## 3 Results and Discussion
### Synthesis and Characterization of Catalysts
Table 4 shows global compositions (both organic and inorganic), including **Zr** and **Zr-S**, for comparative purposes. The addition of zirconia to **N** (**N-Zr**) results in a lower proportion of carbon while increasing the mineral content. Treatment with concentrated sulfuric acid (**N-Zr-S1**), with the aim of sulfonating the carbonaceous precursor, promotes the lixiviation of mineral content, which is consistent with a lower ratio of zirconium and a larger amount of the other elements. When **N-Zr** is treated with diluted sulfuric acid (**N-Zr-S2**), with the idea of sulfating the deposits of zirconia on carbon **N**, lixiviation occurs in a minor extension, and the amount of sulfur increases significantly. With respect to **Zr-S**, although sulfur content is important, it is mainly composed of zirconium and oxygen, as expected. Trace impurities, not included in the table, constituted by silicon and copper, were detected in the two mineral residues of these catalysts. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Catalyst** & **C (\%)** & **H (\%)** & **N (\%)** & **S (\%)** & **O + Ash (\%)** & **Zr (\%) a ** \\ \hline
**N** & 84.4 & 1.3 & 0.5 & 0.5 & 13.4 & 0.0 \\
**N-Zr** & 46.2 & 2.0 & 0.4 & 0.2 & 51.2 & 25.6 \\
**N-Zr-S1** & 66.0 & 3.0 & 0.4 & 0.8 & 29.8 & 1.1 \\
**N-Zr-S2** & 45.6 & 2.8 & 0.3 & 1.4 & 49.9 & 22.3 \\
**Zr** & - & - & - & 0.0 & 26.0 & 74.0 \\
**Zr-S** & - & - & - & 2.5 & 28.1 & 69.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Elemental compositions of zirconium-based catalysts. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Catalyst** & **C (\%)** & **H (\%)** & **N (\%)** & **S (\%)** & **O (\%)** \\ \hline
**N** & 89.1 & 1.4 & 0.5 & 0.5 & 8.6 \\
**N-Zr** & 77.6 & 3.3 & 0.7 & 0.4 & 18.0 \\
**N-Zr-S1** & 70.8 & 3.2 & 0.5 & 0.8 & 24.8 \\
**N-Zr-S2** & 70.7 & 4.4 & 0.5 & 2.2 & 22.3 \\
**N-HT** & 93.5 & 0.8 & 1.4 & 0.5 & 3.8 \\
**N-HT-S** & 86.8 & 0.2 & 0.2 & 0.9 & 11.9 \\
**M** & 91.9 & 0.7 & 0.7 & 0.8 & 6.0 \\
**M-S** & 83.3 & 1.4 & 0.7 & 1.4 & 13.2 \\
**M-N** & 85.6 & 1.0 & 1.0 & 0.7 & 11.7 \\
**HT** & 64.3 & 6.7 & 0.0 & 0.0 & 29.0 \\
**HT-S** & 59.6 & 3.2 & 0.0 & 3.4 & 33.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Elemental analyses of catalysts. Elemental compositions measured by XPS are collected in Table 5. In general terms, these results are in agreement with those obtained by elemental analysis (Table 3): carbon is the main component of carbonaceous materials; the oxygen content of **HT** is much larger than that detected for the rest of the carbonaceous catalysts; treatment with acids gives rise to the oxidation of the carbons with increased content of nitrogen or sulfur, depending on the acid employed; treatment of **N-Zr** with concentrated sulfuric acid leads to a substantial lixiviation of zirconia (**N-Zr-S1**), decreasing the amount of zirconium and oxygen; sulfur is not detected in **Zr**, whereas in **Zr-S**, it is present in a 2.5%. Moreover, the chemical treatment with acids decreases the oxygen content when compared to data obtained by elemental analysis, suggesting that oxygenated groups are degraded further when they are attached to the surface of the catalyst. Deconvoluted C 1s data of carbonaceous catalysts are collected in Table S1. Most of the catalysts display three components: one is detected at 284.8 eV, corresponding to the reference and assigned to the most reduced form of carbon (C-C and C-H bonds); a second component is observed at 286 eV, attributable to carbon atoms partially oxidized (i.e., ether, alcohol, aldehyde); and a third component close to 290 eV, due to highly oxidized forms of carbon like carboxylic or ester groups [12]. Formation of zirconia causes a minor change in the composition of parent carbon **N** since the reduced fraction decreases, whereas the most oxidized forms increase. Such an effect is even more significant after the treatment with sulfuric acid, acting as an oxidant as well.