Steps, direct dissociation

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. 

Two DFT studies have calculated the stepwise addition of adsorbed H to adsorbed N on Ru(0001) terraces to form NH3. The last step is the reverse of NH3 dissociation on the terraces. One study finds W = 1.3 eV and V = 1.4 eV (for NH3 dissociation) [350,351] while the other finds W = 0.89 eV and V = 0.8 eV [352]. The second study is in better agreement with experiments in the direct dissociation regime. However, only Refs. [350,351] study dissociation of NH3 at step sites and find W = 1.76 eV and Ec—Ed = —0.5 eV. This agrees reasonably with experiment for the precursor channel. Therefore, DFT studies do qualitatively support the picture suggested by the experiments. 

As we will discuss later the step effects are found to be quite general, and the role of steps in dissociation reactions has also been observed directly for ethylene activation on Ni surfaces, see Figure 4.20 [69,70]. 

This means that direct dissociation is sterically hindered at the Pt(l 1 1) surface so that it becomes a two-step process. First the molecule is trapped molecularly in the chemisorption well where it equilibrates. At sufficiently high surface temperatures dissociation will then be induced by thermal fluctuations which make the 02 molecules enter the dissociation channel. 

A variety of processes can occur in the interaction of 02 molecules and Ag(l 11). At first scattering from and trapping in the physisorption potential can occur. Secondly, scattering from the chemisorption (02 ) potential occurs, together with transient trapping-desorption. The chemisorption potential well is very shallow. From being transiently trapped the molecule can be captured in the molecular chemisorption well presumably surface imperfections are necessary to stabilize the molecular adsorbate in this case. From the molecular chemisorption well the molecule can proceed to dissociation. In this step ad atoms may be involved on Ag(l 11). Finally, there is a small probability for direct dissociative chemisorption of 02 at Ag(l 11). The formation of added Ag-O rows (fences) at the surface inhibit further sticking at the surface. 

Figure 12 So for D2 on Pt(5 3 3) at T = 300 K and E = 180 meV as a function of incident angle 4>i ( ) [63], scattering in a plane across the step direction. 4>i is defined as positive when scattering into the (100) step edge, with O = 0° corresponding to the (5 3 3) surface normal (Fig. 6). The contribution to the direct channel associated with the (11 1) terraces has been estimated (diamond dotted line), and subtraction from the experimental data yields the direct dissociation contribution of the (100) steps ( ). The latter was fitted with a cos3 0 dependency (square dotted line).

W(1 0 0) surface [164]. A similar very small dependence of So on Ts was also observed elsewhere [165]. This lead to the suggestion [164, 168] that the precursor responsible for dissociation may not be fully accommodated at Ts, but had sufficient lifetime at the surface to undergo dissociation if it encountered a defect (or step) site. It also leads to the series of experiments in which H2 and N2 dissociation was investigated on the W(1 0 0)-c(2 x 2)Cu alloy surface in order to establish the effect of changing the activation barrier to direct dissociation in the surface unit cell, and concomitant effects on the indirect dissociation channel. 

Figure 5.4.3 Molecular modeling simulation of different CO dissociation pathways on an iron (100) surface, along with relative energies in electron volt (1 eV 96 kJ/mol). The direct dissociation of CO (i.e. reaction [10] in the text), yields the lowest activation barrier, but the two-step mechanism in which CO first reacts with H to an HCO intermediate yields an overall barrier that is only slightly higher. Hence, under conditions where the CO is coadsorbed with H atoms, this route might compete with the direct dissociation (adapted from Elahifard et al. [20]).

These results show that one has to discriminate between C—O bond activation through H addition to the C atom in CO (formation of HCO) and bond acfivafion fhrough H addition to the O atom of CO (formation of COH). Acfivafion of CO fhrough COHa s on nickel proceeds with substantially lower barriers than direct CO dissociation, even on stepped and double-stepped surfaces. Dissociation of CO via COHads appears to be rather insensitive to CO coverage. 

On nickel (a metal with a low reactivity for CO activation), hydrogen-assisted CO dissociation is always the most favorable pafhway, independent of the nature of fhe surface site. On ruthenium, two scenarios must be differentiated. On stepped ruthenium surfaces, direcf CO dissociation is the reaction path with the lowest energy barrier, whereas on ruthenium terraces, hydrogen-assisted C—O cleavage is again favored over direct dissociation. 

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