Nickel oxygen chemisorption

The only conclusions that can be drawn from our experiments concerning the role of oxygen in these interactions are (1) oxygen chemisorption is faster than the subsequent interaction, and (2) oxygen can react with hydrogen on the surface of nickel in the adsorbed layer at room temperature. 

The above differences in the catalytic activities of the metal are in good agreement with the data on oxygen chemisorption (Table 2), which indicates that the surface of the metallic nickel is comparable for samples 2 and 4, and about 6 times larger than that of sample 3. The result on the latter sample evidences that the nickel is not easily reducible, which explains very well the negligible activity of the metal in this catalyst. 

Dell and Stone 5) have already discussed this mechanism in their study of the chemisorption of oxygen on oxide-coated nickel powders. These workers found that only a monolayer of oxide formed during activation which followed oxygen chemisorption at 25° in the film thickness range from about 25 to 110 A. This behavior might be expected also with cobalt, although here the film thickness at which this mechanism alone operates is certainly greater than 25 A. 

It has been shown recently that the adsorption equilibrium function (/) can be used to obtain information on the thermodynamics of chemisorption processes as they occur during a catalytic reaction 2). Thus, the free energy, enthalpy, and entropy of oxygen chemisorption on nickel, platinum, and silver surfaces were determined while these surfaces were being used for the catalytic decomposition of water. 

Y-Alumina supported unpromoted catalyst molybdenum sulphide and promoted catalysts cobalt sulphide - molybdenum sulphide nickel sulphide - molybdenum sulphide were prepared by Precipitation From Homogeneous Solution (PFHS) technique using thioacetamide hydrolysis in a single step. Oxygen chemisorption studies, hydrodesulphurisation (HDS) and hydrogenation (HYD) studies were made for these cat alyst s. These catalysts do not need pre-sulphidation prior to HDS reaction. 

The results of Table 2 show that the HDS activity is maximum for unpromoted 8% Mo loading on y-Al20j and 8 5 Mo-Co and 8 5 Mo-Ni catalysts. These results correlate with the oxygen chemisorption values. HDS activity 01 the catalysts prepared by PFHS was higher when compared to commercial catalysts. It is also of interest to note that HDS activities of cobalt and nickel promoted catalysts appear to be higher than those of unpromoted catalysts. Therefore it appears that... 

We ivill discuss the reaction of hydrogen and oxygen on transition metals first. This reaction has been extensively studied in our laboratory 18-32) using evaporated metal films as a catalyst. From our previous considerations it follows that as a consequence of the choice of this particular system we must restrict ourselves to certain problems only. We cannot identify the surface species (we can indirectly indicate only some of them) nor understand completely their role in the reaction. Because of the polycrystalline character of the film, all the experimental results are averaged over all the surface. Several new problems thus arise, such as grain boundaries, and, consequently, the exact physical interpretation of these results is almost impossible it is more or less a speculative one. However, we can still get some valuable information concerning the chemical nature of the active chemisorption complex. The experimental method and the considerations will be shown in full detail for nickel only. For other metals studied in our laboratory, only the general conclusions will be presented here. 

From thermodynamic considerations it is evident that bulk nickel cannot be oxidized by CO2. However, it is not justified to conclude from this that dissociative chemisorption of CO2 will not occur. Consider, for example, the chemisorption of oxygen or hydrogen which on several metals takes place under conditions where bulk oxides or hydrides are not at all thermodynamically stable. Dissociative adsorption of CO2 has indeed been observed by Eischens and Pliskin (35). 

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