R.T.K. Baker

The effect of copper on the structural characteristics of carbon filaments produced from iron catalyzed decomposition of ethylene

Abstract Carbon filaments were produced by the decomposition of ethylene over unsupported iron-copper powders in the presence of varying amounts of hydrogen at temperatures ranging from 500°C to 800°C. The structure and properties of the carbon filaments were found to be dependent on a number of parameters including the composition of the catalyst, the temperature and the percent of hydrogen in the reactant gas mixture. Temperature programmed oxidation in CO 2 of demineralized samples of carbon filaments indicated that the most stable structures were those produced from a bimetallic catalyst containing a large fraction of iron. The addition of hydrogen to the system also caused changes in the structure of the carbon filaments and this aspect was manifested by variations in the graphitic nature of the material. High resolution transmission electron microscopy examination indicated the presence of two distinct morphologies: one in which the filaments appeared to have smooth surfaces and where the graphite platelets were preferentially oriented at an angle with respect to the fiber axis; and a second type consisting of a tubular structure in which the graphite platelets were aligned in a direction parallel to the axis of the fiber. Surface areas of the carbon filaments, as determined by nitrogen adsorption at −196°C, were also found to be dependent on the catalyst composition, the reaction temperature and the fraction of hydrogen in the feed gas.

Carbon deposition on iron-nickel alloy particles

Abstract A combination of flow reactor studies and electron microscopy techniques have been used to investigate the manner by which the composition of iron-nickel alloy particles influence the growth characteristics of carbon deposits formed during the decomposition of ethane at temperatures over the range 815 to 865°C. Major differences in the selectivity patterns of alloys were evident with the amount of catalytically produced solid carbon being significantly higher on a Fe-Ni (5:5) powder than on a Fe-Ni (8:2) sample. Examination of the deposit revealed the existence of two types of structures, carbon nanofibers and a graphite shell-like material, both of which contained associated metal particles. The latter structures appeared to predominate at the higher temperature and were most abundant on the Fe-Ni (5:5) particles. A dramatic change in catalyst activity and selectivity was found when 50 ppm H 2 S was added to the ethane feed. Analysis of the gas phase product distribution showed that the behavior of the two alloy powders was almost identical. On the other hand, the yields of solid carbon were generally higher on the iron-rich sample and tended to consist of the shell-like form on both alloys when sulfur species were present in the reactant.

Influence of copper on the structural characteristics of carbon nanofibers produced from the cobalt-catalyzed decomposition of ethylene

We have used a combination of techniques to examine modifications in the structural characteristics of carbon nanofibers produced from the interaction of cobalt and copper-cobalt powders with ethylene at temperatures over the range 425 to 700{degree}C. The nanofibers generated from the interaction of cobalt with ethylene at 600{degree}C were found to be highly crystalline in nature. Incorporation of as little as 2{percent} copper into the cobalt created a major modification in the conformation of the solid carbon deposit, which was composed of multiple nanofiber limbs emanating from a single catalyst particle, and in this state the carbon structures tended to be disordered. As the composition of the bimetallic was progressively changed to the point where copper became the major component, there was a significant increase in the degree of crystalline perfection of the nanofibers even though they maintained their multidirectional form. The transformation in structural characteristics of the carbon nanofibers is rationalized, according to a concept wherein the crystalline order of the deposit is related to the wetting properties of the bimetallic particles with graphite. {copyright} {ital 1996 Materials Research Society.}