ISO 10678: 2010 – Determination of photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue.
Using artificial ultraviolet (UV) radiation, the photocatalytic activity of surfaces by degradation of the dye molecule methylene blue (MB) in aqueous solution, which is in contact with the potentially photocatalytically active surface, is determined, with the overall result being the decolourization of the solution. The high molar absorptivity of MB+ ensures a striking and easily measured colour change, from blue to colourless, upon photobleaching of the dye by the semiconductor photocatalyst under study. The structure of MB is illustrated below.
Typical test conditions are given below:
|Sample size||10 cm x 10 cm and typically 5 mm thick|
|Suitable sample type||Surfaces covered with photocatalytic coatings|
|Unsuitable sample type||Powder or granular photocatalytic materials; porous materials.|
|Sample pretreatment||24-72 hr UV irradiation at > 1 mW/cm2|
|Test conditions||Abs0 (665 nm, 1 cm cell) = 0.74; pH 5.5; time: ≤ 3 hours; irradiance: 1 ± 0.05 mW/cm2; temperature: 23 ± 2 °C|
|Analytical method||Methylene blue: UV-vis spectrophotometry|
|Information returned||Degradation rate (mol/m2/h) and the associated photonic efficiency, ξMB (%)|
Methylene blue (MB+) is a highly popular test pollutant in semiconductor photocatalysis used in the assessment of such key features as: new photocatalytic materials, photoreactors and light sources. The test method is applicable to evaluation of water purification performance, and less appropriately for the evaluation of self-cleaning activity, of surfaces covered with photocatalytic coatings.
The overall photomineralisation of MB+ by semiconductor photocatalysis is summarised by the following reaction:
Some disadvantages stem from underlying assumptions in this standard, and obviously, it is highly desirable to ensure the conditions are such that these assumptions are likely to hold. With this in mind, we have tightened up some of the specified conditions to improve repeatability and reproducibility. The ISO standard assumes that the MB+ used is of a high purity. However, εMB values quoted in the literature span a wide range which is most likely a reflection of the different purities of commercial sources of MB . Furthermore, the kinetics of MB+ photobleaching are highly dependent, upon the MB+ concentration .
This difficulty of MB+ purity is rectified by making up the MB+ reaction solution to a defined absorbance at 665 nm in a 1 cm cell (0.74), as opposed to a defined concentration as in the original standard. Finally, the initial pH of the reaction solution is not stipulated in the current ISO test, but will be set at pH 5.5, since significant deviation from such set pH will alter the amount of MB+ adsorbed which will alter the measured rate and so the calculated value of ξMB.
A typical set of results for test samples with (●) and without (○) a photocatalyst coating are illustrated below. The solid line is the calculated maximum allowable decay curve observable under standard conditions without a significant mass transfer contribution to the final calculated value of the photonic efficiency, ξMB.
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