**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**

**Alexandra Sandulescu 1, Crina Anastasescu 1, Florica Papa <sup>1</sup> , Monica Raciulete <sup>1</sup> , Anca Vasile 1, Tanta Spataru 1, Monica Scarisoreanu 2, Claudiu Fleaca 2, Cristian N. Mihailescu 2, Valentin S. Teodorescu 3, Nicolae Spataru 1,\*, Maria Zaharescu <sup>1</sup> and Ioan Balint 1,\***


**Abstract:** The specific roles played by both support and noble metals in light absorption, charge separation, and the formation of ·OH and O2 − (ROS) are analyzed for light-triggered oxidation of phenol (Ph) over pristine and over noble metal (Ag, Au, Pt) -loaded TiO2. Experiments show that the supported noble metals act as a light visible absorber, assist the separation of photo-charges and reduction of O2 to O2 −. The O2 − oxidizes mildly Ph to oxygenated products (hydroquinone, benzoquinone, and 1,2-dihydroxibenzene). In a parallel process, ·OH radicals, yielded by TiO2, mineralize Ph to CO2 by fast reaction sequences. Radical quenching and photo electrochemical measurements (surface photovoltage) confirm independently that the production of ·OH and O2 − scale with oxidative conversion of Ph. The selectivity to CO2 and mild oxidation products is the result of the interplay between catalyst activity for ·OH and for O2 − production.

**Keywords:** photocatalytic mild oxidation; photocatalytic mineralization; phenol oxidation; TiO2 modified with noble metals; surface photovoltage; O2 photo reduction; reactive oxygen species; charge recombination
