Photosensitized degradation of dyes in polyoxometalate solutions versus TiO2 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, H4SiW12O40), and compares it with the analogous process in the presence of TiO2. 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 H2O2 and O-2(.-), was also investigated to clarify the details of the reaction pathway. With the Use Of SiW12O404- 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 TiO2. Strong O-2(.-)/HO2.- ESR signals were detected in the TiO2 dispersions, whereas only weak ESR signals for the O-2(.-) radical ion were seen in the SiW12O404- solutions during the irradiation period. Experimental results imply that reduction of O-2 occurs by different pathways in the two photocatalytic systems.