THE EFFECT OF PREPARATION METHOD AND THE CHEMICAL COMPOSITION ON THE CRYSTAL SIZE OF MODIFIED Ni-Al TAKOVITE-LIKE MATERIALS

摘要

Ni-Al takovite-like systems with Ni~(2+)/Al~(3+) ratios of 0.5 (Ni-poor) and 2.8 (Ni-rich) have been synthesized and modified by introducing Cr~(3+) ions into the structure. Two procedures of precipitation at pH 8 from the aqueous solutions of the nitrate salts of the elements (coprecipitation of all the components and separate precipitation) were used to investigate the effects of the preparation method and the chemical composition on the crystal size of the obtained materials used as further methanation catalysts. The coprecipitation method leads to very badly organized Ni-poor materials, whereas the two-step separate precipitation enables the partial preservation of a takovite-like structure. The presence of Cr in all materials increases their specific surface area. The influence of Cr~(3+) ions on the crystallite size in the obtained samples is discussed. The introduction of Cr~(3+) ions in the Ni-Al takovite-like system decreases the crystallite size of the Ni-Cr-Al samples. This effect is more pronounced in the samples obtained by separate precipitation.