Interaction of NO2 with TiO2 Surface Under UV Irradiation: Products Study

The interaction of NO2 with TiO2 solid films was studied under UV irradiation using a low pressure flow reactor (1-10 Ton) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. HONO, NO, and N2O were observed as the products of the reactive uptake of NO2 to the illuminated TiO2 surface with the sum of their yields corresponding nearly to 100% of the nitrogen mass balance. The yield of the products was measured as a function of different parameters such as irradiance intensity, relative humidity (RH), temperature, and concentrations of NO2 and O-2. The yield of N2O was found to be 0.15 +/- 0.05 independent of the experimental conditions. The distribution of the products between NO and HONO was found to be independent of temperature in the range T = 280-320 K and was governed by relative humidity: increase in RH led to lower NO and higher HONO yield, with a maximum of nearly 65% reached at similar to 5% RH. Presence of molecular oxygen was shown to shift the HONO/NO distribution to HONO at low RH (<5%) with no effect at higher RH where the HONO yield is maximum. The following values for the yield of the products of NO2 interaction with pure TiO2 under real atmospheric conditions can be recommended from this work: 0.65 +/- 0.10, 0.05 +/- 0.05, and 0.15 +/- 0.05 for HONO, NO, and N2O, respectively. The mechanism of the photoinitiated heterogeneous reaction and possible atmospheric implications of the obtained results are discussed.