Single kink

In summary, a Pd(l 1 1) single-crystal surface is not sufficient to model the complex adsorption behavior of palladium nanoparticles, even for nanoparticles which mostly exhibit (111) facets. High Miller index stepped or kinked single-crystal surfaces may provide better models of nanoparticles. However, one should remember that CO adsorbed on defects of defect-rich Pd(l 11) became invisible at high coverages Furthermore, it will be demonstrated in a following section that the... 

Structure Sensitivity of Hydrocarbon Conversion Reactions on Platinum Surfaces How does the reaction rate depend on the atomic structure of the platinum catalyst surface To answer this question, reaction rate studies using flat, stepped, and kinked single-crystal surfaces with variable surface structure were very useful indeed. For the important aromatization reactions of n-hexane to benzene and Ai-heptane to toluene, it was discovered that the hexagonal platinum surface where each surface atom is surrounded by six nearest neighbors is three to seven times more active than the platinum surface with the square unit cell [155, 156]. Aromatization reaction rates increase further on stepped and kinked platinum surfaces. Maximum aromatization activity is achieved on stepped surfaces with terraces about five atoms wide with hexagonal orientation, as indicated by reaction rate studies over more than 10 different crystal surfaces with varied terrace orientation and step and kink concentrations (Figure 7.38). 

The sketch includes amcrphcus areas (defects), chain ends, rotated crystals, loose folds, sharp folds, areas of para-crystalline structure, extended-chain sections, voids, sheared regions, chain-kinks, single fibrils, migrating folds, and single-crystal regicns. 

Many surfaces have additional defects other than steps, however, some of which are illustrated in figure A1.7.1(b). For example, steps are usually not flat, i.e. they do not lie along a single low-mdex direction, but instead have kinks. Terraces are also not always perfectly flat, and often contain defects such as adatoms or vacancies. An adatom, is an isolated atom adsorbed on top of a terrace, while a vacancy is an atom or group of atoms missing from an otiierwise perfect terrace. In addition, a group of atoms called an island may fonn on a terrace, as illustrated. 

Resistance to axial compressive deformation is another interesting property of the silk fibers. Based on microscopic evaluations of knotted single fibers, no evidence of kink-band failure on the compressive side of a knot curve has been observed (33,35). Synthetic high performance fibers fail by this mode even at relatively low strain levels. This is a principal limitation of synthetic fibers in some stmctural appHcations. 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

B Surface vacant site C Single vacancy kink D Adatom E Kinked ledge F Terrace... 

In the case of a symmetric (or Just slightly asymmetric) potential the instanton trajectory consists of kink and antikink, which are separated by infinite time and do not interact with each other. In other words, we may change the boundary conditions, namely, suppose that the time spans from — 00 to -t- 00 for a single kink, and then multiply the action in (5.72) by factor 2. 

Fig. 2. Cables of parallel SWNTs thal have self-assembled during oxidative cleanup of arc-produced soot composed of randomly oriented SWNTs imbedded in amorphous carbon. Note the large cable consisting of several tens of SWNTs, triple and single strand tubes bent without kinks, and another bent cable consisting of 6 to 8 SWNTs.

RPs are either constructed from a single layer or built up from multiple layers. The properties of each layer are usually orthotropic, which is a special case of anisotropy. Fibers that remain straight in the single layer are desired. However, with many fabrics, they are woven into configurations that kink the fiber bundles severely. Such fabric constructions may be very practical since... 

Although most of an RNA molecule is single-stranded, there often are some double-stranded regions. Intramolecular base pairing between guanine and cytosine and between adenine and uracil creates loops and kinks in the RNA molecule. The structure of one kind of RNA molecule is shown in Figure 13-30. 

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. 

Last, but not least, one must not forget that steps and kinks appear as structural defects on atomically flat surfaces of single crystals, while edges and vertices are inherent structural elements of metal nanoparticles. 

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. 

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