Adsorption of ethylene

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

The temperature regimes for the stability of intermediates is different for various transition metals. For example on Fe(lll) the adsorbed ethylene decomposes partially at 200 K, while the conversion to surface carbon is complete at 370 K. Similarly, on nickel faces molecular chemisorption of ethylene is restricted to temperatures below ambient. At temperatures between approximately 290 K and 450 K ethylene chemisorption on nickel 

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. 

This is indeed shown in Fig. 2.25 which depicts the effect of K coverage on the TPD spectra ofC2H4, H2 and C2H6 following C2H4 adsorption at Ta=100 K. 

In addition, as shown in Fig. 2.26, the increase in K coverage causes a dramatic decrease in the amount of decomposed ethylene, which is accompanied by a decrease in the total amount of desorbed hydrogen (Fig. 2.25). 

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The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

We have developed a compact photocatalytic reactor [1], which enables efficient decomposition of organic carbons in a gas or a liquid phase, incorporating a flexible and light-dispersive wire-net coated with titanium dioxide. Ethylene was selected as a model compound which would rot plants in sealed space when emitted. Effects of the titanium dioxide loading, the ethylene concentration, and the humidity were examined in batches. Kinetic analysis elucidated that the surface reaction of adsorbed ethylene could be regarded as a controlling step under the experimental conditions studied, assuming the competitive adsorption of ethylene and water molecules on the same active site. 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

Another possibility is that carbene species are generated via the dissociative adsorption of ethylene onto two adjacent chromium sites [71]. A second ethylene molecule then forms an alkyl chain bridge between the two chromium sites this can subsequently propagate via either the Cossee or the Green-Rooney mechanism. 

Adsorption of ethylene as an olefinic species would not be likely to occur on the zinc half of the active site. A rigid ethylene molecule could not approach the sequestered zinc ions because of steric restrictions hence, ethylene would be confined primarily to the oxide part of this layer. In... 

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 shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

Adsorption of ethylene at 90 K on oxide coated thermionic emission cathodes was measured (Wooten Brown, JACS 65 113, 1943). The volume adsorbed is in V ml measured at 1 Torr and 25 C. The vapor pressure is Ps = 30.6(10-3) Torr. 

Correlations between surface species and emitted secondary ions are based on characterization of the surface adlayer by adsorption and thermal desorption measurements. It is shown that the secondary ion ratios RuC+/Ru+ and R CTVRuJ can be quantitatively related to the amount of nondesorbable surface carbon formed by the dissociative adsorption of ethylene. In addition, emitted hydrocarbon-containing secondary ions can be directly related to hydrocarbon species on the surface, thus allowing a relatively detailed analysis of the hydrocarbon species present. The latter results are consistent with ejection mechanisms involving intact emission and simple fragmentation of parent hydrocarbon species. 

Jurinak JJ. 1957. The effect of clay minerals and exchangeable cations on the adsorption of ethylene dibromide vapor. Soil Science Society Proceedings 21 599-602. 

Fio. 24. Geometrical relationships for the dissociative adsorption of ethylene at a single atomic center. 

Brooksby and Fawcett [329] have studied adsorption of ethylene carbonate at Au(llO) electrode from aqueous solutions of HCIO4, NaCl04, and Mt4NCl04, using in situ infrared spectroelectrochemical method. 

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... 

Liu, H. B. and Hamers, R. J. Stereoselectivity in molecule-surface reactions Adsorption of ethylene on the silicon(OOl) surface. Journal of the American Chemical Society 119, 7593 (1997). 

Fig. 5. Adsorption isotherms and composition of the gas phase for the adsorption of ethylene on (a) rhodium—silica and (b) palladium—silica at 20°C. o, Total molecules adsorbed , ethylene , ethane.

It has been concluded from deuterium exchange experiments, using ethylene and heavy water, that the addition of an adsorbed proton to adsorbed ethylene is the actual rate-determining step. It can be seen that the two schemes differ, mainly in that the latter includes dissociative adsorption of water on the surface of the catalyst and does not specify the adsorption of ethylene, but they are consistent in that they assume the formation of a carbonium ion as the rate-determining step. 

There is no quantitative proportionality between degree of adsorption and rate of reaction. Nor is any such close relation to be expected. Indeed, at temperatures where reaction attains a measurable speed adsorption is often quite small. Thus, although the adsorption of ethylene by certain kinds of copper catalyst can be demonstrated at lower temperatures, the velocity of interaction of ethylene and hydrogen only attains an appreciable speed at temperatures where the adsorption becomes almost too small to measure. 

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