TiO₂-Assisted Photodegradation of Dyes. 9. Photooxidation of a Squarylium Cyanine Dye in Aqueous Dispersions under Visible Light Irradiation

Abstract

The TiO 2 -assisted photodegradation of a squarylium cyanine dye (SQ) has been examined under visible light irradiation (λ g 430 nm) by UV-vis, proton-NMR, ESR, and GC-MS spectroscopies, by peroxide assays in the presence of peroxidase and catalase enzymes, and by chemical oxygen demand (COD) methods. Significant results were obtained relevant to the mechanism(s) of TiO 2 -assisted photodegradations. The active oxygen species produced first when an aqueous TiO 2 dispersion containing SQ is irradiated by visible light is the superoxide radical anion, which is stable in methanol solvent but very unstable in aqueous media, and is readily converted to • OH radicals via formation and subsequent reduction of H 2 O 2 . The quantity of H 2 O 2 increased during the photodegradation; though expected, no organoperoxides were detected. Of import, cleavage of the cyanine CdC double bond in the SQ dye dominated over the whole degradation process yielding 1-sulfopropyl-3,3-dimethyl-5-bromoindolenium-2-one, the predominant component in the dispersion. This intermediate is not excited by visible light and degrades no further. A sequence of steps is proposed in the initiation of the TiO 2 -assisted photochemical transformation.

Light-Induced Efficient Molecular Oxygen Activation on a Cu(II)-Grafted TiO2/Graphene Photocatalyst for Phenol Degradation

“…34 These results reveal that both •O 2 -and •OH were generated from the irradiated photocatalyst samples. To further assess ROS generation, temporal evolutions of the ESR signal were displayed in Figure 5a and 5b, which were performed under the same conditions for comparison.…”

Catalysis of organic pollutant photodegradation by metal phthalocyanines immobilized on TiO2@SiO2

“…ESR is a modern separation technique used to determine shortlived free radicals. It can be useful to elucidate the mechanism of photocatalytic reactions [26]. To detect and identify short-lived free radicals, the non-saturated anti-magnetic material (spin trapping agent) DMPO was added to MnPcSTiO 2 @SiO 2 with visible light irradiation.…”

“…This result indicates that the photocatalytic reaction involves ·OH, which is a catalytic reaction intermediate species. In methanol, the characteristic peaks of DMPO-O 2 −· adduct [26] were observed under visible light irradiation ( Figure 12(b)). The observed peaks were similar to those reported by Zhao et al [28].…”

A DFT-based Analysis on H2O Molecule Adsorption and Dissociation on the Rutile TiO2 (110) and (100) Surfaces

“…Research on TiO 2 for photocatalytic water splitting has set its relevance in itsˆeld as much attention have recently been focused in the advancement of artiˆcial photosynthesis technology which is seen to be a good candidate for alternative energy source. Often referred to as the``holy grail'' of heterogeneous photocatalysis, water splitting is viewed as a way to produce H 2 and O 2 from the splitting of the H 2 O molecule through a proper photocatalyst and using the energy of the sun (UV rays or in some cases visible light [5][6][7][8] ) as the bias voltage to initiate the photocatalytic process. Research interest on thisˆeld started from the works of Honda and Fujishima 9) on the photoelectrochemical cell, where they used a TiO 2 photoanode and a platinum photocathode to split H 2 O to H 2 and O 2 under the irradiation of near-UV light.…”