SYNERGISM OF Co and Mo IN THE CATALYTIC PRODUCTION OF SINGLE-WALL CARBON NAOTUBES BY DECOMPOSITION OF C0
Dr. Daniel Resasco
School of Chemical Engineering and Materials Science
University of Oklahoma
Monday, May 14, 2001 3:00 pm
Ben Bandy Conference Center
Center for Applied Energy Research
The catalyst composition and operating conditions for the synthesis of single-walled carbon nanotubes (SWNT) from CO decomposition have been systematically varied in order to maximize the selectivity towards SWNT. A simple quantification method based on the standard Temperature Programmed Oxidation (TPO) technique has allowed us to determine the distribution of the different forms of carbonaceous deposits present on the catalysts after the CO decomposition reaction.
A synergistic effect between Co and Mo has been observed. When both metals are simultaneously present, particularly when Mo is in excess, the catalyst is very effective. However, when they are separated they are either inactive (Mo alone) or unselective (Co alone). To understand this synergistic effect, X-ray absorption (EXAFS/XANES), FTIR, and UV/VIS spectroscopies have been used to characterize the state of Co and Mo on the catalysts before and after the production of SWNT.
The characterization results indicate that, at the beginning of the reaction, Co is in the oxidic state (Co2+), but it is progressively reduced to the metallic form. During this process, a Co metal particle growth takes place. Simultaneously, Mo which is initially in the form of a well dispersed Mo4+ oxidic species is converted to the carbidic form (Mo2C). Co acts as the active species in the activation of CO, while the role of the Mo is dual. It helps stabilizing Co as a well-dispersed Co2+ and acts as a carbon sink to moderate the growth of carbon and to inhibit the formation of MWNT.
The yield and selectivity to SWNT are a strong function of the reaction temperature. If the temperature is much lower than 700oC, the selectivity decreases, while when the temperature is much higher, the catalyst deactivates more quickly. Similarly, the CO concentration in the gas phase is decisive in determining the selectivity. At low CO concentrations, only amorphous carbon is produced. The selectivity towards SWNT greatly increases at CO concentrations above 50 %.