Selective Catalytic Reduction (SCR), a process where a gaseous or liquid reductant (most commonly ammonia or urea) is added to the flue gas stream and is absorbed onto a catalyst. The reductant reacts with NOx in the flue gas to form H2O and N2.
Specialty Catalytic Converters are required to make an SCR system work, the current options being a Vanadium based catalyst, or a catalyst with Zeolites in the washcoat. In some jurisdictions use of a Vanadium based catalyst are frowned on (California) and the Zeolite type preferred, however Vanadium offers better performance.
SCR catalysts function well only within a narrow temperature window, and for OEM installations the Engine Control Unit is specially programmed to keep the exhaust gas temperature in that range