## 3 Results and Discussion
### Synthesis and Characterization of Catalysts
The S 2s signal of **Zr-S** contains two peaks, 226 and 234 eV. The first peak can be associated with the presence of adventitious carbon. The second peak can be attributed to the formation of sulfates on the surface of ZrO\({}_{2}\). Another peak is observed at 169.7 eV in the S 2p spectrum, corresponding to sulfates groups, not detected in **Zr**. Deconvoluted Zr 3d spectrum gives two peaks at 183.0 y 185.5 eV, typical for ZrO\({}_{2}\) and assigned to 3d\({}_{5/2}\) y 3d\({}_{3/2}\) multiplicities, respectively. Likewise, the Zr 3p spectrum shows two peaks at 334 and 338 eV in **Zr** and **Zr-S**. PZC values and quantification of acid and basic sites of catalysts are collected in Table 6. Carbon **N** is slightly basic, as observed for **M** and **Zr. N-HT** also possesses a basic character, suggesting that the hydrothermal treatment with saccharose barely alters the PZC measurement. On the other hand, the carbonaceous material prepared by hydrothermal treatment of saccharose shows high acidity. Chemical treatment with acids leads to a reduction in PZC values, including that of **HT**. The low PZC value of **HT** can be explained by a lower degradation experimented with saccharose when subjected to the hydrothermal process, which agrees with the abundance of hydroxyl and carboxyl groups. Treatment with nitric acid enhances the acidity of the solids due to the formation of oxygenated functional groups with acid behavior, especially carboxylic acids. Likewise, treatment with sulfuric acid drives even more acid materials, a fact that is consistent with the creation of highly acid sulfonic groups. The treatment with acids raises the number of acid groups, whereas basic sites are reduced. FT-IR spectra (Figures S2-S5) show characteristic OH absorption bands at about 3400 cm\({}^{-1}\). Vibrational bands in the range 1500-1600 cm\({}^{-1}\) are assigned to C=C bonds typically detected in graphitic carbonaceous materials. Hydrothermal carbons **HT** and **HT-S** also display bands at 1700 cm\({}^{-1}\), consistent with the presence of carboxylic groups. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Catalyst** & **PZC a** & **Acid Groups b** & **Basic Groups b** \\ \hline
**N** & 9.1 & 0.32 & 0.56 \\
**Zr** & 7.8 & 1.07 & 0.76 \\
**Zr-S** & 3.9 & 1.57 & ND c \\
**N-Zr** & 7.2 & 0.68 & 0.81 \\
**N-Zr-S1** & 4.9 & 0.93 & 0.09 \\
**N-Zr-S2** & 4.7 & 1.34 & 0.13 \\
**N-HT** & 9.2 & 0.43 & 0.47 \\
**N-HT-S** & 4.4 & 0.74 & 0.12 \\
**M** & 10.0 & 0.53 & 0.25 \\
**M-S** & 3.8 & 0.76 & 0.03 \\
**M-N** & 7.4 & 0.82 & 0.12 \\
**HT** & 2.8 & 1.96 & ND c \\
**HT-S** & 1.9 & ND c & ND c \\ \hline \hline \end{tabular} a
[FOOTNOTE:a]Footnote a: footnotetext: Point of zero charge. b
[FOOTNOTE:b]Footnote b: footnotetext: Measured as mEq g\({}^{-1}\). c  
\end{table} 
Table 6: PZC values, acid, and basic sites of catalysts. In general, treatment with sulfuric acid often reveals the detection of absorption bands at \(\sim\)1100 and \(\sim\)617 cm\({}^{-1}\), attributable to sulfonic groups [12; 33]. Figure 5a,b shows the typical spheric morphology of hydrothermal carbons prepared from carbohydrates, with different sphere sizes for both catalysts [34]. Moreover, chemical treatment with sulfuric acid does not alter the characteristic morphology of this material. Nor does it induce any significant change in particle size. Nevertheless, catalysts **N-HT** and **N-HT-S** show both an irregular morphological pattern with a lack of spheres (Figure 5c,d), thereby pointing to a modification of the surface of the catalyst without producing an activated carbon/hydrothermal carbon composite.