Do photocatalytic materials really help to tackle traffic pollution?

Published on Monday, 01 November 2010 15:15

	By Robin Whitlock Follow @RobinWhitlock66

The problem of nitrogen oxide (NOx) pollution in cities has long been an unwelcome side effect of continuing traffic growth, especially with regard to diesel engine vehicles. According to statistics obtained in 2001 by the Department of Environment, Food and Rural Affairs (DEFRA), the problem has grown to such an extent that over 50 per cent of nitrogen oxide pollution can be directly attributed to road traffic. [1]

Nitrogen oxides form as a result of the combination of nitrogen (N2) and oxygen (O2) in high temperature and under pressure during fuel combustion. All existing fuel sources such as gasoline, diesel, biodiesel, propane, coal and ethanol emit nitrogen oxides when burned. [2]

NOx has been identified as a factor in respiratory problems due largely to the build up of photochemical smog and low-level ozone in cities. [3] Long term exposure to low level ozone can reduce bodily resistance to infection, inflammation of the lung lining and general respiratory discomfort. [4]

Some fuels are also carcinogenic, that is to say they cause or help to encourage the development of cancer in humans. [5] Diesel vehicles are particularly to blame, being one of the largest sources of NOx. The problem is often exacerbated by 'street canyons', streets bordered with tall buildings in which the pollutant gases hang close to the ground, trapped by the surrounding buildings. As well as being linked to cancer, traffic pollution has been widely cited as a major cause of asthma among children living in urban environments. [6]

Over the last ten years or so, a technological development involving paving slabs and concrete-based coatings impregnated with titanium dioxide, a substance that reacts with the presence of sunlight to absorb nitrogen dioxide, has become popular and has been trialled and tested in various countries around the world, with various degrees of success.

One such coating is called 'Ecopaint' which is based on a silicon polymer called polysiloxane containing nanoparticles of titanium dioxide (TiO2) and calcium carbonate. NOx particles pass through the polymer and are absorbed by the titanium dioxide which is a photocatalyst, that is to say it absorbs ultra violet radiation from sunlight and uses it to convert the NOx into nitric acid which is then washed away by the rain or absorbed by the calcium carbonate. The rate at which this process occurs varies with the amount of sunlight available. [7]

According to Robert McIntyre of Cristal Global the slabs are coated in a 0.3 millimetre layer of polymer that should last for five years. Polysiloxane was chosen primarily because, unlike other coatings, it is resistant to attack by titanium dioxide.

Over recent years the slabs have been tested in a European-wide European Union programme called Photocatalytic Innovative Coverings Applications for Depollution Assessment (PICADA) which is testing the technology in both lab and micro-scale field tests.

Other field-based experiments have been conducted around the world. One of the most celebrated trials was that conducted in Italy in 2002 during which 7000 square metres of road surface in Milan were covered with photocatalytic cement.[8] The Milan experiment displayed a 60% reduction in NOx levels according to Alberto Ghisalberti of Italian cement company Italcementi. [9] Data collected by the German federal environment ministry has shown that in 2009 Nox levels in German cities exceeded the maximum permitted levels at some 55% of urban monitoring stations. [10]