THE INTERPLAY BETWEEN SULFUR ADSORPTION AND CARBON DEPOSITION ON COBALT CATALYSTS

Sulfur is present as an impurity in various industrial feedstocks and is generally regarded as a catalyst poison; however, controlled poisoning of metals by sulfur can improve both reactivity and selectivity. We have attempted to develop an understanding of the interplay between sulfur adsorption and carbon deposition reactions by monitoring not only the gas-phase products but also the amount of filamentous carbon deposited from the interaction of sulfur-contaminated cobalt particles with ethylene/hydrogen mixtures. Pretreatment of cobalt in low levels (4-100 ppm) of H2S was found to dramatically increase the weight percentage of carbon filaments produced during the reaction, from 4.5 (without sulfur treatment) to 70%. In contrast, when the metal was pretreated at higher H2S levels (>500 ppm) or when the sulfide was added continuously in the hydrocarbon feed at concentrations of >60 ppm the catalytic activity was completely suppressed. Carbon filaments produced on uncontaminated cobalt particles were found to be highly graphitic in nature, and this degree of structural perfection decreased as sulfur was added to the system. In addition, it was apparent that sulfur could induce fragmentation of the powdered catalyst, a feature which may provide some new insights into the understanding of redispersion phenomena of sintered metal particles. © 1993 Academic Press, Inc.