a1 Department of Chemistry, Northeastern University, Boston, MA 02115
Extremely fine, tubular graphite nanofibers of varying geometries and degrees of crystallinity were produced by the decomposition of CO and hydrogen over various compositions of nickel–iron particles supported on silica. High-resolution transmission electron microscopy coupled with temperature programmed oxidation studies revealed that, as the iron content of the catalyst was increased, the bimetallic particles precipitated a chainlike graphitic fibrous structure in a stepwise mechanism. The high-iron-content system Fe–Ni (8:2) yielded a small amount of these chainlike graphite fibers that were extremely resilient to oxidation, suggesting a highly crystalline structure. When the catalyst particles consisted of a nickel–iron mixture, Fe–Ni (5:5), there was a decrease in the degree of crystallinity of the fibers (78% graphite) and a corresponding increase in the amount of amorphous carbon precipitated (22% amorphous) within the structure. The high-nickel catalyst Fe–Ni (2:8) generated the largest amount of the tubular nanofiber product. It was significant that there was an increase in the amorphous carbon content (58%) precipitated as opposed to graphitic carbon (42%) in the structures. In some cases, amorphous carbon was deposited inside the graphite core of the nanofibers. The influence of the catalyst composition and nature of the metal-support interaction are key factors in the continuing development of graphite nanofibers possessing desired structures for potential uses in a variety of applications.
(Received October 02 1998)
(Accepted April 12 1999)