Corinne Lehaut

Comparison of various titania samples of industrial origin in the solar photocatalytic detoxification of water containing 4-chlorophenol

The degradation of 4-chlorophenol (4-CP) was used as a model reaction to investigate the photocatalytic properties of different industrial TiO2 catalysts and to compare their efficiencies in the treatment of contaminated waters. Parallel experiments have been performed using either artificial UV-light in a batch photoreactor in laboratory experiments or solar energy in the flow reactor of the pilot plant at Plataforma Solar de Almeria (PSA) in Spain. Depending on the kinetic criteria chosen for comparison ((i) initial rate of pollutant disappearance; (ii) amounts of intermediate products present in solution at a given time; (iii) time necessary to obtain total mineralization), different classifications of photocatalyst activities were observed. Among various physical characteristics such as particle size, structure or active site density, which may intervene on the photocatalytic activity, the influence of the surface area appeared of prime importance. For lower surface area catalysts, there is a decrease in the readsorption rate of intermediate products and consequently in the overall photodegradation rate. By contrast, for higher surface area catalysts, it was observed a lower final rate of total organic carbon (TOC) disappearance because of a very low coverage in pollutant which favors electron–hole recombination. The comparison of the kinetic results for decontaminating water at PSA and in laboratory experiments indicated the same kinetic order (apparent first order) for 4-CP disappearance and the same apparent quantum yield. However, fewer intermediate products and a faster TOC disappearance were observed in the solar pilot reactor at PSA, which, in addition, worked with a smaller optimum concentration of suspended titania. This was ascribed to the design of the photoreactor.

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.