Nickel catalysts surface area

Vanadium also promotes dehydrogenation reactions, but less than nickel. Vanadium s contribution to hydrogen yield is 20% to 50% of nickel s contribution, but vanadium is a more severe poison. Unlike nickel, vanadium does not stay on the surface of the catalyst. Instead, it migrates to the inner (zeolite) part of the catalyst and destroys the zeolite crystal structure. Catalyst surface area and activity are permanently lost. 

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

The catalytic activity of a powder catalyst should be proportional to its surface area in case its entire surface is equally effectii-e for the catalytic reaction. On the other hand, this correlation does not apply to catalysts whose active sites are located at crystallographically exceptional positions such as edges and corners of its microcrystallies. Schwab and Rudoloph (99) studied as early as 1934 the Topochemistry in Heterogeneous Catalysis, and concluded that the active sites of several catalysts were located at crystal boundaries in their surfaces. The chief evidence represented by these workers is that in various catalytic reactions, e.g., in the hydrogenation of ethyl cinnamate over different specimens of powdered nickel catalysts, the rates are proportional to a power of the catalyst surface areas lying between 1.8 and 4.0. 

The relationship between nickel size, surface area, and catalyst activity has been accepted for many years in regard to commercial reforming catalysts, and... 

In 1923, Schmid [8, 9] improved fuel-cells electrode, and Scharf [10] developed the hydrogen diffusion electrode. A major advance in alkaline electrochemistry was achieved by Raney [11], who developed the weU-known Raney nickel (high surface-area Ni) catalyst. With this high surface-area electrode, it was possible to achieve a technically relevant performance with a simple catalyst material. 

The nickel catalyst surface is reduced after 4.5 h at a current density of 5 mAm. The electrodes are not expected to be reduced after a short activation time because the catalysts are still significantly covered by the PTFE film, observed with XPS and specific surface area measurements. Perforations in the PTFE film are sufficient to allow contact between the electrolyte and the nickel catalyst and therefore to reduce the nickel oxide. During the activation process, the perforations in the PTFE film are increased or a new contact between the electrolyte and the metal catalyst is formed. The contact area may be created by the hydrogen that is formed on the nickel surface and that separates the PTFE film from the catalyst surface. Therefore, the initial nickel signal of the activated electrode (observed by XPS) is increased. The specific surface area increases without the complete removal of the PTFE film. 

TABLE 3.9. Nickel Metal Surface Area of Catalysts Reduced in Hydrogen. 

The available surface area of the catalyst gready affects the rate of a hydrogenation reaction. The surface area is dependent on both the amount of catalyst used and the surface characteristics of the catalyst. Generally, a large surface area is desired to minimize the amount of catalyst needed. This can be accomphshed by using either a catalyst with a small particle size or one with a porous surface. Catalysts with a small particle size, however, can be difficult to recover from the material being reduced. Therefore, larger particle size catalyst with a porous surface is often preferred. A common example of such a catalyst is Raney nickel. 

Nickel. As a methanation catalyst, nickel is presently preeminent. It is relatively cheap, it is very active, and it is the most selective to methane of all the metals. Its main drawback is that it is easily poisoned by sulfur, a fault common to all the known active methanation catalysts. The nickel content of commercial nickel catalysts is 25-77 wt %. Nickel is dispersed on a high-surface-area, refractory support such as alumina or kieselguhr. Some supports inhibit the formation of carbon by Reaction 4. Chromia-supported nickel has been studied by Czechoslovakian and Russian investigators. 

The metal surface area at the inlet end of the catalyst bed in experiment HGR-12 was smaller than that at the outlet end this indicates that a decrease in nickel metal sites is part of the deactivation process. Sintering of the nickel is one possible mechanism, but carbon and carbide formation are suspected major causes. Loss of active Raney nickel sites could also conceivably result from diffusion of residual free aluminum from unleached catalyst and subsequent alloying with the free nickel to form an inactive material. 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

The advanced all-metallic catalysts are believed to be formed by bonding active copper-nickel alloys onto stainless steel wires. Under the scanning electron microscope, it appears that the surface area may be more than twenty times the geometric surface area (42) ... 

The nickel supported catalysts formed in this way have some specific features (144)- The catalysts containing about 3% of Ni are paramagnetic. When varying the nickel content from 0.1 to 20%, all the nickel the reduced catalyst (the exposed surface area of nickel was about 600 m2/g Ni) is oxidized by oxygen. The activity in benzene hydrogenation is very high and increases in proportional to the nickel content in the catalyst. 

Fig. 8. Spectra of CO adsorbed on nickel catalysts B, A0, Af, C, and D taken at 1 Torr CO pressure and room temperature. The extinction per square meter of Ni surface area is plotted along the ordinate. The specific metal surface used in the calculation of the extinction per square meter of Ni surface area was determined from 3VS for the catalysts B, A0, Af, and from d for the catalysts C and D. The length of the arrow is a measure of the number of B5 sites on the surface (see text).

The catalysts used in these experiments included those already employed in the infrared measurements in addition to some others. The results are presented in Tables VI and VII along with some older measurements on Raney-nickel and a nickel-on-kieselguhr catalyst. These older measurements are slightly less accurate because the cyclohexane content of the reaction product was determined by mass spectrometry. The surface area of catalyst E was not determined hence, its reaction rates per unit of surface area could not be calculated. 

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