Photocatalytic Oxidation of Nitrogen Dioxide with TiO2 Thin Films under Continuous UV-Light Illumination

A deactivating behavior of TiO 2 photocatalysts in NO 2 oxidation to HNO 3 was studied with use of TiO 2 nanoparticulate thin films (0.5−1.5 μm thick) under UV light illumination over 10 h. The photocatalytic activity was decreased with accumulation of HNO 3 on the TiO 2 surface. For thicker TiO 2 films, the deactivation rate was found to be slower. The amount of HNO 3 trapped on the TiO 2 surface was increased and finally saturated, at which the largest amount of HNO 3 was proportional to the thickness of the film. On the basis of the results, we concluded that the produced HNO 3, inhibiting the reaction as a physical barrier, must be able to diffuse on the TiO 2 surface at a rate of at least more than 1.5 μm h −1, and finally distributes homogeneously on the whole film. The maximum density of HNO 3 accumulated on the TiO 2 surface was estimated to be ∼2 molecules nm −2 under standard conditions. Finally, when the steady state is reached, the photocatalytic activity remained ∼8% of the initial one.