Catalyst unit for reduction of NOx-compounds

Disclosed is a catalyst unit for reduction of NO x compounds in an exhaust gas stream having a support structure (10) defining channels (9), characterized in that the support structure (10) comprises a layer (11) of silver/alumina material for converting NOx to intermediate gas species and the said structure (10) also comprises a decomposition layer (12) of a material having no sites for oxidation nor any acidic sites, said material preferably consisting of silicon carbide, gold, silicalite, base metal oxide, active carcoal or soot, wherein the decomposition layer (12) accelerates and facilitates decomposition of the intermediate gas species formed in the said layer (11) to N 2 , and a motor vehicle comprising such a catalyst.