Catalyst system for the reduction of NO

x 3x 3x x x x x 33 x x x This catalyst system simultaneously removes ammonia and enhances net NOconversion by placing an NH-SCR catalyst formulation downstream of a lean NOtrap. By doing so, the NH-SCR catalyst adsorbs the ammonia from the upstream lean NOtrap generated during the rich pulses. The stored ammonia then reacts with the NOemitted from the upstream lean NOtrap-enhancing the net NOconversion rate significantly, while depleting the stored ammonia. By combining the lean NOtrap with the NH-SCR catalyst, the system allows for the reduction or elimination of NHand NOslip, reduction in NOspikes and thus an improved net NOconversion during lean and rich operation.