Photosensitized Degradation of Dyes in Polyoxometalate Solutions Versus TiO 2 Dispersions under Visible‐Light Irradiation: Mechanistic Implications

This article examines the photoxidation of a dye (rhodamine‐B, RhB) by visible‐light irradiation in the presence of a polyoxometalate (12‐tungstosilicic acid, H 4 SiW 12 O 40 ), and compares it with the analogous process in the presence of TiO 2 . The photoreaction processes were examined by UV‐visible spectroscopy, fluorescence spectroscopy, high‐performance liquid chromatography (HPLC), liquid chromatography/mass spectral techniques (LC‐MS), and total organic carbon (TOC) assays in order to identify the intermediates produced. Formation of oxygen species, such as H 2 O 2 and O 2 . − , was also investigated to clarify the details of the reaction pathway. With the use of SiW 12 O 40 4− ions as the photocatalyst, the photoreaction leads mainly to N‐dealkylation of the chromophore skeleton. In contrast, cleavage of the whole conjugated chromophore structure predominates in the presence of TiO 2 . Strong O 2 . − /HO 2 . − ESR signals were detected in the TiO 2 dispersions, whereas only weak ESR signals for the O 2 . − radical ion were seen in the SiW 12 O 40 4− solutions during the irradiation period. Experimental results imply that reduction of O 2 occurs by different pathways in the two photocatalytic systems.