Photosensitized Degradation of Dyes in Polyoxometalate Solutions Versus TiO2 Dispersions under Visible‐Light Irradiation: Mechanistic Implications

Abstract

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 404− 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 404− solutions during the irradiation period. Experimental results imply that reduction of O 2 occurs by different pathways in the two photocatalytic systems.