Miller index, high crystal face

TABLE 2.5j. Surface Structures on High-Miller-Index (Stepped) Crystal Faces ... 

STRUCTURES ON HIGH-MILLER-INDEX (STEPPED) CRYSTAL FACES... 

The temperature behavior of low446,491,503 558 as well as high Miller index crystal faces of Au447,448 has been examined in 0.01 M perchloric acid solutions. For all gold surfaces studied, C, was found to decrease and Ea=Q moved to less negative values with increasing t 446-448 491503-558... 

The slice through a bulk crystal can differ from both the 111 plane and the 100 plane by small angles. This produces a kink in the face of the step. By an extension of the analysis that leads to step characterization, these kinks can also be characterized. For example, a plane with Miller indices 10,8,7 has 111 terraces seven atoms wide, 110 steps one atom high, and kinks of 100 orientation every two atoms. Because of the greater thermodynamic stability of the planes of low Miller index, these surfaces of ordered roughness are stable and can be prepared and studied. Since it is sensitive to periodicity over a domain about 20 nm in diameter, LEED sees the pattern associated with terraces of various widths and may be used to characterize these surfaces. Satisfactory LEED patterns do not require absolute uniformity of terrace width but may be obtained with experimental surfaces that display a distribution of widths. 

The chemisorption of over 25 hydrocarbons has been studied by LEED on four different stepped-crystal faces of platinum (5), the Pt(S)-[9(l 11) x (100)], Pt(S)-[6(l 11) x (100)], Pt(S)-[7(lll) x (310)], and Pt(S)-[4(l 11 x (100)] structures. These surface structures are shown in Fig. 7. The chemisorption of hydrocarbons produces carbonaceous deposits with characteristics that depend on the substrate structure, the type of hydrocarbon chemisorbed, the rate of adsorption, and the surface temperature. Thus, in contrast with the chemisorption behavior on low Miller index surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of platinum even at 300 K and at low adsorbate pressures (10 9-10-6 Torr). Hydrocarbons on the [9(100) x (100)] and [6(111) x (100)] crystal faces form mostly ordered, partially dehydrogenated carbonaceous deposits, while disordered carbonaceous layers are formed on the [7(111) x (310)] surface, which has a high concentration of kinks in the steps. The distinctly different chemisorption characteristics of these stepped-platinum surfaces can be explained by... 

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. 

A single crystal surface exhibits terraces (T) with low Miller index crystal faces and high surface density. They are separated by ledges (L) which may have kinks (K). A terrace between two ledges is a step. Defects on terraces consist of vacancies or adsorbed atoms. Thus the smooth surface may indeed be rough on an atomic scale. This model is called as TLK model (Fig. 2.6). The systematic... 

Figure 6 Top views of the atomic arrangement in the first two or three layers of the low-Miller-index faces of fee, hep, and bee crystals. High-symmetry adsorption sites are indicated. (From Fig. 1 in Ref 77.)...

Top views of the atomic arrangement in the first two or three layers of the low-Miller-index faces of fee, hep, and bcc crystals. High-symmetry adsorption sites are indicated. (Adapted from Mate, C.M., Bent B.E. and Somorjai, G.A., in Hydrogen Effects in Catalysis, Z. Paal and P. G. Menon, eds., Marcel Dekker, New York, 1988, Chapter 2, pp. 57-81, Fig. 1.)... 

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