Atmospheric NOx Reduction by Photocatalytic Paints (Industrial Technology Research Institute, MITI/Okitsumo, 1990s)

Atmospheric NOx Reduction by Photocatalytic Paints (Industrial Technology Research Institute, MITI/Okitsumo, 1990s)
In Japan in the 1990s, air pollution in urban areas became increasingly serious, with automobile-derived NOx and photochemical smog in particular becoming social problems. The rate of achievement of environmental standards was stagnant, and there was an urgent need to regulate diesel vehicles and develop technologies to improve the urban environment. Under these circumstances, a photocatalytic paint jointly developed by MITI's Industrial Technology Research Institute and Okitsumo emerged as an innovative technology that transformed urban space itself into a purification system. When sunlight hits the photocatalyst, titanium dioxide, NOx is oxidized to nitric acid, which is then washed away by rain, thereby regenerating the paint film's purification ability. This self-regenerating property is a great advantage over conventional adsorbents and filter systems, and was highly evaluated for its low maintenance and management burden.
In tests, a coating film of approximately 200 square centimeters showed a high removal rate of approximately 80% after 24 hours of treatment with a NOx concentration of 1ppm. This was outstanding performance for an urban air purification technology at the time, and the possibility of a wide range of applications was pointed out, including use around expressways, inside tunnel walls, and on the exteriors of buildings. Photocatalytic research was entering the application stage more than 20 years after the Honda-Fujishima effect, and this NOx-reducing paint attracted international attention as the beginning of its practical application. The frequent occurrence of NO2 alarms in Europe and the deterioration of urban environments provided a tailwind, and photocatalytic building materials were studied worldwide as a technology for improving the environment.
Since the end of the 1990s, the application of photocatalytic building materials has expanded to self-cleaning exterior walls and pavement materials, forming the basis for a new group of materials that purify urban air. The NOx reduction technology using photocatalytic paints was important as the starting point, and was a symbolic link between policy, the building materials industry, and urban environmental engineering.