Alcohols as Scavengers for Hydroxyl Radicals in Photocatalytic Systems: Reliable or Not?

Abstract

The evaluation of the contributions of hydroxyl radicals ( • OH) is of great significance toward the elucidation of photocatalytic mechanisms. Currently, trapping (or quenching) tests have been widely conducted for the determination of reactive species, where alcohols are often selected as • OH scavengers. However, their feasibility and reliability have been under scrutiny by researchers and need to be further explored. For this study, a photocatalytic system was established using P25 as the photocatalyst and bisphenol A (BPA) and oxalic acid as model pollutants under UV light irradiation. The quenching capacities of five alcohols were investigated at concentrations ranging from 0.5 to 100 mM. The alcohols revealed different quenching abilities in the BPA and oxalic acid systems. Further, the amount of H 2 O 2 generated from highly concentrated alcohols was quantified, which significantly influenced the estimation of the contribution of • OH. Unexpectedly, • O 2 − was also formed in the system, whose contribution to the degradation of pollutants was also mistakenly attributed to • OH. This study provides a comprehensive evaluation of the five most commonly used alcohols as scavengers for • OH. Based on the findings, it strongly recommends reconsidering these alcohols and emphasizes the need for developing more reliable methods for detecting radicals.

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“…A similar problem exists when IPA or BQ is added to the reaction solution. It is worth noting that some literature has offered new insights into capture experiments [41,42]. This makes us doubtful about the role of holes, •OH and •O 2 − in photocatalysis by capture experiments.…”