Nickel silica

As in the nickel/silica case, the rate of hydrogen reduction of nickel oxide... 

On the whole, with impregnated catalysts, nickel/alumina is more difficult to reduce than nickel/silica, (with nickel/silica-alumina occupying an intermediate position). 

Fig. 2. (a) Magnetization-volume isotherms for the chemisorption of hydrogen and of benzene on kieselguhr-supported nickel at 150° C (16). (b) Average number of bonds formed by benzene adsorbed on nickel-silica as a function of temperature (17). From J. Amer. Chem. Soc. 79, 4637 (1957) 83, 1033 (1961). Copyright by the American Chemical Society. Reprinted by permission of copyright owner. 

Specific Activities of Various Nickel-Silicas for Benzene Hydrogenation (Ah) and Exchange (A )... 

Rh > Ir > Ni > Pd > Co > Ru > Fe A plot of the relation between the catalytic activity and the affinity of the metals for halide ion resulted in a volcano shape. The rate determining step of the reaction was discussed on the basis of this affinity and the reaction order with respect to methyl iodide. Methanol was first carbonylated to methyl acetate directly or via dimethyl ether, then carbonylated again to acetic anhydride and finally quickly hydrolyzed to acetic acid. Overall kinetics were explored to simulate variable product profiles based on the reaction network mentioned above. Carbon monoxide was adsorbed weakly and associatively on nickel-activated-carbon catalysts. Carbon monoxide was adsorbed on nickel-y-alumina or nickel-silica gel catalysts more strongly and, in part, dissociatively,... 

Alumina, iron, nickel, silica, sodium, and vanadium are examples of compounds which can be found in residual fuel ash. If the vanadium content of residual fuel is high, severe corrosion of turbine blades can occur and exhaust system deposit formation can be enhanced. Vanadium-enhanced corrosion can occur at temperatures above 1200°F (648.9°C). 

Breeder etal. (Ill) have carried out S.P. and magnetization experiments to distinguish between ionic and covalent bonding for the adsorption of H2 and O2 on Ni. Nickel contains 9.4 electrons, 0.6 hole, and 0.6 unpaired electron spin per atom in the d band, the latter being responsible for the magnetic properties of the metal. The S.P. measurements were made on an evaporated Ni film and the magnetization studies on a nickel-silica catalyst, the properties of which were regarded as strictly comparable with the metal film. 

Table I. Isomerization of Saturated Hydrocarbons in the Presence of Nickel-Silica-...

From the changes in magnetic susceptibility of nickel—silica catalysts during ethylene adsorption at room temperature, Selwood [55] has concluded that ethylene exists both as an associatively and a dissociatively adsorbed species. On increasing the temperature, the dissociative adsorption becomes more important. Thus at 100° C, the susceptibility changes are consistent with the formation of six bonds to the surface for each adsorbed ethylene molecule, suggesting the following process... 

The spectra observed for ethylene adsorbed on nickel—silica [75,78] show similar features to those found with platinum—silica. The major difference is the temperature range over which the various bands are observed with nickel—silica, the surface n-butyl groups are present at room... 

Adsorption of hex-l-ene, a mixture of cis- and frans-hex-2-ene, and c/s-hex-3-ene on nickel—silica results in identical infrared spectra [83]. Addition of hydrogen results in an intensification of the spectrum suggesting that the initial spectrum results from dissociatively adsorbed species, a conclusion substantiated by the observation that the gas in equilibrium with the surface during the initial adsorption contains isomerised hexenes. Evacuation of the hydrogen causes a decrease in intensity and the reappearance of the initial spectrum. 

Indications of the mechanism of isomerization of saturated hydrocarbons were obtained by Ciapetta (C3), who observed that olefins were isomerized over nickel-silica-alumina catalyst at appreciably lower temperatures than were the corresponding saturated hydrocarbons, suggesting that olefins were intermediates in the reaction. Ciapetta also suggested that the rearrangement of the carbon skeleton took place via a carbonium... 

In the above diagram, H-D refers to hydrogenation-dehydrogenation centers and A to acidic centers on the catalyst. The reaction sequence involves successive ring contraction and expansion steps, similar to the mechanism proposed by Pines and Shaw (P4) to account for transfer of tagged carbon from the side chain to the ring when ethylcyclohexane was contacted with a nickel-silica-alumina catalyst. 

