| 摘要 |
This invention provides a NOx purification system ( 1 ) having NOx occlusion reduction-type catalysts ( 2, 3 ) comprising 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. In this system ( 1 ), a high-temperature type catalyst ( 2 ) is placed on the upstream side, and a low-temperature type catalyst ( 3 ) is disposed on the downstream side in series with the high-temperature type catalyst ( 2 ). The molar ratio of platinum to rhodium supported on the high-temperature type catalyst ( 2 ) is not less than 2:1 and not more than 1:2. According to the above construction, the NOx purification system ( 1 ) can have a wide NOx activation temperature window by devising the composition and layout of the catalyst. |
