Coumarin as a Quantitative Probe for Hydroxyl Radical Formation in Heterogeneous Photocatalysis

Abstract

In this work, we have assessed coumarin as a quantitative probe for hydroxyl radical formation in heterogeneous photocatalysis. Upon reaction with the hydroxyl radical, coumarin produces several hydroxylated products, of which one, 7-OH-coumarin, is strongly fluorescent. The fluorescence emission is strongly affected by inner filtering due to the presence of coumarin. Therefore, we performed a series of calibration experiments to correct for the coumarin concentration. From the calibration experiments, we could verify that the inner-filtering effect can be attributed to the competing absorption of the fluorescence excitation light between coumarin and 7-OH-coumarin. Through judicious calibration for the inner-filtering effects, the corrected results for the photocatalytic system show that the rate of hydroxyl scavenging is constant with time for initial coumarin concentrations of ≥50 μM under the conditions of our experiments. The rate increases linearly with coumarin concentration, as expected from the Langmuir–Hinshelwood model. Within the coumarin concentration range used here, the photocatalyst surface does not become saturated. Given the fact that the highest coumarin concentration used (1 mM) in this work is quite close to the solubility limit, we conclude that coumarin cannot be used to assess the full photocatalytic capacity of the system, i.e., surface saturation is never reached. The rate of hydroxyl radical scavenging will, to a large extent, depend on the affinity to the surface, and it is therefore not advisable to use coumarin as a probe for photocatalytic efficiency when comparing different photocatalysts.

Confirming the Formation of Hydroxyl Radicals in the Catalytic Decomposition of H2O2 on Metal Oxides Using Coumarin as a Probe

“…Given the strong adsorption of hydroxyl radicals to oxide surfaces demonstrated by both DFT calculations and experiments assessing the reduction potential of the surface‐bound hydroxyl radical compared to the free hydroxyl radical, desorption of the radical would appear to be a very unlikely process. Also, the concentration dependence on the rate of coumarin hydroxylation in heterogeneous photocatalysis differs significantly from that observed in homogeneous gamma radiolysis (where hydroxyl radicals are produced homogeneously in solution) which points in the direction of surface‐bound hydroxyl radicals in the case of heterogeneous photocatalysis …”

“…Also, the concentration dependence on the rate of coumarin hydroxylation in heterogeneous photocatalysis differs significantly from that observed in homogeneous gamma radiolysis (where hydroxyl radicals are produced homogeneously in solution) which points in the direction of surface-bound hydroxyl radicals in the case of heterogeneous photocatalysis. [12,20] Although a high coumarin concentration (resulting in a larger amount of probe being adsorbed on the surface of the catalyst) would provide a higher radical scavenging capacity, the competing absorption of the excitation light by coumarin in the fluorescence measurement of 7-hydroxycoumarin drastically reduces the sensitivity of the method. [20] There are several ways to handle this problem.…”

“…[12,20] Although a high coumarin concentration (resulting in a larger amount of probe being adsorbed on the surface of the catalyst) would provide a higher radical scavenging capacity, the competing absorption of the excitation light by coumarin in the fluorescence measurement of 7-hydroxycoumarin drastically reduces the sensitivity of the method. [20] There are several ways to handle this problem. By far, the easiest way is to perform the calibration of the fluorescence measurements using solutions with the same coumarin concentrations as used in the experiments.…”

“…[20] However, the affinity of coumarin towards the TiO 2 -surface was assessed to be similar to that of tris. [20] Nevertheless, a fraction of the coumarin is adsorbed to the surface and capable of scavenging hydroxyl radicals. In a previous paper, the tris concentration dependence on the formaldehyde production rate in a system where H 2 O 2 was catalytically decomposed on ZrO 2 was studied.…”

Dynamics of Spent Nuclear Fuel Dissolution : Impact of Catalysis, Matrix Composition and Time Evolution

“…However, coumarin itself is poorly fluorescent [108] and due to an inner filtering effect i.e. high absorption of the excitation light by coumarin, the signal of 7-hydroxycoumarin is reduced by the presence of coumarin especially at high probe concentrations [109]. The calibration for 7-hydroxycoumarin was therefore made in the presence of 0.05 mM cou-marin.…”