Z. Q. WANG, J. NAVARRETE 95
catalyst remains a long lifetime. The diameter of CNTs
has a close relation with the Ni particle size; large Ni
particles lead to formation of CNTs with big diameter
(Figure 6(b)). As a result, the inner diameter of the
CNTs is largely determined by the diameter of the Ni
particle. It is also observed that almost all of the CNTs,
no matter whether they have thin or thick tube walls, are
curved in certain degree (Figures 6(a) and (b)).
On the fresh Ni/MCM-41 catalysts, most of the Ni par-
ticles have a shape as pseudo-sphere (Figure 2). How-
ever, after the reaction, the Ni particle shows a shape
diamond-like with a tail inserting into the carbon nano-
tube (Figure 6(c)). It is known that the melting point of
metallic Ni is around 1452˚C, Ni particle may change its
shape only at a temperature above its Tamman point Tm
= 726˚C. Under the present reaction condition, the reac-
tion temperature is 550˚C. It seems impossible for Ni
particle to alter its shape. It is reported that on a spent
Ni/SiO2 catalyst, Ni carbide (Ni3C) is formed [14]. Ni3C
is unstable and it can be decomposed into nickel and
graphitic carbon at a relative low temperature, i.e., 400˚C.
It is also proven that carbon atoms produced from CH4
decomposition over the Ni based catalysts can form
NixCy solid solution as intermediate [15], at such condi-
tion, C atoms are able to move within the bulk of Ni par-
ticles and are then they are released from the NixCy solid
solution to form nanotubes at the interface of the Ni par-
ticle and support. Because methane decomposition is an
exothermic reaction (ΔH = 75 kJ/mol), the temperature
around Ni particle surface is, therefore, at somewhat
overheat state, with respect to its surrounding tempera-
ture. As a result, the Ni-C system may transfer into a
quasi-liquid state at a reaction temperature even lower
than the Tamman temperature of Ni; this provides the
possibility for Ni changing its shape. It is also noted that
a gradient of carbide concentration in the Ni particles
during the reaction leads to pressure built up at the inter-
face of Ni and support. When the graphitic layers are
initially formed in parallel to the Ni crystal surface, car-
bon nanotube growth along the Ni crystal surface may
drive the Ni particles to be squeezed out, changing Ni
shape from pseudo-sphere to diamond-like.
4. Conclusion
The present work confirms that simultaneous production
of hydrogen and CNTs can be realized in a single reac-
tion of methane catalytic decomposition by using Ni/
MCM-41 as catalyst. The Ni/MCM-41 catalysts exhibit
high catalytic stability during 500 min of reaction. The
formed CNTs have 20 - 50 nm in diameters and a few
micrometers in length, depending on the reaction condi-
tion. Large Ni particles usually favour the formation of
CNTs with big diameter. During the reaction, the shape
of Ni particles changes from pseudo-sphere to diamond-
like. All the CNTs consist of multiple layer walls and are
curved in certain degree. The CNTs formation may grow
with a tip-growth mode due to the weak interaction be-
tween Ni and MCM-41 support.
5. Acknowledgements
The authors would like to thank Mr. L. A. Moreno, Dr. J.
Alberto Andraca Adame, and Dr. C. Angeles for their tech-
nical assistance.
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