title: 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(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.
authors.affiliation: Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.
authors.forename: Chuncheng
authors.initials: C
authors.lastname: Chen
authors.forename: Wei
authors.initials: W
authors.lastname: Zhao
authors.forename: Pengxiang
authors.initials: P
authors.lastname: Lei
authors.forename: Jincai
authors.initials: J
authors.lastname: Zhao
authors.forename: Nick
authors.initials: N
authors.lastname: Serpone