Patent ID: 8186149

Claim:
A control method of a NOx reduction system for treating NOx contained in exhaust gas from an internal combustion engine including: a NOx treatment means disposed in an exhaust passage from the internal combustion engine; and an air-fuel ratio control means for adjusting an air-fuel ratio of the exhaust gas to lean or rich; wherein the NOx treatment means comprises at least a first catalyst layer including a solid acid catalyst capable of adsorbing ammonia and a second catalyst layer including a noble metal and a cerium oxide-based material, the second catalyst layer and the first catalyst layer being stacked in that order on a carrier, the first catalyst layer configuring the uppermost layer, and the second catalyst layer is configured such that the content of the noble metal decreases gradually or stepwise from the first catalyst layer to the carrier, and wherein the control method of a NOx reduction system is characterized in that the air-fuel ratio control means is controlled in order to repeat at predetermined intervals the following Steps A and B of: Step A: the air-fuel ratio control means making a lean condition, thereby making the NOx adsorb onto the second catalyst layers while being oxidized by the second catalyst layer (Step A 1 ), and ammonia adsorbed onto the first catalyst layer in Step B being reduced to nitrogen and released (Step A 2 ); and Step B: the air-fuel ratio control means making a rich condition, thereby converting NOx adsorbed in the Step A 1 to ammonia by the second catalyst layer, and adsorbing the ammonia to the first catalyst layer.