Synthesis and Characterization of Highly Ordered Co−MCM-41 for Production of Aligned Single Walled Carbon Nanotubes (SWNT)

Highly ordered cobalt substituted MCM-41 samples were synthesized and characterized for application as catalytic templates for producing aligned single walled carbon nanotubes (SWNT). Highly reproducible Co−MCM-41 samples were successfully synthesized using alkyl templates with 10, 12, 14, 16, and 18 carbon chain lengths by direct incorporation of cobalt into the siliceous MCM-41 framework using a hydrothermal method; the pore size and the pore volume can be controlled precisely. The local environment of cobalt as determined by UV−vis spectroscopy is a mixture of tetrahedral and distorted tetrahedral structures similar to those observed in Co3O4. Cobalt atoms are uniformly distributed in the pores (about 30−40/pore) at nearly atomic dispersion probed by XAFS. Incorporation of cobalt into siliceous MCM-41 improves the structure, most likely by dehydroxylation and/or knitting the defective structure of the amorphous silica polymer. The optimum crystallization temperature and time were 100 °C and 4 days for siliceous MCM-41 and 6 days for Co−MCM-41, respectively. Co−MCM-41 is very stable against reducing and oxidation conditions at temperatures under 750 °C. The catalytic templates showed over 90% selectivity to SWNT with up to 4 wt % carbon yield. The growth of SWNT in the pores of Co−MCM-41 was confirmed by Raman spectroscopy and TEM. The catalytic template maintained its structure after successive reaction cycles, which suggests that Co−MCM-41 is a very stable template for producing SWNT under harsh reaction conditions.