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).

carbon nanotubes; cobalt; catalyst support; chemical vapor deposition; methane