The catalysts used for the production of CNTs are usually transition metals supported on silica, mesoporous silica (MCM-41, MCM 48, SBA-15), calcium carbonate, zeolites or magnesium oxide. All types of catalysts have been studied with the aim of increasing the productivity of CNTs, for improving their properties and modulate their structures. In this work a CVD process was used for the synthesis of CNTs, using different silica-supported catalysts, like Fe, Ni, Co and Mo. The sol-gel method was used for the synthesis of supported metal catalysts.
Nanotubes thus grow through the vapor-liquid-solid (VLS) mechanism in CVD. In this respect, the role of the metal particles is not catalytic as in a normal chemical reaction. However, there are three stages in the nanotube formation process: (i) decomposition of hydrocarbon molecules, (ii) carbon incorporation into and precipitation out of the metal particle, and (iii) formation of a graphite structure. The VLS mechanism is related to step ii. The transition-metal particles act as a catalyst in step i. Step iii might also require the catalytic function of the transient metals.
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