## 3 Results and Discussion
### Characterization of the Char-Based Materials
According to the literature [63], carboxylic acids decompose below 400 \({}^{\circ}\)C, carboxylic anhydrides decompose between 350 and 600 \({}^{\circ}\)C, and lactones reveal a peak above 600 \({}^{\circ}\)C. Phenol groups are registered in a middle-temperature range of 500-750 \({}^{\circ}\)C, while carbonyl/quinone groupsrelease CO above 700 \({}^{\circ}\)C. Finally, pyrone-like groups, if present, are released at 950 \({}^{\circ}\)C. On this basis, the TPD profiles were deconvoluted. The char showed the lowest release of TCD signal, attributable to low oxygenated species on the surface. After CA treatment of the char, the TCD signal rose, as a consequence of the oxygenated groups increasing. The activation of the char with Na\({}_{2}\)CO\({}_{3}\) led to a material with the presence of carboxyl groups, phenol, and carbonyl groups resistant to thermal treatment, close to pyrone behavior. The CA post-thermal modification enriched the variety of carbonyl and quinone groups, with contributions at lower temperatures compared to the non-modified AC. Interestingly, the TPD profiles confirm that the modification with CA destroyed the presence of carboxyl groups as the concentration of CA increased. From the profiles attained, it can be concluded that the modification with CA enriched the variety of contributions ascribed to the range of release of carbonyl and quinone, mainly. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & \multicolumn{3}{c}{**Surface Composition (wt. \%)**} & \multicolumn{3}{c}{**C\({}_{\mathbf{1s}}\)**} & \multicolumn{3}{c}{**O\({}_{\mathbf{1s}}\)**} \\ \hline  & \multicolumn{3}{c}{C\({}_{\mathbf{XPS}}\)} & \multicolumn{1}{c}{N\({}_{\mathbf{XPS}}\)} & **Others** & **Type** & **Peak (\%)** & **Type** & **Peak (\%)** \\ \hline  & & & & & C=C & 44.7 & & \\  & & & & & C-C & 42.7 & & Quinone & 37.6 \\ char & 86.4 & 10.8 & 0.7 & 1.3 (Si) & C-O & 4.5 & C=O & 34.2 \\  & & & & 0.8 (Ti) & C=O & - & C-O & 28.3 \\  & & & & & & \(\mathrm{Sum}\)\(\mathrm{C}_{\mathrm{Ox}}\) & 4.5 & & \\  & & & & & C=C & 65.6 & & \\  & & & & & C-C & 17.2 & & \\ char-CA-1.0 & 71.1 & 18.7 & 1.9 & 3.5 (Si) & C-O & 2.9 & C=O & 43.4 \\  & & & & 3.5 (Ti) & C=O & 8.1 & C-O & 24.0 \\  & & & & & & \(\mathrm{Sum}\)\(\mathrm{C}_{\mathrm{Ox}}\) & 17.2 & & \\  & & & & & & C=C & 68.2 & & \\  & & & & & 10.8 (Si) & C-C & 16.2 & & \\  & & & & & 11.0 (Ti) & C-O & - & Quinone & 35.5 \\ AC & 32.2 & 38.6 & 1.0 & 2.1 (P) & C=O & 10.5 & C=O & 40.3 \\  & & & & & 2.1 (Ca) & COO- & 5.1 & & C-O & 24.2 \\  & & & & 1.9 (Fe) & \(\mathrm{Sum}\)\(\mathrm{C}_{\mathrm{Ox}}\) & 15.6 & & \\  & & & & & C=C & 59.9 & & \\  & & & & 9.9 (Si) & C-C & 22.1 & & Quinone & 28.4 \\ AC-CA-0.5 & 49.3 & 32.3 & 1.0 & 5.1 (Ti) & C-O & 2.4 & C=O & 42.3 \\  & & & & 1.1 (P) & C=O & 10.5 & C-O & 29.2 \\  & & & & & & \(\mathrm{Sum}\)\(\mathrm{C}_{\mathrm{Ox}}\) & 18.0 & & \\  & & & & & & \(\mathrm{C}\)=C & 44.6 & & \\  & & & & & 11.0 (Si) & C-C & 25.3 & & \\ AC-CA-1.0 & 47.7 & 33.0 & 1.3 & 4.2 (Ti) & C-O & 17.2 & Quinone & 28.0 \\  & & & & & 1.3 (P) & C=O & 8.3 & C-O & 49.5 \\  & & & & & & \(\mathrm{Sum}\)\(\mathrm{C}_{\mathrm{Ox}}\) & 4.6 & & \\  & & & & & & \(\mathrm{Sum}\)\(\mathrm{C}_{\mathrm{Ox}}\) & 30.1 & & \\  & & & & & & C=C & 60.5 & & \\  & & & & & 10.8 (Si) & C-O & 15.4 & & \\  & & & & & & C-O & 9.7 & \\  & & & & & 1.2 (P) & C=O & 5.9 & C=O & 45.9 \\  & & & & & & \(\mathrm{COO}\)- & 8.4 & & \\  & & & & & & \(\mathrm{Sum}\)\(\mathrm{C}_{\mathrm{Ox}}\) & 24.0 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: XPS surface chemical composition and C\({}_{\mathbf{1s}}\) contribution after Na\({}_{2}\)CO\({}_{3}\) activation and citric acid modification. Figure 4: TPD profiles of the char and the activated carbon after citric acid modification. Figure 3: High-resolution XPS spectra of C\({}_{18}\) (**left**) and O\({}_{18}\) (**middle**) regions for char, AC, and AC-CA-1.0, and the deconvoluted contribution for all the samples (**right**).