Steps, kinked

As the crystal surface exposed to the atmosphere is usually not ideal, specific sites exist with even much lower co-ordination numbers. This is shown schematically in Fig. 3.5, which gives a model comprising so-called step, kink and terrace sites (Morrison, 1982). This analysis suggests that even pure metal surfaces contain a wide variety of active sites, which indeed has been confirmed by surface science studies. Nevertheless, catalytic surfaces often behave rather homogeneously. Later it will be discussed why this is the case. In short, the most active sites deactivate easiest and the poorest active sites do not contribute much to the catalytic activity, leaving the average activity sites to play the major role. 

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... 

Figure 1.12 Schematic diagram showing the mirror equivalent step-kink arrangements ofthe fee 6 4 3 R and fee 6 4 3 s surfaces (Adapted with permission from Ref. [37]. Copyright 1996, American Chemical Society.)...

Surface faceting may be particularly significant in chiral heterogeneous catalysis, particularly in the N i/P-ketoester system. The adsorption of tartaric add and glutamic acid onto Ni is known to be corrosive and it is also established that modifiers are leached into solution during both the modification and the catalytic reaction [28]. The preferential formation of chiral step-kink arrangements by corrosive adsorption could lead to catalytically active and enantioselective sites at step-kinks with no requirement for the chiral modifier to be present on the surface. 

Since the most active catalytic sites are usually steps, kinks, and surface defects, atomically resolved structural information including atomic distribution and surface structure at low pressure, possible surface restructuring, and the mobility of adsorbate molecules and of the atoms of the catalyst surface at high temperature and high pressure is crucial to understanding catalytic mechanisms on transition metal surfaces. The importance of studying the structural evolution ofboth adsorbates... 

Figure 2.5 Representation of a surface (100) plane of MgO showing steps, kinks, and terraces, which provide sites for Mn+ and low-coordination oxygen anions. (Reproduced from Dyrek, K. and Che, M., Chem. Rev. 1997, 97, 305-331. Copyright 1997, American Chemical Society. With permission.)...

A silicon surface, no mater how well it is prepared, is not perfectly flat at the atomic scale, but has surface defects such as surface vacancies, steps, kinks sites, and dopant atoms. The dissolution of the surface is thus not uniform but modulated at the atomic scale with higher rates at the defects and depressed sites. The micro roughness of the surface will increase with the amount of dissolution due to the sensitivity of the reactions to surface curvature associated with the micro depressed sites. These sites, due to the higher dissolution rates, will evolve into pits and eventually into pores. Depending on the condition, a certain amount of dissolution is required before the initiation of pores on all types of materials. 

Surface defects, such as steps, kinks, and pits, establish surface sites of different activation energy, with different rates of reaction ... 

The structure of real surfaces differs from the structure of ideal surfaces by the surface roughness. Whereas an ideal surface is atomically smooth, a real surface has defects, steps, kinks, vacancies, and clusters of adatoms (Fig. 3.16). 

For the atomistic simulation of the kaolinite interface it is assumed that surfaces are planar. Irregularities such as steps, kinks and ledges, which are present on real surfaces, are omitted for the present treatment. For kaolinite the energy of the 001 basal surface was evaluated using a suitable cell containing 425 atoms. 

Fluctuations of an isolated step are also suppressed by the microscopic energy cost to form kinks. On coarse-graining, this translates into an effective stiffness or line tension that tends to keep the step straight. Standard microscopic 2D models of step arrays incorporating both of these physical effects include the free-fermion model and the Terrace-Step-Kink (TSK) model. Both models have proved very useful, though their microscopic nature makes detailed calculations difficult. 

One has, therefore, a picture of ions from a solution being transferred onto the electrode surface as adions of adions joining steps, kinks, etc. of steps advancing on the surface of screw dislocations yielding growth spirals of the surface advancing... 

