Adsorbed acetylene

Quite recently Yasumori el al. (43) have reported the results of their studies on the effect that adsorbed acetylene had on the reaction of ethylene hydrogenation on a palladium catalyst. The catalyst was in the form of foil, and the reaction was carried out at 0°C with a hydrogen pressure of 10 mm Hg. The velocity of the reaction studied was high and no poisoning effect was observed, though under the conditions of the experiment the hydride formation could not be excluded. The obstacles for this reaction to proceed could be particularly great, especially where the catalyst is a metal present in a massive form (as foil, wire etc.). The internal strains... 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

Perhaps the next simplest molecular adsorbates for which quantitative structural information exists are the unsaturated C2 hydrocarbons, notably acetylene (ethyne, HC CH) and ethylene (ethene, H2C=CH2), adsorbed on a number of metal surfaces (especially, Cu, Ni and Pd), and also on Si(100), studied by LEED, SEXAFS, and PhD. In some systems adsorption of ethylene is accompanied by a surface reaction. In particular, on both Pt(lll) [74] and Rh(lll) [75] ethylene is converted to an ethylidyne species, H3C—C—, which bonds to these surfaces through the C atom with the —C axis essentially perpendicular to the surface, in three-fold coordinated hollow sites. In addition, ethylene adsorbed on Ni(l 11) at low temperature dehydrogenates to produce adsorbed acetylene as the surface is warmed towards room temperature this particular system actually provided the first example of the... 

Fig. 22. Variation in the fractions of various forms of adsorbed acetylene on palladium foil with temperature of annealing [148].

The effects of hydrogen on the infrared spectra of adsorbed acetylene together with evidence from mechanistic studies of alkyne hydrogenation has led to the general conclusion that the acetylenic species active in hydrogenation is associatively bonded to the surface. However, as with monoolefins, there is still doubt as to the precise formulation of the surface—alkyne bonding. In the early work [156], it was assumed that the associatively adsorbed complex was adequately represented as a di-a-bonded olefin, which adopted a cis-configuration. 

Fig. 24. Possible reaction scheme for the direct formation of ethane from adsorbed acetylene.

Interaction of the free radical with either an adsorbed acetylene or a normal vinyl radical could then lead to polymerisation. 

No acetylene exchange was observed with Rh, Pd, Ir or Pt, although the steady state analysis showed that 10% (Pd, Pt) or 30% (Ir, Rh) of the adsorbed acetylene was either C2HD or C2D2. Thus acetylene adsorp-... 

The rate of formation of pairs of vinyls will be proportional to the hydrogen pressure and hence the rate of formation of ethylene by step (8) will be first order in hydrogen. An alternative mechanism for the palladium-catalysed reaction involving the reaction of adsorbed acetylene with molecular hydrogen has been proposed [9]. 

With cation-exchanged zeolites [304], the first-order kinetics [eqn. (18)] is explained by the degeneration of the Langmuir—Hinshelwood equation for monomolecular transformation of adsorbed acetylene in the rate-determining step... 

Fig. 14. Room temperature FTIR spectra of H-ZSM-5 before (dotted line) and after (full line) contact with acetylene for 7 s. The arrow indicates the v(C = C) mode of adsorbed acetylene. (Reproduced with permission from Bordiga et al. (152).)...

Variable temperature MAS NMR was used to characterize the structure and dynamics of hydrogen bonded adsorption complexes between various adsorbates and the Brpnsted acid site in H ZSM-5 the Brpnsted proton chemical shift of the active site was found to be extremely sensitive to the amount of type of adsorbate (acetylene, ethylene, CO and benzene) introduced (105). Zscherpel and coworkers performed maS NMR spectroscopic measurements in order to investigate the interaction between Lewis acid sites in H ZSM-5 and adsorbed CO. A new measure for the "overall" Lewis acidity of zeolites was derived from the C MAS NMR spectroscopic data. In addition, the chemical shift of CO adsorbed... 

There are two routes by which benzene can form from acetylene on Pd(lll). The first found under UHV conditions occurs by a pathway in which benzene is produced via a intermediate formed from two adsorbed acetylenes, that rapidly reacts with a third acetylene to form benzene. An alternative, but much slower reaction occurs between vinylidene and acetylene to form the intermediate as shown in Scheme 1.1. The existence of two distinct pathways accounts for the decrease in activity of the high-surface-area catalysts as a function of time on stream [50]. Reaction initially occurs on the clean surface by the first pathway to rapidly form benzene. However, vinylidene species accumulate on the surface so that reaction then proceeds by the slower pathway depicted in Scheme 1.1. 

The chemisorption of acetylene on the Sn/Pt(100) surface alloys revealed similar chemistry and provided additional information on the structure sensitivity of these reactions [53, 54]. While 15% of the adsorbed acetylene monolayer was converted to gaseous benzene during TPD on the (3V2xV2)R45°-Sn/Pt(100) alloy, no such benzene desorption occurred from related surfaces, as shown in Fig. 2.6. 

From an examination of the tt, 3cTg, 2cr orbitals for gaseous and adsorbed acetylene at 132 K on the (5x1) surface it appears that the perturbation of the acetylene is small and restricted to the It- orbital. The orbital identification for ethylene, gaseous and adsorbed, is more complex, but it appears that the molecule is relatively unperturbed upon adsorption except for the 77-like bin orbital. 

One significant area of uncertainty exists in the knowledge of electron transfer to or from adsorbed hydrocarbons. Fischer et al. point out that in an acetylene-(PPh3)2Pt complex, ESCA indicates transfer of 0.7 electrons to the hydrocarbon. In contrast, adsorbed acetylene causes a decrease in work function usually ascribed to charge transfer to the metal. If the dipole length is chosen from the centre of the Pt atom to the mid-point of the C—C bond, then the net charge transfer is 0.05 electron per molecule from acetylene to Pt. 

Fig. 1.102. The proposed mechanisms are shown schematically for the three atoms. Ag and Pd atoms are decorating exciusiveiy F centers after deposition whereas Rh is trapped at step edges and F centers at 90 K. Ag atoms do not adsorb acetylene and are therefore inert for the reaction. Pd and Rh are forming benzene only when trapped at F centers. Note that the relatively broad temperature range for the formation of CeHe on Rh originates from the fact that Rh is trapped at two defect sites at 90 K and that the reaction occurs oniy after diffusion of the Rh(C2tf2) complexes from steps to F centers...

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