Nickel, chemisorption ethylene

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... 

Selwood (100,101) has also employed this technique to study the chemisorption of ethylene, ethane, benzene, and cyclohexane on supported nickel catalysts. Among the new information and important conclusions derived from these measurements are the following ... 

A few measurements of the changes in magnetization of nickel resulting from the chemisorption of hydrogen, ethylene and acetylene have also been reported by Breeder and his co-workers lOSa, b). Their results and conclusions appear to be qualitatively in accord with those of Selwood. 

When ethylene is adsorbed on bare nickel at 35° C. or on either bare or hydrogen-covered nickel at 150° C., the intensity of the C—H bands, shown as A of Fig. 3, is small compared with those of the associated chemisorbed ethylene shown in Fig. 2. When the species represented by A is treated with H2 at 35° C., the band intensities increased as is shown in B of Fig. 3. This behavior shows that A is due to a dissociatively chemisorbed ethylene in which the number of hydrogens per carbon is low (7). The species obtained by dissociative chemisorption will be referred to as a surface complex. It is doubtful whether the surface complex has a specific stoichiometric composition. Rather it appears that the carbon-hydrogen ratio will depend on the severity of the dehydrogenation conditions. In some cases it appears that a surface carbide, which has no hydrogens, is obtained. Even in this case the carbons appear to be easily rehydrogenated to adsorbed alkyl groups. 

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. 

Beeck, Smith, and Wheeler 298) prepared nonoriented and oriented nickel films, the latter showing (110) planes parallel to the substrate on which they were condensed. They showed that the catalytic hydrogenation of ethylene at 0°C. proceeds five times more quickly on these oriented films than on randomly oriented films. The heats of chemisorption of... 

Ethylene and Acetylene. On nickel (111), both ethylene and acetylene are irreversibly chemisorbed neither can be thermally desorbed. We also find that trimethylphosphine cannot displace ethylene or acetylene from these surfaces. There have been suggestions that ethylene and acetylene are not present on the surface as molecules but as molecular fragments. Many ultra-high vacuum studies of ethylene and of acetylene chemisorption on nickel crystal planes have been reported. Most of these studies seem to implicate states in which C-H bond cleavage reactions have accompanied the basic chemisorption process (19). 

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. 

Keii (33) calculated the activation energy of the chemisorption of ethylene on nickel assuming two carbon atoms respectively bonded with two adjacent nickel atoms and found almost constant activation energies of 4 kcal./mole—assuming spacings between adjacent nickel atoms of 2.49 and 3.52 A, and Ni—C bond strengths to vary from 38.2 to 60 kcal./ mole. It is surprising that the rates calculated in this way are compatible with the experimental observations of Steace and Stovel (34). 

The catalytic hydrogenation of ethylene on nickel, as explained by Horiuti, is based on four consecutive elementary reactions, viz., the chemisorption of the reactants to form adsorbed ethylene (la) and adsorbed hydrogen atoms (Ib), the reaction (II) between these adsorbents to give half-hydrogenated molecules, and the addition of another adsorbed hydrogen atom (III) to form ethane. In this mechanism, step (Ib) is... 

In general, chemisorption will produce new spectral bands which are not characteristic of the adsorbate or the adsorbent. However, absence of such bands cannot be taken as evidence of an absence of chemisorption. A difficulty present in any attempt to make kinetic measurements is that extinction coefficients are often significantly altered as a result of adsorption. These changes, which cannot as yet be interpreted theoretically, make it difficult to correlate the observed absorbance with the coverage of adsorbed molecules. The change in extinction coefficient is dependent on both the adsorbate and the adsorbent. For example, an increase of e was observed with increasing coverage for ethylene adsorbed on copper oxide, whereas the reverse occurred with nickel oxide . ... 

Extensive studies of hydrocarbon chemisorption have been made by Eischens and Pliskin (1). In a series of studies on olefins and paraffins chemisorbed on silica-supported nickel they were able to show that both associative and dissociative adsorption could occur, depending on catalyst pretreatment. Associative chemisorption of olefins is observed when hydrogen is left on the nickel surface dissociative absorption, when the hydrogen has been pumped off at an elevated temperature before chemisorption. The associative mechanism is deduced from the fact that the only absorption bands found when ethylene is added to a hydrogen-covered surface are in the C-H stretching region characteristic of saturated hydrocarbons, and that a C-H deformation band at 1447 cm-1 characteristic of two hydrogens on a carbon is also observed. 

Chemisorption of hydrocarbons on various metals, such as nickel, platinum, copper, etc., was investigated in great detail (9, 90, 91, 92). Information on chemisorption of ethylene, acetylene and methane on various metals may be found in Trapnell s review (93). However, direct application of the relations obtained to metal oxide catalysts would scarcely be justifiable. As a rule, oxygen covers the whole surface of the metal, and chemisorption of hydrocarbons occurs either on a thin layer of the given metal oxide formed as an individual phase, or on oxygen that was sorbed on the surface and has filled the adjacent-to-surface layers. Thus data on chemisorption of hydrocarbons on oxides of these metals may be of use in the above cases. 

These considerations can also be applied to other metals. Thus die (100) planes of metals with larger atomic spacings than nickel (e.g., Pd, Pt, and Fe) should exhibit weaker chemisorption, and the same should also be true of metals with shorter interatomic distances such as tantalmn. Figure 5-19 shows the rate of ethylene hydrogenation as function of metal-metal distance (volcano plot). 

This conclusion was based on the result that when excess ethylene is admitted to a nickel film at room temperatures, ethane appears in the gas phase. Trapnell concluded that the initial chemisorption on tungsten films is a four-site process and the final adsorption self-hydrogenation is a two-site process. Jenkins and RideaP obtained results from self-hydrogenation of ethylene on nickel which fit the equation... 

From magnetic work on nickel powders, Selwood favored a two-bond chemisorption at room temperature. Assuming that the formation of a nickel-carbon bond affects the magnetization of nickel in the same way as a nickel-hydrogen bond, the chemisorption of ethylene on supported nickel at room temperature results in the nickel gaining on the average slightly more than two electrons per molecule. This implies that most of the ethylene is associatively chemisorbed [cf equation (47)], but that a moderate fraction is held... 

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