Photocatalysis of Volatile Organic Compounds in water: Towards a deeper understanding of the role of cyclodextrins in the photodegradation of toluene over titanium dioxide.

HYPOTHESIS: Cyclodextrin-assisted photodegradation of toluene was investigated in water in the presence of a photo-irradiated commercial titanium dioxide photocatalyst. It was expected that cyclodextrins could form water-soluble supramolecular host/guest complexes with the toluene and thus promote the approach of the pollutant on the TiO2 surface and enhance the phototocatalytic oxidation efficiency. EXPERIMENTS: Photodegradation kinetics of toluene were investigated under UV-C and near-visible light radiation in aqueous suspensions of TiO2. Impact of cyclodextrin (CD) on the photocatalytic efficiency of TiO2 was evaluated with different cyclodextrins: α-CD, ß-CD, γ-CD and RAME-ß-CD. Host-guest association constants were determined by static headspace gas chromatography and affinity of cyclodextrins for the TiO2 surface by isothermal adsorption studies. Issue of the cyclodextrin stability during the degradation process was examined using Total Organic Carbon, NMR and MALDI-TOF analyses. FINDINGS: Toluene could be fully mineralized by TiO2 in water within hours, even if the presence of cyclodextrin caused a delay in the photodegradation process. The chemical nature of cyclodextrins was found to exert a significant influence on the extent of inhibitory effect, which was discussed in terms of balance between solubilization efficiency, substrate protection and coverage of active sites of TiO2 by competitive adsorption. The cyclodextrin degradation was also studied and discussed.

Understanding the role of cyclodextrins in the self-assembly, crystallinity, and porosity of titania nanostructures.

A series of mesoporous titania photocatalysts with tailorable structural and textural characteristics was prepared in aqueous phase via a colloidal self-assembly approach using various cyclodextrins (CDs) as structure-directing agents. The photocatalysts and the structure-directing agents were characterized at different stages of the synthesis by combining X-ray diffraction, N2-adsorption, field emission scanning electron microscopy, transmission electron microscopy, UV-visible spectroscopy, dynamic light scattering, and surface tension measurements. The results demonstrate that the cyclic macromolecules efficiently direct the self-assembly of titania colloids, resulting in a fine-tuning of the crystal phase composition, crystallite size, surface area, particle morphology, pore volume, and pore size. Depending on the chemical nature of the substituents in the cyclodextrin ring, synergistic or competitive effects arising from the adsorption capacity of these cyclic oligosaccharides onto titania surface, surface-active properties, and ability to aggregate in water by intermolecular interactions were found to substantially impact the characteristics of the final material. We propose that, in contrast to the native cyclodextrins, which tend to favor the local agglomeration of titania nanoparticles due to the strong intermolecular interactions, the substitution of hydroxyl groups by a relatively large number of methoxyl or 2-hydropropoxyl ones in the ß-CD derivatives allows for creating smoother interfaces, thus facilitating the self-assembly of the colloids in a more homogeneous network. The photocatalytic activity of those titania materials was evaluated in the photodegradation of a toxic herbicide, phenoxyacetic acid, and was correlated to the structural and textural characteristics of the photocatalysts.