Step-edge sites

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. 

Step-Edge Ion-Transfer Mechanism. The step-edge site ion transfer, or direct transfer mechanism, is illustrated in Figure 6.14. It shows that in this mechanism ion transfer from the solution (OHP) takes place on a kink site of a step edge or on any other site on the step edge. In both cases the result of the ion transfer is a M adion in the metal crystal lattice. In the first case, a direct transfer to the kink site, the M adion is in the half-crystal position, where it is bonded to the crystal lattice... 

Thus, in a step-edge site transfer mechanism there are two possible paths direct transfer to a kink site and the step-edge diffusion path. 

Adsorption on a clean surface shows a strong kinetic isotope effect, implying that the initial chemisorption process involves C-H bond scission. Adsorption appears to be facilitated by the presence of step-edge sites, since adsorbed intermediates are seen at low potentials on both polycrystalline Pt and on high-index Pt(335) surfaces. 

Surface reconstruction is driven by stabilization of the adsorbate after adsorption of carbon atoms on more reactive surface atoms. Ciobica et al. (74) demonstrated that an overlayer of Cads leads to the Co(lll) to Co(lOO) reconstruction on fee cobalt (the stable phase of small cobalt particles). Because of the change in metal atom density in the surface layer, the reconstruction may be associated with faceting and hence creation of step-edge sites, which are highly active in the Fischer-Tropsch reaction (5). Hence, surface reconstruction and formation of a stable carbide overlayer may actually be the processes occurring during the initial activation of the catalyst. This phenomenon has been described by Schulz (101) as self-organization. 

The planar structure of surface terraces puts a geometric constraint on the orientation of the molecular or atomic species that are part of the adsorption overlayer. For instance, two coadsorbed CO molecules on a metal surface will not be able to interact with the same surface metal atom. This remains the case even on more open and step-edge sites. The direct repulsive interaction between the adsorbates inhibits their close approach as the direction of the surface adsorbate chemical bond is constrained. Therefore, one will rarely observe the high coordination of several adsorbates to the same surface metal atom as observed in coordination complexes of small metallic clusters. 

It has also been shown on Co that increased adsorption of C atoms on the (111) surface of a small particle with fee bulk structure leads to transformations of the Co(lOO) surface. This surface becomes stabilized because the C atom prefers the fivefold coordination possible on this surface. The reconstmetion will generate step-edge sites on the surface because of the decreased Co metal density on the more open [87], which are potential sites for low-activation CO bond cleavage as... 

When one compares the relative stability of such step-edge sites on a surface of a particle of decreasing size, it appears that on surfaces less then a particular size such step-edge sites cannot anymore be supported [64, 65]. Hence, the low-activation barriers for dissociation as well as recombination disappear on small particles. As a consequence, the rate of reaction controlled by activation of molecular k bonds decreases when the size of the nanoparticle is below this particle size [44]. The behavior of curves I of Figure 10.28 results. 

Figure 10.30 DFT-calculated CO adsorption energies and activation of CO dissociation on Ru(OOOl) terrace and step-edge site (kj/mol). (Adapted from Ref [88].)...

Figure 10.30 illustrates the likeness of the geometrical as well as electronic structure of the CO in its dissociated state compared to that in the transition state [44]. The changes in electron density are shown for a terrace and step-edge site. Especially in the transition state on the step-edge site, with the lower barrier, the electron density between the C and O atoms in the transition state has become very small. 

Similarly as on a step-edge site, dissociation of tr-bonded molecules on the square (100) surface ofan fee crystal becomes substantially lower than on the (111) surface. 

One should note the remarkable similarity in activation energy dependence on surface structure as we discussed in the previous section for activation of CO. Remarkable is the low activation energy of NO on the (100) surface compared to that on the (111) surface and the closeness in its value compared to that at the step-edge site. Figure 10.32b shows that the NO molecules dissociates on the (100)... 

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