Silica system, nickel

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... 

The results of the specific adsorption of nickel onto silica and goethite are in basic agreement with similar work reported for other metals (2, 18), The James and Healy model is capable of describing the interactions well. Similarly, when complexing ligands are added to nickel-silica systems the results can be adequately explained by a competition model in... 

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). 

Torr of CO, the doublet structure was not observed. This result was also confirmed by Harrod et al. (102) on rhodium films evaporated under ultrahigh vacuum conditions. The difference in these results obtained using different methods of sample preparation can be accounted for either in terms of crystallite size or differences in electronic environment at the metal surface. Blyholder (98) believes that crystallite size also accounts for the differences observed in the infrared spectra of CO adsorbed on the nickel-titania system compared with the nickel-silica and nickel-alumina systems (111) as outlined in Table VI (107). 

When silica is added to the nickel-cyanide system, the results are quite predictable as shown in Figure 16. At the lower cyanide concentration (lO M) there is enough cyanide to complex about 10% of the nickel present, so adsorption of NiOH onto silica and precipitation of Ni(OH)2(s) are affected only slightly. As would be expected, at higher cyanide concentrations the nickel is made completely soluble at all pH values studied. 

Gonzalez-Marcos, M.P., et al.. Effect of thermal treatments on surface chemical distribution and catalyst activity in Nickel on silica systems. J. Mol. Catal. A Chem., 120 pp. 185-196, 1997. 

Chitosan has been explored as a catalyst support for two different metals, palladium and nickel. In both cases, stable catalysts have been produced, which display good activity in chosen reactions. Generally, activity appears to be similar to existing silica-based analogues, which should allow more extensive replacement of silica systems with renewable supports. Initial attempts to form films of catalyst appear to be very promising, and further work is planned in this direction. 

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

In the industrial scale of hydrogenation of fats and oils, the most frequently used catalysts are Ni based. The 20-30% Ni is supported on silica. When partial hydrogenation is needed, the temperature applied is between 140 and 200 °C and the pressure between 4 and 10 bar. The total hydrogenation requires higher temperature and pressure (200 °C, 20 bar). Nickel is not a perfect catalyst due to its relative low activity and also due to the formation of Ni-soaps. Recently, a colloidal Pd catalyst was applied successfully in a two-phase system for this type of hydrogenation, at room temperature and atmospheric pressure. The complete conversion of multiunsaturated compounds could be achieved during 15-45 minutes. In dimethylformamide as the second phase solvent, 92% monoene yield with a 70/30 cis/trans ratio could be produced48. 

For the most highly developed processes, maf coal conversion can be as high as 90 to 95 % with a C4+ distillate yield of 60 to 75 wt % and a hydrogen consumption of 5 to 7 wt %. When an external catalyst is used, it is typically some combination of cobalt, nickel, and molybdenum on a solid acid support, such as silica alumina. In slurry hydrogenation processes, catalyst life is typically fairly short because of the large number of potential catalyst poisons present in the system. 

In conclusion, four catalyst systems, Pd-KOH/silica, Pd-CsOH/silica, Ni-KOH/silica, and Ni-CsOH/silica, have been investigated for the conversion of acetone to MIBK. Systems highly selective to MIBK have been obtained (Pd-CsOH/silica, 100 % at 473 K). Both the metal and the base affected the product distributions. The nickel catalysts were generally less selective, with MIBK being further hydrogenated to MIBC and isophorone becoming a major product at high temperatures. Over Ni-CsOH/silica, a selectivity of around 30% was... 

Shortly after the introduction of the bismuth molybdate catalysts, SOHIO developed and commercialized an even more selective catalyst, the uranium antimonate system (4). At about the same time, Distillers Company, Ltd. developed an oxidation catalyst which was a combination of tin and antimony oxides (5). These earlier catalyst systems have essentially been replaced on a commercial scale by multicomponent catalysts which were introduced in 1970 by SOHIO. As their name implies, these catalysts contain a number of elements, the most commonly reported being nickel, cobalt, iron, bismuth, molybdenum, potassium, manganese, and silica (6-8). 

However, even with this a priori knowledge, the actual state of the system still remains somewhat vague on many essential points. For instance, is the nickel present as separate crystals, or is there a film of metal on the silica walls How narrow are the pores An answer to these questions and to many more is necessary before we can hope to gain an insight into the phenomenon of catalysis in general and of the influence of the carrier system in particular. 

The reaction achieved considerable attention over the years, and various alterations have been reported. Behr also reported the combination of carbon dioxide, butadiene and ethylene oxide to give the hydroxyester of the acids depicted in Scheme 19. A nickel-catalyzed analogous system using triphenylphosphine or triisopropylphosphite takes a different route as cyclopentanecarboxylic acids are reported as the main product [126]. A palladium catalyst immobilized on a phosphine-decorated polystyrene polymer [127] or on silica also proved to be active [128]. 

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