Adsorption ethylene

The effect of the presence of alkali promoters on ethylene adsorption on single crystal metal surfaces has been studied in the case ofPt (111).74 77 The same effect has been also studied for C6H6 and C4H8 on K-covered Pt(l 11).78,79 As ethylene and other unsaturated hydrocarbon molecules show net n- or o-donor behavior it is expected that alkalis will inhibit their adsorption on metal surfaces. The requirement of two free neighboring Pt atoms for adsorption of ethylene in the di-o state is also expected to allow for geometric (steric) hindrance of ethylene adsorption at high alkali coverages. 

Figure 2.25. C2H4 (a), H2 (b) and C2H6 (c) TPD spectra recorded after ethylene adsorption on clean and K-covered Pt(l 11). Ta = 100 K. 0K values are relative to the saturation K coverage in the first layer taken as unity. Inset effect of 0k on C2H6 TPD area. The real coverage in monolayers (K adatoms per surface atom) is 3.03 times smaller.74 Reprinted with permission from Elsevier Science.

Figure 6.37. Modes of ethylene adsorption on noble metals.

Fig. 4. Activity versus water adsorption , hydrogen adsorption sequence O, ethylene adsorption sequence.

Ethylene adsorption at room temperature is rapid and reversible. Even after prolonged exposure to the catalyst, the ethylene is recoverable as such by brief evacuation (10). The isotherms are nonlinear and show some evidence of saturation at 0.5-0.6 cm3/gm, a value roughly five times that of the type I hydrogen. Since the adsorption is quite weak, it would seem that this adsorption is, in part, physical adsorption. To investigate this possibility, adsorption of ethylene (boiling point — 104°C) was compared to that of ethane (boiling point — 89°C) (IS). By traditional criteria physical adsorption of ethane should be greater than that of ethylene, and the comparison of the relative adsorption should let us assay what fraction of the ethylene adsorption is physical. 

The first electron spectroscopic study of adsorbed hydrocarbons was that reported by Eastman and Demuth (78) who used He radiation to probe the valence electrons of benzene, acetylene, and ethylene. Figure 17 shows the difference spectrum of C2H4 adsorbed on Ni(lll) at 100 and 230 K compared with the results of Clarke et al. (79) for ethylene adsorption on Pt(lOO) at 290 K, propylene adsorption on Pt(lOO), and ethylene adsorption on Pt(lll). 

Figure 1.7. Plan view of a fcc(l 11) surface, showing the acetylene and ethylene adsorption geometries on Ni(lll), and that of acetylene on Pd(lll). The H atom positions are schematic only and have not been determined experimentally.

Fink, A., Huber, R. and Widdra, W. Ethylene adsorption on Ge(100)-(2 x 1) A combined angle-resolved photoemission and thermal desorption spectroscopy study. Journal of Chemical Physics 115, 2768-2775 (2001). 

From the changes in magnetic susceptibility of nickel—silica catalysts during ethylene adsorption at room temperature, Selwood [55] has concluded that ethylene exists both as an associatively and a dissociatively adsorbed species. On increasing the temperature, the dissociative adsorption becomes more important. Thus at 100° C, the susceptibility changes are consistent with the formation of six bonds to the surface for each adsorbed ethylene molecule, suggesting the following process... 

These results are supported by volumetric studies [52,54]. With both nickel and palladium there is a slow self-hydrogenation at —78°C. This becomes increasingly important as the temperature is increased at 0°C the overall hydrogen/carbon ratio in the surface species is 1.5, falling to 1.0 at room temperature. Field emission microscopic (FEM) studies of ethylene adsorption on iridium [56] and tungsten [57] are also satisfactorily... 

The co-existence of at least two modes of ethylene adsorption has been clearly demonstrated in studies of 14C-ethylene adsorption on nickel films [62] and various alumina- and silica-supported metals [53,63—65] at ambient temperature and above. When 14C-ethylene is adsorbed on to alumina-supported palladium, platinum, ruthenium, rhodium, nickel and iridium catalysts [63], it is observed that only a fraction of the initially adsorbed ethylene can be removed by molecular exchange with non-radioactive ethylene, by evacuation or during the subsequent hydrogenation of ethylene—hydrogen mixtures (Fig. 6). While the adsorptive capacity of the catalysts decreases in the order Ni > Rh > Ru > Ir > Pt > Pd, the percentage of the initially adsorbed ethylene retained by the surface which was the same for each of the processes, decreased in the order... 

C-Propene adsorption on platinum—alumina and platinum—silica [66] differs from ethylene adsorption insofar as a fraction of the initially retained 14C-propene is relatively easily exchanged or removed by hydrogen treatment. This suggests less extensive dissociation of the adsorbed propene and a 7T-allyl species (structure F) has been proposed in this case, viz. 

These studies also showed that, on a surface effectively saturated with 14C-propene, ethylene adsorption could still occur, although the amount... 

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