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Temperature ( C) Unselective catalyst la n ro w g

Figure 26. Resonant Raman spectra obtained for (a) raw HipCO material at room temperature and (b) purified HipCO material before and after subsequent in-situ oxidation steps at 400, 500, and 630 °C. The laser excitation wavelength was 514 nm.

Figure 27. Temperature programmed oxidation profiles of the carbonaceous species present in (a) a SWNT product obtained over a highly selective Co-Mo catalyst and (b) a low quality product obtained over an unselective Co-W catalyst. The profiles were obtained using a mixture of 5% O2 in He at a heating rate of 12 °C/min.

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Figure 26. Resonant Raman spectra obtained for (a) raw HipCO material at room temperature and (b) purified HipCO material before and after subsequent in-situ oxidation steps at 400, 500, and 630 °C. The laser excitation wavelength was 514 nm.

below the temperature in which MWNT, graphite, and carbon fibers are oxidized, but above the temperature at which amorphous and chemically impure carbon species are oxidized [29]. However, an interesting difference is observed in this case on the position of the maximum between the TPO profile obtained for this material and that obtained on the HiPCO raw material (Fig. 25). Even though both profiles exhibit a single peak, the one corresponding to SWNTs obtained on the Co-Mo catalyst appears at more than 100 °C higher temperature than the HiPCO nanotubes. This difference in peak position can only be due to the catalytic effect of the residual metals since the types of carbon species in both samples are similar. The material obtained by the HiPCO process contains an amount of iron catalyst up to 30% in weight [157]. The sample obtained over the Co-Mo catalyst contains less than 5% of Co and Mo metal impurities [28] and, perhaps more importantly, contains a large excess of silica support that may separate the nano-

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Figure 27. Temperature programmed oxidation profiles of the carbonaceous species present in (a) a SWNT product obtained over a highly selective Co-Mo catalyst and (b) a low quality product obtained over an unselective Co-W catalyst. The profiles were obtained using a mixture of 5% O2 in He at a heating rate of 12 °C/min.

tubes from the metal. Consequently, the material obtained by the HiPCO process has a larger amount of metal in close contact with the SWNTs. This metal catalyzes the oxidation of SWNTs during the TPO process and lowers the temperature of the peak.

The corresponding analysis after in-situ oxidation steps at 450 and 500 ° C under the same conditions used to get the TPO is shown in Figure 28. The spectra show that the sample is indeed composed by high quality SWNTs. There also are some differences after the different TPO stages. A decrease in the contribution of the D band to the whole spectra is observed at higher oxidation temperatures. Before the oxidation at 450 °C some amorphous and chemically impure carbon species may be present. Therefore the first oxidation step eliminates the amorphous carbon material from the samples. This results in the decrease of the relative intensity of the D band in the Raman spectra. However, as shown in Figure 28, a subsequent oxidation step (500 °C) does not change the contribution of the D band to the spec tn

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wavenumber(cm-1)

Figure 28. Resonant Raman spectra obtained for the carbon deposits obtained over a highly selective Co-Mo catalyst. The spectra were acquired before and after subsequent in-situ oxidation steps at 450 and 550 °C. The laser excitation wavelength was 514 nm.

tra, since at this temperature all amorphous carbon has been removed.

A contrasting behavior is observed for the TPO profile of the material obtained over the unselective catalyst, as shown in Figure 27b. Similar to the TPO of the purified HiPCO material, three peaks are obtained. In-situ Raman spectroscopy was used to identify the different kinds of carbonaceous species responsible for each of the TPOs. In this particular case, the spectra were acquired at room temperature and then after subsequent in-situ oxidation steps

Figure 29. Resonant Raman spectra obtained for the carbon deposits obtained over an unselective Co-W catalyst. The spectra were acquired before and after subsequent in-situ oxidation steps. The laser excitation wavelength was 514 nm.

at 400, 500, and 600 °C under the same conditions used in the TPO experiments. Figure 29 shows the corresponding Raman spectra. It can be observed that the relative intensity of the D band is much stronger in this case than for any of the other samples. This clearly reveals the low SWNT selectivity this particular catalyst. An interesting variation is observed in the D/G ratio as a function of oxidation temperature. After the first oxidation step (400 °C) the D/G ratio clearly decreases due to the burning of amorphous carbon, similar to the previous case of the selective sample. However, as the oxidation temperature increases, the D/G ratio quickly increases. This increase is to a decrease in the amount of ordered SWNTs compared to the disordered MWNTs and graphite nanofibers that resist oxidation to higher temperatures [158].

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