Nickel-alumina catalyst structure

Alumina is one of the most commonly used supports for nickel catalysts 111, 178,194-204). Ni/Al203 exhibits carbon deposition (180) that depends on the catalyst structure, composition, and preparation conditions. 

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

Data from Bartholomew et al. [23,27] for Ni/alumina catalysts (Table 2 and Figure 8) provide perspective regarding the role of alumina surface area, structure, and pretreatment on the thermal stability of nickel. From Figure 8 it is evident that at any given temperature the rate of sintering is greater for Ni supported on 5-alumina (SA = 98 m2/g) than for Ni on y-... 

The resulting metal membrane has a structure similar to the bulk pores of the anodic alumina but without the dense "skin layer and has straight through-pores. Nickel and platinum membranes having pore diameters in the 15 to 200 nm range can be produced this way. One of the critical steps of this method is depositing palladium catalyst on the surface, but not inside the pores, of the anodic alumina. The catalyst is used to facilitate the deposition of metal from the bottom to the surface of the cylindrical pores. 

The main product in hydrosilation of a,P-unsaturated ketones and aldehydes catalyzed by chloro-platinic acid, platinum on alumina, or metallic nickel is the corresponding silyl enol ether. With nickel catalyst, product distribution is highly dependent on the enone structure. Hydridosilanes add to a, -unsaturated esters, producing the corresponding silyl enolate as well as carbon silylated products. The course of addition depends on substrate structure and the hydridosilane utilized. Thus, triethylsilane undergoes 1,4-addition to methyl acrylate in the presence of chloroplatinic acid, while trichlorosilane with either chloroplatinic acid or Pt/C gives the -silyl ester (Scheme 65). ... 

R. Mann, A. Al-Lamy, and A. Holt, Visualized porosimetry for pore structure characterization of a Nickel/Alumina Reforming Catalyst, Trans, I, Chem, E. 73(A) 47 (1995). 

The ANOF technique proved to be a promising method for obtaining eggshell catalysts with a very good mechanical and chemical resistance. By appropriate choice of the metallic substrate, electrolyte composition and anodization conditions, catalysts with tailor-made pore structure, pore density, pore length, and compositions can be controlled. The nickel catalysts supported on alumina, magnesia or titania were found to be efficient for the selective oxydehydrogenation of cyclohexane to cyclohexene. 

The nickel dispersion of the catalyst on alumina support was less than that on silica support. This may be due to the strong interaction between nickel and alumina and undeveloped support pore structure than that of silica support. However, high catalytic activity and resistance to carbon deposition were obtained on the nickel catalyst supported on alumina. This indicated that metal dispersion was not the decisive factor that influenced the catalyst performance. Actually, the catalytic performance of the catalysts were integrative effect of nickel loading, metal dispersion, support, promoter, preparation and activation. 

Lippens, B. C., Fransen, P., van Ommen, J. G., Wijingaarden, R., Bosch, H., and Ross, J. R. H. 1985. The preparation and properties of lanthanum-promoted nickel-alumina catalysts structure of the precipitates. Solid State Ionics 16 275-82. 

Find et al. [25] developed a nickel-based catalyst for methane steam reforming. As material for the microstructured plates, AluchromY steel, which is an FeCrAl alloy, was applied. This alloy forms a thin layer of alumina on its surface, which is less than 1 tm thick. This layer was used as an adhesion interface for the catalyst, a method which is also used in automotive exhaust systems based on metallic monoliths. Its formation was achieved by thermal treatment of microstructured plates for 4h at 1000 °C. The catalyst itself was based on a nickel spinel (NiAl204), which stabUizes the catalyst structure. The sol-gel technique was then used to coat the plates with the catalyst slurry. Good catalyst adhesion was proven by mechanical stress and thermal shock tests. Catalyst testing was performed in packed beds at a S/C ratio of 3 and reaction temperatures between 527 and 750 °C. The feed was composed of 12.5 vol.% methane and 37.5 vol.% steam balance argon. At a reaction temperature of 700°C and 32 h space velocity, conversion dose to the thermodynamic equilibrium could be achieved. During 96 h of operation the catalyst showed no detectable deactivation, which was not the case for a commercial nickel catalyst serving as a base for comparison. 

Methane or natural gas steam reforming performed on an industrial scale over nickel catalysts is described above. Nickel catalysts are also used in large scale productions for the partial oxidation and autothermal reforming of natural gas [216]. They contain between 7 and 80 wt.% nickel on various carriers such as a-alumina, magnesia, zirconia and spinels. Calcium aluminate, 10-13 wt.%, frequently serves as a binder and a combination of up to 7 wt.% potassium and up to 16 wt.% silica is added to suppress coke formation, which is a major issue for nickel catalysts under conditions of partial oxidation [216]. Novel formulations contain 10 wt.% nickel and 5 wt.% sulfur on an alumina carrier [217]. The reaction is usually performed at temperatures exceeding 700 °C. Perovskite catalysts based upon nickel and lanthanide allow high nickel dispersion, which reduces coke formation. In addition, the perovskite structure is temperature resistant. 

