Photocatalytic Oxidation of NO 2 on TiO 2 : Evidence of a New Source of N 2 O 5 .

N ₂ O ₅ is an important intermediate in the atmospheric nitrogen cycle. Using a flow tube reactor, N ₂ O ₅ was found to be released from the TiO ₂ surface during the photocatalytic oxidation of NO ₂ , revealing a previously unreported source of N ₂ O ₅ . The rate of N ₂ O ₅ release from TiO ₂ was dependent on the initial NO ₂ concentration, relative humidity, O ₂ /N ₂ ratio, and irradiation intensity. Experimental evidence and quantum chemical calculations showed that NO ₂ can react with the surface hydroxyl groups and the generated electron holes on the TiO ₂ , followed by combining with another NO ₂ molecule to form N ₂ O ₅ . The latter was physisorbed on TiO ₂ and had a low adsorption energy of -0.13 eV. Box model simulations indicated that the new source of N ₂ O ₅ released from TiO ₂ can increase the daytime N ₂ O ₅ concentration by up to 20 % in urban areas if abundant TiO ₂ -containing materials and high NOx concentrations were present. This joint experimental/theoretical study not only demonstrates a new chemical mechanism for N ₂ O ₅ formation but also has important implications for air quality in urban areas.
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