Last week that giant multinational of aluminum production Alcoa announced its new "smog-eating" architectural panels - in other words cladding stuck to a building's exterior that can remove pollutants from the surrounding air. The aluminum panels, branded Reynobond with EcoClean technology, have a titanium dioxide coating which breaks down pollutants in direct sunlight.
Of course the purifying properties of titanium dioxide are well known, and have been widely applied, both in so-called self-cleaning applications, such as an experimental cotton treatment; and products such as lightbulbs that purify the air in a room. We've even seen research products to apply titanium dioxide, also known as titania, to water purification systems.
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Titanium dioxide is really rather useful. Because of its brilliant whiteness, the naturally-occurring compound is commonly used as a pigment in paints, plastics and papers, as well as food and medicines. Added to skimmed milk, it's thought to increase palatability. Its ability to absorb ultraviolet radiation make it a commonly used ingredient in sunscreen.
But its titania's photocatlaytic abilities under ultraviolet light that single it out for use in this and similar products. In fact, with the addition of tungsten trioxide or nitrogen ions, titania can act as a photocatalyst under visible light too. Titania is hydrophyllic, as opposed to hydrophobic, so that when used as a self-cleaning material, it is not wholly reliant on water to remove dirt. Instead, organic pollutants are broken down into "harmless matter" when exposed to light, oxidized by free radicals generated by the titania. The remaining matter is simply washed way come the next rainfall.
Alcoa is claiming that 10,000 sq feet (929 sq meters) of its EcoClean panels have "smog removal" capabilities equivalent to about 80 trees - enough to offset the nitrogen oxide emissions of four cars per day. There's no indication of cost - though we imagine Alcoa would quote on an order-by-order basis.