Synthesis and characterisation of carbon nanofibres with macroscopic shaping formed by catalytic decomposition of C2H6/H2 over nickel catalyst

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Abstract

Carbon nanofibres composites with macroscopic shaping were successfully synthesised by chemical vapour decomposition (CVD) using a low loading in nickel as catalyst (≤1 wt.%). A high carbon nanofibres yield, i.e. 100 wt.% weight gain per hour of synthesis with respect to the initial catalyst weight was obtained, which significantly lowered the synthesis cost while the direct macroscopic shaping rendered more reliable the applications of these materials in conventional catalytic processes.

Introduction

Since their discovery as a by-product of the arc-discharge process, carbon nanotubes and their related materials, i.e. nanofibres or onion-like particles, have received an increasing academic and industrial interest due to their exceptional mechanical and electronic properties [1], [2], [3], [4], [5], [6]. Depending on the nature of the metal catalyst, the composition of the carburising mixture, and the synthesis temperature, carbon nanostructures with different shapes, i.e. nanotubes or nanofibres, can be prepared [7], [8], [9].

Among the different potential applications of these materials, catalysis either in the gas or in the liquid phase, seems to be the most promising according to the results recently reported in [10], [11], [12]. Metals supported on carbon nanofibres and nanotubes exhibit unusual catalytic activity and selectivity patterns when compared to those encountered with traditional catalyst supports such as alumina, silica or activated carbon. The extremely high external surface area displayed by these nanoscale materials significantly reduces the mass transfer limitations, especially in liquid phase reactions [13], [14], and the peculiar interaction between the deposited metallic phase and the exposed planes of the support which leads to the formation of active metallic faces [15], [16], [17], were advanced to explain these catalytic behaviours.

However, these materials have only been synthesised in a nanoscopic form, i.e. diameter <100 nm, making difficult their handling and large scale use, especially in a fixed-bed catalytic reactor. The handling of the carbon nanostructures is hampered by the formation of powder and a severe pressure drop along the catalyst bed. It is of interest to find a method allowing: (i) the synthesis of carbon nanostructures on a large scale; (ii) with a macroscopic shape in order to be used as catalyst support. It is expected that the macroscopic shaping of such nanostructured materials will open-up a real opportunity for their use as catalyst support in competition with the traditional catalyst carriers such as alumina, silica or carbon [18]. The macroscopic support should also not alter the physical properties of the carbon nanostructures deposited on it, i.e. high mechanical strength in order to avoid breaking and catalytic bed plugging, high specific volume in order to afford a high space velocity of the gaseous reactants, high thermal conductivity which is essential for catalysts operating in highly exothermic or endothermic reactions and finally, a high chemical resistance in order to be used in aggressive environments, i.e. highly acidic or basic medium.

Recently, the effort about the macrosizing of the carbon nanofibres has been reported by Teunissen [19] and by Jarrah et al. [20]. Teunissen has reported the synthesis of either carbon nanofibres in a sphere shape with millimeters size. On the other hand, Jarrah et al. have reported the synthesis of supported carbon nanofibres on a monolith structure which was previously covered with an alumina wash-coat for liquid phase reactions. However, the weight of carbon nanofibres deposited was relatively low, i.e. 0.0085–0.157 g g−1 monolith, and thus a large amount of the catalyst weight was belong to the inactive support.

The aim of the present article is to report the synthesis of carbon nanofibres, with uniform diameter of ca. 30 nm and length up to micrometers, on a large scale (50 g g−1 metal catalyst h−1) and at relatively low temperature, 700 °C, supported on a macroscopic host such as carbon materials with different sizes and shapes. The present method allows the synthesis of a composite with a large weight of carbon nanofibres per total support weight, i.e. 50%.

Section snippets

Material and catalyst

The macroscopic host structure used was graphite felt (Carbone Lorraine Co.) which was constituted of a dense entangled network of micrometer graphite filaments with a smooth surface (Fig. 1). The starting graphite felt had almost no porosity which was in good agreement with its extremely low specific surface area, i.e. <1 m2 g−1. This felt was cut into a pre-defined shape, i.e. disk with a diameter of 1 cm and thickness of 0.7 cm, before deposition of the nickel active phase.

Nickel was deposited

Nickel/graphite felt characteristics

The starting Ni/graphite felt microstructure, after calcination and reduction, was investigated by SEM and the micrograph is presented in Fig. 2. Because of the low surface area and the chemical inertness of the support the nickel particles were non-homogeneously dispersed on the surface with a wide particle size distribution ranging from 2 to 20 nm with some aggregates with diameters greater than 50 nm.

Carbon nanofibres yields

Supported nickel is well known in the literature to be an active catalyst for carbon nanofibre

Conclusion

Large amounts of carbon nanofibres (50 g g−1 Ni h−1) have been directly synthesised with a macroscopic shaping using a CVD technique and a nickel catalyst with extremely low loading, i.e. ≤1 wt.%. Such a nanomacrocomposite combines the peculiar and interesting properties of the nanostructured carbon material, i.e. high metal dispersion, high surface area and large void fraction with a 3D porous network, and the ease for handling and packaging of a macroscopic structure for subsequent uses in

References (31)

  • N. Krishnankutty et al.

    Catal. Today

    (1997)
  • F. Salman et al.

    Catal. Today

    (1999)
  • C. Pham-Huu et al.

    J. Mol. Chem. A: Chem.

    (2001)
  • N. Jarrah et al.

    Catal. Today

    (2003)
  • D. Ding et al.

    Carbon

    (2003)
  • J.M. Ting et al.

    Carbon

    (2003)
  • R.L. Vander Wal et al.

    Chem. Phys. Lett.

    (2001)
  • J.W. Snoeck et al.

    J. Catal.

    (1997)
  • E. Boellaard et al.

    J. Catal.

    (1985)
  • E. Tracz et al.

    Appl. Catal.

    (1990)
  • P. Chen et al.

    Carbon

    (1997)
  • M.J. Ledoux et al.

    J. Catal.

    (2003)
  • S. Iijima

    Nature (London)

    (1991)
  • M.S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.), Carbon Nanotubes: Synthesis, Structures, Properties and...
  • T.W. Ebbesen (Ed.), Carbon Nanotubes—Preparation and Properties, CRC Press, Boca Raton, FL,...
  • Cited by (0)

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