*2.2. Photocatalytic Test Results*

The experimental data concerning reactant and product distribution, carbon balance, and conversion of phenol (Ph) over bare and metal-modified TiO2 in aqueous media after 6 h of reaction time are presented in Table 2.

**Table 2.** Phenol conversion to gaseous and liquid products and carbon balance measured after 6 h of reaction time.


a—μmoles of carbon contained by inlet phenol, non-reacted outlet phenol (Ph) and formed reaction products (hydroquinone (HQ), benzoquinone (BQ), 1,2-dihydroxibenzene (1, 2 DHBz)), and CO2; b—Phenol conversion after 6 h of reaction time (reaction conditions: 110 mL of 50 mg·L−<sup>1</sup> phenol aqueous solution.0.05 g photocatalyst, T = 18 ◦C, light source AM 1.5).

> The carbon balance ((C(outlet)/C(inlet))x100) in our experiments was better than 92%. The Ph conversion ranged between 8.7% (for bare TiO2) and 15.8% (for Pt/TiO2). From the Ph conversion point of view, metal deposition enhances the activity of TiO2 (Pt/TiO2 > Ag/TiO2 > Au/TiO2 > TiO2). The formation of Ph mild oxidation products at end of reaction time, hydroquinone (HQ), benzoquinone (BQ) and 1,2-dihydroxibenzene (1,2 DHBz), could be observed only on metal-loaded TiO2 (Me = Ag, Au, Pt). In contrast, over bare TiO2, Ph was mineralized directly to CO2. The brief analysis of our results suggests that metal deposition on TiO2 favor the formation of oxygenated products, whereas over pristine TiO2, Ph is mineralized directly to CO2, without intermediate formation. As we shall show in this article, the choice of metal is crucial in controlling the selectivity of oxidation reaction.

> The time course of products formation during photocatalytic oxidative degradation of phenol over bare and metal-modified TiO2 exposed to simulated solar light is presented in Figure 3. It can be observed that the formation of oxygenated products takes place

only on metal-loaded TiO2. The amount of HQ increases rapidly in the first hour of the reaction, then the formation rate is stabilized at ≈ 0.1 <sup>μ</sup>moles h−1. The activity order for HQ formation over metal-loaded TiO2 is Ag/TiO2 < Au/TiO2 < Pt/TiO2. Transient formation of BQ was observed only over Pt/TiO2 and Au/TiO2. The amount of BQ peaked at ≈ 2.3 and 0.4 μmoles for the former and second photocatalysts, respectively, after 2 h of reaction. For a longer reaction time, the amount of BQ decreases progressively, vanishing completely for Au/TiO2 and remaining at low concentration (≈ 0.5 μmoles) in the case of Pt/TiO2.

**Figure 3.** Solar light-driven phenol oxidative conversion to oxygenated compounds (HQ (**A**), BQ (**B**), and 1,2 DHBz (**C**)), CO2 (**D**) and to H2 (**E**) over bare and metal modified TiO2 synthesized by laser pyrolysis. Experimental conditions: 0.05 g suspended in 110 mL of 50 mg·L−<sup>1</sup> phenol aqueous solution, reaction temperature 18 ◦C, simulated solar light AM 1.5.

The evolution of reaction selectivity to oxygenated compounds and CO2 is presented in Figure 4A–E. Selectivity to 1,2 DHBz reaches a maximum at 30 min for all metal-loaded catalysts (78% for Ag/TiO2, 73% for Pt/TiO2, 53% for Au/TiO2) (Figure 4C). Highest selectivity to oxygenated products (1,2 DHBz + HQ + BQ) of ≈95% was measured for Pt/TiO2 after 0.5 h of reaction time, followed by Au/TiO2 (77% at 1 h), Ag/TiO2 (61% at 2 h), and TiO2 (0%) (see Figure 4E).

#### *2.3. Noble Metals Role in Charge Separation and ROS Generation*

The next step of our investigation was to elucidate in more details the role played by metals in photocatalytic oxidation processes, specifically in (i) charge separation and in (ii) ROS generation.

Electron-hole recombination is one of the main energy loss routes through radiative and nonradiative processes [19]. Photoluminescence (PL) experiments were designed to observe whether, in our case, metal deposition is effective to decrease charge recombination by PL emission.

It is documented that, PL emission intensity depends on photogenerated charge concentration [20]. The PL spectra in Figure 5 show that the energy loss by radiative recombination decreases because of metal deposition on TiO2, due to a better separation of photocharges at the metal–oxide interfaces [14,15]. In light of experimental results, the most efficient charge separation takes place on Au/TiO2, followed, in order, by Ag/TiO2 and Pt/TiO2. Improvement in charge separation is expected to enhance photocatalytic activity because a greater number of electrons and holes become available for redox processes associated with photocatalytic reactions. Our results confirm that a higher conversion of Ph is observed over metal-loaded TiO2 compared to bare TiO2 (see Table 2). However, based only on PL emission intensity results, it is difficult to predict the precise order of activity because, beside the important role played by metals in charge separation, metals work as co-catalysts, mediating charge transfer to reacting substrates.

**Figure 4.** Time course of selectivity for phenol photocatalytic oxidative conversion to HQ (**A**), BQ (**B**), 1,2 DHBz (**C**), CO2 (**D**) as well as overal selectivity to oxygenates (**E**) over bare and metal-modified TiO2 exposed to simulated solar light.

**Figure 5.** Comparative PL emission spectra of bare and metal-loaded TiO2. Experimental conditions: 0.5 mg catalysts suspended by ultrasonication in 3 mL of water.
