Ethylene chemisorbed

Fig. XVni-4. HREELS vibrational spectra from ethylene chemisorbed on Rh(lll). There is a sequential dehydrogenation on heating. (From Ref. 13, p. 64.)...

Similar experiments were done at several temperatures starting with either normal or fully deuterated ethylene chemisorbed on... 

Eischens and Pliskin have interpreted the infrared spectra of ethylene chemisorbed on nickel dispersed on silica 32). When introduced to a surface previously exposed to hydrogen, ethylene gave rise to absorption bands which correspond to the C—H stretching frequencies of a saturated hydrocarbon (3.4-3.5 p) and a deformation associated with a methylene group (6.9 p). A weak band at 3.3 p was attributed to an ole-finic C—H. Treatment of the chemisorbed ethylene with hydrogen caused the spectrum to change to one which was interpreted as due to an adsorbed ethyl radical. Apparently in the presence of hydrogen most of... 

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.

Fig. 2. (4) Spectrum of ethylene chemisorbed on hydrogen-covered nickel (B) after treatment with H. ...

The conclusion that ethylene chemisorbed on nickel, containing a pre-... 

The structure and reactivity of ethylene chemisorbed on transition-metal surfaces are of fimdamental importance in surface science and heterogeneous catalysis. HREELS has been foremost among the surface characterization techniques employed in fact, the first vibrational spectroscopic study of ethylene chemisorbed on Pt(lll) was carried out with electron energy-loss spectroscopy (EELS) almost a decade before IRAS was employed. ... 

Davis S, Zaera F, Gordon BE, Somoijai GA (1985) Radiotracer and thermal desorption studies of dehydrogenation and atmospheric hydrogenation of organic fragments obtained from [ C]ethylene chemisorbed over Pt(lll) surfaces. J Catal 92 240... 

The Raman spectrum of ethylene chemisorbed on Ag(llO) is shown in figure 24 and provides an example of catalytic interest. The C=C stretch of adsorbed ethylene is observed at 1580 cm and the symmetric C-H in-plane deformation at 1320 cm ... 

Figure 24 The normal Raman spectrum of ethylene chemisorbed on Ag (110) at submonolayer coverage at 100 K. The silver surface was predosed with Oj at 300...

When ethylene chemisorbs on the (111) face of rhodium, it lies with its C = C bond parallel to the surface at low temperatures, forms ethylidyne (C2H3 —) at 300 K, and dissociates to C2H— and CH— groups at 410... 

S.M. Davis, F. Zaera, B.E. Gordon, and G.A. Somoijai. Radiotracer and Thermal Desorption Studies of Dehydrogenation and Atmospheric Hydrogenation of Organic Fragments Obtained from l C]Ethylene Chemisorbed over Pt(l 11) Surfaces. J. Catal. 92 240 (1985). 

Figure 5 RAIRS spectra of ethylene (C2H4) chemisorbed at 100K, ethylene chemisorbed at 300 K, and ethyl iodide (C2H5I) chemisorbed at 300 K on a R(111) single crystal surface. (Reprinted with permission from Hoffmann H, Griffiths PR, and Zaera F (1992) A FtAIRS study on the surface chemistry of ethyl iodide on R 111). Surface Science 262 141-150 Elsevier.)...

Fig. VIII-10. (a) Intensity versus energy of scattered electron (inset shows LEED pattern) for a Rh(lll) surface covered with a monolayer of ethylidyne (CCH3), the structure of chemisorbed ethylene, (b) Auger electron spectrum, (c) High-resolution electron energy loss spectrum. [Reprinted with permission from G. A. Somoijai and B. E. Bent, Prog. Colloid Polym. ScL, 70, 38 (1985) (Ref. 6). Copyright 1985, Pergamon Press.]...

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

An appreciation of statistical results can be gained from a study conducted to support the first application of computer control for an ethylene oxide production unit at Union Carbide Corporation in 1958. For the above purpose, twenty years of production experience with many units was correlated by excellent statisticians who had no regard for kinetics or chemistry. In spite of this, they did excellent, although entirely empirical work. One statement they made was ... [ethane has a significant effect on ethylene oxide production.] This was rejected by most technical people because it did not appear to make any sense ethane did not react, did not chemisorb, and went through the reactor unchanged. 

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. 

Mixed-valence Ru"-Ru" paddlewheel carboxylate complexes also have potential for oxidation reactions after incorporation in a microporous lattice with porphyrinic ligands. This MOF can be used for oxidation of alcohols and for hydrogenation of ethylene. Both the porosity of the lattice and the abihty of the diruthenium centers to chemisorb dioxygen are essential for the performance of the catalyst [62, 64]. 

To overcome some of the problems associated with aqueous media, non-aqueous systems with cadmium salt and elemental sulfur dissolved in solvents such as DMSO, DMF, and ethylene glycol have been used, following the method of Baranski and Fawcett [48-50], The study of CdS electrodeposition on Hg and Pt electrodes in DMSO solutions using cyclic voltammetry (at stationary electrodes) and pulse polarography (at dropping Hg electrodes) provided evidence that during deposition sulfur is chemisorbed at these electrodes and that formation of at least a monolayer of metal sulfide is probable. Formation of the initial layer of CdS involved reaction of Cd(II) ions with the chemisorbed sulfur or with a pre-existing layer of metal sulfide. 

The dependence of intensity of the observed bands on pressure is significant. Except for a relatively small intercept the intensities of bands at about 3130 and 3060 cm-1 are roughly proportional to pressure between 22 and 240 mm, whereas the intensities of bands at 1451, 1438, 1600, and 2984 cm-1 are insensitive to pressure in this region. (The band at 2993 cm-1 seems to behave similarly to the bands above 3000 cm-1, but its overlap with the band at 2984 cm-1 makes analysis difficult.) Thus, the bands above 3000 cm-1 (and perhaps the band at 2993 cm-1) are primarily due to physically adsorbed ethylene and only in part due to chemisorbed ethylene. By way of contrast the remaining bands stem primarily from chemisorbed ethylene. 

The band at 1600 cm-1 due to a double-bond stretch shows that chemisorbed ethylene is olefinic C—H stretching bands above 3000 cm-1 support this view. Interaction of an olefin with a surface with appreciable heat suggests 7r-bonding is involved. Powell and Sheppard (4-1) have noted that the spectrum of olefins in 7r-bonded transition metal complexes appears to involve fundamentals similar to those of the free olefin. Two striking differences occur. First, infrared forbidden bands for the free olefin become allowed for the lower symmetry complex second, the fundamentals of ethylene corresponding to v and v% shift much more than the other fundamentals. In Table III we compare the fundamentals observed for liquid ethylene (42) and a 7r-complex (43) to those observed for chemisorbed ethylene. Two points are clear from Table III. First, bands forbidden in the IR for gaseous ethylene are observed for chemisorbed ethyl-... 

Observed Bands for Chemisorbed Ethylene and Related Systems... 

Figure 11(a) shows the spectrum of adsorbed species on an active catalyst in a hydrogen-ethylene stream. This spectrum appears and stabilizes within minutes after hydrogen is blended into the ethylene stream. Three new bands appear in the presence of hydrogen at 2892, 2860, and 2812 cm-1. The appearance and location of these bands were verified by expanded scale spectra. Experiments at lower ethylene pressures reveal that there is an additional band at about 2940 cm-1 partially obscured in Fig. 11 by overlap of the ethylene spectrum. On a poisoned catalyst, which does not show the ZnH and OH bands, only the bands characteristic of chemisorbed ethylene are seen. 

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