Mechanisms ethylene hydrogenation

Figure 14. Proposed ethylene hydrogenation mechanism in molten tri-chlorostannate(II) salts (114).

The adsorbates which are more weakly bound to the surface are more likely to interact with other surface species through bond-making processes. An example of this situation will be discussed in Section 3.10.3 where we examine the ethylene hydrogenation mechanism as a function of surface coverage. We specifically analyze the elementary reaction steps for both tt- and cr-bonded ethylene intermediates. 

To apply the long-chain approximation to the preceding ethylene hydrogenation mechanism, we write a rate expression that includes only the rate of ethylene consumption in the propagation steps, i.e., in the steps that carry the chain. 

CO insertion reactions. The catalytic hydrogenation mechanism presented was based on detection of species 64 and HC1 during hydrogenation of ethylene, Eq. (92). Species 65 then catalyzes the reaction by pathways outlined in Eq. (5). 

In the case of ethylene hydrogenation, the mechanism proposed by Parshall [61] involves the coordination of an alkene molecule through a five-coordinate intermediate (Eq. (13)) the subsequent alkene insertion into the Pt-H bond (Eq. (14)) and intervention of a second molecule of H2 (Eq. (15)) leads to the elimination of ethane and restoration of the catalytic active species [PtH(SnCl3)]2. However, in 1976 Yasumori and coworkers reported a kinetic analysis conducted on the hydrogenation of ethylene catalyzed by the Pt-Sn complex [(Me)4N]3[Pt(SnCl3)5] [70], under much milder conditions than those... 

We suspect that the intermediate(s) involved in the conversion of ethylene into ethylidyne are closely related to those intervening in the mechanism for ethylene hydrogenation described above. 

He concludes that the first (associative) mechanism gives values nearest the observed heat of adsorption determined by Beeck (30), and is therefore accepted as nearest the truth (34) (Qo (calculated) = 42 kcal./ mole Qo (observed) = 58 kcal./mole). Experiments on tungsten and nickel films (Beeck (35), Trapnell (36), and more recent work in Rideal s laboratory) have shown that when ethylene is added to a clean metal surface ethane appears in the gas phase. A self hydrogenation mechanism must be operative and at least in these cases dissociation of ethylene must occur on the catalyst. It is suggested that the calculations might be complicated by the energy of bond strain in the adsorption of an ethylene molecule to the fixed lattice distances of the metal. 

Attempts to interpret the mechanism of ethylene hydrogenation over nickel [96—99] and over platinum catalysts [100,101] in terms of a statistical mechanical approach have not met with any substantial success, partly due to the limitations of the model which must be assumed in order to perform the calculations and partly due to the complexity of the calculations themselves. 

An interesting example for which eqn. (169) is valid, but the mechanism has an interaction step of various intermediates, is ethylene hydrogenation on nickel, i.e. the Twigg mechanism [54]... 

Systems (1) enter into class 3 (a PDE point is a PCB). Systems with linear reaction mechanisms belong to both class (2) and class (3) but these classes do not overlap since there are systems without intermediate interactions that do not satisfy the principle of complex balance (e.g. the Eley-Rideal mechanism for CO oxidation on platinum metal). On the other hand, there exist reaction mechanisms containing steps of "intermediate interactions but at the same time always having a PCB (e.g. the Twigg mechanism for ethylene hydrogenation on nickel). 

For reactions that are the same on metal and other catalytic sites (e.g., hydrogenation or total oxidation), the reaction may seem to proceed in a similar fashion on the metallic source of spillover and on the diluent support. Some careful studies may be able to discriminate between activity on the metal and the spillover-induced sites. As an example, hydrogenation of ethylene occurs on Pt (or Ni) and on silica or alumina activated by spillover. The product (i.e., only ethane) is the same, as the kinetics often are (rate = /c[C2H4]°[H2]1), but the specific mechanism is different. Deuteration is able to discriminate between the relative rate of alkyl reversal. Deuteration of ethylene on an activated silica produces d2-ethane as the initial product (137), contrary to the results for metal-catalyzed ethylene hydrogenation (2). 

A simple mechanism that has been proposed for ethylene hydrogenation on metal catalysts is that of Horiuti and Polanyi [J. Horiuti and M. Polanyi, J. Chem. Soc., Faraday Trans., 30 (1934) 1164] ... 

H. Nagamoto, H. Inoue, Mechanism of ethylene hydrogenation by hydrogen permeable palladium membrane, J. Chem. Eng. Japan 14 311 (1981). 

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. ... 

During and after World War II, Horiuti continued his research in chemical kinetics and its applications. His results were compiled in a voluminous paper entitled A Method of Statistical-Mechanical Treatment of Equilibrium and Chemical Reactions (1948). This method is applicable both to heterogeneous and homogeneous systems. Horiuti and his co-workers further attempted to apply the method to the study of a number of chemical syntheses and reactions, such as ammonia synthesis and ethylene hydrogenation. Nearly all of his research papers were published in the Journal of the Institute for Catalysis, of which he was the chief editor. 

Therefore an ylid mechanism satisfactorily accounts for the ethylene, hydrogen, and amide ion which is produced. 

At temperatures of 400 C. or so, the desorption of ethylene and hydrogen will proceed fairly rapidly on most catalysts. There would seem no reason to postulate simultaneous release of two hydrogen atoms to form molecule, i.e., the reverse of the hydrogenation mechanism, since conditions in the chemisorbed layer will be different from those Twigg and Rideal found for the hydrogenation at 100 C. Since chemisorbed hydrogen is expected, bond migration vill probably occur simultaneously. 

The foregoing observations narrow down the likely mechanism of ethylene hydrogenation on alumina and provide another example of the potential usefulness of the TPD technique. However, once again, further work will be required before aU the important aspects of the reaction mechanism are clarified. 

Carbonaceous deposits, formed during the hydrogenation of ethylene (150-450 C) over the cobalt foil model catalyst, were investigated by TPO, TPR and SEM methods. The cataljdic tests performed under the transient conditions provided the information on the mechanism of ethylene hydrogenation. The quantitative results of the deposit oxidation were related to the number of metallic active centers measured by TPO and TPR. The reaction temperature was found to exert the most profound impact on both the deposit forms and the deposit functions in the catalytic reaction. Below 300 C only a fraction of the deposit was found to block the metallic active centers and the other part appeared non-reactive in the hydrogenation. Above 400 C the regeneration of the catalyst activity was associated with the difiusion of cobalt atoms to a deposit surface. The results were confronted with the model of deactivation postulated before for CO2 hydrogenation [1]. 

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