Heterogeneous Reactivity of NO2 with Photocatalytic Paints: A Possible Source of Nitrous Acid (HONO) in the Indoor Environment

Abstract:

There is an increasing concern about the indoor air environment, where we spend most of our time. Common methods of improving indoor air quality include controlling pollution sources, increasing ventilation rates or using air purifiers. Photocatalytic remediation technology was suggested as a new possibility to eliminate indoor air pollutants instead of just diluting or disposing them.

In the present study, heterogeneous reactions of NO₂ were studied on photocatalytic paints containing different size and quantity of TiO₂. The heterogeneous reactions were conducted in a photo reactor under simulated atmospheric conditions. The flat pyrex rectangular plates covered with the paint were inserted into the reactor. These plates have been sprayed with the photocatalytic paints at our industrial partner’s (ALLIOS) facilities using a high precision procedure that allowed the application of a thin layer of a given thickness of the paint. This allows a homogeneous coverage of the surface with the paint and an accurate determination of the exact amount of paint exposed to gaseous NO₂.

We demonstrate that the indoor photocatalytic paints which contain TiO₂ can substantially reduce the concentrations of nitrogen dioxide (NO₂). We show that the efficiency of nitrogen dioxide (NO₂) removal increase with the quantity of TiO₂ in the range 0 - 7 %. The geometric uptake coefficients increase from 5 · 10^-6 to 1.6 · 10^-5 under light irradiation of the paints. On the other hand, during the reactions of NO₂ with this paint (7 % of TiO₂) nitric oxide (NO) and nitrous acid (HONO) are formed. Nitrous acid (HONO) is an important harmful indoor pollutant and its photolysis leads to the formation of highly reactive OH radicals (Gomez Alvarez et al., 2013). Maximum conversion efficiencies of NO₂to HONO and NO of 15 % and 33 % were observed at 30 % RH, respectively.

Thus, the quantity of TiO₂ embedded in the paint is an important parameter regarding the nitrogen oxides (NO_x = NO + NO₂) remediation, but may also influence formation of harmful by-products such as HONO, which should be considered for future optimization of photocatalytic paints aimed for indoor applications.

Reference:

Gómez Alvarez E., Amedro D., Afif C., Gligorovski S., Schoemacker C., Fittschen C., Doussin J.F., Wortham H. Proc. Natl. Acad. Sci. U.S.A, 110(33), 13294-13299, 2013.