Emission Control of Nitrogen Oxides—Current Status and Future Challenges

Oxides of nitrogen (NO_x) represent unwanted by-products from combustion of fossil fuels. NO_x can cause significant harm to living beings through decreased air quality. This review summarizes the sources of NO_x emissions, regulations established to limit these emissions from on-road transportation, and the technologies developed by the automotive industry to meet these requirements. As regulations continue to establish lower limits for NO_x emissions, engine and power technologies are evolving to meet these requirements. Exhaust aftertreatment systems have enabled near-zero tailpipe NO_x emissions using NO_x reduction catalysts. These catalysts are being further developed for improved low-temperature performance and higher durability. Such improvements are enabled through fundamental understanding of the underlying chemistry governing these catalytic processes. Future modifications in exhaust aftertreatment system architectures and catalysts will require robust system control to optimize system performance and ensure high NO_x conversion throughout useful life.