Acetylene, adsorbed state

C-Tracer studies of acetylene adsorption on alumina- and silica-sup-ported palladium [53,65], platinum [66] and rhodium [53] show the coexistence of at least two adsorbed states, one of which is retained on the surface, the other being reactive undergoing molecular exchange and reaction with hydrogen. Acetylene adsorption exhibits the same general characteristics as those observed with ethylene (see Sect. 3.2). However, there are important differences. The extent of adsorption and retention is substantially greater with acetylene than with ethylene. Furthermore, the amounts of acetylene retained by clean and ethylene-precovered sur-... 

In a recent study of the adsorption of acetylene on platinum single crystals by low energy electron diffraction [160], it has been shown that acetylene adsorbs on the (111) planes. These results show that, on a clean Pt (111) surface, acetylene adsorbs at a distance of 1.95 A above the topmost plane of platinum atoms, either in the C2 or, less likely, the Bl mode shown in Fig. 23. No evidence was found for adsorption in the A or A2 modes, which corresponds to a 7r-complex structure or for the B2 mode corresponding to a di-o-complex, although it was stated that such structures may be possible with a less stable overlayer which had been observed. 

Following the proposals of Rooney et al. [85—87], it has generally been assumed that, as with monoolefins, the adsorbed state of an alkyne active in hydrogenation is a 7r-complex formed by the interaction of the 7r-orbitals of the acetylenic bond with two metal atoms. The 7r-complexed alkyne may be represented as structure L. 

The hydrogenations of dialkylacetylenes are of particular interest from a stereochemical viewpoint since, as noted in Sect. 4.3, the adsorbed state of the acetylene is expected to adopt a c/s-configuration and, consequently, upon hydrogenation to yield the cis-olefin. Wide use of this fact has been made in preparative organic chemistry as noted by Burwell [192] and by Campbell and Campbell [193]. Although early studies of the catalytic hydrogenation of disubstituted acetylenes [194—196] revealed the formation of trans- as well as c/s-olefins, it was generally assumed that the trans-isomer was formed by isomerisation of the cis-ole-fin. However, more recent studies have shown that this view may have... 

There is a dearth of information concerning the adsorbed state of acetylene and other alkynes in the context of their reactions with... 

Acetylene is more strongly adsorbed than ethylene, and this must be accommodated in the proposed structure for the adsorbed state. However, both Structures (I) and (II) fulfill this requirement. Di-a-bonded acetylene is expected to be more strongly bound to the surface than di-CT-bonded ethylene because the first 7r-bond in acetylene is weaker than the second (2, 74). For the 7r-adsorbed structures, acetylene, can bond with two metal atoms whereas ethylene may only bond with one, so again acetylene is reasonably expected to be the more strongly adsorbed species. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

The mechanism of the hydropolymerisation of acetylene is not too clear. It has been suggested [9,169] that in the hydrogenation of acetylene to ethylene, the half-hydrogenated state, an adsorbed vinyl species, may exist in either a normal or free radical form, viz. 

Whether an adsorbed species can exist as a free radical on a metal surface is open to some debate. It seems unlikely that the vinyl free radical will exist as a relatively stable surface intermediate although it could be envisaged as a transition state in the formation of adsorbed vinyl from acetylene. An alternative mechanism involving the direct insertion of a vinyl group into an acetylene... 

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-... 

Extremely interesting infrared studies of physically adsorbed molecules were carried out by Sheppard and Yates (52). These workers studied the spectra of methane, ethylene, acetylene, and hydrogen on porous glass. They found that the perturbing effects of surface forces made it possible to detect bands which are found in the Raman spectra but are not observed in the normal infrared spectra. This indicates that the degree of symmetry of the adsorbed molecule is less than in the gaseous state because of the one-sided nature of the surface forces. This effect was discovered independently by Karagounis and Peter (52a) in studies 1,3,5-trichlorobenzene physically absorbed on silica. 

As shown in Figure 12.7, an initial deposition rate that is roughly four times greater than the steady-state deposition rate is observed with styrene, which is an easily condensable vapor (high adsorption onto the surface), by method A. The gaseous monomer acetylene, on the other hand, shows an initial deposition rate identical to the steady state is obtained immediately on the inception of discharge, as shown in Figure 12.8. Whether the adsorbed monomer polymerizes at the surface or... 

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