Photocatalytic oxidation of nitrogen dioxide with TiO2 thin films under continuous UV-light illumination

A deactivating behavior of TiO2 photocatalysts in NO2 oxidation to HNO3 was studied with use of TiO2 nanoparticulate thin films (0.5-1.5 mu m thick) under UV light illumination over 10 h. The photocatalytic activity was decreased with accumulation of HNO3 on the TiO2 surface. For thicker TiO2 films, the deactivation rate was found to be slower. The amount of HNO3 trapped on the TiO2 surface was increased and finally saturated, at which the largest amount of HNO3 Was proportional to the thickness of the film. On the basis of the results, we concluded that the produced HNO3, inhibiting the reaction as a physical barrier, must be able to diffuse on the TiO2 surface at a rate of at least more than 1.5 mu m h(-1), and finally distributes homogeneously on the whole film. The maximum density of HNO3 accumulated on the TiO2 Surface was estimated to be similar to 2 molecules nm(-2) under standard conditions. Finally, when the steady state is reached, the photocatalytic activity remained similar to 8% of the initial one.