## 3 Results and Discussion
### Characterization of the Cu/SiO\({}_{2}\) Catalysts
Taking into account that the metal reducibility strongly depends on the particle size, oxidation state, and its interaction with the support [42; 43; 44], TPR analysis of the fresh Cu/SiO\({}_{2}\) catalysts was performed in order to obtain information about the nature of Cu species and their dispersion onto the support. Figure 5 and Table 3 show the TPR profiles and relative H\({}_{2}\) consumption, respectively. It could be noted that copper oxide was present in both CuOB/SiO\({}_{2}\) and CuAsc/SiO\({}_{2}\) calcined at 500 \({}^{\circ}\)C catalysts and that it was completely reduced at 400 \({}^{\circ}\)C. However, in the CuCTAB/SiO\({}_{2}\) sample an additional reduction effort (51%) was necessary at higher temperature, indicating that after the CuNPs were supported on SiO\({}_{2}\) 49% of the copper remained as Cu(0). Prior to the reaction tests, the catalysts were initially reduced at 400 \({}^{\circ}\)C in the presence of hydrogen. As a consequence, at the beginning of the reaction test, only metallic copper particles could be detected in the CuOB/SiO\({}_{2}\) and CuAsc/SiO\({}_{2}\) catalysts. In contrast, almost 50% of the copper was present as oxidized entities in the fresh CuCTAB/SiO\({}_{2}\) solid. The low reducibility of this sample may be attributed to (i) the presence of strong metal-support interaction that increased the reduction temperature of the  
Figure 5: Temperature programmed reduction (TPR) profiles of the calcined fresh catalysts. Calcination treatment: 500 \({}^{\circ}\)C in air flow, heating rate: 10 \({}^{\circ}\)C min\({}^{-1}\). Solid line: CuCTAB/SiO\({}_{2}\), dash line: CuOB/SiO\({}_{2}\), dash dot line: CuAsc/SiO\({}_{2}\). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**H\({}_{2}\)/Cu:**} \\ \cline{2-5}
**Catalyst** & \multicolumn{2}{c}{**Total molar H\({}_{2}\) Consumption / Cu molar Content**} \\ \cline{2-5}  & **Fresh** & **Fresh Re-Oxidized** & **Used** & **Used Re-Oxidized** \\  & **TPR-I** & **TPR-II** & **TPR-I** & **TPR-II** \\ \hline CuOBS/SiO\({}_{2}\) & 1 & - & 0.06 & 1 \\ CuAsc/SiO\({}_{2}\) & 1 & - & 0.65 & 1 \\ CuCTAB/SiO\({}_{2}\) & 0.51 & 0.76 & 0.22 & 0.31 \\ \hline \hline \end{tabular}  
* TPR measurement conditions: H\({}_{2}\)/Ar 5% v/v ambient, 30 mL-min\({}^{-1}\), heating ramp: 10 \({}^{\circ}\)C min\({}^{-1}\) up to 500 \({}^{\circ}\)C, dry catalyst weight: 50 mg. The H\({}_{2}\) consumption was obtained taking into account the reduction of the CuO standard reagent under similar temperature programmed conditions.  
\end{table} 
Table 3: Reducibility analysis of fresh, used, and re-oxidized catalysts. oxidized copper species, or (ii) the formation of silicalite type species during the calcination treatment evidenced by the turquoise-blue final color (Supplementary Materials Figure S3) [42; 45]. The latter hypothesis could be sustained by the highly exothermic decomposition reaction during the calcination of the sample, which could generate high temperature spots leading to the formation of such Cu-Si complexes (Supplementary Materials Figure S4). In order to verify the presence of metallic copper species and their interaction with SiO\({}_{2}\), the sample resulting from the TPR-I experiments was further calcined in air flow up to 500 \({}^{\circ}\)C. Then a subsequent H\({}_{2}\)-TPR (TPR-II) was performed. The TPR-I profile of the CuOB/SiO\({}_{2}\) catalyst after reaction indicated that the main copper species supported on SiO\({}_{2}\) was in metallic state Cu(0) (94%) (Figure 6A, Table 3). An analogous effect could be observed in the CuASC/SiO\({}_{2}\) used catalyst with ca. 65% of reduced copper entities (Figure 6B). However, in the CuCTAB/SiO\({}_{2}\) catalysts, 22% of the initial CuO remained as Cu\({}^{+\text{n}}\). Not only the H\({}_{2}\) consumption was modified after all catalytic tests, but also the maximum reduction temperatures (Figure 6C). The H\({}_{2}\) uptake profiles of the used catalysts prepared from the CuNP suspensions shifted to lower temperatures.