A Methodology to Discriminate Between Hydroxyl Radical-induced Processes and Direct Charge-transfer Reactions in Heterogeneous Photocatalysis

Abstract

Abstract A method to assess the ability of a photocatalyst to induce reactions with free or trapped hydroxyl radicals versus direct charge-transfer processes is here proposed, based on the use of phenol and 2-hydroxybenzoic acid (salicylic acid) as test substrates. The rationale is that phenol degradation would be preferentially (although not exclusively) induced by hydroxyl radicals, while salicylic acid would mainly undergo direct charge-transfer oxidation. The use of t-butanol as selective ·OH scavenger is helpful to understand how much each substrate is a selective indicator of the intended reaction pathway in the presence of a given semiconductor oxide. Phenol and salicylic acid should be used at low concentration (e.g. 25 μmol L-1) to limit the occurrence of the back-reactions, the importance of which can be highlighted by using higher initial concentration values (e.g. 1 mmol L-1). The method was optimized with the well-studied photocatalysts Evonik P25 and Wackherr's “Oxyde de titane standard”, and it was then applied to study the behavior of two TiO2/Al2O3 binary oxide systems (where TiO2 occurs as a mixture of anatase and rutile). The latter photocatalysts were poorly efficient toward the degradation of phenol, but they performed better with salicylic acid. These findings, which are coherent with the results of t-butanol addition, suggest that the two binary oxide systems would induce charge-transfer rather than ·OH reactions.