Terrace plane

A kinked surface, like fee (10,8,7), can also be approximately expressed in this fomi the step plane (h k / ) is a stepped surface itself, and thus has higher Miller indices than tlie terrace plane. However, the step notation does not exactly tell us the relative location of adjacent steps, and it is not entirely clear how the terrace width M should be counted. A more complete microfacet notation is available to describe kinked surfaces generally [5]. 

Table 5.5. Miller indices, stepped surface designations and angles between the macroscopic surface and terrace planes for fee crystals...

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. 

A notation for these surfaces, the compact-step notation, devised by Lang and Somoijai [3], gives the surface structure in the general form w h,k,l,) X (h kj,), where hjc,l,) and h kj,) are the Miller indices of the terrace plane and the step plane, respectively, while w is the number of atoms that are counted in the width of the terrace, including the step-edge atom and the in-step atom. Thus, the fcc(755) surface is denoted by 7(111) x 1(100), or also by 7(111) x (100) for simplicity. A stepped surface with steps that are themselves high-Miller-index faces is termed a kinked surface. For example, the fcc(10, 8, 7) = 7(111) X (310) surface is a kinked surface. The step notation is, of course, equally applicable to surfaces of bcc, hep, and other ciystals, in addition to surfaces of fee crystals. Stepped surfaces of several orientations are listed in Table 2.2 (p. 88). Here the crystal faces are denoted both by their Miller indices and by their stepped-surface notation. 

Another special case of weak heterogeneity is found in the systems with stepped surfaces [97,142-145], shown schematically in Fig. 3. Assuming that each terrace has the lattice structure of the exposed crystal plane, the potential field experienced by the adsorbate atom changes periodically across the terrace but exhibits nonuniformities close to the terrace edges [146,147]. Thus, we have here another example of geometrically induced energetical heterogeneity. Adsorption on stepped surfaces has been studied experimentally [95,97,148] as well as with the help of both Monte Carlo [92-94,98,99,149-152] and molecular dynamics [153,154] computer simulation methods. 

Au vapor deposited on mica at 300-400° is known to form large [111] terraces [117-122], Several attempts to use these surfaces in the flow-cell (Figure 3C) generally resulted in delamination, due to the constant rinsing of the cell. Ideally, the top of the mica consists of a single plane of the compound. This is not generally the case, however, and defects lead to cracks, solution infiltration and then delamination. Some success in using Au on mica in the flow-cells has been had when a photo-resist was patterned on the surface. It is possible that the resist helps to seal defects. 

The properties of defects of this type are difficult to determine experimentally, although absorption spectra do give information about electron or hole binding energies. Much information is obtained by calculation, using density functional or other quantum computational methods. In this way, the relative stabilities of defects on plane faces, steps, terraces, and corners can be explored. 

We can create surfaces from the fee, hep and bcc crystals by cutting them along a plane. There are many ways to do this Fig. A. 1 shows how one obtains the low-index surfaces. Depending on the orientation of the cutting plane we obtain atomically flat surfaces with a high density of atoms per unit area or more open surfaces with steps, terraces and kinks (often referred to as corrugated or vicinal surfaces). Thus, the surface of a metal does not exist one must specify its coordinates. 

Figure 2.5 Representation of a surface (100) plane of MgO showing steps, kinks, and terraces, which provide sites for Mn+ and low-coordination oxygen anions. (Reproduced from Dyrek, K. and Che, M., Chem. Rev. 1997, 97, 305-331. Copyright 1997, American Chemical Society. With permission.)...

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

Fig. 3-6. Ions on the surface and in the interior of solids 0=occupied or vacant lattice site - surface kink site (S> = surface adsorption site = surface lattice vacancy, (f) = step plane = terrace ai = unitary level of occupied or vacant lattice site ions a = unitary level of surface kink site ions.

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