Reaction of -picoline with a nickel-alumina catalyst has been reported to give a mixture of four isomeric dimethylbipyridines, one of which has been identified at 6,6 -dimethyl-2,2 -bipyridine. With palladium-on-carbon, 2,4-lutidine was found to be more reactive than pyridine,and the isolated biaryl has been assigned the structure (2). However, some confusion arises from the statement that this... 

Several products other than 2,2 -biaryls have been isolated following reaction of pyridines with metal catalysts. From the reaction of a-picoline with nickel-alumina, Willink and Wibaut isolated three dimethylbipyridines in addition to the 6,6 -dimethyl-2,2 -bipyridine but their structures have not been elucidated. From the reaction of quinaldine with palladium-on-carbon, Rapoport and his co-workers " obtained a by-product which they regarded as l,2-di(2-quinolyl)-ethane. From the reactions of pyridines and quinolines with degassed Raney nickel several different types of by-product have been identified. The structures and modes of formation of these compounds are of interest as they lead to a better insight into the processes occurring when pyridines interact with metal catalysts. 

It is thus likely that the carbon coating on the alumina support lessened the interaction between AI2O3 and P and inhibited the formation of nickel phosphate after calcination and also lowered the reduction temperature. Structural models of the supported Ni2P catalysts are shown in Figure 4. 

Nickel, on the other hand, on alumina and on silica supports was found to have only five nearby sulfurs (square pyramidal) with Ni-Mo coordination numbers from 1 to 1.5. Ni-Mo-S supported on carbon was observed to have Ni-S coordination numbers of 6 in a trigonal-prismatic configuration. In addition, Ni (at low Ni concentrations) was found to have one nearby Ni, which could indicate that, in some catalysts, Ni is present as pairs on the MoS2 surface. The overall structure of the Ni-Mo-S was believed to be similar to that of millerite (i.e., Ni is located in the center of the MoS edge in a square-pyramidal configuration, with one sulfur extending perpendicular to the surface) (62-64). 

Kruissink, E.C., Van Reijen, L.L. and Ross, J.R.H. (1981) Coprecipitated nickel—alumina catalysts for mefhanation at high temperature. Part 1. Chemical composition and structure of the precipitates. J. Chem. Soc., Faraday Trans. 1, 77, 649. 

Find et al. [42] developed a nickel-based catalyst for methane steam reforming. As material for the micro structured plates, AluchromY steel, which is an FeCrAlloy (see Section 2.10.7) was applied. This steel forms a thin layer of alumina on its surface, which is less than 1 pm thick. This layer was used as an adhesion interface for the catalyst. I ts formation was achieved by thermal treatment of micro structured plates for 4 h at 1 000 °C. 

Metallic monoliths made of both rhodium ([HCR 1]) and FeCrAlloy (72.6% Fe, 22% Cr and 4.8% Al ([HCR 3]) carrying micro channels of 120 pm x 130 pm cross-section at various length (5 and 20 mm) were applied. The monoliths were prepared of micro structured foils by electron beam welding. After bonding, the FeCrAlloy was oxidized in air at 1 000 °C for 4 h to form an a-alumina layer, which was verified by XRD. Its thickness was determined as < 10 pm by SEM/EDX. The alumina layer was impregnated with rhodium chloride and alternatively with a nickel salt solution. The catalyst loading with nickel (30 mg) was much higher than that with rhodium (1 mg) (see Table 2.4). The amount of rhodium on the catalyst surface was determined as 3% by XPS. 

This article is focused on HDN, the removal of nitrogen from compounds in oil fractions. Hydrodemetallization, the removal of nickel and vanadium, is not discussed, and HDS is discussed only as it is relevant to HDN. Section II is a discussion of HDN on sulfidic catalysts the emphasis is on the mechanisms of HDN and how nitrogen can be removed from specific molecules with the aid of sulfidic catalysts. Before the discussion of these mechanisms, Section II.A provides a brief description of the synthesis of the catalyst from the oxidic to the sulfidic form, followed by current ideas about the structure of the final, sulfidic catalyst and the catalytic sites. All this information is presented with the aim of improving our understanding of the catalytic mechanisms. Section II.B includes a discussion of HDN mechanisms on sulfidic catalysts to explain the reactions that take place in today s industrial HDN processes. Section II.C is a review of the role of phosphate and fluorine additives and current thinking about how they improve catalytic activity. Section II.D presents other possibilities for increasing the activity of the catalyst, such as by means of other transition-metal sulfides and the use of supports other than alumina. 

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