California Institute for Energy and Environment (CIEE)

In this project, the potential for passive cleaning of outdoor air using photocatalytic titanium dioxide particles was investigated. Small particles (nanoparticles) of anatase titanium dioxide can serve as photo-catalysts to assist in the oxidation of atmospheric pollutants such as oxides of nitrogen (NOx) and volatile organic compounds (VOCs). The energy enabling this catalysis is contained in the ultraviolet part of the solar spectrum. By-products may be produced, but the ultimate fate of the pollutant molecules is to be oxidized ("mineralized") to H2O, CO2, and acids such as HNO3 and H2SO4. Active indoor air cleaners using this technology are now commercially available. They can, for example, eliminate malodorous compounds in the air. Research and development in this area has been ongoing for more than a decade in Japan, and recently a major multi-institutional project has been completed in Europe. Despite the extensive research to date, the practical feasibility of passive photo-catalytic air cleaning is still a controversial subject. Some individuals claim that pollution can be halved with straightforward measures; others are unconvinced. This project has clarified the technology potential by focusing on measured values of catalytic activity. Catalytic activity is the rate at which air can be cleaned by a given area of photocatalyst. For example, one square meter of catalytic film or plate can clean a certain number of cubic meters of air per day. The units of catalytic activity are m d-1. Laboratory data show that a high-quality TiO2 catalyst has an activity of about 200 m d-1 for NOx, about 60 m d-1 for typical volatile organic compounds, and approximately 0 m d-1 (i.e., not useful) for carbon monoxide. Prior to widespread deployment of passive photo-catalytic air-cleaning technology, large-scale meteorological simulations are needed to validate deployment strategies.