Nickel high-area-supported

In this account of work in progress, we report that the kinetics of CH4 production over initially clean Ni(lOO) are in excellent agreement with previous data for polycrystalline nickel foil and high-area-supported nickel catalysts. Traces of surface impurities such as iron act as poisons, causing a marked lowering of the reaction rate. 

The values of Nch4 determined in the present work are plotted in Arrhenius form in Figure 3 the activation energy determined from the slope of this line is 24.6 kcal/mol. For comparison, the values of Nch4 measured for both polycrystalline nickel foil and high-area-supported nickel catalysts are also shown. The rates are all normalized to a 4 1 H2 C0 mixture at a total pressure of 120 Torr. Generally speaking, a... 

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... 

The dramatic increase in irreversible CO adsorption on presulfided supported nickel catalysts at moderate pressures (162) has significant, practical implications in regard to the use of CO chemisorption to measure nickel dispersion. For example, it is often desirable to determine nickel surface areas for catalysts used in a process where sulfur impurities are present in the reactants. Substantial differences in the measurements of nickel surface area by H2 or CO adsorption are possible depending upon the catalyst history and choice of adsorption conditions. In view of the ease with which catalysts may be poisoned by sulfur contaminants at extremely low concentrations in almost any catalytic process, and since large CO uptakes may be observed on supported Ni not necessarily representative of the unpoisoned nickel surface area, the use of CO adsorption to measure nickel surface areas is highly questionable under almost any circumstance. 

Raney nickel is an alternative to dispersing nickel on a support to obtain high surface area particles. It is made by treating of a Ni-Al alloy with a concentrated alkaline solution. Aluminium is selectively dissolved, forming soluble aluminates, and leaving porous nickel metal that retains, at least in part, the structure of the starting alloy with channels easily accessible to the reactants. 

Since catalytic oxidation is a surface reaction, an inexpensive support material is normally coated with the noble metal. The support material can be made of ceramic, such as alumina, silica-alumina, or of a metal, such as nickel-chromium. The support material is arranged in a matrix shape to provide high surface area, low pressure drop, uniform flow of the waste gas through the catalyst bed, and a structurally stable surface. Structures that provide these characteristics are pellets, honeycomb matrices, or mesh matrices. 

But with metals of higher oxidisability, in particular with aluminium, the atomic ratio percentage is much lower that the initial salt ratio. One could assume that aluminium is leached out by the alcoholate ion which results from the large excess of sodium naphthalene i.e the alcoholate ion would act in a way similar to that of the hydroxide ion in the preparation of Raney nickel. The very high value measured for the nickel surface area might support this assumption. 

Metal salt mixtures (NiXa, MXn) deposited on graphite can be reduced with naphthalene sodium into bimetaiiic supported cataiysts. The process aiiows one to prepare cataiysts with metals as highly reducible as aluminium at low temperature. It is important to notice that the nickel surface area of supported Ni-M catalysts is always larger ( up to seven times ) than that of the unsupported ones prepared by the same procedure. 

R. G. Nuzzo, L. H. Dubois, N. E. Bowles, and M. A. Trecoske, 1984, Derivatized, high surface area, supported nickel catalysts, J. Catal., 85,267-271... 

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 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. 

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