Disordered overlayer

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 ... 

Plane waves are often used when two conditions are satisfied 1) many (but not necessarily all) atomic layers of the surface have a two-dimensional periodicity, and 2) either a plane-wave incident electron beam is present or angle-resolved electron detection is applied. Computation based on the plane-wave expansion are often much more efficient than those based on the spherical-wave expansion. This explains their frequent use even in problems that do not involve strict two-dimensional periodicity, as with disordered overlayers on an otherwise periodic substrate. 

Zheng T, Stacchiola D, Poon HC, Saldin DK, Tysoe WT (2004) Determination of the structure of disordered overlayers of ethylene on clean and hydrogen-covered Pd(lll) by low energy electron diffraction. Surf Sci 564 71... 

Fig. 7. Schematic illustration of the adsorption geometry for Ni(lll)-(2x2)-C2H2 and the disordered overlayers of Cii(10(>) C2Ha and Cu(I00)-CiH4. The hydrogen positions were not determined and therefore only the carbon atom locations arc shown.

Although in the earlier sections we have omitted mention of the chemisorption of benzene and other aromatic molecules so as not to muddy the waters unnecessarily, there have been a number of structure determinations of chemisorbed benzene that are conveniently described at this point. On surfaces of trigonal symmetry (i.e. fee (111) and eph (001)) the molecule sometimes forms disordered overlayers, which can be forced into long-range order by co-adsorbing carbon... 

The restrictions on the adsorption system are minimal ordered or disordered overlayers can be examined, as can either well-structured single crystal samples or optically rough surfaces. Hence, chemisorption on evaporated films can be studied, as can the nature of metal overlayer-semiconductor interactions. In addition, coadsorbed atoms and molecules can be studied without difficulty. 

In the heterogeneous catalytic system, the reacting molecules, and the metal-surface atoms, can be quite mobile. This leads to locally ordered structures when synchronization or self organization phenomena are present however, disorder in the surface layer prevails when these phenomena are absent. For example, it has been proposed that the catalytic oxidation over mixed metal oxides, discussed in Section 2.3.5, actually occurs in the disordered overlayer that forms at the surface under reaction conditions. 

The interfacial thickness. Lb, for crystallites formed in dilute solution is about 10 A, independently of the molecular weight [226]. Under these crystallization conditions, chain entanglements are minimal and there is no significant chain-mobility restraint on the process of crystallization. In addition to the interface, there is also a substantial disordered overlayer associated with crystals formed in solution [227]. Thus it is not surprising that results obtained using many different experimental methods indicate that crystals formed in dilute solution are only 85 %-90% crystalline [3]. They are not completely crystalline. 

Ge(100)- (2xl)+Cs (disordered) XRD yes yes 0.057 0.792 19 94M3 disordered overlayer on reconstructed relaxed substrate adatoms occupy T3 site (on top of 3rd layer Ge) asymmetric Ge-dimer remain with 2 equivalent random orientations... 

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