Etude quantitative des équilibres réversibles de striation de l'argent dans les atmosphères faiblement sulfurantes

Abstract

Although it is well known that sulfur acts as a poison for many nickel catalyzed reactions, it is now being recognized that certain benefits can be derived from following a partial and well controlled sulfidation treatment of the metal. The current investigation was undertaken to ascertain the influence of sulfur on the formation of carbon filaments arising from the nickel catalyzed decomposition of ethylene/hydrogen mixtures. We have determined not only variations in the concentrations of the gaseous products, but also the amount and characteristics of the solid carbon deposit, as a function of several parameters. It is suggested that the observed enhancement in carbon filament growth following pre-treatment of nickel in low levels of H2S (4–50 ppm) is related to a reconstruction of the metal surface. The inhibition of catalytic activity resulting from a more severe pre-treatment in H2S is ascribed to the formation of a 2-D or 3-D bulk sulfide. Pre-adsorption of sulfur on nickel induced some major perturbations in the morphological characteristics of the filamentous carbon deposit, which are generated in the form of regularly coiled structures. In contrast, those produced from unadulterated metal particles were found to be relatively straight. This change in growth mode indicates that sulfur is modifying the diffusion characteristics of carbon species through the catalyst particle.