Dye decolorization test for the activity assessment of visible light photocatalysts: Realities and limitations

The development of photocatalysts with visible light activity has been extensively investigated. Their activities are usually tested by measuring the degradation rate of different organic compounds. Among these organic substrates, dyes are the most widely employed due to their rapid decolorization and simple kinetic analysis using a spectrophotometric method. However, the dye test has much uncertainty in the evaluation of photocatalytic activity. To assess the validity of the dye test, six visible-light photocatalysts (N-TiO2, C-TiO2, C-60(OH)(x)/TiO2, Pt/WO3, BaBiO3, and Bi2WO3) were tested and compared for the degradation of five organic dyes (anionic: acid orange 7, indigo carmine, and new coccine; cationic: methylene blue and rhodamine B) in this study. This study aimed to assess how the measured activities depend on the kind of test dyes and how reliable the dye test is as an activity evaluation method. The activities determined by the dye test were highly specific to the kind of dye and photocatalyst. For example, N-TiO2 is the most active photocatalyst for the degradation of acid orange 7 at pH 3 but is one of the least active at pH 9; Pt/WO3 is the best photocatalyst for the degradation of methylene blue but not much active for the degradation of acid orange 7. This is ascribed to the fact that the dye test is significantly influenced by various factors such as the dye sensitization of catalyst particles, the absorption spectral overlap between dyes and photocatalysts in the visible region, the electrostatic interaction (attractive or repulsive), and the properties of dye degradation intermediates. In general, the dye decolorization efficiency was poorly correlated with the dye mineralization efficiency, which limits the practical value of the dye test. Therefore, the practice of dye test for the activity assessment of visible light photocatalysts should not be recommended and the activity results obtained for a specific combination of a dye and a photocatalyst should not be generalized. (C) 2013 Elsevier B.V. All rights reserved.