Diesel Exhaust Fluid is also commonly referred to as DEF.
It is a composition of 32.5% urea (an organic compound with chemical formula CO(NH2)2 ) and 67.5% de-ionized water (water that has had almost all of its mineral ions removed, such as cations like sodium, calcium, iron and copper, plus anions such as chloride and sulfate). It is used in diesel transportation applications to reduce the amount of nitrogen oxides (NOx) emitted into the air by diesel engines.
Diesel Exhaust Fluid is produced to an international standard (ISO 22241). In order to meet the 2010 EPA Diesel Emission Standards to reduce the amount of NOx emitted by diesel engines, several engine manufacturers employed Selective Catalytic Reduction (SCR) as an after-treatment technology combined with Exhaust Gas Recirculation (EGR).
In order to reduce NOx (Nitrogen Oxides), the fluid is injected directly into the exhaust upstream of a catalyst. The DEF vaporizes and decomposes to form ammonia (NH3), which in conjunction with the SCR catalyst, reacts with NOx to convert the pollutant into Nitrogen (N2) and Water (H2O).