Why inorganic salts decrease the TiO2 photocatalytic efficiency

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.