Semi-automated EPR system for direct monitoring the photocatalytic activity of TiO2 suspension using TEMPOL model compound

The photocatalytic activity of TiO2 nanoparticles in aqueous solutions is commonly evaluated by monitoring the rate of methylene blue 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 TiO2 suspensions without the need for filtration. The results showed that TEMPOL aqueous solution is in the dark and under UV-A illumination, does not absorb UV-A and visible light, and has negligible dark adsorption. The influence of TEMPOL concentration, light intensity, and TiO2 loading on the photocatalytic deactivation rate has been investigated. The mechanisms of TEMPOL deactivation in the presence and absence of oxygen as well as in the presence of methanol (OH)-O-center dot radicals' scavenger have been discussed. The photocatalytic deactivation products have been analyzed using EPR, H-1-NMR, and C-13-NMR spectroscopies. It is found that the deactivation of TEMPOL is initiated by (OH)-O-center dot radicals and alpha-H abstraction from the 4-piperidine position followed by the formation of TEMPONE (4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl) and 4-oxo-2,2,6,6-tetramethylpiperidine). In the presence of methanol, the formed alpha-hydroxyl radicals ((CH2OH)-C-center dot) attack the nitroxide side of TEMPOL and produce 4-hydroxy-tetramethylpiperidine. Same activity trends have been observed for the photocatalytic methanol oxidation and 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. [GRAPHICS] .

Automated summary

This study investigated the potential use of electron paramagnetic resonance (EPR) and TEMPOL to directly monitor the photocatalytic activity of TiO2 suspensions without the need for filtration. The results showed that TEMPOL had negligible dark adsorption and could effectively evaluate the activity of photocatalysts. The mechanisms of TEMPOL deactivation and the formation of deactivation products were analyzed.