Acetylene adsorption complexes

Aeetylacetone, 32 248-249 hydrogenation, 32 259-262 Acetylcholine esterase, 20 344, 367 Acetylene adsorption complexes, 31 6-7 potential dependence, 30 258 catalytic oxidation of, for oxygen manufacture, 3 107... 

Clearly the situation of acetylene adsorption on Si(100) is a complex problem, presumably related to the higher potential reactivity and capacity for bonding of the C=C species. Fortunately, the situation for ethylene appears to be more straightforward, and it seems to be the interaction of Si(100) with C=C species that underpins most of the potential applications in creating suitable inorganic/organic electronic interfaces [126]. 

This postulate has several implications regarding the mechanism of alkyne hydrogenation these will be discussed in Sect. 4.3. It should be noted, however, that there is as yet little or no direct evidence for structure L, although analogous structures are known to exist with organometallic complexes [161], Such a structure is also consistent with the positive surface potentials observed for acetylene adsorption on evaporated nickel films [88]. 

For selective acetylene adsorption from other hydrocarbons (e.g., ethylene and ethane), NiCl2 supported on alumina or silica can form reversible jr-complexation bonds with acetylene but not olefins. Pure component acetylene-ethylene ratios of up to 3 were obtained (Kodde et al., 2000). The bonding between acetylene and NiCl2 is reasonably understood (Huang and Yang, 1999). 

In recent years Seff and co-workers (9) have extensively studied cation siting in zeolite A using single-crystal X-ray diffraction techniques. In favorable cases these workers have also been able to obtain detailed information on the interactions between cations and absorbate molecules. Two examples are shown in Fig. 4, where the adsorption complexes formed when acetylene and NO are adsorbed in Co(II)A have been resolved. In the former case it is proposed that a weak complex is formed via an induced dipole interaction with the polarizable 7i-orbitals of the acetylene molecule. For the NO complex there is good evidence for electron transfer resulting in a complex between CO(III) and NO. In both cases the organic molecules... 

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... 

The IR spectra of methylacetylene adsorbed on zeolites are similar to those of acetylene. Adsorption bands at 3250 (with a shoulder at 3150 cm-i) are assigned to the C-H---0 complexes and are characterized by frequency shifts of 85 cm ( 185 cm"i for the shoulder) relative to the gas-phase frequency, which agrees with the corresponding frequency shifts found for the acetylene adsorption. 

Acetylene adsorption was IR spectroscopically studied by Howard and Kadir [833], who dealt with the uptake of this unsaturated compound into silver-exchanged zeoHte A. They could evidence the formation of acetylides and the formation of two different adsorption complexes of n-bonded acetylene, indicated by bands at 1955 and 1912 cm for CjHj and 1740 and 1710 cm for CjDj imder an adsorbate pressure of 13.3 kPa. 

Padin, J., and Yang, R.T., Tailoring new adsorbents based on pi-complexation Cation and substrate effects on selective acetylene adsorption, Ind. Eng. Chem. Res., 36(10), 4224-4230 (1997). 

To illustrate these points. Fig. 2 shows the structures of adsorption complexes formed by contacting acetylene and NO with a Co(II)A zeolite [8]. In both cases, the Co(ll) cation moves outside the plane of the six-membered ring window to achieve a more tetrahedral-like coordination with the sorbate. In the case of acetylene, a weak complex is formed which involves its polarizable ir-orbitals. For NO sorption, however, electron transfer occurs, and the complex is best described as Co(lII)-N0". Although zeolite A is a small-pore material and has restricted applications to catalysis, model studies such as these are relevant to large-pore zeolites as well. 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

So far, only a very few adsorbed molecular structures have been analyzed by surface crystallography. The first system studied in detail was acetylene adsorbed on the (111) crystal face of platinum. We shall discuss the complex adsorption and structural characteristics of this small organic molecule in some detail as it reveals the unique surface bonding arrangements that are possible and points to the importance of the use of additional techniques to complement the diffraction information. 

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. 

Selective and reversible adsorption of gaseous molecules such as dioxygen, carbon monoxide, ethylene, acetylene, and dinitrogen have been performed by the use of suitable macromolecule-metal complexes. Selective adsorption of metal ions such as UO has also been studied using polymeric ligands. 

Reversible adsorption and desorption of the acetylene is possible under suitable conditions. However, using the corresponding low molecular weight manganese complexes such as MeCpMn(CO)3 and CpMn(CO)3 (Cp = cyclopentadienyl ring), the acetylene complex could hardly be prepared due to instability of the acetylene complex. 

On F centers the mechanism is similar. The two acetylene molecules bind easily to the Pd/Fsc complex, they form C4H4 with a barrier of about 1 eV, then add a third acetylene which reacts with C4H4 to form benzene with a barrier of 0.9 eV. Thus, the barriers are at most of 1 eV, consistent with a reaction temperature of 300 K. Also in this case, once formed, benzene desorbs easily. It should be noted that the barriers for the reaction occurring on Pd atoms adsorbed on low-coordinated F or F centers are always significantly higher, hence inconsistent with the TPR experiment. This suggests that the most likely Pd adsorption sites, at least from the point of view of the reaction barriers, are the F andp"" centers located at the terraces of the MgO surface. 

The adsorption mode of acetylene on a Cu(llO) surface is more complex and the available experimental information concerning this system, is fairly scarce. In a recent paper [9], an experimental and a theoretical study of this adsorption system was presented. Using HREELS and ARUPS the authors concluded that acetylene adopts a low symmetry (most likely C ) adsorption geometry on Cu(llO) (IX on Fig. 

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. 

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