Page 26 - Carbon Nanotubes

Catalytic production and purification of nanotubules 17

Fig. 1. Carbon filaments grown after acetylene decomposition at 973 K for 5 hours on
(a) Co(2.5%)-graphite; (b) Fe-graphite; (c) Ni-graphite; (d) Cu-graphite.

by different methods. Both metals showed very similar models proposed earlier[4,18,22]. The metal outside
catalytic behaviour. Carbon was deposited on these of the support is saturated by the carbon produced
catalysts mostly in the form of filaments. TEM images by hydrocarbon decomposition, possibly in the form
of the tubules obtained on these catalysts are given of “active” carbides. The latter then decomposes on
in Fig. 3. Most of the filaments produced on silica- the surface of the metal, producing graphite layers.
supported catalysts were tubular, with well-resolved Such a situation is typical for catalysts with a weak
graphite layers. Nevertheless non-tubular filaments metal-support interaction, as in the case of graphite.
also grow in these conditions. We observed that the The zeolite support was used to create very finely
relative quantity of well-graphitized tubules was dispersed metal clusters. Metals can be localized in
higher on Co-silica than on Fe-silica catalyst. the solid-state exchanged zeolites in the small cages,
As in the case of graphite-supported catalysts, supercages or intercrystalline spaces. In fact, in accor-
some metal particles were also encapsulated by the dance with previously observed data [ 231, hydrogen-
deposited carbon (Fig. 4). However, the amount of ation of as-made catalysts led to the migration of
encapsulated metal was much less. Differences in the metal to the outer surface of the zeolite HY. The
nature of encapsulation were observed. Almost all sizes of metal crystallites varied in our catalyst from
encapsulated metal particles on silica-supported cata- 1 to 50 nm. We suppose that because of steric limita-
lysts were found inside the tubules (Fig. 4(a)). The tions only the metal particles at the outer surface and
probable mechanism of this encapsulation was pre- in supercages could be available for filament growth.
cisely described elsewhere[ 213. We supposed that The hydrocarbon decomposition over Co-HY pro-
they were catalytic particles that became inactive vides the formation of different graphite-related struc-
after introduction into the tubules during the growth tures (it should be noted that only a small amount
process. On the other hand, the formation of graphite of amorphous carbon was observed). Similar to the
layers around the metal in the case of graphite- previous catalysts, nanotubules of various radius and
supported catalysts can be explained on the basis of metal particles encapsulated by graphite were found