Chemisorption of ethane

To give an idea of the wide rai e of catalytic systems that have been investigated where chemisorption data were essential to interpret the results, some of the author s papers will be discussed. Measurements were reported on the surface areas of a very wide range of metals that catalyze the hydrogenation of ethane. In the earliest paper, on nickel, the specific catalytic activity of a supported metal was accurately measured for the first time it was shown also that the reaction rate was direcdy proportional to the nickel surface area. Studies on the same reaction... 

The answer to the first question is undoubtedly a positive one. The classical papers by Beeck et al., Rideal et al., and others have shown that ethylene disproportionates upon chemisorption into ethane and carbonaceous (adsorbed) residues (see 162). This disproportionation takes place at relatively low temperatures at room temperature and lower (see 162 for review). Moreover, the intensity analysis of LEED data has shown that upon chemisorption of ethylene, ethylidyne structures are formed. Similar structures are also formed by dissociative adsorption of higher olefins (181,182). There is thus no doubt with regard the first question. 

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 ... 

The other example to be discussed in this context comes from Pettit s group. Simultaneous treatment of the iron complex (/u.-CH2)[Fe(CO)4]2 (35) with hydrogen and ethylene gives both methane (66%) and propylene (6%), the expected products from the two separate reactions. In addition, ethane (—600%) is formed, with the actual hydrogenation catalyst still to be determined (72). Because simple diazoalkanes provide the cleanest method to metal-attached alkylidenes, and with the expectation that dissociative chemisorption of diazomethane to absorbed CH2 and free N2 would occur, the reactions of CH2N2 with and without H2 over various transition metals were examined in a careful study with regard to the product ratio (73). It was found, that gas-phase decomposition of the parent diazoalkane upon passage over active Ni, Pd, Fe, Co, Ru, or Cu-... 

The dissociative adsorption of alkanes on Ir(l 1 0) surface was investigated in a series of studies performed by the Madix group [13, 32, 33]. A study performed by Hamza et al. [32] probed the dynamics of the dissociative chemisorption of methane, ethane, propane, and n-butane on the Ir(l 1 0)-(l x 2) surface. These investigations were complemented by a later study of propane dissociation on the Ir(l 1 0)-(l x 2) surface by Soulen and Madix [13]. Shown in Figs 6 and 7 are plots of S0 vs. E obtained for propane at various surface temperatures [32] and a plot of experimental and theoretical values of S0 for propane (at an incident translational energy of 50kcal/mol) on Ir(l 1 0)-(l x 2) as a function of surface temperature [13]. 

A quantum tunneling mechanism was proposed by Yerhoef et al. to describe these trends, and an attempt was made to develop a tunneling model to predict the chemisorption of methane and ethane over the entire range of normal incident energies studied. This was accomplished by modeling the... 

Conclusions. Submonolayer deposits of titania grow on the surface of Rh in the form of two-dimensional islands until a coverage of nearly a monolayer is achieved, at which point some three-dimensional growth of the islands is observed. The titania islands exclude CO chemisorption on Rh sites covered by the titania. The Ti + ions in the overlayer are readily reduced to TP+. This process begins at the perimeter of the islands and extends inwards as reduction proceeds. Titania promotion of Rh enhances the rate of CO hydrogenation by up to a factor of three and increases the selectivity to C2+ hydrocarbons. By contrast, the activity of Rh for the hydrogenolysis of ethane decreases monotonically with increasing titania promotion. 

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... 

Ruthenium-copper aggregates of the type described have been studied with chemical and physical probes. Chemical probes that have been very informative include hydrogen chemisorption and the hydrogenolysis of ethane to methane. Physical probes useful in these characterizations include X-ray diffraction and electron spectroscopy. 

In addition to hydrogen chemisorption and ethane hydrogenolysis, the reactions of cyclohexane provide a useful chemical probe for investigating ruthenium-copper aggregates. On pure ruthenium, two reactions of cyclohexane are readily observed dehydrogenation to benzene and hydrogenolysis to lower carbon number alkanes. The product of the latter reaction is predominantly methane, even at very low conversions. 

Fig. 7. Rates of ethane hydrogenolysis measured in a pulse reactor over a series of 2-wt% Rh/Ti02 catalysts as a function of metal dispersion (H/Rh) as measured by H2 chemisorption after reduction at 473 K. (After Ref. 46.)...

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