Selective catalytic reduction (SCR) is the aftertreatment technology used on modern diesel engines to reduce NOx emissions. DEF (also called AdBlue) is injected into the hot exhaust stream, where ammonia reacts with NOx on the surface of the SCR catalyst and is converted to harmless nitrogen and water vapour.
The chemistry — in plain terms
DEF is a 32.5% solution of urea in demineralised water. In the hot exhaust it decomposes into ammonia (NH₃). On the SCR catalyst, ammonia reduces NOx to N₂ and H₂O. This is the "selective" part — only NOx is targeted, and the rest of the exhaust stream is not disturbed.
Role of the SCR catalyst
The SCR catalyst is the ceramic or metallic element inside the SCR system where the NOx + ammonia reaction actually happens. Without a functional SCR catalyst, the DEF doser and pump can be perfect and NOx will still not be reduced.
DEF dosing and NOx monitoring
A DEF pump supplies pressurised DEF from the tank to the DEF doser (also called a DEF injector or doser valve), which meters the correct quantity into the exhaust. NOx sensors upstream and downstream of the SCR catalyst tell the ECU whether the reaction is efficient.
Where SCR sits relative to the DPF
On Euro VI trucks, off-highway equipment and modern passenger diesels, SCR sits downstream of the DPF system — the DPF removes particulate matter, then SCR removes NOx.