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

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