Abstract
Small scale bed flow photoreactor experiments were performed to assess the photocatalytic performance of cement-based TiO2-containing materials for NOx reduction through the determination of kinetic parameters under variation of the experimental conditions (relative humidity, flow rate, mixing ratio and light intensity) and monitoring of potential reaction products in the gas phase and the aqueous extract of the surface.The results clearly demonstrated the general potential of the tested material to photocatalytically remediate gas phase NOx by conversion into nitrite and nitrate as identified reaction products at the surface. The measured uptake coefficients for NO and NO2 under atmospheric relevant conditions were in the range of 5 × 10−5 with a corresponding surface deposition velocity of about 0.5 cm s−1. However, it became also clear that the photocatalytic activity is in part significantly dependent on the experimental conditions. The relative humidity and the mixing ratio of the air pollutant were identified as the most important parameters. In addition, under certain conditions, a renoxification process can occur.The comprehensive results of the present study are discussed in detail to develop recommendations for a possible future application of this technique to improve urban air quality.
Highlights
Clean air is essential to human health and for the intactness of the environment
The results clearly demonstrated the general potential of the tested material to photocatalytically remediate gas phase NOx by conversion into nitrite and nitrate as identified reaction products at the surface
The reaction with oxygen and water lead to the formation of highly reactive radicals such as O2−/HO2 and other hydroxyl radicals (OH) proven by measurements using, e.g., electron paramagnetic resonance (EPR) [24], cavity ring down spectroscopy (CRDS) [25] or laser-induced fluorescence (LIF) [26,27,28,29]
Summary
Clean air is essential to human health and for the intactness of the environment. Since the industrial revolution, the quality of the air that people breathe has deteriorated considerably as a result of human activities. The reaction with oxygen and water lead to the formation of highly reactive radicals such as O2−/HO2 and OH proven by measurements using, e.g., electron paramagnetic resonance (EPR) [24], cavity ring down spectroscopy (CRDS) [25] or laser-induced fluorescence (LIF) [26,27,28,29] These reactive oxygen species (ROS) enable irradiated TiO2, especially in its most active form Anatase [30], to degrade adsorbed NOx and VOCs [31,32]. Renoxification originating from adsorbed nitrate, proposed by the photocatalytic formation of the nitrate radical (NO3) on TiO2, was recently observed in laboratory experiments [40,41,42,43,44] Both results are dependent on the material used and the applied experimental conditions, but would potentially contradict the application of photocatalytic surfaces to improve urban air quality. The obtained kinetic parameters, experimental dependencies and observed reaction products are evaluated in the context of improving urban air quality and recommendations for a future application of this technique are discussed
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