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Photocatalysis
water adsorbed onto its surface from the
material that can capture and absorb light and use this energy to produce reactive chemical species to increase the rate of a reaction. In the presence of UV light, the semiconductor TiO2
NO2 is a contributor to
respiratory diseases such as asthma as it affects airways
Field trials using CristalACTiV reveal reductions as high as 60% for NO and 20% for NO2
in the immediate reacts with
vicinity of the treated surface. If such technology were applied to buildings, including surfaces in homes, schools and hospitals, and roads and pavements in cities and towns, more pollutants could be converted to harmless products, generating cleaner air. In a laboratory test, 6m2
of external
in the reaction so the reaction has a very long lifetime, and TiO2
atmosphere, generating reactive hydroxyl and peroxy radicals. These radicals can oxidise pollutant molecules – such as NOx, SOx, and VOCs contained in synthetic foams, glues or plastic materials – in close proximity to the catalytic surface, rendering them harmless. In the case of NOx and SOx, soluble nitrates and sulfates are formed, respectively, and VOCs are converted to fatty acids and soaps. Other organic deposits such as grease and bacteria can also be oxidised to carbon dioxide and water. As a catalyst, TiO2
is not consumed is non-toxic. Since
artificial and reflected light have been proven to drive the catalytic reaction, the technology can also be used in car parks and road tunnels. The potential of this technology to improve air quality in cities is immense.
surface covered or treated with CristalACTiV – about the equivalent to an average passenger car – removed on average the NOx emissions of one Euro 4 car (diesel and petrol/gasoline)/day: a car that travels on average 12miles/20km and emits around 3.3g of NOx/day. These figures were reproduced in field trials in Manila, Philippines, (2010) and in a four- year trial in London, in 2011 by scientists at Kings College London, UK. There are other advantages. Photocatalytic surfaces are self-cleaning and therefore economically viable – any dirt that falls on the surface will be oxidised to carbon dioxide and water, leaving it clean. This makes these materials extremely useful in building maintenance programmes to reduce the costs associated with keeping buildings clean. The time between cleanings can be extended and the time required to perform the maintenance can be reduced as the surface grime becomes easier to remove
owing to the photocatalytic process. Polymer chemist Tony Ryan at the University of Sheffield, UK, has recently shown that this technology has a ‘smart’ application. In collaboration with London fashion designer Helen Storey and scientists at Cristal, Ryan showed that nanoparticles of TiO2
Jeans sprayed with TiO2
polluting NOx
clean the air of
sprayed onto fabrics
such as cotton can purify the air in the local vicinity. In theory, any available surface can be treated in this way. However, before this technology can have a significant impact on air pollution, there needs to be a change in the way people think – large numbers of people need to embrace the technology if it is to make a difference. The Sheffield scientists, for example, estimate that 1m2
of coated fabric can take out 0.5g
of NOx/day, so to make any significant reduction on NOx emissions in the city of Sheffield would require at least half the population to wear treated clothing. Thus, driving behavioral change is becoming increasingly important. Air pollution levels must be addressed by multiple stakeholders, including businesses, local authorities, as well as individuals. Everyone has a stake in improving air quality and needs to play a part to de-pollute the air in major cities.
Brian Pickett is business director for the performance chemicals unit at Cristal, in Maryland, US.
Chemistry&Industry • November 2012 23
TiO2
photo-
catalysts remove NOx, SOx and VOCs from the atmosphere