Chemisorption acetylene

Chemisorption of benzene at 297°C on Ni(110) occurred in a rather different manner. Several patterns, some streaked, were observed, and they followed the same sequence and showed the same behavior as those obtained when acetylene was chemisorbed on this surface (29). These structures have not been fully elucidated, but the streaked patterns suggest (i) that the mobility of adsorbed species along the "furrows of the (110) face is easier than their mobility across them, and (ii) that dissociation of the carbon skeleton of benzene and the formation of other structures occurs. 

The use of dispersed or immobilized transition metals as catalysts for partial hydrogenation reactions of alkynes has been widely studied. Traditionally, alkyne hydrogenations for the preparation of fine chemicals and biologically active compounds were only performed with heterogeneous catalysts [80-82]. Palladium is the most selective metal catalyst for the semihydrogenation of mono-substituted acetylenes and for the transformation of alkynes to ds-alkenes. Commonly, such selectivity is due to stronger chemisorption of the triple bond on the active center. 

A few measurements of the changes in magnetization of nickel resulting from the chemisorption of hydrogen, ethylene and acetylene have also been reported by Breeder and his co-workers lOSa, b). Their results and conclusions appear to be qualitatively in accord with those of Selwood. 

Liu, Q. and Hoffmann, R. The bare and acetylene chemisorbed Si(001) surface, and the mechanism of acetylene chemisorption. Journal of the American Chemical Society 117, 4082 (1995). 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

The divalent catalyst is highly coordinatively unsaturated and therefore exhibits some unusual chemistry (33-42). It has a light green color but quickly truns blue when exposed to N2, indicating a weak chemisorption. Carbon monoxide is adsorbed to yield a violet color, and of course it poisons the polymerization. Up to two molecules can be adsorbed. Olefins also adsorb in a 2 1 ratio, and acetylene is converted to benzene. Polar compounds like alcohols, ethers, or amines are strongly held. Nitric oxide (NO) attaches in a 2 1 ratio. 

The chemisorption of acetylene, ethylene, benzene, and cyclohexane were also studied on the Ir(lll) and stepped Ir[6(111) x (100)] crystal surfaces (30). Chemisorption characteristics of the Ir(lll) and Pt(lll) surface are markedly different. Also, the chemisorption characteristics of the low Miller index Ir(l 11) surface and the stepped Ir[6(l 11) x (100)] surface are markedly different for each of the molecules studied. The hydrocarbon molecules form only poorly ordered surface structures on either the Ir(l 11) or stepped iridium surfaces. Acetylene and ethylene (C2H2 and C2H4) form surface structures that are somewhat better ordered on the stepped iridium than on the low Miller index Ir(l 11) metal surface. The lack of ordering on iridium surfaces as compared to the excellent ordering characteristics of these molecules on... 

The Rh(lll) surface was covered with carbon by decomposing 5 x 10 7 torr of either acetylene or ethylene at 1100 K for 10 minutes and subsequent flashing to 1200 K (230. Pre-adsorbed carbon had a very strong inhibiting effect on carbon monoxide chemisorption. This is the same effect it had on the methanation rate (36). The low inelastic scattering intensity indicated relatively small CO coverages while the broad elastic peak and... 

ELS Studies of Acetylene Chemisorption on Rh(lll). The vibrational spectrum of the (2x2) hydrocarbon surface structure formed from the chemisorption of C2H2 on Rh(lll) between 210 and 270 K is shown in Figure 5a. The peak positions and their... 

We simply point out that the stable hydrocarbon overlayer formed from the chemisorption of either ethylene or acetylene and hydrogen on both Pt(lll) and Rh(lll) yield identical vibrational spectra (Figure 7). A more complete discussion of the similarities between the chemisorption of ethylene on Rh(lll) and Pt(lll) is presented elsewhere (24). 

Figure 7. Comparison of the vibrational spectra for ethylene chemisorbed on (a) Pt(lll) (93) and (b) Rh(lll) (24). A discussion of the similarities between acetylene and ethylene chemisorption on Rh(lll) and Pt(lll) is presented in Ref. 24.

The chemisorption of acetylene and ethylene on Bh(lll) yields a series of ordered structures ... 

These adsorbed alkyl radicals can be dehydrogenated by pumping at 35° C. The species obtained during chemisorption of acetylene and by hydrogenation of chemisorbed acetylene can not be easily dehydrogenated by pumping. The intensity of the bands obtained after hydrogenating... 

The surface of the catalyst is, therefore, practically covered with acetylene only, and when a mixture of the two gases is hydrogenated it is only acetylene that is converted to ethylene until practically all acetylene has disappeared. The selective hydrogenation of nonsubstituted and substituted acetylene and ethylene mixtures may be ascribed to the selective chemisorption of the gases (391). 

Prototype Examples The Chemisorption of Ethylene and Acetylene to the Silicon(lOO) Surface... 

The reactions of ethylene and acetylene with Si( 100)-(2 x 1) were initially described as being [2 + 2] cycloadditions, with the di-configuration predicted by this mechanism believed to provide the dominant reaction products. A variety of alternate reaction products could actually be formed as a result of the chemisorption, with, e.g., the organic molecule spanning silicon atoms in different dimer rows, adhering above a row oriented perpendicular to the silicon dimers, or adhering above a row and parallel to the dimers. As reviewed in Sec. 3, a variety of alternate structures have now indeed been found for chemisorbed acetylene. Hence, while the [2 + 2] cycloaddition mechanism appears apt for ethylene chemisorption, its applicability to similar processes in acetylene is questionable. 

In the case of chemisorption of acetylene on Pd(lll), high-resolution electron energy loss spectroscopy (HREELS) data suggest that ethylidyne coexists with vinylidene ( =C=CH2) at this surface (31). In their ultra-... 

Turning to the sp2 vs. sp3 hybridization at carbon issue, acetylene has one C—C 7t bond left after chemisorption. The remaining Si—Si a bond and the C—C it bond may undergo a further pericyclic reaction (Figure 3c) releasing four-membered ring strain... 

Hexynol or Ethyl Octynol. The mechanism of inhibition by acetylenic alcohols consists of chemisorption followed by subsequent polymerization. For maximum inhibition the hydroxyl group has to be in the (I- position to the acetylenic function and the acetylenic function should be terminal. 

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