Emission Reduction Fluid and Emission Reduction Fluid Handling: Chemistry

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Diesel engines run at overstoichiometric air-to-fuel ratio to ensure the combustion of soot and prevent unburned fuel from being emitted. The excess of oxygen necessarily leads to generation of nitrogen oxides (NOx), which are harmful pollutants, from the nitrogen in the air.
Selective catalytic reduction is used to reduce the amount of NOx released into the air using DEF. Diesel exhaust fluid (from the DEF tank located in large trucks) is injected into the exhaust as it moves through the engine where it vaporizes and decomposes to form ammonia and carbon dioxide. With the SCR catalyst, the NOx are catalytically reduced by the ammonia (NH3) into water (H2O) and nitrogen (N2), which are both harmless and are released through the exhaust.