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

Abstract

A deactivating behavior of TiO2 photocatalysts in NO2 oxidation to HNO3 was studied with use of TiO2 nanoparticulate thin films (0.5−1.5 μm 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 μm h−1, and finally distributes homogeneously on the whole film. The maximum density of HNO3 accumulated on the TiO2 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.

Interaction of NO<sub>2</sub> with TiO<sub>2</sub> surface under UV irradiation: measurements of the uptake coefficient

“…A deactivating behaviour of TiO 2 in the reaction with NO 2 under UV irradiation observed in the present study supports the previous observations of Ohko et al (2008). These authors have observed that the photocatalytic activity was decreased with accumulation of HNO 3 on the TiO 2 surface and concluded that the produced HNO 3 inhibited the photocatalytic heterogeneous reaction as a physical barrier.…”

“…In several previous studies (e.g. Lin et al, 2006;Ohko et al, 2008) it was observed that the photocatalytic activity of the TiO 2 film can be fully regenerated after rinsing with water. In the present work, we have observed similar behaviour: the deactivated TiO 2 sample was regenerated after being immersed in water.…”

“…Henderson, 2011). In addition, TiO 2 is known to transform nitrogen oxides (NO/NO 2 ), via catalytic heterogeneous reactions, to HNO 3 , which remains on the TiO 2 surface (Ibusuki and Takeuchi, 1994;Dalton et al, 2002;Devahasdin et al, 2003;Negishi et al, 1998;Ohko et al, 2008). Due to these photocatalytic properties TiO 2 is widely used in a variety of so-called de-polluting building materials aimed to remove the nitrogen oxides from the atmosphere.…”

Light intensity dependence of the kinetics of the photocatalytic oxidation of nitrogen(ii) oxide at the surface of TiO2

“…75 A maximum density of B2 molecules of HNO 3 nm À2 TiO 2 was reported. 74 With the maximum amount of oxidized NO found during this work (n rem = 9.25 Â 10 À6 mol) and the geometric surface area of the sample a maximum density of B1 molecule of HNO 3 per 1000 nm 2 is calculated to be produced during a single experimental run performed in this work indicating that the TiO 2 surface is far from being saturated by adsorbed HNO 3 . It is therefore suggested that a dynamic equilibrium between NO, HNO 2 /NO 2 À , and NO 2 is established comprising surface reactions as well as adsorption-desorption equilibria, but that the final oxidation product is continuously accumulated on the photocatalyst surface during the entire experimental run, thus blocking the NO adsorption sites.…”

“…The respective photocatalytic oxidation pathway of NO, where water and all nitrogen compounds present at the photocatalyst surface are in equilibrium with the gas phase, is outlined in Table 2. [62][63][64] Profound objections have been raised against this pathway by Ohko and co-workers, 24,25,74 who claimed the contribution of a terminal (bridging) oxygen species, TiO 2 (O s ), and oxygen vacancies, TiO 2 (&), at the TiO 2 surface to the photocatalytic NO oxidation. The NO reaction pathway suggested by this group is presented in Table 3.…”