Di-o-bonded ethylene

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. 1.11 Depiction of di-o-bonded ethylene and ethylidyne species on Pd(l 11). (Reprinted from [103] with permission from Elsevier.)...

In contrast to the behavior of di-o-bonded ethylene and ethylidyne, the appearance of tu-bonded species is directly correlated with the reaction rate. It is most likely the key intermediate in ethylene hydrogenation. It should be emphasized that the other surface species present, di-o bond ethylene and ethylidyne, are spectators and do not contribute to the turnover rate in any significant way [29]. Interestingly, only these strongly chemisorbed species are normally detectable by studies in UHV, while the weakly bond Jt-bonded ethylene is more readily found at atmospheric reaction conditions. 

UHV spectroscopic measurements were conducted to calibrate the concentration of 7t-bonded ethylene on Pt(lll) under reaction conditions. The calibration was achieved by exposing the clean Pt(l 11) surface to a near-saturation coverage of oxygen at room temperature, followed by exposure to ethylene at 120 K [30]. This results in a mixture of jt-bonded and di-o-bonded ethylene on the surface at a concentration of 6% of a monolayer and 10% of a monolayer respectively [31]. Using this spectrum as a reference, the intensity of the 3000 cm peak correspond to approximately 4% (error bars from 2-8%) of a monolayer of reactive Jt-bonded ethylene (the error bars for this result are a factor of 2 at the 90% confidence level using worst case assumptions about C-H bond reorientation). This means that the turnover rate for ethylene hydrogenation is actually 25 times faster per reactive intermediate species than when estimated per exposed platinum atom. Therefore, the absolute turnover rate of physisorbed ethylene is approximately 275 ethane molecules formed per surface intermediate per second under the above conditions. 

During ethylene hydrogenation over Pt(lll) the reaction intermediate appears to be weakly bound 7t-bonded ethylene which produces most of the ethane, while ethylidyne and di-o bonded ethylene are spectators during the catalytic process. The surface concentration of k-bonded ethylene is 4% of a monolayer during the turnover, which yields an absolute turnover rate 25 times higher than the turnover rate per platinum atom. 

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. 

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. 

Our calculations also support the picture, already suggested by several experimental studies, of a significantly distorted adsorbate on the three metal surfaces there is a lengthening of the CC bond and a rehybridization of the carbon atoms from sp toward sp On these three surfaces, the a-donation is stronger than the d-rt backdonation, leading to positively charged species on the surface. The results obtained, also show that on platinum, palladium and nickel (100) surfaces the ethylene molecule adsorbs preferentially on the di-o mode. This conclusion is based not only on the adsorption energies (whose calculation is known to be the major problem of the cluster model approach) but also on a comparision between the calculated vibrational frequencies and the available experimental results. 

A second example involves the adsorption of ethylene on transition metal surfaces and offers an interesting challenge, in that it can bind via n or di-o adsorption modes. Complete structural optimizations were performed for ethylene in both coordination geometries (Fig. 3). In the 7t-mode, the TZ orbital on ethylene interacts with the dz orbital on the metal center. There is a backdonation of electron density into the antibonding 7C orbital of ethylene which leads to a small weakening of the C-C bond length. This is noted by the slight increase (0.05 A) in the C-C bond from the gas phase value 1.34A. There is considerably more backdonation of electron... 

More complex is the adsorption of hydrocarbons and particular ethylene that forms different intermediate species. Cassuto et al. [10] showed a n adsorption form of ethylene on flat Pt[lll] surface at 40 K, without significant changes of the structure in the gas phase. However, at 90 K forms a di-o complex with hybridization sp over a flat Pt[lll] surface, which then passes to the ethylene-ethylene stretched bonding structure from 1.34 A up to 1.49 A. 

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