Temperature Programmed Oxidation

The detailed quantification of the different forms of carbon on a SWNT sample is a difficult task. Electron microscopy can only provide a qualitative description of the type of carbon species produced. Many times, it is hard to determine how representative of the overall production is a given micrograph. At the same time, as mentioned previously, the D/G band ratio in a Raman spectrum can be used as a semiquantitative indication of the presence of unde-sired (disordered) carbon species. However, Raman spec-troscopy does not provide a precise quantification of the relative amounts of each carbon species present in the sample. In an attempt to make this quantification more precise, Kitiyanan et al. [29] employed temperature programmed oxidation (TPO), a standard technique in catalysis research used to quantify and characterize carbonaceous deposits on heterogeneous catalysts [153, 154]. In a typical TPO experiment, a continuous flow of 5% O2/He is passed over the catalyst containing the carbon deposits while the temperature is linearly increased (11 °C/min). The evolution of CO2 produced by the oxidation of the carbon species is monitored by a mass spectrometer. Alternatively, the use of a flame ionization detector (FID) provides a higher carbon sensitivity than the mass spectrometer. In this detection mode, the CO2 and CO produced during the oxidation can be quantitatively converted to methane in a methanator [155], in which the stream coming from the TPO is mixed with a 50 cm3/min stream of H2 over a Ni/y-Al2O3 catalyst at 400 °C. The methane produced in the methanator, which exactly corresponds to the CO2 and CO generated in the TPO, is monitored in a FID. Calibration of the evolved CO and CO2 with pulses of pure CO2 or oxidation of known amounts of graphite gives a direct measurement of the amount of carbon that gets oxidized at each temperature. TPO is particularly suitable for the quantitative characterization of SWNTs because SWNTs are oxidized in a relatively narrow temperature range, which lies above the temperature of oxidation of amorphous carbon and below the oxidation of MWNT and graphitic carbon. Thermo-gravimetric analysis (TGA) studies [156] have shown that, in the absence of a catalyst, the ignition temperature of SWNTs is 100 °C higher than that of C60 fullerenes and 100 °C lower than that of MWNTs.

Figure 22 illustrates the TPO profiles of the carbon species produced on a Co:Mo/SiO2 catalyst, which exhibited high selectivity toward SWNT [29]. This sample presented a small oxidation peak centered at around 330 °C, which can be ascribed to amorphous carbon, and a major peak marked in the figure with an arrow, centered at about 510 °C, which is ascribed to the oxidation of SWNT. Two reference samples

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