**1. Introduction**

The sun light driven photo oxidation processes of organic matter are of great importance for several practical reasons: (i) imply low material and operational costs, (ii) are potentially able to clean water and air by mineralization of organic pollutants to CO2 [1–4], and (iii) are attractive alternative routes for selective synthesis of high-added value oxygenated products [5–7].

The thermodynamic of organics oxidation is downhill process (ΔG < 0), the light being used to speed up chemical reaction via generation of charge carriers. The general accepted steps in photocatalytic processes are: (i) light absorption by photocatalysts followed by generation of e<sup>−</sup> and h+ charges; (ii) charge transfer to reactant substrate intermediated by reactive oxygen species, ROS; (iii) development of redox processes with participation of e−, h+, and ROS on surface and vicinity of photocatalysts. Metals are involved actively in all essential reaction steps, determining the final overall photocatalytic efficiency [8,9].

The prevalent reaction mechanism in liquid phase depends on a series of factors including nature of photocatalyst, reaction media, and reacting organic substrate. The reac-

**Citation:** Sandulescu, A.; Anastasescu, C.; Papa, F.; Raciulete, M.; Vasile, A.; Spataru, T.; Scarisoreanu, M.; Fleaca, C.; Mihailescu, C.N.; Teodorescu, V.S.; et al. Advancements on Basic Working Principles of Photo-Driven Oxidative Degradation of Organic Substrates over Pristine and Noble Metal-Modified TiO2. Model Case of Phenol Photo Oxidation. *Catalysts* **2021**, *11*, 487. https://doi.org/ 10.3390/catal11040487

Academic Editor: Ewa Kowalska

Received: 16 March 2021 Accepted: 7 April 2021 Published: 10 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

tion mechanism, described well by the Langmuir–Hinshelwood adsorption equation [10], implies the interaction of photogenerated charges with the adsorbed species, at a time scale of 10−<sup>10</sup> to 10−<sup>5</sup> s [11]. The photo generated charges (e<sup>−</sup> and h+) react first with hydroxyl groups of adsorbed H2O and O2 to yield reactive ·OH (H2O+h<sup>+</sup> → ·OH + H+) and ·O2 <sup>−</sup> (O2 + e<sup>−</sup> → O2 −) ROS [9]. The oxidative conversion of organic compounds is intermediated by formation and diffusion of ROS to reaction scene [12], which can be remote from the illuminated surface [13].

Metals dispersed on surface of active materials (i) help separation of photo generated charges, (ii) work as cocatalyst by mediating the charge transfer to reacting substrates, (iii) favor the formation of O2 −, (iv) control the selectivity of oxidation process, (v) bend the energy bands of photocatalysts at solid-liquid interfaces, (vi) modify the light absorbing property of materials, (vii) contribute to enhancement of photocharge production in visible wavelength domain by surface plasmon resonance (SPR) phenomenon [9]. The Schottky regions built at metal-oxide interfaces contribute to electron and hole separation, leading to increased efficiency of photo-driven redox processes. The charge separation efficiency is validated experimentally by the comparing the PL (Photoluminiscence) emission intensity of metal-loaded photocatalysts with pristine oxide [14,15]. The bending of valence band (VB) and conduction band (CB) depends on nature of metal and pH of solution. Metals shift the light absorption edge, in many cases with beneficial effects on efficiency. Noble metals exhibit visible light absorption peaks due SPR phenomenon, which is a collective electron oscillation in metal nanoparticles induced by visible light absorption. The SPR was reported to have in some cases favorable effects on photo-driven redox processes performed in visible light [16–18].

In spite of a large number of published researches, essential issues in photocatalysis remain to be elucidated. For example, the specific contribution of metal and support to formation of O2 <sup>−</sup> and ·OH is, in many cases, controversial, although it is of crucial importance. The predominance of one reaction pathway over the other affects the selectivity of photocatalytic oxidation reaction. The reaction of an organic matter with ·OH is unselective, yielding CO2, whereas oxidation pathway with ·O2 − on surface of solid or on its premises proceed apparently with high selectivity to oxygenated products.

The general aim of this research is to gain a deeper insight on particularities showed by metal-modified oxide photocatalysts compared to pristine semiconductor oxides in what concerns ROS generation, charge separation, reaction mechanism of organic compounds oxidative conversion. The role of active participants in the oxidative degradation pathways are analyzed in detail. The light-driven (sun or visible light) oxidation of phenol over pristine and metal (Ag, Au, Pt)-modified TiO2 was chosen as model reaction. To uncover the complex reaction network associated to photo-driven oxidation of organic compounds, we analyzed comparatively: (i) the relative efficiency of supported noble metals in separation of photo charges and implicitly the impact on photocatalytic activity, (ii) the specific contribution of metal and of oxide support to ROS (·OH and O2 −) formation, (iii) the relationship between O2 − formation and reaction selectivity to oxygenated products, (iv) the correlation between the activity of catalyst for mineralization of organic substrate to CO2 and the amount of ·OH generated under light irradiation, and (v) the relationship between nature of supported metals and surface photovoltage (SPV) generated under light irradiation in connection with material capacity to generate O2 −, with obvious implications in reaction mechanism.

### **2. Results and Discussion**
