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Figure 22. Temperature programmed oxidation of all the carbonaceous species formed over a CoMo/SiO2 catalyst after disproportionation of CO at 700 °C, compared to similar TPO profiles of a graphite reference and commercial SWNTs obtained from [email protected] Also included is the TPO of the blank solution, containing a surfactant, in which the commercial SWNTs are dispersed. All the references were physically mixed with bare CoMo/SiO2 catalyst. Reprinted with permission from [29], B. Kitiyanan et al., Chem. Phys. Lett. 317, 497 (2000). © 2000, Elsevier Science.

were compared to this TPO and their profiles are included in the figure. The first reference is a graphite powder physically mixed with the bare Co:Mo catalyst. The oxidation of this form of carbon occurred at very high temperatures, starting at about 700 °C, and completed after holding for 30 minutes at 800 °C. The second reference is a commercial sample of purified SWNTs, obtained from [email protected] (Rice University). This sample came in a liquid suspension of 5.9 grams/liter, containing a nonionic surfactant Triton X-100. To conduct the TPO experiment, this suspension was impregnated on the Co:Mo/SiO2 catalyst to get 0.6 wt% SWNTs on the sample. As shown in Figure 2, the TPO of this impregnated sample exhibited two peaks, a low-temperature one that corresponds to the oxidation of the surfactant and a second one centered at 510 °C, which corresponds exactly to the position ascribed to the oxidation of SWNTs. To corroborate that the first peak was indeed due to the oxidation of Triton, a sample was prepared with a blank solution containing only the surfactant in the same concentration. The TPO shows that indeed that was the case. The quantification of the amount of SWNT in the sample from the CO2 produced gave a value of 0.64 wt%, in good agreement with the amount of SWNT loaded in the sample (0.6 wt%).

TPO is a quick test for different catalyst formulations. For example, Figure 23 shows the clear synergistic effect exhibited by Co and Mo. In the first place, Mo alone does not produce carbon nanotubes and only exhibits a small low-temperature peak corresponding to amorphous carbon. The term "amorphous carbon" implies carbonaceous deposits, which are not in the form of ordered nanotubes or graphite. The claim that the Mo-alone sample only produced amorphous carbon was substantiated by the absence of graphite or nanotubes in the TEM observations. On the other hand, Co alone is not selective for the production of SWNTs and generates mainly graphitic carbon and MWNTs. Again, these observations were corroborated by TEM. By contrast, the combination of the two metals in appropriate ratio results in high selectivity for SWNTs. TPO has also been used to study the effect of varying the Co:Mo molar

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