Photocatalytic Oxidation of NO₂ on TiO₂ : Evidence of a New Source of N₂ O₅

N₂ O₅ is an important intermediate in the atmospheric nitrogen cycle. Using a flow tube reactor, N₂ O₅ was found to be released from the TiO₂ surface during the photocatalytic oxidation of NO₂ , revealing a previously unreported source of N₂ O₅ . The rate of N₂ O₅ release from TiO₂ was dependent on the initial NO₂ concentration, relative humidity, O₂ /N₂ ratio, and irradiation intensity. Experimental evidence and quantum chemical calculations showed that NO₂ can react with the surface hydroxyl groups and the generated electron holes on the TiO₂ , followed by combining with another NO₂ molecule to form N₂ O₅ . The latter was physisorbed on TiO₂ and had a low adsorption energy of -0.13 eV. Box model simulations indicated that the new source of N₂ O₅ released from TiO₂ can increase the daytime N₂ O₅ concentration by up to 20 % in urban areas if abundant TiO₂ -containing materials and high NOx concentrations were present. This joint experimental/theoretical study not only demonstrates a new chemical mechanism for N₂ O₅ formation but also has important implications for air quality in urban areas.