## 3 Results and Discussion
### Characterization of the Cu/SiO\({}_{2}\) Catalysts
This could be attributed to a lower interaction of the CuO particles with the support or to the particle size changes on the support surface [46]. For the CuASC/SiO\({}_{2}\) fresh catalyst, TPR-I revealed a maximum at 286 \({}^{\circ}\)C with a shoulder at 327 \({}^{\circ}\)C, which suggests the reduction of CuO in two different steps: Cu(II) to Cu(I) and Cu(I) to Cu(0), respectively [46]. However, this former peak could be also associated with particles with different sizes [47]. It could be observed that at 400 \({}^{\circ}\)C all copper species were reduced. Regarding the CuCTAB/SiO\({}_{2}\) catalyst, the TPR profile of the fresh catalyst (Figure 5C) showed a main peak around 365 \({}^{\circ}\)C with a shoulder at 441 \({}^{\circ}\)C. A second TPR after re-oxidation (not shown) showed a 20% increment in the H\({}_{2}\) consumption higher than in the fresh catalyst, thereby suggesting that part of the copper remained in the metallic state even after being supported and calcined or that Cu-Si like entities formed during the pretreatment were less prone to reduction. The TPR-I profile of the used solid CuOB/SiO\({}_{2}\) catalyst (Figure 6A) showed that H\({}_{2}\) consumption decreased notably, suggesting that after the reaction a great percentage of the metal remained reduced [44]. After this TPR-I, the sample was re-oxidized up to 500 \({}^{\circ}\)C during 30 min under pure oxygen flow, and a new TPR-II was obtained. In this second profile two peaks appeared, one at 263 \({}^{\circ}\)C and other at 317 \({}^{\circ}\)C,  
Figure 6: TPR profiles of the used (TPR-I) and re-oxidized catalysts after TPR-I (TPR-II). (**A**) CuOB/SiO\({}_{2}\), (**B**) CuASC/SiO\({}_{2}\), (**C**) CuCTAB/SiO\({}_{2}\). Catalyst weight: 50 mg. Re-oxidation treatment: 500 \({}^{\circ}\)C in air flow, heating rate: 10 \({}^{\circ}\)C min\({}^{-1}\). confirming that after reaction, copper particles did not oxidize completely and probably the dispersion of the same ones changed over the support [47; 48; 49]. For the CuAsc/SiO\({}_{2}\) used catalyst, a similar profile was observed that resembled the calcined sample, but with a lower H\({}_{2}\) consumption. This indicates that a fraction of copper remained as metallic particles (Cu(0)), in agreement with the XRD pattern (Figure 4). The profile of the used and re-oxidized catalyst (TPR-II, Figure 6B) revealed two well-defined peaks (217 and 328 \({}^{\circ}\)C) whose H\({}_{2}\) consumption corresponded to the total reduction of copper (Table 3). These differences suggest that the oxidation treatment promoted some reorganization of copper species, which gave rise to two well defined reduction peaks at lower temperatures, indicating an increment of the reducibility of the sites and a decreasing metal-support interaction [50; 51]. In the case of the CuCTAB/SiO\({}_{2}\) after reaction, the reduction area (Figure 6C) turned out to be much lower with a single peak centered at 340 \({}^{\circ}\)C. This suggests that a fraction of the copper particles remained like Cu(0) after the reaction. On the other hand, the total reduction of the copper entities took place at higher temperatures (350 \({}^{\circ}\)C) than the other catalytic counterparts (CuOB/SiO\({}_{2}\) and CuAsc/SiO\({}_{2}\)). Consequently, it is possible to infer that the lower reducibility of this catalyst was due to a higher metal-support interaction. Moreover, it could be associated with the formation of a chrysocolla type phase, a kind of copper hydrosilicate [39; 45; 52]. From the evaluation of our Cu/SiO\({}_{2}\) catalysts by transmission electron microscopy (TEM) (Figures S6 and S7) it could infer the amorphous and nanometric morphology of the silica selected as support. For the grinded CuOB/SiO\({}_{2}\), both fresh and used, large CuO particle aggregates were mainly detected.