Invited Paper SE-TuA11
Rules for Efficient Titania Coatings. Applications to Water, Air and Material Cleaning
Tuesday, October 21, 2008, 5:00 pm, Room 204
Photocatalysis concerns Fine, Green and Environmental Chemistry. Titania coatings must follow the rules of photocatalysis with the influence of 5 basic parameters governing the activity (mass of catalyst, wavelength, concentrations or partial pressure of reactants, temperature and radiant flux). Besides mild selective oxidations obtained by generating active neutral atomic O* species in the absence of water, photocatalysis can detoxify inorganics in water by oxidizing them in their harmless upper oxidized state. Similarly, organic pollutants are totally oxidized in CO2. Complex molecules (pesticides, dyes, herbicides, insecticides) are totally destroyed as well as bacteriae (E.Coli, streptococcus faecalis) in water without re-growth. Air pollutants (VOC’s, odors) can also be destroyed by cracking OH° radicals produced by UV-irradiated titania-coated supports. The applications concern domestic anti-odor refrigerators, electronics with a “molecular” purity of the ambient working atmosphere, the destruction of odors emitted by water treatment plants and solid waste landfills. Eventually, Virus H5N2, a model virus close to H5N1, responsible for the aviar flu, was totally destroyed in a contaminated air flux of 40 m3/h with a 99.93% efficiency in a single pass. Photocatalytic self-cleaning technology consists in depositing sub-micronic thin layers of titania at the surface of glass, metals, concrete, etc, whose photocatalytic activities are calibrated by test-reactions. In the real world, dirty materials, such as glasses, to be “self-cleaned” are soiled by greasy and sticky deposits, which, in addition, induce a strong adherence of ambient dusts. The mechanism of self-cleaning glasses was elucidated by the mineralization of palmitic acid (n-C15H31-COOH), present in the human sebum and in the natural varnish of plants. It disappeared by successive “photo-Kolbe” reactions with photo-produced holes h+. The linear hydrophobic aliphatic chains are progressively “peeled off,” carbon atom after carbon atom, via successive photo-Kolbe reactions. Palmitic acid degradation would require 16 of them. Fortunately, the overall self-cleaning glass process can be strongly accelerated by by-passing many intermediates via the release of several VOC’s in the atmosphere, quite innocuous for the environment. The efficiency of titania demands two requirements: (i) a good cristallinity (anatase phase) and (ii) a high (cationic) purity.