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International Journal of Photoenergy
Volume 7 (2005), Issue 1, Pages 1-9
http://dx.doi.org/10.1155/S1110662X05000012

Why inorganic salts decrease the TiO2 photocatalytic efficiency

1Laboratoire d'Application de la Chimie à l'Environnement (LACE), Université Claude Bernard Lyon 1, 43 bd du 11 nov. 1918, Bât Raulin, 3ème étage, Villeurbanne Cedex F-69622, France
2Laboratoire de Catalyse et Environnement, Ecole Nationale d'Ingénieurs de Gabés (ENIG), Gabès, Tunisia

Copyright © 2005 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Methylene Blue (MB) has been chosen as a model molecule to evaluate the impact of inorganic salts, present in textile waste waters, on the adsorption properties and on the photocatalytic efficiency of TiO2. No OH radical scavenging by anions such as NO3, Cl, SO42, PO43, and CO32 was observed at neutral and basic pH, while this phenomenon can be suggested at acidic pH for some anions except carbonate anions which are totally neutralized and/or eliminated as CO2 in these conditions. The decrease in the rate MB photocatalytic degradation in the presence of inorganic salts was shown to be due to the formation of an inorganic salt layer at the surface of TiO2, which inhibits the approach of MB molecules. The correlation between the amount of MB adsorbed and the rate of its photocatalytic degradation, whatever the nature of the salt, its concentration and the pH of the solution, indicates (i) that photocatalysis occurs at the surface and not in the solution and (ii) that OH ions added at basic pH do not participate to the increase in the photocatalytic efficiency by inducing an increase in OH formation. They increase the surface density in adsorption sites TiO. The effect of various salts is similar on various titania samples of industrial origin (Millennium TiO2 PC 500, PC 50, and Degussa P 25). It is however more important on Millennium PC 10 probably because of its smaller surface area.