Chemisorption ethylene

Adapted from Marcinkowsky Berty, J. Catalysis, 29, 3, 1973 Academic Press. Figure 7.4.2 Ethylene chemisorption on silver. 

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... 

It is tempting to associate directly the absence of ethylene oxide over catalysts with more than 40% Pd with the appearance of holes in the d-band. It could be assumed that ethylene is chemisorbed directly on Pd-rich alloys and rapidly decomposed, whereas on Ag-rich alloys ethylene is only adsorbed on top of an oxygen-covered surface leading to selective oxidation. However, the general conclusion from earlier kinetic studies (143) is that the rate-determining step over pure palladium also involves the latter mode of ethylene chemisorption. 

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.

Recent work by Selwood (9), based on changes in the magnetization of nickel during chemisorption of ethylene, indicates that ethylene is associatively adsorbed on bare nickel. He suggests that the discrepancy between this result and the dissociative chemisorption indicated by the infrared experiments is due to factors such as the relative activity of the sample surfaces and temperature effects caused by the heat of chemisorption. Low-temperature infrared experiments in which ethylene is studied at —78° C. are expected to provide evidence on the importance of the above factors in determining the course of ethylene chemisorption. 

The reactions of ethylene and acetylene with Si( 100)-(2 x 1) were initially described as being [2 + 2] cycloadditions, with the di-configuration predicted by this mechanism believed to provide the dominant reaction products. A variety of alternate reaction products could actually be formed as a result of the chemisorption, with, e.g., the organic molecule spanning silicon atoms in different dimer rows, adhering above a row oriented perpendicular to the silicon dimers, or adhering above a row and parallel to the dimers. As reviewed in Sec. 3, a variety of alternate structures have now indeed been found for chemisorbed acetylene. Hence, while the [2 + 2] cycloaddition mechanism appears apt for ethylene chemisorption, its applicability to similar processes in acetylene is questionable. 

Ethylene chemisorption should be enhanced at Cu(I) sites, which result from the chlorination of ethylene. Cu2Cl2 is reoxidized by HC1 and oxygen to CuCl2 ... 

It had been found that removal of hydrogen from nickel at 350°C, instead of at room temperature, produced a profound difference in the properties of the nickel with respect to ethylene chemisorption (4). Therefore the chemisorption of carbon monoxide was repeated using nickel which had been degassed at 350°C. This produced a startling difference compared to the results shown in Fig. I now most of the carbon monoxide was chemisorbed in the linear structure (5). Similar experiments have not been made with palladium but it is reasonable to predict that the ratio of linear to bridged carbon monoxide would be increased by a more thorough removal of hydrogen from this metal. 

The fractional order of the dependence upon ethylene suggests a Langmuir type of mechanism with respect to ethylene (chemisorption of ethylene on the catalytic surface), while the nearly first order dependence with respect to hydrogen suggests that hydrogen reacts from the gas phase with chemisorbed ethylene. 

Figure 2.26. Bonding geometry of ethylidyne on the Rh(lll) and Pt(lll) crystal faces. Ethylidyne forms as a result of ethylene chemisorption.

The adsorption of ethylene on these sites is so strong that the gas saturates them at a pressure as low as 2 x 10 mm, and cannot be removed by evacuation at room temperature (at least for evacuation times of up to 1 hour). Alumina prepared from aluminum isopropoxide gives the same desorption chromatogram, showing that these active sites for ethylene chemisorption are an intrinsic property of alumina catalysts. 

The mechanism of the dehydrogenation of cyclohexane, which may be taken as typical, is therefore pictured as follows. Chemisorption of both saturated and unsaturated hydrocarbons takes place with dissociation of hydrogen. With cyclohexane, this probably occurs with formation of a benzene nucleus, bound to the surface by six bonds similar to those postulated by Twigg and Rideal (45) for ethylene chemisorption,... 

Ethylene chemisorption on isolated Pd atoms by means of matrix-isolation techniques (metal vapor, 10-30 K, xenon matrix, high vacuum) detection of chemisorption complexes... 

Kessmodel LL, Dubois LH, Somorjai GA. LED analysis of acetylene and ethylene chemisorption on the Pt(lll) surface evidence for ethylidyne formation. J Chem Phys. 1979 70 2180. 

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