Research Interview
A
ir pollution is a growing problem and has a negative impact on our environment. Exposure to high levels of air pollution can cause skin irritation, and asthma attacks, it can also lead, in more extreme cases, to cardiovascular disease. As a result, cities are developing novel ways of tackling this problem head-on. One of these cities is Sheffield, whose air has been above the legal limit of nitrogen dioxide allowed since January 2010. Because of this, 500 deaths per year are associated with the air pollution in Sheffield. The council decided to team up with The University Of Sheffield in 2014 to help find a solution. With the help of Prof. Tony Ryan from the Department of Chemistry, a 10 m by 20 m poster coated in photocatalytic nanoparticles of titanium dioxide was manufactured. This was displayed on the Alfred Denny building between 2014-2017 facing the concourse viaduct. Its surface incorporated a poem, In Praise Of Air written especially for the project by the, now, poet laureate, Prof. Simon Armitage from the School of English. Over the course of 4 years the poem removed over two tonnes of air pollution. Titanium dioxide is a widely used photocatalyst due to its high photocatalytic activity, stability, low cost, non-corrosivity, non-toxicity, and high availability. It was used in this project to convert nitrous oxides, NOx, and volatile organic compounds, VOCs, from harmful air pollutants to harmless alternatives. When TiO2 particles are exposed to light of sufficient energy, a valence electron is promoted to a higher energy state,
‘In Praise of Air’ in situ on the Alfred Denny Building in 2014.
leaving a positively charged “hole”, h+ on the Ti atom. Atmospheric oxygen, O2 is reduced by the excited electron to form a superoxide anion, •O2- and water in the air is oxidised by the “hole” to form hydroxide radicals, • OH. Nitrous oxides are oxidised by these molecules to a harmless soluble nitrate, whereas VOCs are oxidised to harmless fatty acids which are all washed away.
NO2 + •OH → NO3- + h+
enabling more pollution-depleting reactions to occur at once. The poster was situated facing the sun during the day and street lighting during the night. As the photocatalysis requires light to oxidise the pollutants, the poster was able to remove pollution 24 hours a day. Furthermore, as it faced over the concourse it was exposed to a higher amount of car exhaust fumes as they drove over the busy road. The poem was able to remove the pollution of 20 cars per day throughout the 4 years it was on display. Although now the poem is no longer on display and was sold at a charity auction in 2017, the science behind it remains.
The success was because of a number of factors. The nano-size of the particles meant a greater surface area
1. https://catalyticpoetry.org/ 2. J.S. Dalton et al, 2002, Environmental Pollution, 120, 115–422.
O2 + e- → •O2OH + h+ → •OH
-
The team (left to right): Simon Armitage, Joanna Gavins, Prof Tony Ryan & David Walker.
The University of Sheffield || Resonance Issue 13
4
