2.4.2. TGA

Figure 17 shows the results of TGA analysis. For the DR sample, at T up to 450 ◦C, a mass gain of 0.25% was detected. Between 450 ◦C and 765 ◦C, there was a mass loss of about 60%; finally, between 765 ◦C and 890 ◦C, a mass gain of 0.05% was observed.

**Figure 17.** TGA analysis of Ni-UGSO 13% after CC reaction at 750 ◦C and after DR reaction at 650 ◦C for 2 h TOS.

For the CC sample, up until 500 ◦C, a mass gain of 0.23% was measured. From 500 ◦C to 750 ◦C, there was a mass loss of 88.62% and, finally, between 750 ◦C and 900 ◦C, a mass gain of 0.05% was observed.

The higher the temperature of oxidation, the higher the degree of structural order. Thus, as can be seen in Figure 17, the oxidation of CNF produced by DR (CNF-DR) begins at a temperature lower than that in the case of CNF formed by CC (CNF-CC) (450 ◦C vs. 500 ◦C). In the literature, it has been reported that the oxidation of graphite and C60 in TGA occurs at 645 ◦C and 420 ◦C, respectively [37]. The oxidation temperature of CNF-CC is similar to that reported for CNT [38] and higher than that reported by Sui et al. [39]. They are all lower than the graphite oxidation temperature. Serp et al. [40] have confirmed that CNT and CNF are more reactive than graphite. They have shown that CNF samples with 10% of remaining metal (produced from ethylene on Fe/SiO2 catalysts) present a maximum gasification rate at 650 ◦C. The single-wall carbon nanotube (SWCNT), which is the carbon nanostructure that has the least remaining metal percentage (less than 1% of metal) and the least defects on its surface, presents a maximum gasification rate at 800 ◦C. MWCNT, with 3% and 7.5% of residual metal, presents a maximum rate at 650 ◦C and 550 ◦C, respectively. According to these

findings, the presence of defects on the CNF surface and the presence of residual metal within the carbon nanostructures that can catalyze carbon gasification cause a shift to lower temperatures. While the oxidation resistance of the DR carbon and the CC carbon is different, we can say that either CNF-DR carbon is more structured than CNF-CC, or that it contains more metal, or even that their surfaces are not the same, which means that they are two distinct types of CNF. The TEM analyses reported below help us to identify the type of CNF produced. It is well known that there are different types of CNF depending on the arrangemen<sup>t</sup> of the graphene plans. Accordingly, CNF are classified into three categories: platelets, fishbone, and stacked-cup CNF [2].
