Optimization of carbon nanotubes formation and the role of water vapor in catalytic decomposition of methane
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Date
2011
Authors
Kim Yang, Lee
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Abstract
Carbon nanotubes (CNT) were synthesized over CoOx-MoOxf AhOJ catalyst
via decomposition of methane in a horizontal quartz tube reactor system. In order to
have a systematic process study and process optimization, this research study was
carried out by using statistical design of experiments (DoE). The effects of the six
process variables (reaction temperature, reaction time, catalyst amount, metal loading,
methane flow rate and nitrogen flow rate) on the carbon yield and the lr/10 ratio of
as-produced CNT were investigated using Resolution III fractional factorial design
(FFD) coupled with response surface methodology (RSM), i.e. Box-Behnken design
(BBD). Reaction temperature, reaction time and metal loading were identified to be
the decisive process variables influencing the carbon yield and the intensity _radio of
D~peak and G-peak in Raman spectra (Jolla ratio), as distinguished using a 2fj/3 FFD;
then BBD was exploited to construct a response surface from the decisive process
variables. The optimum parameter set was found at a reaction temperature of 762°C,
2.3 h reaction time and metal loading of 27%, with the carbon yield and Idla of 350%
and 0.595, respectively. The effect of water vapor on catalytic activity and catalyst
lifetime was investigated afterwards, based on analysis of the effluent gas. The
introduction of an appropriate amount of water vapor, i.e. 133.3 ppm into the
reaction ambient enhanced and sustained the catalytic activity by etching the
amorphous carbon coated on active sites surface, led to higher methane conversion
and carbon yield as well. The catalytic activity was sustained after 2 h reaction and
the carbon yield increased to 1076%. Furthermore, it was found that the CNT
exhibited better structure and morphology without the adherence of amorphous
carbon, as observed from HRTEM image. However, the CNT growth is suppressed
when the water vapor was over supplied, due to the water induced oxidation of active
sites. A continuous supply of controlled amount water vapor is preferable in order to
produce CNT with higher crystallinity. This research work is ended with the
demonstration of natural gas decomposition over the said catalyst at optimized
conditions for the production of CNT. The catalyst lifetime was found longer
compared to· decomposition of methane, with the formation of filamentous carbon,
i.e. a mixture of CNT and carbon nanofibers (CNFs). The findings suggest an
alternative cost effectively route for co-producing CNT, CNFs and hydrogen without
using high purity carbon source.
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Keywords
Carbon nanotubes , Catalytic decomposition