## 3 Results and Discussions
### Characterization of the Plastic Pyrolysis Char after Activation
Micrographs of the char before and after activation are depicted in Figure 2. Despite the high heterogeneity of the char produced after pyrolysis, a consequence of the heterogeneity of the rejected plastic mixture, the SEM depicted micrographs suggested the presence of irregular flakes with low roughness onto the surface. These images support the already deduced morphology of the mesoporous described during the hysteresis desorption loop of N\({}_{2}\) isotherms, as flaky aggregates are easily recognized. Compared to the native char, the stabilization after HCl washing and the latter thermal treatment is likely to increase the superficial irregularities, not as much as those monitored after chemical activation, though. Thus, the thermal chemical activation with CO\({}_{2}\), NaOH, or KOH considerably produced microscopic cavities and raised the imperfections of the surface [45], which explains the higher porosity registered in these samples. This porosity development is mainly due to the release of excess gases, i.e., CO and CO\({}_{2}\), during the activation process [41]. The composition of the superficial chemical groups of all the samples was assessed by FTIR in general terms analysis due to the complexity of the mixture pyrolyzed. The results are depicted in Figure 3. An important modification of the chemical structure can be observed after the chemical activation of the native char. The original char displayed broadband of maximum ca. 3400 cm\({}^{-1}\), which is attributed to superficial hydroxyl moiety [46]. After the stabilization with HCl, the -OH groups were completely removed. No signals corresponding to aliphatic groups of aliphatic hydrocarbons of alkanes or alkenes, peaks that should appear within 2800-3000 cm\({}^{-1}\)[47], were detected. The peaks at 1600 cm\({}^{-1}\) and 1700 cm\({}^{-1}\) were assigned to aromatic C=O and aromatic COOH/C=O stretching, respectively [47]. These oxygenated groups considerably decreased in intensity after stabilization with HCl washing and practically disappeared after thermal chemical activation. Moreover, a doubled peak characteristic of n-ring C=C stretch and conjugated C=C stretch (around 1400 cm\({}^{-1}\)) [46] were well defined in the original char and also were partially destroyed during HCl washing and, finally, almost depleted after thermal treatment. The C-O-C peak located at \(\sim\)1300 cm\({}^{-1}\), which was not observed in the original char, is linked to stretching mode in acids, ethers, and esters [48], increasing its intensity in the stabilization process with HCl to almost disappear after chemical activation. Finally, in the region of lowest wavelength values, a peak located at 880 cm\({}^{-1}\) appeared due to the presence of C-H of methyl groups or C-H out-of-plane bend [49]. In this case, after chemical activation, a  
Figure 1: N\({}_{2}\) adsorption–desorption isotherms of the char from plastic pyrolysis before and after activation. small definition of this peak can be observed. Also, the aromatic C-H was registered in the original char at around 750 cm\({}^{-1}\)[50], lowering the intensity after the stabilization and disappearing after the thermal treatment displayed an important intensity after the stabilization with HCl. The thermal treatment led to the decrease in this peak in the sample treated with N\({}_{2}\), and it was not observed after chemical activation in the rest of the samples. In summary, it can be deduced that the thermal treatment was mainly responsible for the removal of all the characteristic groups, especially the oxygenated ones. Figure 2: SEM micrographs of the char from plastic pyrolysis before and after activation. The metallic composition of the solid samples before and after activation is summarized in Table 2. Diverse metals have historically been used in plastic production, either as pigments or stabilizers.