Spherical Multi-Walled Carbon-Nanotube Architectures: Formation Mechanism and Catalytic Performance

The chemical-vapor-deposition (CVD) process is sufficiently well developed to allow the large-scale production of carbon nanotubes (CNTs). Metal nanoparticles (Fe, Co, Ni, etc.) are usually immobilized on a solid support and then decompose the gaseous hydrocarbon feedstocks into fragmented carbon units at elevated temperatures of 700-1100°C. Metal carbides are formed and then these dominate the growth of filamentous carbon as a structural template. The produced CNTs cannot stay firmly with an inert support and so inevitably detach away, especially during liquidphase purification or mechanical treatment. The biggest difficulties when handling the loose powder are filtration in the slurry-phase operations and the large pressure drops in fixed-bed reactors. These problems very much limit the application of CNTs on a large scale. The fabrication of carbon filaments on a structured support (e.g., activated carbon, carbon fibers, porous frameworks, graphite) has also suffered from discontinuities in the structure. The future use of CNTs has also been restricted by the technical requirements and the necessary high energy consumption to operate the intense and complicated CVD process. To avoid any mechanical post-treatment of powder and the risk of explosion during CVD synthesis, we have explored a simple method to produce monolithic CNTs directly as millimeter-scale spheres, featuring an integral continuity in the structure from microscopic to macroscopic scales. The whole process of CNTs growth has been conducted in a flow of ultrapure N2, involving a set of elementary steps in the solid phase being less complex than the widely used CVD process. The growth of CNTs is based on a solid reaction mechanism and well-graphitized nanotubes can be obtained even at a temperature as low as 400C. Direct observation of all growth events is made by using in-situ transmission electron microscopy (TEM), X-ray diffraction (XRD), IR, Raman spectroscopy and thermogravimetric analysis. A commercial styrene-divinylbenzene copolymer resin was used as the carbon precursor. According to Scheme 1, the ion-exchanged resin spheres were calcined at 800C in a flow of N2 or vacuum, after which the average diameter decreases from 0.77 to 0.49 mm, while the bulk density increases from 0.55 to 0.91 g ml. A novel macroscopic structure can be clearly seen, especially after the removal of the Fe nanoparticles and the amorphous carbon species by refluxing the spheres in concentrated HNO3. As seen in Fig. 1a, the twisted nanotubes construct the monolith while keeping a spherical shape. The cross-section views of the hand-broken spheres show no difference in the structure between the bulk and the surface, indicating a homogeneous distribution of the nanotubes in the millimeter-scale spheres (Fig. 1b). The nitrogen adsorption/ desorption isotherm shows a hysteresis loop to identify a typical mesoporous feature. The measured BET surface area and the pore volume are 223.7 m g and 0.42 ml g, respectively. The distribution of the pores ranges between about 2 and 15 nm.