Multi-walled carbon nanotubes (MWCNTs) are prepared by chemical vapor deposition (CVD) in C2H6 + H2 mixture over 20þ/SiO2 catalysts. The effects, in terms of yield and selectivity of the growth process, of the changes in composition (H2 at 0-75%) and flow-rate (100-150 sccm) of gas-mixture, catalyst reduction-temperature (400 °C and 650 °C), temperature (750-850 °C) and duration (0.5-6 h) of the synthesis-reaction are investigated. Although yield to C deposits improves by increasing gas flow-rate, temperature or duration of the synthesis-reaction, and by diminishing H2 volume fraction in growth atmosphere, it never exceeds 90% due to the heavy catalyst encapsulation. Moreover, the results of morphological and structural analyses show that a loss of selectivity towards nanotubes and a deterioration of CNT crystalline quality generally accompany the yield enhancement. This suggests the unsuitability of the ethane decomposition over silica-supported Fe-catalysts to the large-scale production of CNTs. Nevertheless, the nanotubes attained might be profitably used in all applications exploiting the magnetic properties originating from the encapsulated iron.
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