Revisiting terephthalic acid and coumarin as probes for photoluminescent determination of hydroxyl radical formation rate in heterogeneous photocatalysis

We investigated in detail the applicability of two radical scavengers – coumarin (COUM) and terephthalic acid (TA) – as probe compounds in a widely used photoluminescent (PL) determination of OH∙ radical rate formation of photocatalysts. The study reveals that precautions must be taken when using either of the two compounds as neither allows a direct determination of the absolute OH∙ radical formation rate. Chromatographic analyses of irradiated COUM and TA solutions revealed that both probe compounds reacted via more than one pathway, out of which, only one pathway in each case yielded the measured photoluminescent 7-hydroxycoumarin (7-OHC) and 2-hydroxyterephthalic acid (TAOH), respectively. Applicability of both probes was also tested on three model TiO2-based catalysts (anatase TiO2 nanorods (TNR), anatase TiO2 nanoparticles (TNP), amorphous TiO2 nanorods (a-TNR)). Regardless of the probe compound used, the order of relative OH∙ radical formation rates determined for these catalysts was the same (TNR < TNP < a-TNR) and in good correlation with the order of bisphenol A (BPA) degradation rates. This demonstrates that (i) formation of OH∙ radicals is the predominating criterion when probing a photocatalyst’s activity (but not the only one) and that (ii) TA and COUM potentially enable a relative evaluation of photocatalytic materials, despite the numerous shortcomings of these probes. However, in order to allow for general inter-laboratory comparisons the photoluminescence method should clearly be standardized with precisely defined experimental parameters.