Catalyst unit for reduction of NOx compounds

x ab, c, d, ea, b, c, d, e; a, b, c, d, ea, b, c, d, e2 a, b, c, d, ea, b, c, d, ea, b, c, d, ex A catalyst unit configured for the reduction of NOcompounds in a gas stream and which includes a catalytically active material of the silver/alumina type. The catalyst unit has at least one first reaction zone containing the catalytically active material and at least one second reaction zone which is arranged to permit breakdown of a gas species which is formed in the first reaction zone, with Nbeing formed, and in that the gas stream is conducted through the first reaction zone and the second reaction zone. The invention results in an improved catalyst unit which in particular provides high-grade reduction of NOcompounds in an exhaust gas stream from a combustion engine.