Photocatalytic oxidation of nitrogen monoxide using TiO 2 thin films under continuous UV light illumination

Abstract

An apparent deactivating behavior of TiO 2 photocatalysts in NO (1 ppm) oxidation in air was examined using TiO 2 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 HNO 3 accumulation on the TiO 2 surface. At the final steady state, the rate of NO oxidation was one-third of the initial one, and NO 2 was released into air at the equivalent rate. The amount of HNO 3 trapped on the TiO 2 film was increased and finally saturated, at which the largest amount of HNO 3 was proportional to the thickness of the film, and then the maximum density of HNO 3 on the TiO 2 surface was estimated to be ∼0.5 molecule nm −2. The value was much smaller than the previously reported one in the NO 2 oxidation (∼2 molecule nm −2). The discrepancy is explained by the consumption of HNO 3 during the photocatalytic reaction, thus HNO 3 reacts with NO and produces NO 2 on the TiO 2 surface under UV light illumination. On the basis of the results, we concluded that the maximum surface density of HNO 3 on TiO 2 in the NO oxidation is determined by the balance between the accumulation amount and the consumption amount of HNO 3 on the TiO 2 surface.