II. Texture and Structure of Nickel-Silica Catalysts 1. Introduction... 

The first question in the study of the properties of binary catalysts such as the nickel-silica system concerns its texture. A priori, we can hazard a reasonable guess as to the building system that we are going to encounter. It is well known that silica forms a porous system and one may therefore... 

Nicolaides, C. P., Scurrell, M. S. and Semano, P. M. Nickel silica-alumina catalysts for ethene oligomerization - control of the selectivity to 1-alkene products. Appl. Catal., A, 2003, 245, 43-53. 

It is also clear that double-bond-shift is relatively facile on the nickel-silica-alumina catalysts. Double-bond shift may occur on the nickel cation centers or on the silica-alumina support or on both14. The hexene products formed from ethene are as expected for a reaction sequence involving (1) dimerization of ethene to but-l-ene etc, (2) double-bond-shift of but-l-ene to a but-2-ene mixture, and (3) reaction of but-2-ene with a further ethene... 

P4-22g Alkylated cyclohexanols are important intermediates in the fragrance and perfume industry [Jnd. Eng. Chem. Res., 28, 693 (1989)]. Recent work has focused on gas-phase catalyzed hydrogenation of o-cresol to 2-methylcyclo-hexanone, which is then hydrogenated to 2-methylcyclohexanol, In this problem we focus on only the first step in the reaction (Figure P4-22). The reaction on a nickel-silica catalyst was found to be zero-order in o-cresol and first-order in hydrogen with a specific reaction rate at 170°C of 1.74 mol of o-cresol/(kg cat - min - atm). The reaction mixture enters the packed-bed reactor at a total pressure of 5 atm. The molar feed consists of 67% Hj and 33% o-creso at a total molar rate of 40 mol/min. 

Traher AD and Kittrell JR. Immobilization of catalase on nickel-silica alumina. Biotechnol. Bioeng. 1974 16 413M-17. 

Metal-support interaction was observed in two nickel-silica catalysts, but they were prepared either by precipitating a complex carbonate from nickel nitrate solution containing Si02 as a slurry or co-precipitating the carbonate from a solution of nickel nitrate and sodium silicate. The similar spectra from the unreduced catalysts resembled in shape and binding energy a NiSi03 standard and were quite distinct from NiO. The interaction was... 

It was found originally by Swift and Bozik in an early study of supported bimetallic catalysts that the addition of tin to a nickel-silica catalyst greatly promoted the activity and gave a longer catalyst life for the dehydrogenation of cyclohexanol or cyclohexanone to phenol, especially with a... 

Purified diets analysed over many years have indicated that the following elements are found in our diet oxygen, carbon, nitrogen, calcium, magnesium, phosphorus, iron, sodium, potassium, chlorine, sulfur, iodine, zinc, copper, selenium, manganese, molybdenum, vanadium, fluorine, cobalt, chromium, nickel, silica, aluminium, tin, and possibly traces of arsenic, are essential or beneficial to human health. 

Direct evidence for the participation of any of the foregoing species in hydrogenation reactions is scant. Structure (A) was believed to be form-ed when ethylene is admitted to hydrogen-covered nickel-silica (8). Evidence for Structure (B) is provided by the observation that the surface potential of ethylene on nickel film is -f 0.83 volts (24) and not negative as would be expected for Structure (A) on electronegativity considerations. Until such time as experimental methods for the direct observation of adsorbed species under reaction conditions are perfected, we must discuss possible reaction mechanisms in the most general way. 

The reaction of 1-hexene with deuterium over nickel-silica at 105° yields hexanes distributed as shown in Table VI (49) no observations on isomerization or exchange were made. The hexanes become progressively more deuterated as the D /hexene ratio is raised. Isomerization of n-hexenes has been observed during their hydrogenation over Raney nickel (50). 

Transport may occur over the surface or through the atmosphere, with ihe latter most likely to involve molecular intermediates. This is true with platinum-alumina, for example, where oxidizing atmospheres during regeneration produce volatile PtO molecules. In nickel-silica catalysts exposed to carbon monoxide, nickel carbonyl serves the same purpose. For surface transport, atomic migration is favored, but depends on the substrate composition. 

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