materials. In this context, it is relevant to mention results obtained with adatom-modified Pt stepped surfaces, prior to discussing the effect of adatom modification on electrocatalysis. 

It is concluded that the occupation of the step and kink sites plays a crucial role in the promotion of the Pt catalyst. The cyclic voltammetry results can be used to explain the conversion trends observed in Figure 2. For unpromoted 5%Pt/C the Pt step and kink sites are unoccupied and available for adsorption of reactant and oxidant species. During reaction these sites facilitate premature catalyst deactivation due to poisoning by strongly adsorbed by-products (5) and (or) the formation of a surface oxide layer (6). The 5%Pt,0.5%Bi/C catalyst has a portion of these Pt step and kink sites occupied and the result is a partial reduction in the catalyst deactivation and a consequent increase in alcohol conversion. As the Bi level is increased to lwt.% almost all of the Pt step and kink sites are occupied and the result is a catalyst with high activity. As more Bi is introduced onto the catalyst surface a bulk Bi phase is formed. Since the catalyst activity is maintained it is speculated that the bulk Bi phase is not involved in the catalytic cycle. 

It was realized at an early stage that the adsorption of two enantiomers at chiral step-kink sites was likely to occur with slightly different adsorption energies. In... 

Such a possibility has been recognized by early workers,9 but in spite of this intriguing possibility, only recently has such a metal surface been created. Chiral kink sites were created on Ag single crystal surfaces to produce the enantiomeric surfaces Ag(643)s and Ag(643)R however, no differences between (R)- and (S)-2-butanol were observed for either the temperature-programmed desorption from the clean surfaces or the dehydrogenation (to 2-butanone) from preoxidized surfaces.10 Unfortunately, Ag exhibits few catalytic properties, so only a limited array of test reactions is available to probe enantioselectivity over this metal. It would be good if this technique were applied to a more catalytically active metal such as Pt. 

Fig. 23 The bonds that constitute crystalline domains must lie nearly parallel to the jy-axis with an angle 6 of less than 20°. Furthermore, the bonds must have at least three neighbors that satisfy 0.7a < Jr + r < 1.3a and ry < r0/2. Note that the crystalline stems deep inside the crystal (black spheres) have six neighbors, while those on the free sin-faces (hatched spheres) have four neighbors. The stems at the half-crystal site, or at the kink site, (white sphere) have three neighbors. Stems attached on the free surface, and those floating in the melt phase have less than three neighbors...

Fig. 27 Shape of crystalline domains at 330 K after 6.4 ns (grey spheres). The crystalline lamellae are found to have rather flat 100 surfaces. Also shown are newly added stems (black spheres) during the next 0.128 ns of simulation. The addition of the stems starts preferentially at kink sites... 

Receptor sites. The intercalation and kinked sites in DNA used in this study are listed in Table V. Three theoretically determined intercalation sites (I, II and III) permit the study to be conducted with the DNA unwound by 7°-12°, lU-l8° and 25°-32° with parallel base pairs separated by 6.76 A and with alternating (a) sugar puckers (67,68) Attention will be confined to site I because it was found to be the most favorable in the present studies. Several kink (K) sites have been identified (66). The constraint that proper hybridization exists about N2(g) and CIO of BPDEs stimulated an investigation in kinked DNA. In an idealized structure the pyrene moiety is approximately parallel to one of the base pairs as shown... 

Table IX. Binding to N6(A) via trans Addition of BPDEs in the 3 -and 5 -Orientations in a Kinked Site (Kma)a...

Although we postulate that this receptor site results in stereoselectivity, it may not be the final state. The orientation of the long axis of the pyrene moiety is approximately 80°-90° and this implies quasi-intercalation of site IQ (56-58). The kinked site proposed by Hogan et al. (50) and studied by Taylor and Miller (MO for l(+)-N2(G) binding in retrospect appears to represent different binding sites. The orientation of the pyrene moiety of a(BPDE) = 1+3° and the local DNA axis in the kink of y(DNA) = 29° (50) will be explained by external binding in the next section. The intercalative covalent binding in a kinked site is an intermediate step between intercalation and the final structure for the externally bound BPDE-N2(G) adduct, but it becomes the final structure for the quasi intercalated BPDE-N6(g), 06(g) and N +(c) adducts. 

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