Patent ID: 8017086

Claim:
A catalyst unit configured for reducing NO x compounds in an internal combustion engine's exhaust gas stream, comprising: at least one first zone having a support structure with a first catalytically active material of the silver/alumina type supported by the support structure, and at least one second zone which is located downstream of an immediately preceding first zone, wherein the catalyst unit is arranged such that the exhaust gas stream is conducted through the first zone or zones and the second zone or zones, wherein interaction of the exhaust gas stream with the first catalytically active material produces an unstable intermediate gas species that breaks down into N 2 , and wherein each second zone is a space that is 1) free of any device that acts on the unstable intermediate gas product in such a way as to foster or promote breakdown of the unstable intermediate gas product into N 2 , and 2) that is long enough such that the majority of the unstable intermediate gas species that is formed by interaction of the exhaust gas stream with the first catalytically active material in the immediately preceding first zone will have broken down into N 2 by the time the exhaust gas stream has reached the downstream end of the second zone.