The kinetics of carbon nanotube (CNT) synthesis by decomposition of CH 4 over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH 4, as well as to the formation of Mo 2 C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis isproportional to the CH 4 pressure, indicating that the dissociation of CH 4 is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation rate of the graphene network is smaller than the carbon deposition rate by decomposition of CH 4 .

The functions and structures of Mo/Ni/MgO catalysts in the synthesis of carbon nanotubes (CNTs) have been investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Thin 2-5-walled CNTs with high purities (over 90%) have been successfully synthesized by catalytic decomposition of CH(4) over Mo/Ni/MgO catalysts at 1073 K. It has been found that the yield of CNTs as well as the outer diameter or thickness correlates well with the contents of these three elements. The three components Mo, Ni, and MgO are all necessary to synthesize the thin CNTs at high yields since no catalytic activity was observed for CNT synthesis when one of these components was not present. The outer diameter of the CNTs increases from 4 to 13 nm and the thickness of graphene layers also increases with increasing Mo content at a fixed Ni content, while the inner diameter stays at 2-3 nm regardless of their contents. Furthermore, the average outer diameter is in good agreement with the average particle size of metal catalyst. That is, the thickness or the outer diameter can be controlled by selecting the composition of the Mo/Ni/MgO catalysts. XRD analyses have shown that Mo and Ni form a 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 only small Mo-Ni alloy particles 2-16 nm in size catalyze CNT synthesis, with larger particles over 15 nm exhibiting no activity. Mo carbide and Ni should play different roles in the synthesis of the thin CNTs, in which Ni is responsible for the dissociation of CH(4) into carbon and Mo(2)C works as a carbon reservoir.

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