Abstract
The nature of the nickel metal phase supported on a range of alkali metal cation-exchanged Y zeolite catalysts was characterized by X-ray diffraction line broadening. The sintering process is shown to occur via a crystallite migration mechanism. Nickel crystallite size was measured and correlated with such diverse factors as reduction time, reduction temperature, heating rate, sample precalcination, and ammonia adsorption. The nature of the alkali metal co-cation (Li+, Na+, K+, Rb+, or Cs+) is shown to strongly influence the dimensions of the supported metal, yielding the following order of decreasing crystallite size: NiLiY > NiNaY > NiKY > NiRbNaY > NiCsNaY. In the case of the CeNiNaY and CeNiKY samples, the introduction of an additional level of acidity, due to the presence of the highly polarizing Ce3+ ions, during thermal activation, serves to suppress crystallite growth. For comparative purposes, data on the sizes of nickel metal crystallites formed on the reduction of nickel-impregnated Y zeolites and silica and alumina supports are also presented; the nature of the alkali metal co-cation has a much greater influence on the dispersion of nickel supported on the impregnated zeolites. Under identical reduction conditions, the amorphous alumina and silica carriers exhibit the smallest supported crystallite sizes.
Published Version
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