Photocatalytic oxidation of nitrogen monoxide using TiO2 thin films under continuous UV light illumination

An apparent deactivating behavior of TiO2 photocatalysts in NO (1 ppm) oxidation in air was examined using TiO2 nanoparticulate thin films (0.5–1.4 μm thick) under continuous UV light illumination (1 mW cm−2). The rate of NO oxidation decreased with HNO3 accumulation on the TiO2 surface. At the final steady state, the rate of NO oxidation was one-third of the initial one, and NO2 was released into air at the equivalent rate. The amount of HNO3 trapped on the TiO2 film was increased and finally saturated, at which the largest amount of HNO3 was proportional to the thickness of the film, and then the maximum density of HNO3 on the TiO2 surface was estimated to be ∼0.5 molecule nm−2. The value was much smaller than the previously reported one in the NO2 oxidation (∼2 molecule nm−2). The discrepancy is explained by the consumption of HNO3 during the photocatalytic reaction, thus HNO3 reacts with NO and produces NO2 on the TiO2 surface under UV light illumination. On the basis of the results, we concluded that the maximum surface density of HNO3 on TiO2 in the NO oxidation is determined by the balance between the accumulation amount and the consumption amount of HNO3 on the TiO2 surface.