Ethylidyne hydrogenation

C-C scission in the unsaturated acyl formed from acrolein would release vinyl (CH2=CH-) ligands to the surface. Isomerization of these would lead to stable ethylidynes, hydrogenation, to volatile ethylene and ethane. The observation that acrolein decarbonylates at lower temperature than does propanal suggests that these hydrogenation steps must follow C-C scission propanal cannot be an intermediate in the acrolein decarbonylation sequence. 

Fig. 1.7 Plot of the rate constant k ) for ethylidyne hydrogenation on Pd(lll) as a function of hydrogen pressure (Reproduced from [102] with permission from Elsevier.)...

It was also observed that the ethylene hydrogenation reaction occurred at the same rate regardless of the presence of ethylidyne on the surface. In addition, preadsorbed ethylidyne groups do not effect the reaction rate. This indicates that ethylidyne is not directly involved in the hydrogenation reaction. The same conclusion was reached from the results of a transmission infrared spectroscopy study of C ethylidyne hydrogenation in hydrogen [28]. 

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. 

Figure 8.10 shows the application of SFG on adsorbed hydrocarbons [35], Ethylene was adsorbed on the (111) surface of platinum at 240 K, and subsequently heated to different temperatures. The spectra monitor the conversion of di-G bonded ethylene to ethylidyne (=C-CH3), via an intermediate characterized by a frequency of 2957 cm-1 attributed to the asymmetic C-H stretch of a CH3 group in the ethylidene (=CH-CH3) fragment. Somoijai and coworkers have demonstrated the usefulness of the SFG technique for in situ work with studies of ethylene hydrogenation and CO oxidation at atmospheric pressure [36]. 

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. 

However, due to the difficulties in calculating ion yields in SIMS, quantitation of the data is not very reliable, and their work was not conclusive. We have determined here that the reaction of chemisorbed ethylene to form ethylidyne is first order in ethylene coverage. A noticeable isotope effect was observed, with activation energies of 15.0 and 16.7 Kcal/mole for C H and 02 respectively. These values are smaller than those calculated from TDS, but the differences can be reconciled by including the recombination of hydrogen atoms on the surface in the interpretation of the thermal desorption experiments. 

We have measured the kinetics of ethylidyne formation from chemisorbed ethylene over Pt(lll) surfaces. The rates of reaction display a first order dependence on the ethylene coverage. There is an isotope effect, since the reaction for CjH is about twice as fast as for CjD. We obtain values for the activation energy of 15.0 and 16.7 Kcal/mole for the normal and deuterated ethylene, respectively. These values are lower than those obtained from TDS experiments, but the differences can be reconciled by taking into account the hydrogen recombination when analyzing the thermal desorption data. 

Species D is most likely an ethylidyne complex which forms from self-hydrogenation on the palladium surface. Such species along with species E have been suggested to be part of the compounds formed from platinum and acetylene. 

Fig. 7.3. Perspective view of ethylidyne on Pt(l 11), the stable structure reached after acetylene adsorption with hydrogen addition...

The complete conversion of C2H2 to ethylidyne requires the presence of surface hydrogen atoms and proceeds rapidly only at 350 K. By comparison with reported reaction mechanisms on related transition metal clusters it seems likely that vinylidene... 

With the advent of sophisticated experimental techniques for studying surfaces, it is becoming apparent that the structure of chemisorbed species may be very different from our intuitive expectations.10 For example, ethylene (ethene, H2C=CH-2) chemisorbs on platinum, palladium, or rhodium as the ethylidyne radical, CH3—C= (Fig. 6.2). The carbon with no hydrogens is bound symmetrically to a triangle of three metal atoms of a close-packed layer [known as the (111) plane of the metal crystal] the three carbon-metal bonds form angles close to the tetrahedral value that is typical of aliphatic hydrocarbons. The missing H atom is chemisorbed separately. Further H atoms can be provided by chemisorption of H2, and facile reaction of the metal-bound C atom with three chemisorbed H atoms dif-... 

Ethylidyne occurs on the triangular threefold sites on fee (111) or hexagonal close-packed (hep) (0001) faces and is formed at lower temperatures on Pd(lll) and Pt(lll) in the presence of coadsorbed hydrogen. Its spectral signature also occurs on Ru(0001) at 330 K and on Ir(lll) at 300 K. Ni(lll) is exceptional in not giving spectroscopic evidence for the ethylidyne species derived from adsorbed ethyne (or from ethene, 1). 

Investigating the same C2H4/Pt(lll) system, McDougall and Yates (373) determined the temperature dependence of the di-cr to ethylidyne conversion and then ethylidyne decomposition, giving the approximate energies of activation of 17.5 and 29 kcal/mol, respectively. The same authors investigated the slow hydrogenation of the ethylidyne species (presumably at ambient temperature) and showed that this accelerated at a pressure of... 

In this step, the well-established dehydrogenated C2H3(a) species is ethyl-idyne, CCH3. The hydrogenation of ethylidyne to give ethane is clearly an unfavored reaction on a Pt(lll) surface unless enough adsorbed hydrogen is available from the presence of H2 in the gas phase. 

The lack of any important effect of ethylidyne on the ethene hydrogenation has presented something of a dilemma because (CCH3)Pt(lll) models imply that there is little room left for the adsorption and reaction of ethene (400). Surface hydrogen (or deuterium) atoms which could be present despite the high coverage of ethylidyne would somehow have to be transferred to ethene weakly adsorbed on top of the ethylidyne adlayer (399), or, alternatively, the ethylidyne species would need to move apart from each other under reaction conditions to allow ethene to reach the surface for reaction (400). 

---