Adsorbed ethylene

Yang W H, Hulteen J 0, Schatz G G and Van Duyne R P 1996 A surface-enhanced hyper-Raman and surface-enhanced Raman scattering study of trans-1,2-bis(4-pyridyl)ethylene adsorbed onto silver film over nanosphere electrodes. Vibrational assignments experiments and theory J. Chem. Phys. 104 4313-26... 

Neurock and coworkers [M. Neurock, V. Pallassana and R.A. van Santen, J. Am. Chem. Soc. 122 (2000) 1150] performed density functional calculations for this reaction scheme up to the formation of the ethyl fragment, for a palladium(lll) surface. Figure 6.38(a) shows the potential energy diagram, starting from point at which H atoms are already at the surface. As the diagram shows, ethylene adsorbs in the Jt-bonded mode with a heat adsorption of 30 kj mol and conversion of the latter into the di-a bonded mode stabilizes the molecule by a further 32 kJ mol . ... 

Figure 6.38. Potential energy diagram for the hydrogenation of ethylene to the ethyl (C2H5) intermediate on a palladium(m) surface. The zero of energy has been set at that of an adsorbed H atom, (a) Situation at low coverage ethylene adsorbed in the relatively stable di-cr bonded mode, in which the two carbon atoms bind to two metal atoms. In the three-centered transition state, hydrogen and carbon bind to the same metal atom, which leads to a considerable increase in the energy...

Thus, the rate should be half-order in hydrogen pressure and proportional to the amount of ethylene adsorbed on the oxide half of the active site under reaction conditions. 

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. 

A variety of radical ions have been produced by photochemical reactions involving adsorbed species. One of the simplest of these is the radical ion which is formed upon y irradiation of ethylene adsorbed on silica gel (92). The resulting nine-line spectrum has been attributed to (CH2 = CHi-) + however, it may be due to the corresponding negative ion. The neutral ethyl radical is also formed under these conditions. 

Figure 2.16 Temperature programmed reaction between O atoms and ethylene adsorbed on Rh(l 11). The majority of the adsorbed ethylene decomposes in several steps to H and C atoms, which react with the adsorbed O atoms to form H2, H20, CO and C02. Because there is insufficient oxygen, the surface still contains carbon at the end of the experiment (adapted from [36],...

Figure 8.10 Sum frequency generation spectra of ethylene adsorbed on Pt( 111) at 200 K after heating to the temperature indicated. The spectra indicate the conversion of di-o bonded ethylene to ethylidyne via an intermediate attributed to ethylidenc (adapted from Cremer et al. [35].)...

Fig. 3. Time dependence of the conversion of normal ethylene adsorbed on Pt(lll) to ethylidyne at four different temperatures.

Acetylene forms spontaneously an ordered (2 X 2) surface structure on the Pt(l 11) surface at 300 K, at low exposure under ultrahigh vacuum conditions. The intensity profiles reveal that this structure is metastable, and upon heating to 350-400 K for one hour, it undergoes a transformation to a stable structure with the same (2 X 2) unit cell. Ethylene adsorbs on the Pt(l 11) surface and at 300 K, it forms an ordered (2 X 2) surface structure that is identical to the stable acetylene structure as shown by the intensity profiles. 

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... 

Figure 2.34. Symmetry-resolved experimental C Is XAS of ethylene adsorbed on Cu(l 10) (solid lines) and Ni(110) (dashed lines). The coordinate system and molecular geometry used for the calculations are shown in the inset. From Ref. [85].

Methane dehydroaromatization on zeolites Mo/HZSM-5 was also investigated by solid-state MAS NMR spectroscopy 162. Both variation of the state of the transition metal component and products (such as ethane, benzene, and ethylene) adsorbed in zeolite were observed after reaction at high temperature (900-1000 K). Molybdenum carbide species, dispersed on the external surface or in the internal channels of the zeolite catalysts, had formed during the reaction 162. ... 

The spectra observed for ethylene adsorbed on nickel—silica [75,78] show similar features to those found with platinum—silica. The major difference is the temperature range over which the various bands are observed with nickel—silica, the surface n-butyl groups are present at room... 

Introduction. The structure of acetylene and ethylene adsorbed on transition metal surfaces is of fundamental importance in catalysis. An understanding of the interaction of these simple molecules with metal surfaces may provide information on possible surface intermediates in the catalytic hydrogenation/dehydrogenation of ethylene. High resolution ELS is a particularly useful... 

Fig. 6. Optimized DFT structures of ethylene adsorbed on four different sites of the surface Ag1830 oxide. Light grey balls and sticks represent Ag atoms. O and C atoms are represented by dark grey and black balls, respectively. On one specific site, Ag-3, two orientations of ethylene relative to the Ag-O linkage (3 and 3") are compared.

Table 1. Adsorption energy and interaction distances for ethylene adsorbed on different oxide atomic sites, dc-oxide corresponds to the closest distance between the adsorbate and the oxide substrate, namely between one C atom of the ethylene and a Ag or Ou atom of the substrate.

Finally at this stage we point out several important issues that remain to be resolved. In particular, existing STM data indicates that ethylene adsorbs exclusively at Ag-3 sites. As shown above DFT, however, indicates that ethylene should also adsorb at Ag-1 sites in addition to the Ag-3 sites. This apparent discrepancy is as yet unresolved. In addition the contrast of the ethylene molecule in the simulated STM images (not shown) is severely overestimated (by about a factor of two). See [50] for a brief discussion on this later issue. 

Reaction mechanisms with ethylene adsorbed at the second type of ethylene adsorption site identified (Ag-1/2) were also investigated. However after several unsuccessful attempts to identify a stable oxametallacy-cle associated with this ethylene adsorption site we concluded that ethylene was not reactive at this site. This is to be anticipated given that the ethylene molecule is a considerable distance from the O atoms in the oxide ring (around 4 A). 

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