Nickel catalysts ethane hydrogenolysis

The first reported work on the kinetics of hydrogenolysis reactions of simple hydrocarbons appears to be that of Taylor and associates at Princeton (2-4, 14, 15), primarily on the hydrogenolysis of ethane to methane. The studies were conducted on nickel, cobalt, and iron catalysts. More recently, extensive studies on ethane hydrogenolysis kinetics have been conducted on all the group VIII metals and on certain other metals as well (16,28-83). 

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).

Hydrogenolysis of ethane over a commercial nickel catalyst was studied in a rotating basket reactor containing 40 g of catalyst. 

Fig. 7. Arrhenius plot of the rate constant for hydrogenolysis on nickel/MgAl2O4 and silver/nickel/ MgAl2O4. The rate constant (L) for ethane hydrogenolysis is one order of magnitude lower on the silver/ nickel catalyst than on the nickel catalyst, and the activation energies (determined by the slopes of the Arrhenius plots) are similar for the two.

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.

The presence of a second metal can also have a significant impact on the selectivity of a reaction. Recall the data shown in Fig. 3.1 illustrates the effect of adding copper to a nickel catalyst used to promote both cyclohexane dehydrogenation and ethane hydrogenolysis. 39 It was found that the addition of... 

As an attempt in this direction, a hierarchy was recently developed for nickel catalysts (6). The basic idea is to monitor the chemical properties of a catalyst as probed by hydrogen chemisorption, ethane hydrogenolysis, and carbon monoxide hydrogenation. The hierarchy, originally developed for Ni/I O catalysts, was later extended to nickel supported on phosphate-containing materials and a niobia-silica surface phase oxide. In this paper the usefulness of the hierarchy will be illustrated by its ability to differentiate between support effects of niobia and phosphate, and to establish the intermediate degree of interaction of niboia-silica. 

Ethane Hydrogenolysis. Table II summarizes the kinetic results of ethane hydrogenolysis over nickel catalysts on phosphate (15), niobia (6 ), and niobia-silica (9) supports. As a point of jjefer ... 

Table II. Kinetic Results of Ethane Hydrogenolysis over Nickel Catalysts...

The effect on activity for the dehydrogenation reaction is very different from that for the hydrogenolysis reaction. In the case of ethane hydrogenolysis, adding only 5 at.% copper to nickel decreases catalytic activity by three orders of magnitude. Further addition of copper continues to decrease the activity. However, the activity of nickel for dehydrogenation of cyclohexane is affected very little over a wide range of composition, and actually increases on addition of the first increments of copper to nickel. Only as the catalyst composition approaches pure copper is a marked decline in catalytic activity observed. 

It is known that nature and quality of a catalyst carrier material is important to the performance of a catalyst. The carrier impacts the catalytic reaction and process by its chemical, physical or mechanical properties and/or provides co-catalytic function. An example for a typical support effect is the well-investigated standard reaction of ethane hydrogenolysis over nickel or cobalt on different support materials (Figure 1) [1] ... 

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). 

The hydrogenolysis of ethane on supported nickel catalysts is a good example for the influence of the degree of dispersion of the metal (Table 5-33). It is known that nickel is more highly dispersed on Si02 than on AI2O3, and at the same time there is an influence on the crystallite form. A further influence is due to the acid centers of aluminum oxide, which lead to more extensive coke formation, deactivating the nickel catalyst. 

Table 5-33 Hydrogenolysis of ethane on supported nickel catalysts (10 % Ni) [T35]...

Research on nickel catalysts was mainly related to hydrogenolysis [311] [462] and later methanation [37] [314] became of interest in SNG from coal in the late 1970s. Sinfelt s work on bimetallic catalysts [462] was of interest to steam reforming, in particular as a direct correlation was found [379] [407] between the activity for steam reforming of ethane and that of hydrogenolysis of ethane as illustrated in Figure 6.1. 

P7-13 The ethane hydrogenolysis over a comtnercia) nickel catalyst was studied in a xtirred lank solids reactor... 

Characterization of the Surfaces of Catalysts Measurements of the Density of Surface Faces for High Surface Area Supports. - It has always been a tenet of theories of catalysis that certain reactions will proceed at different rates on different surface planes of the same crystal. Experiments with metal single crystals have vindicated this view by showing that the rate of hydrogenolysis of ethane on a nickel surface will vary from one plane to another. In contrast the rate of methanation remains constant for the same planes.4 Because of this structure sensitivity of catalytic processes there is a requirement for methods of determining the number of each of the different planes which a catalyst and its support may expose at their surfaces. Electron microscopy studies of 5nm Pt particles supported upon graphite show them to be cubo-octahedra with surfaces bound by (111) and (100) planes.5 Similar studies of Pd and Pt prepared by evaporation reveal square pyramids of size 60-200 A bounded by incomplete (111) faces.6... 

K. Tanaka, T. Miyazaki and K. Aomura, Intermediates and carbonaceous deposits in the hydrogenolysis of ethane on a nickel-alumina catalyst, J. Catal. 81(2) (1983) 328-334. 

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