Photodegradation of methylene blue in water, a standard method to determine the activity of photocatalytic coatings?
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The degradation of methylene blue (MB) in aqueous solutions has been re-examined as a method to characterize the photocatalytic activity of transparent TiO2 coatings. Increasing irradiation intensities leads to a change in the observed kinetic behavior from zero-order to pseudo-first-order regarding the concentration of MB. This is due to a diffusion inhibition of MB. In order to obtain data within a zero-order kinetic regime at an initial MB concentration of 10 μmol/l and, thus, to avoid the diffusion control, irradiation intensities below E=5 W/m2 for substrates comprising higher photonic efficiencies than ζ=0.09% have to be applied. Recommendations for a standard protocol are given.
KeywordsPhotodegradation methylene blue photocatalysis TiO2 coatings kinetics
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