*3.2. Detection of* ·*OH Radicals*

Coumarin was used as selective trap for the ·OH radicals formed under photocatalysts exposure to light [12,27]. The 0.001 g of powder catalysts were first suspended in 40 mL of 11 mM coumarin (Merck) aqueous solution and then exposed either to simulated solar light AM 1.5 or to visible light. A cut off filter (L42, Asahi Spectra, California, USA) was in the

case of visible light (λ > 420 nm). Aliquots of 1.5 mL solution were sampled at 10 min time interval for fluorescence measurements (Cary Eclipse Fluorescence Spectrophotometer, Agilent Technologies, Santa Clara, CA, USA) to monitor umbelliferone, formed by reaction between coumarin and ·OH radicals. Umbelliferone gives a specific fluorescence peak at ≈ 450 nm for λexc = 330 nm.

#### *3.3. Detection of O2* −

In a typical experiment, 0.004g of catalyst was suspended into 4 mL of 3 mM solution of XTT sodium salt (2, 3-bis(2-methoxi-4-nitro-5sulfophenyl)-2H-tetrazolium-5-carboxanilide) (Alfa Aesar). Then, the samples were exposed to simulated solar or visible light for 10, 20, and 30 min to induce the formation of O2 −. XTT reduction by O2 − produces XTTformazan, which can be evidenced by a broad absorption peak at ≈ 470 nm [12,44]. The measurements were carried out with UV-VIS spectrophotometer (Analytik Jena Specord, 200 Plus, Jena, Germany).

For the surface photovoltage (SPV) measurements, a sandwich-like photovoltaic cell was built, according to a method previously described in the literature [30]. Briefly, a small amount (ca. 12 mg) of pristine or metal-modified titanium oxide was firmly pressed in between two ITO electrodes, to obtain a confined film composed of the investigated powder sample. The upper face of the cell was irradiated (under chopped conditions) with a monochromatic light (the light beam of 300 W Xe lamp of Asahi Spectra MAX-350 (Tokyo, Japan) light source was passed through high transmission bandpass filters with FWHM = 11 nm) and the SPV signal was measured by means of a computer-driven Keithley 2425 source-meter (Cleveland, Ohio, USA). The energy of the monochromated light beam was measured with Newport optical power meter (Model 1830-R, Irvine, CA, USA) equipped a calibrated photodiode detector (Newport, 918D series). For the chronoamperometric experiments, the same cell was used and the measurements were carried out in an air-tight reactor by means of a PAR 273A (Princeton Applied Research Walpole, MA, USA) potentiostat, both under pure O2 and Ar atmospheres.

### **4. Conclusions**

This study gives a comprehensive view on the light-initiated photocatalytic oxidation pathways of a model organic substrate with an aromatic ring (Ph) over bare and noble metal-loaded TiO2. The analysis of complex phenomena associated with photocatalytic reaction focuses on particular roles played by oxide support and by noble metals on light absorption, charge separation, formation of ROS (·OH and O2 −), as well as on reaction mechanism of oxidative conversion of Ph.

We have found out that TiO2 support generates only ·OH as ROS when it is exposed to light with λ < 400 nm. These radicals are responsible for deep oxidation of Ph directly to CO2, apparently without the formation of detectable long-lived intermediates. The formation of ·OH, and consequently the photocatalyst activity, cease in visible light domain.

Deposited noble metals (Ag, Au, Pt) (i) adsorb the visible light (SPR phenomenon), (ii) assist effectively the charge separation, and the (iii) O2 reduction to O2 −. The deposited metal raises the Fermi level of TiO2 allowing the reduction of adsorbed O2 to O2 −. The O2 − produced on metals oxidizes mildly Ph to oxygenated products (HQ, BQ, 1,2 DHBz). In a parallel process, ·OH radicals produced by TiO2 support mineralize Ph directly to CO2 by fast reaction sequences. At this stage, it is not clear the precise function of metals in the reaction between organic substrate (Ph) and O2 −. With the exception of Au, the hot electrons produced by SPR at λ > 400 nm are not active to produce measurable amounts of O2 −.

This study demonstrates, by two complementary experimental methods (radical quenching and photo electrochemical measurements), that production of ·OH and O2 − over the investigated catalysts correlates well with the activity showed for oxidative conversion of Ph. According to our data, the oxidation of Ph by photo charges is intermediated by ROS.

In light of our results, the bare TiO2 suits the best the photocatalytic depollution purposes, where the aim is to mineralize the harmful organic substrate to CO2. When noble metals are deposited on TiO2, intermediate oxygenated compounds are formed by mild oxidation of organic substrate(s) by O2 −, via photo induced electron transfer from metals to O2. Thus, from a depollution point of view, the modification of TiO2 with noble metals is not beneficial. In addition, the metal-modified photocatalyst in powder form dispersed in water can be harmful to the environment. Same assessment can be made for photo water splitting, where the consumption of photo-generated electron by adsorbed O2 hinders H<sup>+</sup> reduction. On the other hand, should be the practical aim of valuable oxygenated compounds synthesis by mild selective oxidation of organic compounds, the use of catalytic metals is mandatory.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/catal11040487/s1, Figure S1: High resolution XPS spectra of TiO2 in O1s and Ti2p binding energy regions, Figure S2: High resolution XPS spectra of Ag/TiO2 in O 1s, Ti 2p and Ag 3d binding energy regions, Figure S3: High resolution XPS spectra of Au/TiO2 in O 1s, Ti 2p and Au 4f binding energy regions, Figure S4: S4 High resolution XPS spectra of Pt/TiO2 in O 1s, Ti 2p and Pt 4f binding energy regions, Figure S5: Comparative XRD difraction patterns of simple and metal-modified TiO2. •-anatase, +-rutile, Table S1: Elemental composition obtained from EDAX analysis of simple and metal -modified TiO2, Table S2: XPS survey of elemental composition of simple and noble metal-modified TiO2, Table S3: Chemical state of titanium in the investigated materials, Table S4: Crystalline phase composition and average crystallite size of simple and metal-modified TiO2.

**Author Contributions:** Conceptualization, I.B., Investigation, A.S., C.A., F.P., M.R., A.V., T.S., M.S., C.F., C.N.M., V.S.T., N.S., M.Z. and I.B., Methodology, A.S., C.A., F.P., M.R., A.V., T.S., M.S., C.F., C.N.M., V.S.T., N.S., M.Z. and I.B., Resources, A.S., C.A., F.P., M.R., A.V., T.S., M.S., C.F., C.N.M., V.S.T., N.S., M.Z. and I.B., Supervision, N.S. and I.B., Visualization, I.B., Writing—original draft, N.S. and I.B., Writing—review & editing, N.S. and I.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was supported by Grants 46 PCCDI/2018 MALASENT.

**Conflicts of Interest:** The authors declare no conflict of interest.
