Abstract
The adsorption of various gases (H 2, H 2S, alcanes, alcenes and alcynes containing up to five carbons, benzene, cyclohexane, cyclopentane, and some mixtures) on Ni-SiO 2 catalysts has been studied by measuring variations of saturation magnetization. The adsorption has been performed at −78 °C and the system heated in steps. When coverage is small intermediate adsorbed compounds thermally stable have been detected. The observed bond number is even (2, 4, 6, 8) and it corresponds to strongly dehydrogenated states for alcanes, alcenes, cyclanes and benzene, and to an associative model for acetylene and propyne. Complete cracking occurs at relatively low temperatures (50 °C to 200 °C) with formation of Ni 3C and NiH: the greater the saturation of the hydrocarbon, the smaller its carbon number, and the lower is its cracking temperature. Sulfur of H 2S is rapidly and irreversibly dissolved into the bulk of nickel. These results which are not always in good agreement with those obtained by low field techniques, strongly supports the following view: nickel atoms, on which a molecule is adsorbed cease to participate to the collective magnetism.
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