Alloys ethane hydrogenolysis

Fig. 6. Activities of copper-nickel alloy catalysts for the hydrogenolysis of ethane to methane and the dehydrogenation of cyclohexane to benzene. The activities refer to reaction rates at 316° C. Ethane hydrogenolysis activities were obtained at ethane and hydrogen pressures of 0.030 and 0.20 atm., respectively. Cyclohexane dehydrogenation activities were obtained at cyclohexane and hydrogen pressures of 0.17 and 0.83 atm, respectively (74).

Summary of Kinetic Parameters for Ethane Hydrogenolysis on Copper-Nickel Alloys (74)... 

Figure 8.20 Activity of Ni-Cu alloys towards cyclohexane dehydrogenation and ethane hydrogenolysis. (Following Sinfelt, 1977.)...

In having a low activity with respect to C-C bond fission and in promoting isomerization, the Ni-Cu alloys are more reminiscent of platinum than nickel. The explanation given is similar to that proposed for the suppression of ethane hydrogenolysis. Hydrogenolysis requires multicenter adsorption and is therefore more sensitive to alloying than reactions needing fewer centers. This was examined in detail by Ponec et al. (60) in a study of the... 

Variations in the activation energies for ethane hydrogenolysis on Cu-Ni alloys (147) were relatively small and, while the compensation plot showed appreciable scatter, the line found (Table IV, G) was close to that for cracking... 

Figure 5.2.6 I Effect of alloy composition on the rates of ethane hydrogenolysis and cyclohexane dehydrogenation on Ni-Cu catalysts. (Figure from Catalytic Hydrogenolysis and Dehydrogenation Over Copper-Nickel Alloys by J. H.

In their study of alloys the authors say that the average size of surface-exposed ensembles will decrease with dilution this may result in changes in selectivity as has been observed for Group VIII/Group IB metals. Thus, for example, two Ni atoms seem to be required for ethane hydrogenolysis. In their study of alloys the authors claim that they have avoided carbonaceous residues by working at appreciable H2 pressure and high H2/hydrocarbon ratios. 

At the same time that our work on ethane hydrogenolysis and cyclohexane dehydrogenation on nickel-copper alloys was published, a paper by Ponec and Sachtler on the reactions of cyclopentane with deuterium appeared (9). These workers reported data on the rates of formation of deuterocyclopen-tanes via exchange, and of CD4 by hydrogenolysis. The exchange reaction occurred at about the same rate (per surface nickel atom) on nickel-copper alloys as on pure nickel, while the rate of formation of CD4 was substantially decreased. 

Cusumano et al. (128) studied the reaction over Pt on alumina and on silica supports and concluded that the TOF was about the same for both catalysts, which did show quite different atomic rates AR. The later work of Sinfelt et al. (269) on reactions over copper-nickel alloys led also to the suggestion that cyclohexane dehydrogenation over Ni does not require a large ensemble of surface atoms and thus may be structure insensitive on a geometric basis. For ethane hydrogenolysis studied on the same CuNi alloys, it was found that the activity decreased much more rapidly than did the fraction of Ni atoms on the surface of the alloys. This implies that ethane hydrogenolysis requires an ensemble of surface atoms and should show antipathetic structure sensitivity. This reaction will be discussed in connection with Fig. 15 (below). 

Thus in our discussion of the role of the multiatom sites (ensembles) usually considered important in antipathetic structure sensitivity we shall have occasion to refer to related results on catalysis over alloys. For example, the hydrogenolysis of ethane over Ni is inhibited as Cu is added and the necessary ensembles are broken up. 

These Ni/Cu alloys exhibit special selectivity effects, which has been demonstrated in the hydrogenolysis of ethane to methane (Eq. 5-36) and the dehydrogenation of cyclohexane to benzene (Eq. 5-37). 

Fig. 5-28. Specific activity of copper-nickel alloys for the dehydrogenation of cyclohexane and the hydrogenolysis of ethane to methane at 316 °C...

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