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On the national (DIN) and international (ISO) level the determination of the photonic efficiency of the light-driven decolorization of methylene blue (MB) solutions in contact with a photocatalyst is currently being discussed as a standard method for testing and comparison of photocatalytic coatings. In this context it is necessary to make sure that the photocatalytic reaction of MB is a simple and robust method that meets the requirements of non-specialised laboratories in industry. Therefore, the photocatalytic degradation of methylene blue was studied using a fixed layer of Degussa P25 on soda lime glass as the photocatalyst substrate. The influence of the initial concentration of MB and of the intensity of the incident UV(A) light on the initial reaction rate and on the photonic efficiency was investigated in detail. It was observed, that both the initial concentration of MB as well as the incident light intensity strongly influences the degradation rate. Increasing the initial concentration of MB resulted in an increase of the initial reaction rate with a characteristic curvature in the direction of the concentration axis. The initial reaction rate was found to be not directly proportional to the incident light intensity. An increase of the photonic efficiency with decreasing initial MB concentration and decreasing light intensity was observed. A Langmuir-Hinshelwood rate law is used to discuss the implications of these observations with regard to the proposed standard method for testing and comparison of photocatalytic coatings.

Photocatalytic Degradation of Methylene Blue on Fixed Powder Layers: Which Limitations are to be Considered?