Ethylene on metal surfaces

Figure 2,24. n- and a- coordination of ethylene on metal surfaces.73 Reprinted with permission from Elsevier Science. 

FIGURE S. Structures resulting from the adsorption of ethylene on metal surfaces. 

Low-Temperature Ethylene Adsorption on Ni(lll) The adsorption of ethylene on metal surfaces can be understood in terms of the Dewar-Chatt-Duncanson (DCD) model [73], put forward to rationalize ethylene coordination in organometaUic clusters. Essentially, the interaction is controlled by donation of electron density from the highest occupied molecular tt level and backdonation from the metal into the empty tt level. Both types of electron transfers increase the metal-molecule bond strength and decrease the molecular carbon-carbon bond. On the basis of this model, three extreme descriptions of adsorbed ethylene can be formulated, depicted as the di-a, the metallocyclopropane, and the tt-adsorbed species [67, 74], Figure 3.4.1.8a. The vibrational fingerprints for these species have been reviewed [22, 23, 52, 67] and fall within three main patterns. Figure 3.4.1.8b... 

Uses. Besides polymerizing TFE to various types of high PTEE homopolymer, TEE is copolymerized with hexafluoropropylene (29), ethylene (30), perfluorinated ether (31), isobutylene (32), propylene (33), and in some cases it is used as a termonomer (34). It is used to prepare low molecular weight polyfluorocarbons (35) and carbonyl fluoride (36), as well as to form PTEE m situ on metal surfaces (37). Hexafluoropropylene [116-15-4] (38,39), perfluorinated ethers, and other oligomers are prepared from TEE. 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

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. 

The coadsorption of oxygen as well as of other electronegative additives on metal surfaces favors in general the 7t-bonded molecular state of ethylene, as the latter exhibits, compared to the di-o bonded state, a more pronounced electron donor character and a negligible backdonation of electron density from the metal surface. 

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. 

The chemistry of the 1 1 and 1 2 complexes differs with respect to hydrogenation (84,89). The 1 2 derivatives are inert to hydrogenation, while the 1 1 compounds are smoothly transformed into an ethylidene complex (see Scheme 1). This difference in behavior may well reflect the cause of differences in behavior of olefins on metal surfaces toward hydrogenation. The ethylidene complex may be converted back to the olefin adduct by reaction with trityl ion. The ethylidene adduct was first obtained for ruthenium by interaction of ethylene with H RujfCO) (89), and is structurally related to the corresponding cobalt derivatives, Co3(CO)9RC. As discussed above, the structure has been established in detail and involves a capping of the metal triangle... 

Stevenson, R. W Poly condensation rate of poly (ethylene terephthalate) - II. Antimony trioxide catalyzed polycondensation in static thin films on metal surfaces, J. Polym. Sci., PartA-1, 1, 395-407 (1969). 

The eight- and twelve-electron reductions of CO2 to methane and ethylene, are very exciting results for two reasons. First, these results demonstrate that complex multielectron reductions are possible. Second, they illustrate the close relationship that exists between reactions observed for discrete molecular complexes and reactions observed on metal surfaces. 

Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

The largest proportion of TFE is used for the polymerization into a variety of PITH homopolymers. It is also used as comonomer in the copolymerization with hexaflu-oropropylene, ethylene, perfluorinated ether, and other monomers and also as a comonomer in a variety of terpolymers. Other uses of TFE are to prepare low-molecular-weight polyfluorocarbons, carbonyl fluoride oils, as well as to form PTFE in situ on metal surfaces,13 and in the synthesis of hexafluoropropylene, perfluorinated ethers, and other oligomers.14... 

The adsorption and subsequent reaction of ethylene on Group VIII metal surfaces provides a rich chemistry, which depends upon the adsorption temperature and the Group VIII metal. 13c NMR offers great potential to investigate the adsorbed states and to follow the reaction of ethylene on the surface. The study of the reactions of ethylene on Ru surfaces by 13c NMR has been reported by Gerstein and coworkers.(7,2) They have shown that ethylene on silica-supported ruthenium is converted to ethane, n-butane, 2- butenes and strongly adsorbed alkyl groups. 

The temperature dependence of ethylene s reaction on metal surfaces was studied. Figure 4A is a spectrum of a sample of Ru- Ca-Y exposed to ethylene at room temperature then heated at 373 K for 3 days. While ethane and n-butane are still... 

The density functional theory and the cluster model approach enable the quantitative computational analysis of the adsorption of small chemical species on metal surfaces. Two studies are presented, one concerning the adsorption of acetylene on copper (100) surfaces, the other concerning the adsorption of ethylene on the (1(X)) surfaces of nickel, palladium and platinum. These studies support the usefulness of the cluster model approach in studies of heterogeneous catalysis involving transition metal catalysts. 

It is known that in theoretical studies of the adsorption of chemical species on metal surfaces, the adsorption properties of the adsorbates, are influenced by the methodology, by the cluster size and shape and by the basis sets used in the corresponding study. So a reliable comparative theoretical study of the adsorption of ethylene on the Ni, Pd and Pt surfaces can only be obtained if the same conditions in terms of methodology, cluster size and shape and basis sets are used in the study of the adsorption on the three metal surfaces.The aim of this study was to compile theoretical results for the adsorption of ethylene on the di-o and rt-top adsorption sites on the (100) surfaces of platinum, palladium and nickel under the same conditions mentioned above, in order to compare and understand the trends in the adsorption of this species on these three metal surfaces. 

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