Cobalt Catalyst Characterization for Methane Decomposition and Carbon Nanotube Growth

Several analytical techniques, such as N2 physisorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), temperature programmed reduction (TPR) and chemisorption were employed to characterize the structure of Co/γAl2O3 catalysts used for the production of carbon nanotubes by methane vapor deposition. The catalysts were studied after the calcination step and presented three main cobalt species, CoAl2O4, CoO and Co3O4. The CoAl2O4 species were well dispersed and were reduced only at high temperatures, rendering them inactive for the carbon nanotube production. In the case of the 1%Co/Al2O3 catalyst, the main cobalt species found was CoAl2O4. However, increasing the cobalt content in the catalysts led to a higher formation of Co3O4 as compared to CoO and CoAl2O4 species. The 2 and 3%Co/Al2O3 catalysts showed particle agglomeration during the pretreatment step that decreased selectivity towards nanotube production. The 4%Co/Al2O3 catalyst did not show particle agglomeration and presented a higher selectivity to carbon nanotube production, 71%, mainly multi-walled carbon nanotubes (MWNT).