Page 25 - Carbon Nanotubes

16 V. IVANOV al.
et
was mixed in an agath morter with HY zeolite. The ite sheets on the latter catalysts. The tubular filaments
product was pressed, crushed, dried overnight at on Fe- and Co-graphite sometimes possessed well-
403 K and calcined in air for 1 hour at 793 K, then crystalline graphite layers. In the same growth batch
for 1 hour at 1073 K and after cooling for 30 minutes we also observed a large amount of non-hollow
in flowing nitrogen, the catalyst was reduced in a filaments with a structure similar to that observed
flow of 10% H2 in N2 for 3 hours at 673K. The on Cu and Ni catalysts.
concentration of COO was calculated in order to In general, encapsulated metal particles were
obtain 8 wt% of Co in the zeolite. observed . on all graphite-supported catalysts.
The list of studied catalysts and some characteris- According to Ref. [4] it can be the result of a rather
tics are given in Table 1. weak metal-graphite interaction. We mention the
The samples were examined before and after catal- existence of two types of encapsulated metal particles:
ysis by SEM (Philips XL 20) and HREM by both a those enclosed in filaments (Fig. 1) and those encap-
JEOL 200 CX operating at 200 kV and a JEOL 4000 sulated by graphite. It is interesting to note that
EX operating at 40OkV. The specimens for TEM graphite layers were parallel to the surface of the
were either directly glued on copper grids or dispersed encapsulated particles.
in acetone by ultrasound, then dropped on the holey As was found in Ref. [ 131, the method of catalytic
carbon grids. decomposition of acetylene on graphite-supported
'H-NMR studies were performed on a Bruker catalysts provides the formation of very long (50 pm)
MSL-400 spectrometer operating in the Fourier tubes. We also observed the formation of filaments
transform mode, using a static multinuclei probehead up to 60pm length on Fe- and Co-graphite. In all
operating at 400.13 MHz. A pulse length of 1 ps is cases these long tubules were rather thick. The thick-
used for the IH 90" flip angle and the repetition time ness varied from 40 to 100 nm. Note that the disper-
used (1 second) is longer than five times T,, ('H) of sion of metal particles varied in the same range. Some
the analyzed samples. metal aggregates of around 500 nm in diameter were
also found after the procedure of catalyst pretreat-
ment (Fig. 2). Only a very small amount of thin
3. RESULTS AND DISCUSSION
(20-40 nm diameter) tubules was observed.
3.1 Catalyst support The as-produced filaments were very strongly
The influence of the support on the mechanism of covered by amorphous carbon produced by thermal
filament formation was previously described [ 1-41. pyrolysis of acetylene. The amount of amorphous
The growth process was shown to be strongly depen- carbon varied with the reaction conditions. It
dent on the catalyst-support interaction. In the first increased with increasing reaction temperature and
stage of our studies we performed the acetylene with the percentage of acetylene in the reaction
decomposition reaction over graphite supported mixture. Even in optimal conditions not less than
metals. This procedure was reported in Ref. [ 131 as 50% of the carbon was deposited in the form of
promising to obtain a large amount of long nano- amorphous carbon in accordance with[ 131.
tubes. The reaction was carried out in the presence As it was established by Geus et aL[l8, 191 the
of either Cu, Ni, Fe or Co supported particles. All of decrease of the rate of carbon deposition is a positive
these metals showed a remarkable activity in filament factor for the growth of fibres on metal catalysts.
formation (Fig. 1). The structure of the filaments was SiO, is an inhibitor of carbon condensation as was
different on the various metals. We have observed shown in Ref. [20]. This support also provides possi-
the formation of hollow structures on the surface of bilities for the stabilization of metal dispersion. Co
Co and Fe catalysts. On Cu and Ni, carbon was and Fe, i.e. the metals that give the best results for
deposited in the form of irregular fibres. The detailed the tubular condensation of carbon on graphite
observation showed fragments of turbostratic graph- support, were introduced on the surface of silica gel

Table 1. Method of preparation and metal content of the catalysts
Metal particle
Poae 0 Method of Metal diameteP
Sample (A) preparation (wt%) (nm)
Co-graphite - Impregnation 0.5-10 1Cb100
F-phite - Impregnation 2.5 2 100
Ni-graphite - Impregnation 2.5 2 loo
-aphite - Impregnation 2.5 2 loo
Co-SiO, 90 Ion exchange precipitation 2.1 2-2Ob
CO-HY 7.5 Solid state ion exchange 8 1-50
Co-Si0,-1 40 Pore impregnation 2.5 10-100
Co-Si02-2 90 Pore impregnation 2.5 10-100
Fe-SiO, 90 Pore impregnation 2.5 10-100
"Measured by SEM and TEM.
%e distribution of the particles was also measured (Fig. 6).