| 摘要 |
PROBLEM TO BE SOLVED: To prevent deterioration of nitrogen oxides adsorbing (trapping) ability of an exhaust emission control catalyst containing zeolite trapping nitrogen oxides in exhaust in a low temperature state. SOLUTION: A catalyst tube 18 of a cold nitrogen oxides trapping catalyst 11 comprises three moisture permeable layers of a catalyst bed 33 containing zeolite, a moisture permeating layer 32, and a retaining layer 31 from an inner side where the exhaust circulates, and it is composed so that a side of the moisture permeating layer 32 not adjacent to the catalyst bed 33 containing zeolite is exposed to external air via the retaining layer 31. Since trapped water molecules are sufficiently smaller than pores of the catalyst bed 33 containing zeolite, they can move inside the catalyst. Since concentration of the water molecules near the water permeating layer 32 is higher at a catalyst 33 side and it is lower at a retaining layer 31 side on the opposite side because it is exposed to external air, the water molecules move toward the retaining layer 31 side and they are carried away by the external air. |
