Controlled growth of single-walled carbon nanotubes by catalytic decomposition of CH4 over Mo/Co/MgO catalysts

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Abstract

It was found that the addition of molybdenum to Co/MgO catalysts could remarkably increase the yield and also improve the quality of single-walled carbon nanotubes (SWNTs) from catalytic decomposition of methane. The generation rate of SWNTs was raised at least 10 times and the formation of amorphous carbon was suppressed. But there is an optimum content of Mo and Co, beyond which multi-walled carbon nanotubes (MWNTs) were formed. In other words, the relative amount of SWNTs and MWNTs could be controlled by the composition of catalysts. The obtained SWNTs showed a very high BET surface area. The promotion role of molybdenum was discussed.

Introduction

Since carbon nanotubes were discovered by Iijima in 1991 [1], many studies on their synthesis and application have been engaged worldwide. Carbon nanotubes possess unique electrical and mechanical properties [2]. For example, a single-walled carbon nanotube (SWNT) can be either metallic or a semiconductor depending on its helicity and diameter [3], [4]. It may bring a promising application in molecularly electronic devices. The highest Young's modulus and tensile strength among all known materials make carbon nanotubes ideal components for high strength composite materials [5], [6]. In addition, it has been reported that SWNTs showed an unusual hydrogen storage ability [7], [8].

In the synthesis of SWNT, three techniques have been used, including arc discharge between two graphite electrodes [9], laser ablation of graphite target [10] and catalytic decomposition of hydrocarbons over metal catalysts [11]. Although the former two methods can produce high quality SWNTs, the available quantity from both arc discharge and laser ablation is limited. The catalytic method may be the best choice for the large-scale production of SWNTs, in tens of gram or even kilogram scale for the future industrial application such as hydrogen storage. This method has been successful in the large-scale fabrication of multi-walled carbon nanotubes (MWNTs) [12]. However, it is more difficult to produce high quality SWNTs with ideal yield using the catalytic method. Dai et al. [13] first reported the synthesis of isolated SWNTs from CO decomposition on Mo/Al2O3 nanoparticles and Peigney et al. [14] produced a mixture of SWNTs and MWNTs by the decomposition of H2/CH4 on Fe/Al2O3 nanocomposites. SWNTs were also prepared by the catalytic pyrolysis of benzene [15] or ethylene [16], [17].

More recently, the synthesis of bulk amounts of high quality SWNTs has been reported from CH4 decomposition over Fe/Mo/SiO2–Al2O3 hybrid [18], aerogel Al2O3-supported Fe/Mo [19] catalyst, MxMg1−xO (M=Co, Fe) catalysts [20], [21], and CO decomposition over Co/Mo/SiO2 catalyst [22]. As revealed in these studies, the quality and yield of SWNTs are very sensitive to catalyst supports, metal loading and reaction conditions. Among various supports, MgO possesses the advantage to be easily removed. Especially as high as 6000 cm3/min flow rate of CH4 was used by Cassell et al. [18] although it was not emphasized whether it is a key factor for their synthesis of SWNTs. Bacsa et al. adopted a moderate total flow rate of H2/CH4 (250cm3/min), but the yield of SWNTs was low. Recently we have found that MoOx additive to Co/MgO catalyst can obviously increase the yield of SWNTs and improve the quality of SWNTs, with conditions for catalyst reduction and CH4 decomposition similar to those taken by Flahaut et al. [23]. In order to achieve the highest yield of SWNTs, in the present Letter the effect of cobalt and molybdenum contents on the yield and quality of SWNTs grown has been investigated in detail by TEM, TGA and Raman Spectroscopy. The promotion role of molybdenum has also been explored.

Section snippets

Experimental

The catalysts were prepared by a wet mechanical mixing and combustion synthesis [24], with citric acid in place of urea as foaming and combustion additive. The weighted amounts of compounds (Mg(NO3)2·6H2O, Co(NO3)2·6H2O and (NH4)6Mo7O24·H2O) in the molar ratio as in MoxCoyMg1−xyO were mixed together. Some citric acid and several drops of distilled water whose amount depended on the weight of the mixture were added to it [25]. The mixture was ground in a bowl until it was uniform. It was

Results and discussion

Fig. 1 shows TGA curves of the as-prepared materials (without HNO3 purification), synthesized on several catalysts without (Fig. 1a) or with different amounts of Mo doping (Figs. 1b–d). The weight loss is due to the combustion of carbon in O2, and therefore corresponds to the carbon content in the samples. The carbon contents obviously increase with the increase of molybdenum loading. The yields of carbon materials, calculated from TGA curves as described in the experimental section, are given

Conclusion

The addition of molybdenum to Co/MgO could largely increase the yield of SWNTs (at least 10 times). But there was a limit to the amount of molybdenum, beyond which MWNTs would be formed. 114.3 wt% yield of SWNTs with good quality was achieved over Mo0.025Co0.05Mg0.925O catalyst and as high as 248.9 wt% yield with some MWNTs over Mo0.045Co0.09Mg0.865O. The amount of SWNTs and MWNTs may be controlled by the change of the cobalt and molybdenum loading. The BET surface of obtained SWNTs is very

Acknowledgements

This work is supported by the National Science and Technology Board of Singapore (GR6773).

References (27)

  • T. Guo et al.

    Chem. Phys. Lett.

    (1995)
  • H. Dai et al.

    Chem. Phys. Lett.

    (1996)
  • E. Flahaut

    Chem. Phys. Lett.

    (1999)
  • M. Su et al.

    Chem. Phys. Lett.

    (2000)
  • J.-F. Colomer et al.

    Chem. Phys. Lett.

    (2000)
  • R.R. Bacsa et al.

    Chem. Phys. Lett.

    (2000)
  • B. Kitiyanan et al.

    Chem. Phys. Lett.

    (2000)
  • L. Alvarez et al.

    Chem. Phys. Lett.

    (2000)
  • S. Iijima

    Nature

    (1991)
  • B.I. Yakobson et al.

    Am. Sci.

    (1997)
  • N. Hamada et al.

    Phys. Rev. Lett.

    (1992)
  • J.W.G. Wildoer et al.

    Nature

    (1998)
  • M.M.J. Treacy et al.

    Nature

    (1996)
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