Environmental Profile of NO_x Reduction by a Photocatalytic Surface Coating and a Vehicle Catalytic Converter

Nitrogen oxides (NO_x) in urban air close to ground have significant health implications. Restrictions in traffic, mandatory use of catalytic converters on vehicles, and novel photocatalytic coatings on surfaces contribute to reducing the level of NO_x in cities. The aim of this study is to establish environmental profiles of NO_x removal by a Three-Way Catalyst (TWC) car converter and by a photocatalytic surface coating (for asphalt and concrete pavements) for fostering technological development in reducing the levels of NO_x in urban air. We assessed the environmental performance for the removal of 1 kg NO_x by the two technologies with Life Cycle Assessment (LCA; EF.3 impact assessment method). In order to do so, we established Life-Cycle-Inventory (LCI) data representing production, operation and end-of-life of the two technologies based on data from literature and industry. The production of photocatalytic surface coatings, used on concrete and asphalt, has environmental loads two orders of magnitude lower than the environmental benefits of NO_x reduction expressed as a reduction in Photochemical Ozone Formation (POF), Acidification (A), and Terrestrial Eutrophication (TE). The vehicle catalytic converter shows similar results except that the use of rare earth elements in the production constitutes a significant load to Freshwater Ecotoxicity (FET) and that additional use of fuel during operation induces a modest Climate Change (CC) impact. For both technologies, the environmental benefits of reducing NO_x far exceed any adverse environmental aspects of the production of the technologies.