Degradation of palmitic (hexadecanoic) acid deposited on TiO2-coated self-cleaning glass:: kinetics of disappearance, intermediate products and degradation pathways

Self-cleaning glass was prepared by depositing anatase nanoparticles as a transparent film onto glass previously coated by a barrier layer. A photoreactor was built to evaluate the efficiency. Palmitic acid, chosen as a compound representing stains from various sources and sprayed over these plates to form an uniform layer of ca. 580 nm thickness, was shown to disappear at a rate of ca. 60 nm h(-1) under UV irradiation corresponding to the average solar radiant power at midlatitude. That clearly demonstrated that this glass is efficient enough, at least with this type of grease stain, to implement its use. Our main efforts explored the identification of palmitic acid degradation products under these conditions by use of chromatographic methods, various solvents for recovering products on the glass, and various adsorbents (cartridges or SPME) for gas-phase analysis. The 39 products identified revealed the gradual splitting of the palmitic acid chain yielding the whole series of linear aldehydes and carboxylic acids, some alkanes and two alcohols. In the closed photoreactor, complete mineralization was achieved. When the photoreactor atmosphere was renewed every hour, formaldehyde, acetaldehyde and acetone reached the highest concentrations in the gas phase. Mechanisms involving the initial attack of palmitic acid either by photogenerated holes or by hydroxyl radicals with subsequent formation of alkylperoxy radicals, hydroperoxides and tetroxides are discussed to account for the products.