Ethylene, adsorbed hydrogenation

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

Figure 6.38. Potential energy diagram for the hydrogenation of ethylene to the ethyl (C2H5) intermediate on a palladium(m) surface. The zero of energy has been set at that of an adsorbed H atom, (a) Situation at low coverage ethylene adsorbed in the relatively stable di-cr bonded mode, in which the two carbon atoms bind to two metal atoms. In the three-centered transition state, hydrogen and carbon bind to the same metal atom, which leads to a considerable increase in the energy...

Spectroscopic evidence that ethylene preferably hydrogenates directly via the jt-bonded adsorbate and not via the more stable di-<7 bonded state has been provided by Somorjai and coworkers by in situ sum-frequency generation measurements [P.S. Cremer, X. Su, Y.R. Shen and G.A. Somorjai,y. Am. Chem. Soc. 118 (1996) 2942]. 

Second, catalytic reactions do not necessarily proceed via the most stable adsorbates. In the ethylene case, hydrogenation of the weakly bound Jt-C2H4 proceeds much faster than that of the more stable di-cr bonded C2H4. In fact, on many metals, ethylene dehydrogenates to the highly stable ethylidyne species, =C-CH3, bound to three metal atoms. This species dominates at low coverages, but is not reactive in hydrogenation. It is therefore sometimes referred to as a spectator species. Hence, weakly bound adsorbates may dominate in catalytic reactions, and to observe them experimentally in situ spectroscopy is necessary. 

Adsorbed hydrogen atoms which are the intermediates of reaction 8 can be used for the hydrogenation of ethylene and acetylene. The light driven reaction occurs according to the following scheme ... 

Thus, the rate should be half-order in hydrogen pressure and proportional to the amount of ethylene adsorbed on the oxide half of the active site under reaction conditions. 

These results are best interpreted in terms of the proposed mechanism. The rate of the reaction in the steady state is the rate of either formation of the ethyl radical or its reaction with adsorbed hydrogen (steps 13 and 14). Accordingly, in the steady state, the concentration of the intermediate, I, should be constant if the ethylene is suddenly removed but hydrogen is still present, the concentration of I should decrease, and the initia-rate of decrease of I should be equal to the steady state conversion to ethane. 

Eischens and Pliskin have interpreted the infrared spectra of ethylene chemisorbed on nickel dispersed on silica 32). When introduced to a surface previously exposed to hydrogen, ethylene gave rise to absorption bands which correspond to the C—H stretching frequencies of a saturated hydrocarbon (3.4-3.5 p) and a deformation associated with a methylene group (6.9 p). A weak band at 3.3 p was attributed to an ole-finic C—H. Treatment of the chemisorbed ethylene with hydrogen caused the spectrum to change to one which was interpreted as due to an adsorbed ethyl radical. Apparently in the presence of hydrogen most of... 

Nickel films completely covered with oxygen will not adsorb hydrogen when exposed to this gas immediately after oxidation, but will regain their ability to adsorb hydrogen after several hours. Hydrogen thus adsorbed is not able to hydrogenate ethylene. Heats of adsorption measurements of this type of hydrogen adsorption have not been made. 

As soon, however, as one or other of the reacting gases is rather strongly adsorbed, the reaction kinetics become much more complicated. In the combination of ethylene and hydrogen on copper Peasef found that at 0° and 20° C. the simple bimolecular course is no longer followed. The... 

Introduction. The structure of acetylene and ethylene adsorbed on transition metal surfaces is of fundamental importance in catalysis. An understanding of the interaction of these simple molecules with metal surfaces may provide information on possible surface intermediates in the catalytic hydrogenation/dehydrogenation of ethylene. High resolution ELS is a particularly useful... 

Noble metals such as Pt, Pd or Rh act also as catalysts in hydrogenation processes of olefin or acetylenes. Hydrogenation of ethylene by molecular hydrogen on a metal surface involves a stepwise mechanism outlined in Eq. (34-37). In fact, dissociation of molecular hydrogen and formation of surface-adsorbed hydrogen, Eq. (34), corresponds to the intermediate catalytic species formed in the photo-... 

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