Hydro-dehydrogenating sites

These reactions proceed essentially through a bifimctional mechanism involving two types of catalytic sites hydro-dehydrogenating sites and acidic sites. A saturated hydrocarbon (paraffin... 

In A, a Raney copper catalyst would be able to hydro-dehydrogenate alcoholic functions (-H, +H) on metallic copper sites. About 10 to 15% of the copper would be hydroxylated copper able to catalyze the degradation reactions DOH, RC, RM. These sites would be more reactive than Cu towards Mn + in the oxido-reduction modification of the initial Raney copper, so that, beeing first exchanged, the rates of DOH, RC, RM decrease. 

Bifunctional catalytic reactions involve a series of catalytic steps over acidic and hydrogenating-dehydrogenating sites with formation of intermediate compounds. Thus n-hexane (hydro)isomerization involves successively n-hexane dehydrogenation in n-hexenes (metal catalyzed), skeletal isomerization of n-hexenes into isohexenes over protonic acid sites followed by the (metal catalyzed) hydrogenation of isohexenes into isohexanes (Figure 1.4). 

Hydroisomerization reactions are generally intimately associated with hydrocracking reactions. The overall scheme is rather complex. It involves the independent action of both types of catalytic sites and the existence of a transport mechanism for olefins between these sites. Therefore, the catalyst must be designed according to this bifunctional mechanism. The relative strength of the hydro-dehydrogenation and acidic components must be adjusted for the desired operation. 

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. 

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