Titanium Dioxide Photocleans Polluted Air

Could titanium-clean air be how advertisers one day try to sell us the virtues of paint or even clothing that reduces the pollution caused by automobiles? The technology behind such products involves the well-known ability of titanium dioxide (TiO2) to photocatalytically split water to form hydroxyl and peroxyl radicals, a reaction that has already been harnessed to make self-cleaning windows. Pacific Paints (Boysen) Philippines, Inc., is now promoting a paint it claims cleans air of automobile-produced nitrogen oxides (NOX)—known respiratory irritants and precursor molecules of groundlevel ozone. In the presence of sunlight, TiO2 nanoparticles in the paint form hydroxyl and peroxyl radicals, which then react with NOX in the air to produce nitric acid. This reacts with calcium carbonate in the paint matrix to produce minute quantities of calcium nitrate, water, and carbon dioxide (CO2). Another company, Alcoa, is marketing a coating for aluminum building panels that relies on the same kind of photochemistry, while Boral Roofing is using it to make pollution-reducing roof tiles. Cristal Global produces the CristalACTiVTM TiO2 nanoparticles currently used by Pacific Paints in their Boysen KNOxOUTTM paint. In one trial in Manila’s Guadalupe train station, Cristal Global painted 4,100 m2 of exterior wall and found the paint removed about 26 g of NOX per 100 m2 of painted surface. The company claims each painted square meter could remove 80 g of NOX per year. In another trial that ran over four years, a 135-m2 wall in London was treated with another Cristal product and the NOX levels in its vicinity measured. Compared with control areas in other parts of the city, the company reported reductions as great as 60% for the NOX species nitric oxide and 20% for the species nitrogen dioxide. The photocatalytic paint is even reported to work in the low-light environment of multistory parking lots, capturing 2.2 g of NOX per square meter per year. 6