## 3 Results and Discussion
### Synthesis and Characterization of Catalysts
Hydrothermal treatment of **N** to give **N-HT** barely modifies the chemical composition of the surface, but treatment with sulfuric acid drives the oxidation of the material (Figure 4). A similar trend is observed when **M** is treated with acids. As expected, the oxidation is larger with nitric acid (**M-N**). In this case, four components are distinguished: the peak at 285.7 eV can be assigned to C-O bonds; the peak at 287.6 eV is assigned to carboxyl groups mainly, and the peak at 290.7 eV is attributable to carboxyl or derivatives. Hydrothermal carbon derived from saccharose showed a different spectrum since it possesses the highest oxygen content and only two components are present: the reduced form is minor, whilst the major component is highly oxidized (component 3, 289 eV). However, after the treatment with sulfuric acid, the latter component disappears, showing instead the intermediate component at 286.5 eV (Figure 4). This suggests that the acid reacts with the most oxidized functional groups, removing them from the catalyst surface. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Catalyst** & **C (\%)** & **O (\%)** & **N (\%)** & **S (\%)** & **Zr (\%)** \\ \hline
**N** & 95.5 & 4.5 & 0.0 & 0.1 & - \\
**N-Zr** & 72.5 & 18.0 & 0.5 & 0.0 & 7.1 \\
**N-Zr-S1** & 88.6 & 8.5 & 0.6 & 0.3 & 0.2 \\
**N-Zr-S2** & 68.7 & 22.2 & 0.3 & 1.3 & 7.5 \\
**N-HT** & 95.0 & 4.8 & 0.0 & 0.1 & - \\
**N-HT-S** & 93.2 & 5.6 & 0.0 & 0.4 & - \\
**M** & 94.1 & 5.0 & - & 0.1 & - \\
**M-S** & 93.1 & 5.7 & - & 0.3 & - \\
**M-N** & 91.2 & 7.1 & - & 0.3 & - \\ HT & 78.0 & 22.0 & - & - & - \\
**HT-S** & 73.8 & 24.6 & - & 1.6 & - \\ \hline \end{tabular}
\end{table} 
Table 5: Elemental compositions obtained by XPS. The interpretation of O 1s XPS spectra in the range of materials studied, namely, activated or hydrothermal carbon, along with other carbonized substrates (Table S2), is complex indeed as one functional group gives rise to more than one peak and, in addition, different functional groups often lead to close signals. Therefore, no clear-cut interpretations can be formulated accurately. However, it is worth pointing out that the signal observed near 531 eV is consistent with the presence of both oxygen and metal in the **N-Zr**, **N-Zr-S1**, **N-Zr-S2**, **Zr**, and **Zr-S** samples. Deconvoluted N 1s data (Table S3) show one main component coming from the pristine carbon (**N** or **M**). These nitrogen atoms are not completely oxidized (like nitro, nitrox, or nitrate groups), so a peak above 405 eV should be detected. S 2p deconvoluted components (Table S4) can be divided into two groups. Reduced forms of sulfur, like organic and inorganic sulfides, are assigned to the peak at 164 eV. On the other hand, the peak at 168 eV is due to oxidized forms such as sulfates, sulfones, or sulfonic acids. The treatment of **N-Zr** with sulfuric acid increases the amount of sulfur in an oxidized form due to the presence of sulfonic groups (**N-Zr-S1**) or sulfates (**N-Zr-S2**), where the presence of four oxygen atoms around the sulfur agrees with the shift of the peak to higher values of binding energies. Figure 4: XPS spectra: (**a**) N-HT (C 1s); (**b**) HT (C 1s); (**c**) HT-S (C 1s); (**d**) N-HT-S (C 1s); (**e**) N-HT-S (S 2p); (**f**) HT-S (S 2p). The hydrothermal treatment of **N** with saccharose modifies the ratio of the two types of sulfur atoms. This change is more favorable toward oxidized forms of sulfur after the treatment with sulfuric acid. The same behavior is observed for **HT-S**, although, in this case, the precursor does not contain sulfur, thereby proving that the sulfur present in this catalyst comes from sulfuric acid. Interestingly, a small amount of sulfur is reduced in this reaction as a peak at about 164 eV is also detected. The duplicity of some peaks (163.4 and 164.5 eV; 167.4 and 165.8 eV in **HT-S**) is due to the existence of 2p\({}_{3/2}\) y 2p\({}_{1/2}\) configurations. In most cases, however, the low relation signal/noise of the spectrum hampers the visibility of both multiplicities.