One-step preparation of highly dispersed metal-supported catalysts by fluidized-bed MOCVD for carbon nanotube synthesis

Abstract

A new technique of fluidized-bed metal–organic chemical vapour deposition (FB-MOCVD) is developed as a one-step method to prepare highly dispersed metal-supported catalysts for carbon nanotube synthesis. By using ultrafine powder of gamma-alumina (70 nm Sauter mean in size) as the support with Fe(CO)5 and Mo(CO)6 as the metal precursors, Fe/Al2O3, Mo/Al2O3 and Fe–Mo/Al2O3 catalysts have been prepared in an FB-MOCVD reactor. Compared with the conventional catalyst-preparation methods such as impregnation, ion exchange, co-precipitation and co-crystallization, the one-step FB-MOCVD technique is advantageous in many aspects. These include eliminating the solid–liquid separation and the subsequent operations of drying and high-temperature calcination/reduction, thus minimizing the aggregation or the crystalline size-growing problem for the supported metal particles caused by these operations. The metal-supported catalysts obtained by FB-MOCVD are characterized with various techniques including ICP-AES, SEM-EDX, XRD and nitrogen isothermal adsorption. Some catalysts are selected and used for carbon nanotube synthesis by CVD from acetylene (C2H2) in a fluidized bed at 650 or 850 °C. The formation of the entangled multi-walled carbon nanotubes (MWNTs), around 50 nm in outer diameter and 10 nm in inner diameter, and several to tens of microns in length, has been confirmed by the TEM and SEM analyses. High CNT selectivity () with the carbon yield ranging widely from about 10% to over 60%, depending on the type of catalyst used and the CNT deposition temperature, has been demonstrated with TGA tests.