Deactivation and regeneration of visible light active brookite titania in photocatalytic degradation of organic dye

This study was performed to investigate deactivation and regeneration of predominantly brookite titania powders. Visible light active (VLA) polymorphic titania particles were prepared by a water-based ambient condition sol (WACS) process followed by a solvent-based ambient condition sol (SACS) process with N-methylpyrrolidone (NMP) as the solvent. Deactivation of all samples was evaluated by degradation of methyl orange (MO) dye under either UV or VL irradiation for four reaction cycle runs. The photocatalytic activity (PCA) under VL exposure of NMP samples calcined at 200 °C for 2 h in air (NMP-200) was approximately 3.3 times higher than that of a commercial VLA titania (Kronos VLP7000), despite the much lower surface area of the NMP-200. Nonetheless, PCA of all titania samples under VL gradually decreased with an increase in testing time and number of runs. The cause of the deactivation of the titania samples in this study was identified as the deposition of the decomposed MO on the titania nanoparticle surface. Among the possible regeneration or reactivation procedures for the used samples, methanol washing was shown to be the most effective up to ∼80% of the PCA recovery. The used NMP-200 samples could not completely be recovered, since a regeneration process would possibly remove some of nitrogen species responsible for the VLA.