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In the present study, problems of different methods used for quantifying the air remediation activity of photo-catalytic active surfaces are described. It is demonstrated that in bed photo-reactors (e.g. ISO), transport limitations can lead to underestimation of the activity, if fast heterogeneous reactions are investigated. In contrast, in stirred tank photo-reactors (e.g. UNI), complex secondary chemistry may lead to an overestimation of the photo-catalytic remediation of NOx, if NO2 is also present. In addition, the quantities, used for ranking the photo-catalytic air remediation activity in the different methods are not independent of the applied experimental conditions, and thus, make any intercomparison between the different methods or the extrapolation to atmospheric conditions very difficult. Furthermore, unrealistic high NOx levels are used, for which the chemical kinetics may already be affected by surface saturation problems. Finally, it is shown that the use of only nitrogen monoxide (NO) will not enable users to judge about the quality and effectiveness of a photo-catalytic surface for improving air quality, since surfaces which are active toward NO may be completely non-reactive toward other important atmospheric pollutants. A modified method for quantifying the air remediation activity of photo-catalytic surfaces is proposed here to overcome these problems. |
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