Miller index, high

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 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... 

Figure 1.5 The surface structures of (i) several high-Miller index stepped surfaces with different terrace widths and step orientations (ii) several high Miller-index surfaces with differing kink concentrations in the steps. From G.A. Somorjai, Chemistry in Two Dimensions, Cornell University Press. London, 1981, pp. 160and 161. Used by permission ofCornell University Press.

Copper high Miller index, 26 12 Copper oxide, 27 184-187, 199 as adsorbent, 21 44 on alumina, 27 80-85 -manganese oxide, 27 91, 92 oxidation of CO over, 24 86 -platinum catalyst, 27 86-88 propylene oxidation, 30 141 Coprecipitation, perovskite preparation, 36 247-250... 

Gadolinia, conversion rates, 27 35 Gallium arsenide, high Miller index, 26 12 Gallosilicate zeolite, Si MAS NMR studies of, 33 233-236... 

More recently, the question of thermal roughening has also been addressed in the study of metal surfaces. Detailed He difiraction studies from the high Miller index (113) surface of Cu and Ni proved the existence of a roughening transition on these surface. These studies were performed by means of He scattering. Let us make first two short comments. 

We should mention here a special notation used for describing high-Miller-index surfaces. Such surfaces can often be more usefully described as stepped surfaces involving relatively close-packed terraces of low-Miller-index orientation separated by steps whose faces have also a low-Miller-index orientation. For example, the fcc(755) surface can be more easily visualized with the notation fcc(S)-[6(l 11)X (100)], where (S) means stepped , since this indicates that the surface is composed of terraces of (111) orientation and 6 atoms wide, separated by steps of (100) orientation and 1 atom high. A list of such correspondences of notation for stepped fee surfaces is included in Sect. V. 

Rainbow scattering has been detected from high Miller Index stepped platinum surfaces. Typical rainbow scattering patterns are shown in Fig. 4.7. The increase in intensity of the surface rainbows, as displayed by this figure, for an increase in the angle of incidence, qualitatively follows the trend predicted from calculations by McClure... 

In the last few years LEED studies of high Miller index or stepped surfaces have become more frequent. Almost all of these studies have been on fee metals, where the atomic structure of these surfaces consists of periodic arrays of terraces and steps. A nomenclature which is more descriptive of the actual surface configuration has been developed for these surfaces, as described in Section III. In Table 5.5 the stepped surface nomenclature for several high Miller index surfaces of fee crystals has been tabulated. In Fig. 5.1 the location of these high Miller index surfaces are shown on the... 

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. 

II. The Atomic Structure of Surfaces. Structures of Low and High Miller Index Crystal Surfaces. 5... 

D. The Atomic Surface Structure of High Miller Index Surfaces.. 12... 

IV. Chemisorption of Hydrocarbons on Low and High Miller Index Surfaces of... 

C. Hydrocarbon Chemisorption on High Miller Index (Stepped) Platinum... 

Platinum crystal surfaces that were prepared in the zones indicated by the arrows at the sides of the triangle are thermally unstable. These surfaces, on heating, will rearrange to yield the two surfaces that appear at the end of the arrows. There is reason to believe that the thermal stability exhibited by various low and high Miller index platinum surfaces is the same for other fee metals. There are, of course, differences expected for surfaces of bcc solids or for surfaces of solids with other crystal structures. 

Fig. 6. A stereographic triangle of a platinum crystal depicting the various high Miller index surfaces of platinum that were studied.

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... 

These studies are beginning to produce important kinetic information only in recent years. We shall summarize the experimental information available from studies of these reactions on the stepped [6(111) x (100)] and low Miller index (111) crystal surfaces and compare the low- and high-pressure rates when these data are available. 

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