The relevance of crystal faces to the subject of electrociystalhzation comes up as follows Each of the crystal faces just described contains all the microfeatures that have been described in previous sections, steps, kinks, etc. Further, the same phenomena of deposition—the ions crossing the electrified interface to form adions, the surface diffusion, lattice incorporation of adions, screw dislocation, growth spirals, etc.—occur on all the facets. 

Smooth With steps, kinks Agglom- erate On flat surface On high surface area support... 

Davydov et al.44 report on the basis of e.s.r. and i.r. studies that CO adsorption takes place on V3+and V4+-ions. Mori etal.61,119 suggest that the oxidation of CO takes place on active sites such as steps, kinks, or vacancies and that (V=0) groups are much less active. This also explains the observation that Ti02-supported catalysts are less active for this reaction than are unsupported ones, in contrast to the promoting effect observed in hydrocarbon oxidation. Their results do not agree, however, with those of Roozeboom etal.,113 who find a promoting effect also for CO oxidation. 

Figure 2.9. BCF model for surface with adsorbed ions (black spheres). The terms step, kink, hole and nucleus refer to different types of surface sites, and the numbers refer to the number of chemical bonds likely.

The catalyst particle is usually a complex entity composed of a porous solid, serving as the support for one or more catalytically active phase(s). These may comprise clusters, thin surface mono- or multilayers, or small crystallites. The shape, size and orientation of clusters or crystallites, the extension and arrangement of different crystal faces together with macrodcfects such as steps, kinks, etc., are parameters describing the surface topography. The type of atoms and their mutual positions at the surface of the active phase or of the support, and the type, concentration and mutual positions of point defects (foreign atoms in lattice positions, interstitials, vacancies, dislocations, etc.) define the surface structure. 

Fig. 11. Representation of a surface <100) plane of MgO showing surface imperfections such as steps, kinks, corners, etc., which provide sites for ions of low coordination.

Surface Crystallography and Composition. Platinum (11) and nickel (8,9,12) have been the metal surfaces examined in our surface science studies to date. The surface coordination chemistry has been examined as a function of surface crystallography and surface composition. Surfaces specifically chosen for an assay of metal coordination number and of geometric effects were the three low Miller index planes (111), (110) and (100) as well as the stepped 9(lll)x(lll) and stepped-kinked 7(lll)x(310) surfaces (both platinum and nickel are face centered cubic). 

The stepped nickel 9(lll)x(lll) and stepped-kinked Ni 7(lll)x(310) surfaces displayed a benzene coordination chemistry that was quantitatively and qualitatively identical with that of the Ni(lll) surface except that not all the benzene was displaced by trimethylphosphine indicating that either a small percentage ( 10%) of the benzene on these surfaces either was present in different environments or was dissociatively (9) chemisorbed see later discussion of stereochemistry. Benzene chemisorption behavior on Ni(110) was similar to that on Ni(lll) except that the thermal desorption maximum was lower, vl00°C, and that trimethylphosphine did not quantitatively displace benzene from the Ni(110)-C H surface. In these experiments, no H-D exchange was observed with CgHs + C D mixtures. 

Figure 10. An exaggerated representation of benzene molecules chemisorbed near stepped or stepped-kinked sites where C—H hydrogen atoms may closely approach metal atoms in the next plane. This can account for the higher reactivity (C—H bond breaking) observed for benzene on such irregular stepped or stepped-kinked...

Acetonitrile and Methyl Isocyanide (8). Acetonitrile forms an ordered chemisorption state on the fully flat nickel surfaces, a p(2x2) and a c(2x2) on Ni(lll) and Ni(100), respectively. Acetonitrile thermal desorption from these two surfaces was nearly quantitative (a small amount of acetonitrile decomposed at the temperatures characteristic of the reversible thermal desorption from these surfaces). Importantly from an interpretive context, acetonitrile was quantitatively displaced from these two flat low Miller index planes by trimethylphosphine (8). However, the displacement was not quantitative (only 90-95% complete) from the stepped and stepped-kinked surfaces. For the super-stepped (110) surface, chemisorption was nearly irreversible and no acetonitrile could be displaced from this surface by trimethylphosphine. 

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