Enhanced methylene blue photodegradation with propylene carbonate as a solvent

The role of water in the TiO2-based photocatalytic phenomenon is not yet fully understood. The photocatalysis of methylene blue dissolved in propylene carbonate and different concentrations of water was studied. It was observed that the photocatalytic activity of TiO2 (P25 from Evonik) peaks when propylene carbonate solvent is used with minute amounts of water; the maximum photodegradation rate was ca. 6.5 times higher than when just water solvent was used. The conventional interpretation of the methylene blue photooxidation intermediated by free radical OH cannot explain these results. Alternately, the experimental results were interpreted based on the recently proposed “direct-indirect” (D-I) model and a mathematical model was successfully developed and fitted to the experimental results. Finally, new insights on the role of water in the photocatalytic phenomenon were withdrawn.