Nitrogen oxide (NOx) can be reduced in lean-burn-gas- engines through a SCR-Catalyst (SCR= Selective Catalytic Reduction). This system operates at an oxygen surplus in the exhaust gas.
Following basic reactions take place:
6 NO + 6 NH₃ + 1½ O₂ → 6 N₂ + 9 H₂O
4 NO₂ + 4 NH₃ → 4 N₂ + 6 H₂O + O₂

SCR-systems are considerably more complex than the usual catalysts, due to a urea-solution that is feed into the exhaust gas flow. With the right temperature, ammoniac can accumulate which in turn, reduces the nitrogen oxide (NOX).
In addition to the actual catalyst, the SCR-system consists of a dosing system and a mixing pipe as well as a control system and a urea-tank-filling-system.