Steps and kinks

Figure Bl.21.2. Atomic hard-ball models of stepped and kinked high-Miller-index bulk-temiinated surfaces of simple metals with fee lattices, compared with anfcc(l 11) surface fcc(755) is stepped, while fee...

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

Recenl work has defined more carefully ihe nature of active sites. Metal surfaces are thought to contain three main types of sites terraces, ledges (or steps) and kinks, which correspond to one, two. and three coordinatively unsaturated sites of organometallic chemistry. These sites display differing activities toward saturation, isomerization, and CKChiingQ 7 J0,68 JO 1.103,104,105). 

The notation is also applicable to surfaces containing both steps and kinks, which results in a rough step. Thus, a (775) surface is equivalent to 6(lll)x(lll). In this manner all sorts of surfaces may be constructed. Of course, the question is whether these are stable or break up into facetted structures. 

It has been often stressed that low eoordinated atoms (defeets, steps, and kink sites) play an important role in surfaee ehemistry. The existenee of dangling bonds makes steps and kinks espeeially reaetive, favoring the adsorption of intermediate species on these sites. Moreover, smdies of single-crystal surfaces with a eomplex geometry have been demonstrated very valuable to link the gap between fundamental studies of the basal planes [Pt( 111), Pt( 100), and Pt(l 10)] and applied studies of nanoparticle eatalysts and polycrystalline materials. In this context, it is relevant to mention results obtained with adatom-modified Pt stepped surfaces, prior to discussing the effect of adatom modification on electrocatalysis. 

Last, but not least, one must not forget that steps and kinks appear as structural defects on atomically flat surfaces of single crystals, while edges and vertices are inherent structural elements of metal nanoparticles. 

It was mentioned on page 306 (see Fig. 5.24) that, even at room temperature, a crystal plane contains steps and kinks (half-crystal positions). Kinks occur quite often—about one in ten atoms on a step is in the half-crystal position. Ad-atoms are also present in a certain concentration on the surface of the crystal as they are uncharged species, their equilibrium concentration is independent of the electrode potential. The half-crystal position is of basic importance for the kinetics of metal deposition on an identical metal substrate. Two mechanisms can be present in the incorporation of atoms in steps, and thus for step propagation ... 

It is concluded that the occupation of the step and kink sites plays a crucial role in the promotion of the Pt catalyst. The cyclic voltammetry results can be used to explain the conversion trends observed in Figure 2. For unpromoted 5%Pt/C the Pt step and kink sites are unoccupied and available for adsorption of reactant and oxidant species. During reaction these sites facilitate premature catalyst deactivation due to poisoning by strongly adsorbed by-products (5) and (or) the formation of a surface oxide layer (6). The 5%Pt,0.5%Bi/C catalyst has a portion of these Pt step and kink sites occupied and the result is a partial reduction in the catalyst deactivation and a consequent increase in alcohol conversion. As the Bi level is increased to lwt.% almost all of the Pt step and kink sites are occupied and the result is a catalyst with high activity. As more Bi is introduced onto the catalyst surface a bulk Bi phase is formed. Since the catalyst activity is maintained it is speculated that the bulk Bi phase is not involved in the catalytic cycle. 

Fig. 6. A model of a platinum surface on which 5/6 of the atoms are on terraces (the 111 and 1/6 of the atoms are at step and kink sites. Reproduced with permission from Ref.1 ...

Exposure of the clean stepped platinum surface to oxygen caused saturation of the step and kink sites (no adsorption occurred on a 111 surface under identical conditions). The oxygen atom-saturated surface was then exposed to varying amounts of carbon monoxide. Both carbide carbon and CO carbon C Is peaks formed, with a one-to-one correspondence between the growth of carbide and the decrease of surface oxygen atoms. These data are consistent with threee possible reaction schemes ... 

The first scheme was ruled out by showing that, at room temperature, a surface formed by very brief exposure of the oxygen-saturated surface to carbon monoxide is stable after removal of the carbon monoxide from the reaction chamber. In other words, no further surface carbide formed by lateral reactions of adsorbed carbon monoxide with surface oxygen atoms. The second scheme was ruled out by showing that exposure of the surface formed in the latter experiment to oxygen had no effect. Consequently the third scheme is believed to represent the mechanism of oxidation of carbon monoxide at the step and kink sites of platinum. 

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

The sites most active and also surviving the longest are on all metals the steps and kinks (lowest coordination sites). 

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. 

The idea that catalyst surfaces possess a distribution of sites of different energies has been around since the 1920s, but it has not been possible until fairly recently to show that adsorption sites on terraces, steps, and kinks differ in energy. For example, hydrogen shows stronger bonding to steps and kinks on platinum than on the 111 terraces. In addition, the activation energy for H2 dissociation is about zero on the step face and about 8.4 kJ mole-1 on the terrace plane. In addition, carbon monoxide is adsorbed with dissociation on the kinks of Pt, but in the molecular form on the steps and terraces. 

The Pt(lll) surface continues to show undetectable activity as the oxygen content increases, although the activity passes through a maximum for both the stepped and kinked surfaces with increasing oxygen content. 

With respect to the atomic arrangements of the surfaces, the adsorption of cyclohexane occurs very similarly on (111) and (110) planes, in the former case as a nondissociative complex of symmetry C3v as is often the case, the results on the (100) face [of Pt(100)] are qualitatively different. On Ni the (111) face is less reactive for cyclohexane dehydrogenation than the stepped and kinked [5(111) X (110)] plane. 

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 ... 

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