Photocatalytic Oxidation of NO₂ on TiO₂: Evidence of a New Source of N₂O₅

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.