## 3 Results and Discussion  
### Synthesis and Characterization of Catalysts  
The catalysts employed in this study (including their precursors) are listed in Table 1. Synthetic details can be found in Section 2. Parent-activated carbons Merck (**M**) and Norit (**N**) are essentially microporous. According to the IUPAC classification, they display type IV isotherms [31], which suggests the presence of mesopores, some of which have narrow apertures (Figures 2 and 3). Both solids display H4 loops, which are closed at P/P\({}_{0}\) = 1, consistent with the presence of slit-shaped mesopores. The porosity of these solids is not altered substantially after treatment with acid oxidants. Hydrothermal carbons derived from saccharose (**HT** and **HT-S**), however, showed very low adsorption capacity as evidenced by their poor porous development (Figure 3). Hydrothermal heating of **N** in the presence of saccharose to give **N-HT** notably increases the porosity. Further treatment of this material with sulfuric acid to form **N-HT-S** reduces its porosity. The behavior is different for those carbons constituted by zirconia (Figure S1). The deposition of zirconia on Norit (**N-Zr**) hampers the access of N\({}_{2}\) into the pores, probably due to their blockage, thereby reducing the porosity of this material. Treatment with diluted sulfuric acid (**N-Zr-S2**) promotes the lixiviation of this mineral, resulting in a higher porosity. This effect is even more significant when concentrated sulfuric acid is used (**N-Zr-S1**) since a higher amount of ZrO\({}_{2}\) is lixiviated from the solid. **Zr** and **Zr-S** also show type IV adsorption isotherms with a lower proportion of micropores. Furthermore, the higher slope of the curves in the middle range of pressure values suggests that the volume of mesopores is larger for **Zr** and **Zr-S** compared to the carbonaceous materials grafted with zirconia. Table 2 gathers the specific surface values of the catalysts obtained after applying the BET model to their N\({}_{2}\) adsorption isotherms [32]. As expected, the higher the porous development, the larger the specific surface since the physic structure of the catalyst does not change after the treatment employed. Activated carbons possess higher surfaces; zirconia displays medium-range values, and hydrothermal carbons exhibit almost negligible specific surfaces. \begin{table}
\begin{tabular}{c c} \hline \hline
**Catalyst** & **Specific Surface (m\({}^{2}\) g\({}^{-1}\))** \\ \hline
**N** & 1249 \\
**Zr** & 352 \\
**Zr-S** & 311 \\
**N-Zr** & 888 \\ N-Zr-S1 & 1248 \\ N-Zr-S2 & 952 \\ N-HT & 1412 \\ N-HT-S & 1036 \\
**M** & 839 \\
**M-S** & 847 \\
**M-N** & 895 \\
**HT** & 27 \\
**HT-S** & 0.031 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specific surface values of catalysts. Figure 3: N\({}_{2}\) adsorption isotherms of **N**, **N-HT**, **N-HT-S**, **HT**, and **HT-S**. Elemental analysis data of the catalysts are collected in Table 3 (excluding mineral matter). Oxygen content was calculated by difference. As expected, carbonaceous materials are mainly composed of carbon. Treatment of **N-Zr, M, N-HT**, and **HT** with sulfuric or nitric acid raises the oxygen content, a fact consistent with the oxidation of the solids. The percentage of sulfur and nitrogen also increases depending on the acid used in this process. Hydrothermal treatment with saccharose of carbon **N** to give **N-HT** leads to a lower content of hydrogen and oxygen, whilst the ratio of other elements is enhanced, in agreement with a higher graphitization of the solid. Hydrothermal carbon **HT** shows a minor amount of C and a higher content in H and O than that showed by activated carbons such as **M** or **N** since the hydrothermal process is conducted at lower temperatures than pyrolytic carbonization and, therefore, oxygenated groups are degraded in a minor extent. The amount of oxygen and sulfur is also increased when **HT** is treated with sulfuric acid, as stated above for other catalysts. Zirconium-based carbons show a different behavior.