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A. Bachinger, G. Kickelbick:
"Degradation Kinetics of Organic Molecules Covalently Connected to TiO₂ Photocatalyst via Phosphonate or Phosphate Groups under UV-irradiation";
Poster: 3^rd EuCheMS Chemistry Congress, Nürnberg; 29.08.2010 - 02.09.2010; in: "Programme - Abstracts", (2010), 1 S.

Titanium dioxide photocatalysts have gained considerable attention recently due to their optical and electronic properties, chemical stability and low cost. Their applications are mainly found in the degradation of organic compounds, for example in wastewater treatment or for self-cleaning surfaces. Numerous studies on the degradation of organic molecules in the presence of titania photocatalysts under UV irradiation could elucidate the degradation mechanism [1-3]. It is nowadays well known, that on the surface of the photocatalyst hydroxyl-radicals are formed, which can subsequently degrade any organic molecule. However, to our knowledge, no intense studies on the degradation behavior of organic molecules with a covalent connection to the photocatalyst surface have been conducted. We assume that the degradation mechanism and kinetics of organic molecules that are covalently connected via stable phosphonate and phosphate coupling groups differs from free organic molecules. For this reason, photocatalytically active anatase nanoparticles were modified with organic phosphonates and phosphates. The P-O-Ti bond is known to be highly stable [4] and thus adequate for our studies. The modified particles were dispersed in different solvents at different concentrations and illuminated with two 9 W UVA black light lamps or a 150 W mercury lamp. Samples were taken after different times and analyzed by FT-IR, CPMAS NMR, TGA and elemental analysis. We found the P-O-Ti bonds to be stable under illumination with 9 W and 150 W, while the organic moiety was degraded already at 9 W UV-illumination. The degradation rate was found to depend on the particle concentration, the solvent as well as the irradiance. The degradation kinetics of phenylphosphonic acid was compared with phenylphosphoric acid and dodecylphosphonic acid. The aliphatic moiety was found to be degraded faster than the aromatic one. Spectroscopic investigations revealed that the degradation proceeds rather sequentially than via breakage of the P-C or P-O-C bond. Illumination experiments in water-free environments revealed that no water is required for the degradation. Thus we could show that the UV-degradation mechanism of organics that are covalently connected to the photocatalyst does not mainly proceed via hydroxylradicals but via radical transfer and thus is different from the degradation mechanism of free organic molecules.

Projektleitung Guido Kickelbick:
Chemisch massgeschneiderte Grenzflächen in Nanokompositen