Cyclohexane, dehydrogenation, sites

Examples 22-25, all for cyclohexane dehydrogenation over Pt, are similar to the examples just discussed in that the data are all for surface reactions. Also with each of them the log L value calculated for Step 5 is between 8 and 10. We reported similar results and log L calculations (109) and suggested, using independent evidence of Boudart (110), that the site density is not as low as our TST calculations indicate. Thus, the log L calculation can be used to demonstrate the existence of a complexity that might not otherwise be detected, in this case, the possibility that cyclohexene is an intermediate. 

We have used our Single Turnover (STO) reaction sequence to characterize dispersed metal catalysts with respect to the numbers of alkene saturation sites, double bond isomerization sites, and hydrogenation inactive sites they have present on their surfaces (ref. 13). Comparison of the product composition observed when a series of STO characterized Pt catalysts were used for cyclohexane dehydrogenation with those observed using a number of instrumentally characterized Pt single crystal catalysts has shown that the STO saturation sites are comer atoms of one type or another on the metal surface (ref. 10). 

Values of total metal dispersion of flesh and regenerated catalysts are reported in Table 1. It can be seen that the rejuvenation treatment improved the metal dispersion of the catalysts. The dispersion values of the catalysts subjected to a single buming-off step (no rejuvenation) are about 30-35 % for all the samples studied (results not shown). To further confirm this behavior some additional runs were perfcnmed using cyclohexane dehydrogenation as a test reaction of the activity of the metallic sites (11). A firesh laboratory prepared Pt-Re/AbOs... 

This is an SIR. The reaction rate is directly proportional to the surface active sites, and the intrinsic activity or TOF does not depend on particle diameters. The cyclohexane dehydrogenation (CHD) reaction forms only benzene and hydrogen as products, according to Figure 13.1, and occurs under atmospheric pressure and temperatures varying between 250°C and 300°C. 

As pointed out earlier, the dehydrogenation of cyclohexanes to aromatics over a supported platinum catalyst requires only platinum sites. The properties of the support do not appear to be critical, provided that the platinum is well dispersed. 

We have been able to identify another active site by studying the ratio of the dehydrogenation rate to hydrogenolysis rate of cyclohexane to benzene and /i-hexane, respectively (36a). While the benzene /j-hexane ratio is 3 1 on a stepped surface (with roughly 17% of the surface atoms in step positions), the ratio decreases rapidly with increasing kink density (Fig. 21b). Using a set of catalyst surfaces that were cut to maintain the same terrace width (step density equal to 2.5 x 1014/cm2), but with variable kink density in the steps, we have found that the hydrogenolysis rate increases linearly with kink... 

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 ... 

Facile dehydrogenation is consistent with kinetic models derived from catalytic conversion studies of cyclohexane to benzene. These models predict an ensemble size for the active site of only one atom. On the other hand, surface science studies propose a model where several metal atoms, on the order of seven, are required, and suggest that specific orientation with respect to subsurface metal atoms is needed. Theoretical studies suggest that the key is to bring cyclohexane sufficiently close to the metal such that strong orbital overlap will occur. Small clusters may be even more effective than surfaces. Further experiments are needed to identify the chemical state of the products of the cluster reactions in order to connect the results with the surface science and catalysis results. 

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