## 3 Results and Discussion
### Photocatalytic Activity under Solar Conditions  
To conclude the experimental campaign of the present study, a set of experiments are performed under direct solar irradiation by using the apparatus schematically represented in the Material and methods section (Figure 1B). Figure 10 reports the main results from the photocatalytic runs conducted under direct solar irradiation.  
The hydrogen production rate is about 1 umol-h\({}^{-1}\cdot\)cm\({}^{-2}\) at the temperature of 40 \({}^{\circ}\)C and in the presence of visible light irradiation only. Hydrogen generation increases by about 1.6 times when all wavelengths of the solar spectrum are used. The result indicates that the contribution of the visible light portion of the solar spectrum supports about 60% of hydrogen production under sunlight radiation, whereas the improvement in hydrogen generation obtained without any UV cut-off filter may be ascribed to the UV component. Furthermore, as the temperature rises to 70 \({}^{\circ}\)C, the hydrogen generation rate increases to around 3 umol\(\cdot\)h\({}^{-1}\cdot\)cm\({}^{-2}\), demonstrating the favourable influence of temperature on photocatalytic activity. The efficiencies of the catalytic system adopted in the present investigation for hydrogen generation are calculated by using the irradiance values data collected during the experimental runs. The ranges of irradiation considered are 315-400 nm and 380-550 nm, where the mean values of the specific irradiances are about 15.56 W\(\cdot\)m\({}^{-2}\) and 95.75 W\(\cdot\)m\({}^{-2}\), respectively. Table 2 reports the values of the light to chemical energy efficiency. A value of about 6.5% is obtained considering the contribution of the UV range at a system temperature of 70\({}^{\circ}\)C. As regards the light-to-chemical energy efficiency in the solar range, the low calculated values may result from an overestimation of the specific irradiance collected under the visible range (380-550 nm). Indeed, wavelengths higher than about 400 nm are completely useless for the activation of the adopted photocatalyst due to its bandgap value (i.e., 3.1 eV).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Experimental Conditions**} & \multicolumn{2}{c}{**\(\eta\) (\%) in the UV Range**} & \multicolumn{1}{c}{**\(\eta\) (\%) in the Solar Range Explored**} \\ \hline T = 40 \({}^{\circ}\)C; P = 1 atm; pH \(\approx\) 8.5; C (1% wt.)Cu\({}_{2}\)O/TiO\({}_{2}\) = 700 ppm; & 3.33 & 1.16 \\
[Methanol] = 2.5 M; Solar light & 3.33 & 1.16 \\ T = 70 \({}^{\circ}\)C; P = 1 atm; pH \(\approx\) 8.5; C(1% wt.) Cu\({}_{2}\)O/TiO\({}_{2}\) = 700 ppm; & 6.47 & 2.22 \\
[Methanol] = 2.5 M; Solar light & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mean hydrogen production rate and light-to-chemical energy efficiency (in the UV and the visible range) obtained using the milled catalyst (C\({}_{(17\mathrm{\times wt.)Cu_{2}O/TiO_{2}}}\)) under direct sunlight irradiation.  
Figure 10: Hydrogen production rate during experimental runs conducted under direct sunlight irradiation. Experimental conditions: V = 0.30 L; P = 1 atm; pH \(\approx\) 8.5; [MeOH] = 2.5 M; C\({}_{(15\mathrm{\times wt.)Cu_{2}O/TiO_{2}}}\) = 700 ppm.