This project investigated the potential for passive cleaning of outdoor air using nanoparticles of anatase titanium dioxide, which have been shown to photocatalyze the oxidation of atmospheric pollutants such as oxides of nitrogen (NOx) and volatile organic compounds (VOCs). Active indoor air cleaners using this technology are commercially available for eliminating malodorous compounds. Research has been ongoing for more than a decade in Japan, and a major multi-institutional project was recently completed in Europe. Still, the practical feasibility of passive photocatalytic air cleaning remains controversial. Some claim that pollution can be halved with straightforward measures; others are unconvinced.
This project clarified the technology potential by focusing on measured values of catalytic activity—that is, the rate at which air can be cleaned by a given area of photocatalyst (if 1 m2 of catalytic film or plate can clean 100 m3 of air per day, it has an activity of 100 m/day). Laboratory data show that a high-quality TiO2 catalyst has an activity of about 200 m/day for NOx, about 60 m/day for typical VOCs, and approximately 0 m/day (not useful) for carbon monoxide.
Prior to widespread deployment of passive photocatalytic air cleaning technology, large-scale meteorological simulations are needed to validate deployment strategies.