| Abstract
| - The functions and structures of Mo/Ni/MgO catalysts in the synthesis of carbon nanotubes (CNTs) have beeninvestigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectronspectroscopy (XPS), and Raman spectroscopy. Thin 2−5-walled CNTs with high purities (over 90%) havebeen successfully synthesized by catalytic decomposition of CH4 over Mo/Ni/MgO catalysts at 1073 K. Ithas been found that the yield of CNTs as well as the outer diameter or thickness correlates well with thecontents of these three elements. The three components Mo, Ni, and MgO are all necessary to synthesize thethin CNTs at high yields since no catalytic activity was observed for CNT synthesis when one of thesecomponents was not present. The outer diameter of the CNTs increases from 4 to 13 nm and the thicknessof graphene layers also increases with increasing Mo content at a fixed Ni content, while the inner diameterstays at 2−3 nm regardless of their contents. Furthermore, the average outer diameter is in good agreementwith the average particle size of metal catalyst. That is, the thickness or the outer diameter can be controlledby selecting the composition of the Mo/Ni/MgO catalysts. XRD analyses have shown that Mo and Ni forma Mo−Ni alloy before CNT synthesis, while the Mo−Ni alloy phase is separated into Mo carbide and Ni.These alloy particles are supported on MgO cubic particles 15−20 nm in width. It has been found that onlysmall Mo−Ni alloy particles 2−16 nm in size catalyze CNT synthesis, with larger particles over 15 nmexhibiting no activity. Mo carbide and Ni should play different roles in the synthesis of the thin CNTs, inwhich Ni is responsible for the dissociation of CH4 into carbon and Mo2C works as a carbon reservoir.
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