Preparation of carbon nanotubes from CO and in situ formed nano-sized Pd particles
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Carbon nanotubes, typically of 0.5-m length and 20-nm diameter, were prepared with good selectivity by disproportionation of CO in He over a Pd/La2O3 catalyst. The catalyst was formed in situ by reduction of a La4PdO7 precursor. The obtained nanotubes had the so-called fishbone structure with the graphitic planes inclined at an angle to the long axis of the nanotube. The effect of CO concentration was studied at 673 °C, and it was found that, among the concentrations studied, 20 vol% CO in the gas was suitable for nanotube growth. The obtained nanotube/La2O3/Pd composite contained typically only 3 wt% nanotubes after 8 h of CO treatment. A process for selective dissolution of La2O3 and Pd was applied, and a product containing 85 wt% nanotubes was obtained. The nanotubes were characterized by high-resolution, transmission, and scanning electron microscope studies, combined with element analyses using energy dispersive spectrometers, x-ray powder diffraction studies, and thermogravimetric analysis.
I. INTRODUCTION 1
Carbon nanotubes were first discovered by Iijima in 1991. They were prepared in an arc discharge apparatus similar to that used for fullerene synthesis, and they accumulated at the negative end of the carbon electrode. Iijima’s findings initiated much research on the synthesis and properties of these materials. Caron nanotubes are expected to have great potential for use in the field of nanoelectronics due to their both predicted2–4 and recently experimentally verified5,6 ability to be either metallic or semiconducting, depending on the helicity of the tube network and on its diameter. Furthermore, the predicted unusual mechanical properties of carbon nanotubes7 make them interesting as reinforcement materials. Kuzumaki et al. 8 have thus produced an Al/carbon nanotube composite and found that its room-temperature strength was just slightly lowered after heating at 873 K, as opposed to a pure Al reference specimen. Today, most carbon nanotubes are produced by one of the following three methods: (i) arc discharge, (ii) modified arc-discharge, and (iii) catalyzed decomposition/ disproportionation of a carbon-containing gas.9 In this article we will discuss the formation of carbon nanotubes by catalytic disproportionation of CO, which relies on the following reaction: 2CO(g) → CO2(g) + C(s) 1822
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J. Mater. Res., Vol. 15, No. 8, Aug 2000 Downloaded: 26 Mar 2015
Ni supported on oxides such as alumina–silica mixtures or magnesia10–13 is often used as catalyst, but other metals, e.g., Mo14 or Co,15 catalyze the reaction given above, yielding carbon nanotubes. In a number of previous articles,16–20 we have described the thermodynamic, structural, and catalytic properties of the Ln4PdO7-type oxides with Ln ⳱ La, Nd, Sm, Eu, or Gd and concluded that these materials all decompose upon reduction in gas mixtures containing carbon monoxide, whereby nano-sized particles of palladium, located on the surfaces of micron-sized grains of Ln2O3, are formed. The Ln4PdO7 materials were
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