Isomerization surface-structure sensitivity

Fe(lll)> Fe(100)> Fe(110), with relative activities of 430 32 1 and for Re the order is Re(1120)> Re (1010 > Re (0001), with Re (1120) more than 1000 times more active than Re (0001). For both metals it is proposed that the active site is a metal atom in the second layer of an open surface structure, i.e., an atom in the bulk (having a high density of electron holes near the Fermi level) which is accessible to a gaseous molecule because of the open structure of the surface. This model emphasizes the unique electronic rather than structural sensitivity of this reaction. It is possible that similar electronic effects may contribute to structure sensitivity for other reactions (c.f. skeletal isomerization reactions, see later). 

Skeletal rearrangement reactions over Pt single crystals have been studied for methyl cyclopentane, 2- and 3-methylpentane350 and for n-hexane.3sl One conclusion351 is that whereas aromatization reactions are very sensitive to surface structure [Pt(l 11)> Pt(100)], isomerization, Cs-cyclization, and hydrogenolysis reactions display little dependence on structure. Temperature and H2 pressure are more important in affecting the selectivity. 

From their work on methylcyclopentane and methylpentanes Garin et al.3S0 conclude that BS is at a maximum over a Pt(557) surface. On the basis of this they propose that more than 1 metal atom must operate in the BS isomerization. They present a variety of models in which at least 2 metal atoms at a surface step are involved in the initial adsorption process. Davis et al.351 conclude also that BS is probably structure sensitive. However, they... 

The former phenomenon is usual referred to as particle-size effect and is pronounced for structure-sensitive reactions [1,2], i.e., catalytic reactions where the rate and/or selectivity is significantly different from one crystallographic plane to another. Structure-sensitive reactions (e.g., isomerizations) frequently occur on catalytic sites consisting of an ensemble of surface atoms with specific geometry. It is thus reasonable to expect that as the active-phase crystallite size decreases, there will be a different distribution of crystallographic planes on the catalyst surface, with the possible disappearance of ensemble sites, so that both the catalyst activity and... 

For example, Pt single crystals have been cleaved to expose various surfaces exhibiting steps, kinks, and terraces, as shown in Fig. 1. The surfaces have been rigorously cleaned in a vacuum chamber, their compositions determined by Auger electron spectroscopy, their structures determined by LEED, and their catalytic activities measured for reactions of w-heptane. The results demonstrate the structure sensitivity of the hydro-genolysis reaction, which is associated with kink sites, and the structure insensitivity of the dehydrogenation and isomerization reactions. The structures illustrate the nonuniformity that is typical of catalyst surfaces. The sites at which catalysis occurs are called active centers, and these may be a small fraction of the surface they are usually unidentified. 

More recently, the authors have further extended this work to include a comparison of the Au-Pt(lll) versus Au-Pt(lOO) alloy surfaces in n-hexane reactions (157). As shown in Fig. 12, the same general decreases in reaction rates and increases in isomerization selectivity with Au addition were seen for both Pt surface orientations, except for the fact that, in contrast to Pt(100), the rate of methylpentane production actually increases with Au addition. Other more subtle differences were also observed, demonstrating clearly the existence of structural sensitivity in alloy catalysis. Again, ensemble effects were used to explain the results. 

The reactions of C labeled hydrocarbons on platinum catalysts under hydrogen atmosphere are structure-sensitive (14) and isomerization reactions are very sensitive to the crystallographic planes as observed on the platinum stepped surfaces where the bond shift mechanism is favored compared with the cyclic mechanism (15). 

Our laboratory reactor experiments have shown that the surface structure of the bimetallic used are composed with a random mixture of Pt and Ni or Co sites. This structural information has been obtained by using the chemical probes (isomerization and hydrogenolysis of 2-methylpentane and methylcyclopentane respectively and oxidation) which are more sensitive than the physical techniques. 

Figure 7.39. The structure sensitivity of light alkane isomerization and hydrogenolysis. Shown here are the reaction rates of isobutane catalyzed at 570 K and atmospheric pressure over four platinum surfaces shown in Figure 7.37. Isomerization is favored over Pt surfaces that have a square atomic arrangement. Hydrogenolysis rates are maximized when kink sites are present in high concentrations on the platinum surface [155].

Obviously, a great deal more information could be obtained if the isomeric ions could be probed spectroscopically. Vibrational states of the various isomers are not generally well known, but some structural information is available. Thus, the rotational structure of vibrational transitions may provide a better signature for particular isomers. Certainly, insufficient data are available about the potential surfaces of electronically excited states for electronic excitation to be used as a probe, e.g., as in the very sensitive laser induced fluorescence. At present, there are sensitivity limitations in the infrared region of the spectrum, but this may well be an avenue for the future. The study of isomeric systems and their potential surfaces has just begun ... 

Some reactions such as alkene isomerization, alkene hydrogenation, and H2 -I- D2 exchange can be used as sensitive chemical probes of the coordination environment of metal atoms associated with surface-bound metal clusters. Other catalytic reactions such as CO -I- H2 and alkane hydroge-nolysis, which are sensitive to metal ensemble sizes, are applied as a further structural probe. Several attempts have been made to stabilize cluster frameworks in such a way that catalytic activity is maintained. One of the more promising approaches involves the introduction of a capping group into the... 

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