The photocatalytic activity of TiO2 nanoparticles in aqueous solutions is commonly evaluated by monitoring the rate of methylene blue (MB) bleaching and phenols degradation, but both substrates suffer from many drawbacks, e.g. the high capacity of dark adsorption, self-degradation, and photosensitization. Besides,filtration is always required to separate the particulate photocatalyst before the analysis. Herein, we investigated the potential use of electron paramagnetic resonance (EPR) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) to directly monitor the photocatalytic activity of different types of TiO2 suspensions without the need for filtration. TEMPOL does not absorb UV-A and visible light, and thus the photosensitization problem associated with the MB-dyes and phenolic compounds is readily overcome. The dark adsorption results indicated that the adsorption of TEMPOL on TiO2 is negligible. Unlike MB dye, the TEMPOL can be directly used without spending a long-time establishing adsorption equilibrium before the photocatalytic test. The mechanisms of TEMPOL deactivation, in the presence and absence of oxygen (electron acceptors) as well as in the presence of methanol (•OH radicals’ scavenger) have been discussed. Moreover, the photocatalytic deactivation products have been analysed using EPR data simulation, 1H-NMR, and 13C-NMR spectroscopies. Finally, TEMPOL deactivation over different types of TiO2 photocatalysts evincing that the proposed method has a potential for direct monitoring of the activities of photocatalyst suspensions.
Audience Take Away:
- By monitoring TEMPOL-EPR signal decay, the rate of the photocatalytic reaction can be instantly examined without the need for the filtration of the photocatalyst particles.
- A direct semi-automated EPR-photocatalytic setup could be developed to examine the photocatalytic activity of different catalysts’ suspensions relative to Degussa P25-TiO2 photocatalyst as a reference.
- In addition to the kinetic exploration of the reactive oxygen species (ROS’s), the EPR could be used to detect a trapped ROS, such as •OH radicals, using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a conventional spin trapping agent.
- The unavailability of the EPR machine in some labs (e.g. SpinScan X of Linev Systems, of a price comparable to a UV-Vis spectrometer) might limit the widespread use of this method.