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NIRE Annual Report
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1998

Development of Photocatalytic Paints for Passive Air Purification

Photoenergy Application Division
Global Warming Control Department
Objectives
Since we found that semiconductor photocatalyst containing titanium dioxide (TiO2) can efficiently remove low-level atmospheric pollutants such as nitrogen oxides (NOx), efforts have been made to produce practical materials that are used as building and road construction materials. Developing paints that can coat existing structures is one of our targets.
Results
As the photocatalytic reactions take place on the TiO2 surface and the NOx removed from air remain on the surface as nitrate, large specific surface area is indispensable to the photocatalytic air purifying materials. The previously developed fluorocarbon polymer sheets and hardened cement paste were porous materials.
Fig.1.
Our first attempt was to use silicone resins as the binder, because the resins are permeable to gas molecules. However, red curves in Fig. 1 show that the NOx removal by the silicone-based painted films was not very good without depending on the film thickness, though it was dependent on the TiO2 content.
We chose the sol-gel method to obtain porous painted films. The starting material is alkoxysilanes [(R1)4-n-Si-(OR2)n], which undergo hydrolysis in the presence of water and catalyst, and then form a porous network structure by condensation. Photocatalytic TiO2 is mixed during the condensation process. The resultant paint mixture was coated on glass or stainless steel plates and dried at 180 ºC.
As shown by blue curves in Fig. 1, the NOx removal percentage increased with increasing TiO2 content and film thickness. Porous structure was also confirmed by SEM images of the film cross section.
The painted films can remove NOx up to 0.01 mol/m2 under a UV intensity of 1 W/m2 and the activity is restored by rinsing with water. The paint is being tested on busy traffic streets by the Itabashi Ward, Tokyo and other local governments. This study was carried out in collaboration with Okitsumo Inc.
Selected Publications
1) Hikarishokubai no Sekai, Kogyo Chosakai, Tokyo (1998).
2) Performance of plastic-based photocatalysts for passive reduction of air pollutants at highway roadside, Proc. 2nd Int. Symp. Environ. Appl. Adv. Oxid. Technol., EPRI, Rep. CR-107581, pp. 4-145-156 (1997).
3) Air purifying paints based on photocatalysis, Toso Kogaku, 32, 490-495 (1997).

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