*2.4. ROS Formation on Bare and Metal-Loaded TiO2*

We have considered three main reaction pathways for the oxidative conversion of Ph: (i) straight charge injection to adsorbed organic substrate on catalyst surface, (ii) reaction of organic substrate with ·OH or with O2 −. In case of oxidative degradation reaction mechanism, the photogenerated charges are shuttled to Ph by intermediation of ROS (·OH and O2 <sup>−</sup>). It is well documented that ·OH is a powerful, non-selective, oxidant, whereas O2 − is a weak oxidant [21].

To get further information on the relationship between ROS formation and the photocatalytic behavior of our materials, we have assessed the formation of ·OH and O2 − under light irradiation by using selective radical quenchers.

The formation of free ·OH radicals was probed by monitoring the development of fluorescent umbelliferone resulted in the reaction between non-fluorescent coumarin and ·OH radicals. The amount of ·OH raises gradually in time, for all photocatalyst exposed to solar light (see Figure 6A,B). From data presented in Figure 6B, the estimated amounts of ·OH formed in 6 h of irradiation time in 110 mL of solution of reactor are: Pt

TiO2 (168 μmoles g−<sup>1</sup> cat) > TiO2 (156 μmoles g−<sup>1</sup> cat) ≈ Au/TiO2 (155 <sup>μ</sup>moles g−<sup>1</sup> cat) > Ag/TiO2 (131 μmoles g−<sup>1</sup> cat). From comparison with photocatalytic data, it comes out that the ·OH quantity is proportional with that of CO2. The experimentally measured CO2, after of 6 h of reaction time, over 0.05 g of photocatalysts, was: Pt/TiO2 (430 μmoles g−<sup>1</sup> cat) > TiO2 (310 μmoles g−<sup>1</sup> cat) ≈ Au/TiO2 (276 <sup>μ</sup>moles g−<sup>1</sup> cat) > Ag/TiO2 (128 μmoles g−<sup>1</sup> cat) (see comparatively Figures 6B and 4D). Taking into account that the probability of ·OH trapping by coumarin or by Ph vary as a function of experimental conditions [22], it can be observed, based on the good matching between radical quenching and photocatalytic results, that CO2 formation relates to ·OH production. The plots in Figure 6C show a clear correlation between relative amounts of ·OH and CO2 formed over the investigated materials. Therefore, we assume that ·OH radicals are responsible for the mineralization of Ph to CO2 (non-selective oxidation route). The formation of CO2 cannot be prevented over TiO2-based materials dispersed in aqueous media because the formation of ·OH radicals is unavoidable.

The formation of O2 − over metal loaded TiO2 was evidenced indirectly by detection of formazan, which is the product of reaction between XTT and O2 − The specific absorbance peak of formazan is at 485 nm (Figure 7A). The reduction efficiency of O2 to O2 −, estimated from the amount of formazan, decreases in the order Pt/TiO2 > Au/TiO2 > Ag/TiO2 (Figure 7B). The formation of O2 − could not be evidenced via formation of formazan on the bare TiO2 sample, prepared by laser pyrolysis. There are, however, reports claiming that O2 − is formed on TiO2. For example, Goto et al. [23] detected the formation of O2 −

on rutile particles, suggesting that electron transfer takes place from an organic moiety (2-propanol) to O2.

**Figure 6.** Time course of umbelliferone PL (**A**) and evolution in time of ·OH concentration (**B**) over the investigated photocatalysts exposed to solar light, as well as the relative amounts of CO2 and ·OH formed over the photocatalysts exposed to simulated solar light for 6 h (**C**). The formation of ·OH radical was evidenced by observing PL peak of umbelliferone at ≈ 450 nm for λexc = 330 nm (coumarin traps selectively ·OH to form umbelliferone). Inset of figure B represents the calibration curve obtained by plotting the PL response against umbelliferone concentration. Experimental conditions: 1 mg catalyst was dispersed by ultrasonication in 40 mL of 11 mM coumarin solution and then exposed to simulated solar light AM 1.5.

The quantity of oxygenated products resulted by photocatalytic oxidation of Ph scale with the relative amounts of O2 − (see Figure 7C). From here, it comes that, O2 − is the ROS responsible for Ph mild oxidation. The main outcomes from O2 − quenching experiments are: (i) supported metals catalyze O2 − formation and (ii) O2 − is the main player in Ph mild oxidative route. The eventual role played by O2 − for degradation of oxygenated compounds to CO2 should not be completely disregarded, although O2 − is a significantly weaker oxidant compared to ·OH. The BQ is indicated as an effective O2 − quencher [1]. We have observed indeed the rapid degradation of BQ, formed only over Pt/TiO2 and Au/TiO2 (see Figure 3B).

The formation of ROS was checked also in visible light domain (λ > 420 nm).

The results of Figure 8 show that, the formation of ·OH radical does not proceed under visible light for any of the investigated materials (Figure 8), which is in line with the absence of CO2 formation during photocatalytic tests conducted in visible light. Our selective radical quenching results demonstrate that CO2 formation is due to ·OH appearance.

**Figure 7.** Time course of formazan absorbance (**A**), formed in the reaction between O2 − and XTT probe molecule, and evolution of O2 − relative concentration over the catalysts exposed to solar light (**B**). Relative amounts of oxygenates (HQ + BQ) measured at end of reaction in comparison with that of O2 − (**C**). Experimental conditions: 4 mg of catalysts, dispersed into 3 mL of XTT sodium salt solution, were exposed to simulated solar light to induce the formation formazan, which was put in evidence by the UV-VIS absorption peak at ≈ 470 nm.

