Nickel catalysts coprecipitation

Supported and coprecipitated nickel catalyst represent an interesting alternative, from the economical point of view, to other selective hydrogenation catalysts such as Pt or Pd, of higher performance but also with a higher price. In the present study, a coprecipitated NiO/NiAl 04 catalyst has been selected to cany out the selective hydrogenation of acetylene to ethylene as a test reaction. One important characteristic of this process is the large amount of coke which may be generated [l]. 

Alumina-supported nickel catalysts are an excellent example for the advantages of and the problems associated with coprecipitation processes for the manufacture of catalysts. Such catalysts are accessible via several pathways, as impregnation, deposition/precip-itation, coprecipitation from alumina gels, and more conventional coprecipitation routes. Also, for coprecipitation, different routes are possible, the first examples originating from the 1920s [48]. Starting from the nitrate solutions of nickel and aluminum, there are at least three different routes ... 

Four supported nickel catalysts NifCr203, NijSi02, NifMgO and NilZr02 were investigated by Isotopic Transient Kinetics Method [2]. The catalyst samples were prepared by coprecipitation method. B.E.T. surface area and metal dispersion were measured by krypton adsorption and hydrogen chemisorption, respectively [3, 4]. The main measured values of the metallic surface area and the total surface area of the catalysts are reported in Table 1. 

Chen, Y., Xu, H., Wang, Y. and Xiong, G. (2006) Application of Coprecipitated Nickel Catalyst to Steam Reforming of Higher Hydrocarbons in Membrane Reactor. Chinese Journal of Catalysis, 27, 772-777. 

With nickel/alumina catalysts (cf. 4 ) preparation by coprecipitation or by the decomposition of a high dispersion of nickel hydroxide on fresh alumina hydrogel, yields nickel aluminate exclusively. On the other hand, when, as in impregnation, larger particles of nickel compound are deposited, the calcination product is a mixture of nickel oxide and nickel aluminate. The proportion of nickel oxide increases when occlusion of the impregnation solution leads to a very nonuniform distribution (49). 

Garcia, L., Sanchez, J.L., Salvador, M.L., Bilbao, R., and Arauzo J. (1996). Hydrogen-rich Gas from Steam Gasification of Biomass Using Coprecipitated Nickel-alumina Catalysts, Bioenergy 96. The Seventh National Bioenergy Conference, pp. 859-865. 

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. 

The NiO/NiAl30< catalyst was prepared in our laboratory by coprecipitation of nickel nitrate hexahy drate and aluminium nitrate nonahydr te following a method described elsewhere [3]. The samples were calcined in an oxidant atmosphere at 1023 K for a period of time of 3 hours. 

If catalysts are prepared by coprecipitation, the relative solubilities of the precipitates and the possibility for the formation of defined mixed phases are essential. If one of the components is much more soluble than the other, there is a possibility that sequential precipitation occurs. This leads to concentration gradients in the product and less intimate mixing of the components. If this effect is not compensated by adsorption or occlusion of the more soluble component, the precipitation should be carried out at high supersaturation in order to exceed the solubility product for both components simultaneously. Precipitation of the less soluble product will proceed slightly faster, and the initially formed particles can act as nucleation sites for the more soluble precipitate which forms by heterogeneous precipitation. The problem is less crucial if both components form a defined, insoluble species. This is for instance the case for the coprecipitation of nickel and aluminum which can form defined compounds of the hydrotalcite type (see the extensive review by Cavani et al. [9] and the summary by Andrew [10]). 

The catalysts studied were prepared and purified by methods previously described1. Briefly, the silica-alumina support was prepared by a coprecipitation method on to the support and possessed a Si/Al ratio of 25. Ni2+ ions were introduced from an aqueous solution of nickel chloride by ion exchange at reflux. The final catalysts contained about 1.5 mass% Ni, with some residual sodium and chlorine, corresponding to a CI Ni mol ratio of <0.1, and a Ni Al mol ratio close to 1.0 (calculated with the assumption that all residual sodium is associated with some of the Al sites in the material (Na/Al mol ratio typically ca. 0.5)). 

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