Patents filed last week show how the novel coating works. The paint's base is polysiloxane, a silicon-based polymer. Embedded in it are spherical nanoparticles of titanium dioxide and calcium carbonate 30 nanometres wide. Because the particles are so small, the paint is clear, but pigment can be added. The first paint to go on sale will be white.
The polysiloxane base is porous enough to allow NOx to diffuse though it and adhere to the titanium dioxide particles. The particles absorb ultraviolet radiation in sunlight and use this energy to convert NOx to nitric acid.
The acid is then either washed away in rain, or neutralised by the alkaline calcium carbonate particles, producing harmless quantities of carbon dioxide, water and calcium nitrate, which will also wash away.
In a typical 0.3-millimetre layer, there will be enough calcium carbonate to last five years in a heavily polluted city, says Robert McIntyre of the British company Millennium Chemicals, based in Grimsby, Lincolnshire, which developed the paint. When the carbonate has been exhausted, the titanium dioxide will continue to break down NOx, but the acid this produces will discolour the paint.
The breakthrough, says McIntyre, was finding a robust base material. Previous attempts to use titanium dioxide in paints to break down NOx faltered because it attacked the base material as aggressively as it did the pollutants. Polysiloxane is resistant to attack by titanium dioxide, though the developers are not yet sure why.
Ecopaint is being lab tested as part of the Europe-funded Photocatalytic Innovative Coverings Applications for Depollution Assessment programme (PICADA). It has yet to be put the test in the field, but the companies say their experience with another catalytic coating shows how air quality can be improved.
In 2002, after 7000 square metres of road surface in Milan, Italy, were covered with a catalytic cement, residents reported that it was noticeably easier to breathe - with the concentration of nitrogen oxides at street level cut by up to 60 per cent.
Dimitrios Kotzias, who runs PICADA's test programme at the EU's Joint Research Centre in Ispra, Italy, says that the coating is effective because air turbulence is constantly carrying the gases over the surface, yet molecules stick to the surface long enough for the oxidation reaction to break them down.
The paint could cover a much greater surface area than cement, since every building and piece of street furniture could be painted with it.
Photocatalytic cements and paving slabs are already used in Japan, where the market for such building materials is growing. And EU member states are required to monitor NOx levels and ensure that by 2010 they have fallen below an annual average of 21 parts per billion. But current levels in cities are often tens of times that.
"There certainly is a need for new technologies," says Mike Pilling, chair of the Air Quality Expert Group, which advises the UK government.
Millennium Chemicals http://www.millenniumchem.com/
NOx gases http://www.ghgonline.org/othernox.htm