Step and kink densities

Another feature of the spiral tip is that it has an abnormally high step and kink density and perhaps the tip has a higher exchange-current density for deposition than the corresponding planar surface. If this were so, the activation overpotential would be much less at the tip of the spiral than around its base. 

Primary structure sensitivity resulting from the effect of changing particle size on step and kink density appears therefore to be present here at short reaction times. Secondary structure sensitivity (including the effect of carbonaceous poisoning on the reaction rate) appears not to be present here. Thus Somorjai has reported that the dehydrogenation reaction of cyclohexane to cyclohexane is insensitive to both structural featureSt whereas the dehydrogenation of cyclohexene to benzene la very sensitive to the densities of atomic steps and kinks and the order of the carbonaceous overlayer on the platinum crystal surface. 

FIGURE 9.4 Measured turnover frequencies of C-C (hydrogenolysis) and C-H (dehydrogenation) rates as a function of step and kink density on Pt surfaces. The much stronger structure dependence of the hydrogenolysis that involves C-C bond formation is clearly seen. Adapted from Blakely and Somoijai (1976). 

Density of surface atoms at steps and kinks relative to surface atomic density... 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

Liu ZP, Hu P (2003) General rules for predicting where a catalytic reaction should occur on metal surfaces A density functional theory study of C-H and C-O bond brealdng/making on flat, stepped, and kinked metal surfaces. J Am Chem Soc 125 1958... 

A different view of CO oxidation comes from computational studies of extended Au(l 11). Steps and kinks play a critical role in reducing the activation energy for O2 dissociation. The enhanced activity of thin islands may be related to step density (geometric effects) rather than to quantum size effects. The literature suggests a key role for such defects. Gold is an effective catalyst because it can bind CO and O2 but weakly enough so that subsequent processes have achievable activation barriers. 

Davis et al. (23) also studied the isomerization and hydrogenolysis of isobutane, n-butane, and neopentane over flat, stepped, and kinked Pt single-crystal surfaces. The rates and selectivities of butane isomerization and consecutive rearrangements were maximized on (100) portions of the surfaces. Competing hydrogenolysis reactions were most rapid on surfaces containing the greatest step- and kink-site densities. 

The ad-atoms are an intermediate in the mechanism of metal deposition. On one hand, they are in exchange with the metal ions in the electrolyte and, on the other, with atoms in step and kink positions. The density of step atoms and kink sites on a step is also called the step roughness. 

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