Visible – Light Driven Systems: Effect of the Parameters Affecting Hydrogen Production through Photoreforming of Organics in Presence of Cu₂O/TiO₂ Nanocomposite Photocatalyst

1.1. Preparation of the Photocatalyst

The synthesis of the photocatalyst is performed by following the procedure reported by Muscetta et al. [28], in which the catalyst is completely characterized. A fixed amount of TiO₂ (1500 mg) is mixed in the agate milling tank (PM100, RETSCH), with agate balls and a fixed amount of Cu₂O (150 mg), fixing the rotation rate to 200 rpm and the milling time to 1 minute. In the following sections, the photocatalytic material used will be named (1%wt) Cu₂O/TiO₂.

1.2. Photocatalytic Tests and Analytical Determination

All the materials used in the present investigation are reported in the Supplementary materials. Photocatalytic tests are performed as in [28] (see Figure 1A,B for the schematic illustration of the experimental setup). The Supplemental materials also include a full explanation of the reactor. A UV cutoff solution (NaNO₂ 0.5 M) is pumped through the quartz sleeve to allow the investigation of the photocatalytic activity under visible light conditions (λ>400 nm), as reported by others [29]. To perform a typical experimental run, 210 mg of Cu₂O-TiO₂catalyst are suspended in a doubly distilled aqueous solution (V=0.3 L), at fixed pH and concentration of the sacrificial species (0 ÷ 2.5 M). A N₂ stream is bubbled into the solution starting 40 minutes before inserting the photocatalyst to prevent the interaction between dissolved oxygen and copper species or photogenerated electrons. The effect of the temperature was assessed by increasing the temperature of the system up to 80°C (at this temperature a condenser was located on the reactor to prevent the evaporation of the organic species). To test the effect of pH, in some runs the solution pH is corrected to selected values by adding a diluted solution of KOH or HClO₄.

The suspension, containing a fixed concentration of the sacrificial agent (2.5 M) and a fixed amount of the photocatalytic material (700 ppm), was exposed to solar radiation (See the Figure 1B for the schematic illustration of the experimental set – up) to evaluate the photoefficiency of the system under solar light irradiation in selected runs. These runs were carried out in early September 2021 in Naples (40° 50′ 0″ N, 14° 15′ 0″ E), between the hours 11.00 and 13.00 under clear sky conditions. To evaluate the separate contribution of the UV and visible radiation ranges in hydrogen production, some photocatalytic runs are carried out by employing a cut-off filter to eliminate radiations with wavelengths less than 400 nm.

Figure 1. Schematic illustration of the annular batch reactor experimental set-up for the experiments conducted (A) under artificial UV and/or Visible light source and (B) under solar irradiation.

Figure 1. Schematic illustration of the annular batch reactor experimental set-up for the experiments conducted (A) under artificial UV and/or Visible light source and (B) under solar irradiation.

Hydrogen estimation is carried out as reported elsewhere [28,30] The pH of the solution is measured using an Orion 420 p pH–meter (Thermo).

2. Proposed Mechanism under Solar Radiation

An attempt to explain the mechanism of hydrogen generation is herein proposed and discussed (See Figure 11). As previously reported, cuprous oxide in the p–n heterojunction systems based Cu₂O/TiO₂ photocatalysts acts as an energy antenna [27], being able to absorb energy within the visible spectrum. Under direct sunlight radiation, in which the UV and the visible components are present, the activation of both semiconductors (i.e., Cu₂O and TiO₂) can be considered (See the reactions r. 2 and r.3):

$$\mathrm{C}{\mathrm{u}}_2\mathrm{O}\stackrel{\mathrm{h}\mathsf{\nu }}{\to }{\mathrm{Cu}}_2\mathrm{O}\left({\mathrm{h}}^{+}\right)+ {\mathrm{Cu}}_2\mathrm{O}({\mathrm{e}}^{-})$$

(r. 2)

$$\mathrm{Ti}{\mathrm{O}}_2\stackrel{\mathrm{h}\mathsf{\nu }}{\to }{\mathrm{TiO}}_2\left({\mathrm{h}}^{+}\right)+{\mathrm{TiO}}_2({\mathrm{e}}^{-})$$

(r. 3)

After charge carrier generation, an electric field from TiO₂ to Cu₂O and band bending are established. Positive holes can react with the adsorbed organic species (mainly on the titania surface), following the reactions r.4 and r.5, thus forming byproducts and protons, which in turn can react with the photogenerated electrons to form ${\mathrm{H}}_{2\left(\mathrm{g}\right)}$ (through the reaction r.6).

$$\mathrm{Org}+{\mathsf{\theta }}_{{\mathrm{TiO}}_2}\stackrel{{\mathrm{K}}_{\mathrm{ads}}}{\leftrightarrow }{\mathrm{Org}}_{\mathrm{ads}}$$

(r. 4)

$${\mathrm{Org}}_{\mathrm{ads}}+{\mathrm{TiO}}_2\left({\mathrm{h}}^{+}\right)\stackrel{{\mathrm{k}}_{{\mathrm{h}}^{+}}}{\to }{\mathrm{Org}}_{\mathrm{ads}}^{•}+{\mathrm{TiO}}_2\left({\mathrm{h}}^{+}\right)+{ \mathrm{H}}^{+}\stackrel{\mathrm{Fast}}{\to }\mathrm{P}+{2\mathrm{H}}^{+}$$

(r. 5)

$${2\mathrm{H}}^{+}+2\mathrm{Ti}{\mathrm{O}}_2({\mathrm{e}}^{-})\stackrel{\mathrm{Fast}}{\to }{2\mathrm{H}}^{•}\stackrel{\mathrm{Fast}}{\to }{\mathrm{H}}_2$$

(r. 6)

Obviously, electron-hole pairs may recombine to produce heat and light (through reactions r.7 and r.8), despite the p-n heterojunction formation boosts photogenerated charge carrier separation, resulting in better photocatalytic efficiencies with respect to those observed with bare materials.

$${\mathrm{TiO}}_2\left({\mathrm{h}}^{+}\right)+\mathrm{Ti}{\mathrm{O}}_2({\mathrm{e}}^{-})\stackrel{{\mathrm{k}}_{\mathrm{r}}}{\to }\mathrm{Q}+\mathrm{light}$$

(r. 7)

$${\mathrm{Cu}}_2\mathrm{O}\left({\mathrm{h}}^{+}\right)+{\mathrm{Cu}}_2\mathrm{O}({\mathrm{e}}^{-})\stackrel{{\mathrm{k}}_{\mathrm{r}}}{\to }\mathrm{Q}+\mathrm{light}$$

(r. 8)