Ethylene hydrogenation structure

Figure 6.39. Structure of the transition states for ethylene hydrogenation, corresponding to Fig. 6.38 see text for details. [Adapted from M. Neurock, V. Pallassana and R.A. van Santen.J. Am. Chem. Soc. 122 (2000) 1150.]...

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

EXAFS data characterizing Si02-supported molybdenum species made from [Mo2(allyl)4] have led to precise structural models, including those of surface dimers (with Mo-Mo bonds) and pair sites (without Mo-Mo bonds) (Iwasawa, 1987), but the structures seem to be sensitive to undefined chemistry of the Si02 support surfaces and could be difficult to reproduce. Catalytic data for ethylene hydrogenation and butadiene hydrogenation with these samples and with samples expected to have isolated Mo sites point to a catalytic role of the neighboring sites (Iwasawa, 1987). 

The chemisorption of water on ZnO has been investigated by Nagao and Morimoto (147). Dent and Kokes (148) explained H2 chemisorption and ethylene hydrogenation on the basis of a model in which the (0001), (0001), and non-close-packed faces such as (1010) planes of the wurtzite crystal structure are assumed to form the external crystal surface. Chemisorption of H2 was suggested to occur on Zn—O pair sites, since the Zn—H and O—H stretching... 

Kukolich, S.G., Read, W.G., and Aldrich, P.D. (1983) Microwave spectrum, structure and quadrupole coupling for the ethylene-hydrogen cyanide complex./. Chem. Phys., 78, 3552-3 556. 

As a matter of convenience, it is useful to give a name to reactions that, on a given metal, exhibit a lack of sensitivity to details of surface structure. We have proposed to call these reactions facile. Another name for them would be structure-insensitive. The concept is probably as old as the concept of active centers and can be found in Taylor s 1925 paper in which he wrote There will be all extremes between the case in which all the atoms in the surface are active and that in which relatively few are so active and .. . the amount of surface which is catalytically active is determined by the reaction catalyzed (42). Similar ideas have been presented by Crawford et al. (43), who found that specific rates of ethylene hydrogenation on nickel evaporated films change only by a factor of 3 when, as a result of sintering, crystallite sizes change from 625 to 21,000 A. The authors concluded that studies of sintering should be conducted with more structure-sensitive mechanisms. ... 

Zaera et al. (366) have emphasized that Pt and Rh single-crystal surfaces during ethylene hydrogenation are largely covered with ethylidyne species (CH3-C). Considering this reaction to be structure insensitive (however, see Fig. 9), they then explain this behavior by proposing that the catalytic reaction takes place on top of this overlayer. Thus the reaction should be insensitive to the structure of the underlying metal. However, the nature of the overlayer itself may be structure sensitive, especially for FE > 0.5. More experimental proof is needed before this explanation can be accepted, even as it is for the other explanations already discussed. 

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