## 3 Results and Discussion
### Effect of the Sacrificial Agent  
Many researchers proposed hydrogen production through photoreforming of organics in the aqueous phase [14, 15, 16]. Organic species may derive from renewable sources or  
Figure 6: Hydrogen evolution rate at varying the temperature of the system. Experimental conditions: V = 0.30 L; P = 1 atm; pH \(\approx\) 8.5; [Methanol] = 2.5 M; C\({}_{\text{(1\%wt,JCu2O/TiO_{2})}}\) = 700 ppm; visible range.  
Figure 7: Arrhenius plot for (1%wt.) Cu\({}_{2}\)O/TiO\({}_{2}\) photocatalyst.  
organic-rich effluents [63; 64; 65]. Due to (i) the possibility of linking hydrogen generation with water treatment and (ii) the cheap cost of the feedstock, the potential use of organic contaminants found in wastewater is particularly appealing [66; 67; 68; 63]. Evaluating the effect of the scavenger used on photocatalytic hydrogen production is thus necessary. To this aim, some experimental runs are carried out at varying the sacrificial species (i.e., alcohols or carboxylic acids). The main results collected during this investigation are reported in Figure 8. Glycerol and 1,2-Ethandiol result in the greatest options for hydrogen generation in presence of Cu\({}_{2}\)O/TiO\({}_{2}\) photocatalyst, according to literature findings in which Cu-based TiO\({}_{2}\) was used to generate hydrogen through photoreforming [52]. On the contrary, no hydrogen production is herein detected in the case of pure water and seawater. Both the nucleophilicity of the groups in the molecular structure of the sacrificial agents and the capacity of the organics to adsorb on the catalyst surface affect the reactivity of the organics with photogenerated holes and can be used to explain these findings. The four species exhibiting the higher photoefficiency for hydrogen evolution (i.e., methanol, ethanol, glycerol and 1,2-ethanediol) have similar nucleophilicity values, as reported in Table 1. As regards the adsorption phenomenon, the development of the bidentate (and tridentate) complexes can be proposed for glycerol and 1,2-ethanediol. On the other hand, the creation of weaker monodentate complexes can be supposed for ethanol and methanol, according to their structures.  
By contrast, the almost negligible photo efficiency for hydrogen evolution recorded in the presence of formic acid and lactic acid can be related to the following phenomena: (i) acidic conditions (See Table 1) reduce the stability of copper species, as previously discussed; (ii) under acidic conditions, the photocatalyst surface is positively charged, thus exhibiting a weak interaction with the organic compounds. Based on these considerations, glycerol is the best sacrificial agent tested in this study. In addition, a beneficial effect of the  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Organic Species** & **pKa Value** & **Natural pH of the Solution** & **Reference** \\ \hline Ethanol & 15.9 & 7.0 & [69] \\ Methanol & 15.3 & 8.5 & [54] \\
1,2-ethanediol & 15.1 & 6.0 & [54] \\ Glycerol & 14.4 & 6.5 & [54] \\ Lactic acid & 3.86 & 2.0 & [70] \\ Formic acid & 3.75 & 3.0 & [69] \\ \hline \hline \end{tabular}
\end{table}
Table 1: pKa values of several compounds used in the experiments.  
Figure 8: Hydrogen evolution in the presence of various organic species. Experimental conditions: V = 0.30 L; P = 1 atm; T= 35 \({}^{\circ}\)C; [Organic] = 2.5 M; C\({}_{(1\%wt)\text{Cu}\text{O}/\text{TiO}_{2}}\) = 700 ppm; visible light radiation.  
temperature on hydrogen evolution is observed also in the case of glycerol. Indeed, when glycerol is used as a scavenger and the system temperature of the system is set at 80 \({}^{\circ}\)C, the hydrogen generation rate achieves a value of 260 \(\upmu\)mol\(\cdot\)h\({}^{-1}\).