Nitrogen oxide purification system

摘要

An NOx purification system having NOx occlusion reduction-type catalysts including an occlusion material and a metal catalyst, which occludes NOx in a lean state in terms of a fuel-air ratio of an exhaust gas and releases the occluded NOx in a rich state. A high-temperature type catalyst is placed on the upstream side, and a low-temperature type catalyst is disposed on the downstream side in series with the high-temperature type catalyst. The molar ratio of platinum to rhodium supported on the high-temperature type catalyst is within a range of 2:1 to 1:2. This NOx purification system is capable of a wide NOx activation temperature window.