## 3 Results and Discussion
### Catalytic Evaluation in the NO Reduction with Hydrogen  
The catalytic measurements were carried out in triplicate for each catalyst. No significant differences were found, thereby confirming a good reproducibility of the materials and the robustness of the methodology (Table 4). Although no long-term stability studies were conducted, all the catalysts exhibited stable NO conversion levels at each specific reaction temperature under evaluation for an accumulated experiment of six consecutive hours (two hours at 300 \({}^{\circ}\)C, two hours at 400 \({}^{\circ}\)C, and then two additional hours at 500 \({}^{\circ}\)C, respectively; see Figure 9 for comparison) Regarding the activity of the solid mixture CuOB/SiO\({}_{2}\) catalyst, Figure 9A and Table 4 show the conversion of NO in the presence of H\({}_{2}\). Total NO conversion was not observed at any evaluated temperature and total selectivity towards N\({}_{2}\) solely was achieved at 400 \({}^{\circ}\)C, at the expense of lower NO conversion levels. Nevertheless, the activity of this material was negligible at 300 \({}^{\circ}\)C for the reduction with H\({}_{2}\). Figure 9B and Table 4 also summarize the catalytic performance of the CuAsc/SiO\({}_{2}\). In this case, the NO conversion was total for the full temperature ranges. At 500 \({}^{\circ}\)C the reduction reaction showed N\({}_{2}\) and NO\  
Concerning the catalytic activity of CuCTAB/SiO\({}_{2}\), both the conversion of NO and the selectivity towards N\({}_{2}\) was 100% at temperatures of 400 \({}^{\circ}\)C and 500 \({}^{\circ}\)C, respectively (Figure 9C and Table 4), while no reaction occurred at lower reaction temperatures of 300 \({}^{\circ}\)C.  
These results clearly indicate that each catalyst presented different copper species interacting particularly with the support and with a specific dispersion that led to distinct catalytic performance. The following reactions could take place between NO and H\({}_{2}\)[53; 54; 55; 56]. \[\mathrm{NO}+\mathrm{H}_{2}\rightarrow\frac{1}{2}\,\mathrm{N}_{2}+\mathrm{H}_{ 2}\mathrm{O} \tag{1}\]  
\[\mathrm{2}\,\mathrm{NO}+\mathrm{5}\,\mathrm{H}_{2}\to 2\,\mathrm{NH}_{3}+2\, \mathrm{H}_{2}\mathrm{O} \tag{2}\]  
\[\mathrm{4}\,\mathrm{NO}\rightarrow\mathrm{N}_{2}+2\,\mathrm{NO}_{2} \tag{3}\]  
\[\mathrm{2}\,\mathrm{NO}+\mathrm{H}_{2}\rightarrow\mathrm{N}_{2}\mathrm{O}+ \mathrm{H}_{2}\mathrm{O} \tag{4}\]  
\[\mathrm{2}\,\mathrm{NO}+\mathrm{H}_{2}\rightarrow\mathrm{N}_{2}\mathrm{O}+ \mathrm{H}_{2}\mathrm{O} \tag{5}\]  
The only end-products observed in the catalytic reaction with all the prepared solids were NO, N\({}_{2}\), NO\({}_{2}\), and H\({}_{2}\)O. Neither NH\({}_{3}\) nor N\({}_{2}\)O were detected as reaction products, as it was observed when the test was performed with the support (SiO\({}_{2}\)) without copper at the same reaction conditions. This result indicated the selective role of the copper towards nitrogen regardless of the reaction temperature. A simple redox cycle for the selective reduction of NO with H\({}_{2}\) was proposed as a catalytic mechanism, as seen in Figure 10[23]:  
Figure 10: Simple redox cycle for the selective reduction of NO with H\({}_{2}\). Figure 9: Catalytic activity of A: CuOB/SiO\({}_{2}\), B: CuAsc/SiO\({}_{2}\), and C: CuCTAB/SiO\({}_{2}\) for the NO reduction with H\({}_{2}\). Reaction conditions: [NO]\({}_{0}\) = 2000 ppm NO, [H\({}_{2}\)]\({}_{0}\) = 3% H\({}_{2}\), balanced at 1 atm with He, GHSV: 2250 h\({}^{-1}\). In this redox cycle, metallic copper is formed under reductant atmosphere forming the active site. Nitrogen oxide is adsorbed and dissociated, forming nitrogen and copper oxide on the catalyst. In a final step, this oxidized site is regenerated under a reducing atmosphere. The presence of NO\({}_{2}\) could be associated with a labile Cu-O surface species that contributes to a fast NO oxidation [57]. This mechanism agrees with the behavior exhibited by the studied catalysts.