Environmental photocatalysis: Perspectives for China
Résumé
Photocatalysis is based on the double aptitude of the photocatalyst (essentially titania) to simultaneously adsorb reactants and absorb efficient photons. Heterogeneous photocatalysis is able to be efficient in Fine, "Green" and Environmental Chemistry. Photocatalysis induces mild oxidations in the absence of water by generating active neutral atomic O* species. For instance, the oxidation of 4-tert-butyl-toluene is 100% selective in 4-tert-butyl-benzaldehyde. In water treatment, many toxic inorganic ions are oxidized in their harmless upper oxidized state. The elimination of organic pollutants is the main field of water photocatalytic decontamination. Most of aliphatic and aromatic pollutants are totally mineralized into CO2 and innocuous inorganic anions. More complex molecules, such as pesticides (herbicides, insecticides, fungicides, etc.) or dyes, are totally destroyed. Another example of green chemistry is the total degradation of dyes in water, in particular for the azo-dyes, with 100% selective degradation of -N=N- azo-groups into di-nitrogen. Photocatalysis is also active in the "bio-world" by killing bacteria (E. Coli, streptococcus, etc.) in water without re-growth. Air pollutants can also be destroyed, especially all the VOC's (volatile organic compounds), providing certain air humidity enabling titania to produce cracking OH center dot radicals. For chemical engineering reasons, the photocatalyst has to be fixed on a photo-inert support. In these conditions, UV-irradiated titania-based photocatalysis could be applied to the elimination of air pollutants, VOC's, solvents, odors, chemicals, etc. Air treatment has to be associated with water and solid waste treatment because of odors. This is conducted by covering water treatment ponds or lagunas by rafts on which large sheets of Ahlstrom paper are deposited, supporting titania associated with activated carbon. Eventually, virus AH5N2, a model virus close to H5N1, responsible for the avian flu could be totally inactivated. Photocatalysis is directly connected with all 12 principles of Green Chemistry defined by Anastas (1998) and possesses open perspectives for China.