Edge site

The rate and mechanism are different on the basal plane and edge sites of carbon. The reactions involving oxygen are two to three orders of magnitude slower on the basal plane than on the edge sites, because of the weak adsorption of oxygen molecules on the basal plane surface [34]. 

The basic building block of carbon is a planar sheet of carbon atoms arranged in a honeycomb structure (called graphene or basal plane). These carbon sheets are stacked in an ordered or disordered manner to form crystallites. Each crystallite has two different edge sites (Fig. 2) the armchair and zig-zag sites. In graphite and other ordered carbons, these edge sites are actually the crystallite planes, while in disordered soft and hard carbons these sites, as a result of turbostratic disorder, may not... 

Figure 1.14 Energetics (kilojoules per mole) and structure of CO dissociating from Ru step-edge site [15].

The activation energy of such molecules depends strongly on the structure of the catalytically active center. The structures of reactant, transition state as well as product state at a step-edge site are shown for CO dissociation in Figure 1.14. 

Whereas the adsorption energies of the adsorbed molecules and fragment atoms only slightly change, the activation barriers at step sites are substantially reduced compared to those at the terrace. Different from activation of a-type bonds, activation of tt bonds at different sites proceeds through elementary reaction steps for which there is no relation between reaction energy and activation barrier. The activation barrier for the forward dissociation barrier as weU as for the reverse recombination barrier is reduced for step-edge sites. 

Interestingly, when the particle size of metal nanoparticles becomes less than 2 nm, terraces become so small that they carmot anymore support the presence of step-edge site metal atom configurations. This can be observed from Figure 1.15, which shows a cubo-octahedron just large enough to support a step-edge site. 

Class 111-type behavior is the consequence of this impossibihty to create step-edge-type sites on smaller particles. Larger particles wiU also support the step-edge sites. Details may vary. Surface step directions can have a different orientation and so does the coordinative unsaturation of the atoms that participate in the ensemble of atoms that form the reactive center. This wiU enhance the activation barrier compared to that on the smaller clusters. Recombination as well as dissociation reactions of tt molecular bonds will show Class 111-type behavior. 

Figure 1.15 Cubo-octahedron with step-edge sites [18].

In this expression, Xi and Xi are the fractions of terrace versus step-edge sites, ri is net rate of conversion of adsorbed CO to methanol on a terrace site, and t2 is the rate of CO dissociation at a step-edge-type site. Increased CO pressure will also enhance the selectivity, because it will block dissociation of CO. 

Figure 1.22 schematically summarizes the principle of the preferred transition states without sharing of a common metal atom. Whereas we have earher discussed surface sensitivity as a function of the relative ratio of particle surface edge sites and surface terrace atoms, the discussion given above provides a principle for particle size shape differences. 

The advantage of the explanations of Equation (3) is its simplicity. It seems certain that the bulk (r ) and surface (r ) terms exist. The simplest way to get a minimum in AG is to include the linear term. The problem with this explanation is that it leaves us with the question why would edge sites have negative hence favorable energies when face sites have positive, disfavorable energies We do not know. We do know, however, that the system with surface-adsorbed ligands is complex. Perhaps that complexity can lead to such an unexpected consequence. 

It is believe that the HDS sites (rim sites and edge sites) are different than the olefin hydrogenation sites (rim sites) opening an opportunity for the development of selective HDS catalysts [45 171. Another concept to exploit in catalyst development is the competitive adsorption, by which the sulfur compounds inhibit olefins hydrogenation [48]. 

Edge sites have thus a higher specific activity for deuteration than sites on the faces of the crystal, and their isotopic distribution pattern shows higher proportions of cU and dz. The isotopic distribution pattern of the cyclohexane on the Ir-8 catalyst does not reflect the continuing trend... 

In view of the mean particle size of Ir-8 we have good reason to expect that with this catalyst the influence of corner sites becomes noticeable. Obviously, the specific rate of deuteration on these corner sites must be lower than that on the edge sites in addition, the cyclohexane formed on them must contain a larger proportion of the more highly deuterated species than the deuteration product coming from either the edge sites or the sites at the faces. 

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