**Figure 8.** Coumarin formation survey, indicative of ·OH radical formation, upon exposure to visible light (λ > 420 nm) of catalysts dispersed in aqueous media.

The survey conducted in visible light (λ > 420 nm) reveal that O2 − formation does not take place over the scrutinized materials with exception of Au/TiO2 (Figure 9). The formation of O2 − takes place by reaction between hot electrons of Au plasmon and adsorbed O2 in vicinity of Au nanoparticles [14]. Participation of TiO2 in O2 − formation, via Au plasmon electron injection in TiO2 conduction followed by O2 reduction on TiO2, was

also suggested [24]. However, the very short lifetime of plasmons of 2–10 fs associated with the low energy of electrons [25,26] decrease the probability of O2 reduction. We have observed experimentally only tinny amounts of O2 − formed under visible light exposure of Au/TiO2, which are not enough to react with Ph at rates high enough to make possible the identification of mild oxidation reaction products by HPLC. The catalytic test results, carried out over all photocatalysts at λ > 420 nm, evidenced the formation of small amounts of H2 only over Au/TiO2 (≈ 2.5 μmoles in 5 h of reaction). The experiments performed in visible light show that ROS are not produced, because visible light (λ > 400) nm is not absorbed by TiO2.

In the UV region, both ·OH radicals and O2 − are produced, the former on TiO2 and the second on metals. The ·OH oxidizes non-selectively the organic substrate(s) to CO2 and H2O. The reactions implying ·OH participation are important for environmental applications, where the scope is to mineralize rapidly the organic pollutant to non-harmful CO2. In our case, the mild oxidant O2 −, is produced only on supported noble metal particles, only under exposure to UV light. In addition to radical trapping experiments, the results of our photocatalytic tests evidence that, CO2 is the only reaction product of phenol oxidation over bare TiO2. The metals mediate the transfer of the photogenerated electrons from TiO2 to adsorbed O2. The activity order for O2 − formation is Pt/TiO2 > Au/TiO2 > Ag/TiO2 (TiO2 shows no activity). The interplay between material activity for ·OH and O2 − production determines the catalyst selectivity to oxygenated products and CO2. Pt/TiO2 is the most active to produce both ·OH and O2 −, thus it will give finally the best phenol conversion. This result proves that Pt/TiO2 generates the highest amount of photogenerated charges ready to participate in redox processes. The Ag/TiO2 is the less active generator of ·OH and O2 − and consequently shows the smallest phenol conversion among metal-loaded photocatalysts. The supported metals have certain influence on activity of TiO2 support to produce ·OH: Pt/TiO2 > TiO2 ≈ Au/TiO2 > Ag/TiO2. Pt on TiO2 enhances the formation rate of ·OH compared to bare TiO2, whereas Ag depresses it. Supported Au seems to have no influence on activity of TiO2 for ·OH formation.

**Figure 9.** Survey of O2 − production, by monitoring formazan specific absorbance, over the catalysts exposed to visible light (λ > 420 nm).

In visible region (λ > 420 nm), both catalyst types (bare and metal-loaded TiO2) show negligible photocatalytic activity because neither ·OH nor O2 − are produced. The tiny amounts of O2 − generated on Au/TiO2 are originate from Surface Plasmon Resonance (SPR) shown by Au nanoparticles. Hot electrons on surface of Au particles reduce small amount of O2. The absence of ROS production in visible light is most likely due to the fact that the light absorption edge is at 400 nm (see the UV-VIS spectra in Figure 2), consistent

with a band gap of ≈ 3.1 eV. The visible light absorbed by Ag/TiO2 and Au/TiO2 is capable of triggering the formation of tiny amounts of O2 − by electron donation to adsorbed O2, only in case of Au/TiO2 (see Figure 9).

To get additional experimental evidence on the nonselective Ph degradation route by ·OH radicals, we have designed a new series of experiments, aiming to hinder the formation of umbelliferone from coumarin. The concentration of Ph was chosen to be high enough (2 mM) to consume the majority of ·OH radicals formed in 30 min of exposure to light, thus lowering the probability of coumarin to quench ·OH radicals. In this way, the photoluminescence of umbelliferone was expected to diminish in presence of Ph.

The results of Figure 10 confirm that the ·OH radicals produced by TiO2 are able to react with Ph. When the concentration of Ph is small (0.2 mM), the ·OH radicals react preferentially with coumarine, yielding the photoluminescent umbelliferone. When Ph concentration is raised to 2mM, the formation of umbelliferone is depressed by the competing reaction between ·OH and Ph (see orange trace in Figure 10). In case of Au/TiO2 and Ag/TiO2 catalysts, the small residual PL maxima indicate that tinny amount of ·OH radicals are still able to react with coumarine even in presence of Ph in high concentration. Other studies [27] reported that the addition of alcohols have only a limited influence on umelliferone formation because the alcohols are preferentially adsorbed and oxidized by holes on the surface of the photocatalyst, without significant interference of ·OH radicals.

**Figure 10. The** survey of ·OH formation in low (trace 0.2 mM Ph) and high Ph concentration (trace 2 mM Ph) over photocatalyst exposed to simulated solar light for 30 min. Experimental conditions: 0.6 mg photocatalyst was dispersed by ultrasonication in 40 mL solution of coumarin (11 mM)-Ph (0.2 mM) (blue trace 0.2 mM). In second case, the concentration of Ph in 11 mM coumarine solution was increased to 2 mM (orange trace 2 mM).
