Patent ID: 7063818

Claim:
An HC adsorbent for an internal combustion engine, which is mounted in one of a pair of exhaust passages which are provided in parallel to each other in an exhaust pipe downstream of an exhaust emission control catalyst for purifying an exhaust gas of the internal combustion engine, and through which the exhaust gas is permitted to flow selectively by a switchover valve, wherein said adsorbent is formed of a mixture of a plurality of types of zeolites having pores of different small diameters, a zeolite having pores of a small diameter equal to or larger than 3.5 Å and smaller than 6 Å and no pore of a small diameter equal to or larger than 6 Å and smaller than 7 Å and a zeolite having pores of a small diameter equal to or larger than 7 Å and smaller than 10 Å and no pore of a small diameter equal to or larger than 6 Å and smaller than 7 Å being contained in a total amount equal to or higher than 80% by weight in the mixture, and a zeolite having pores of a small diameter equal to or larger than 6 Å and smaller than 7 Å being also contained in an amount not exceeding 20% by weight in the mixture, wherein a ferrierite-type zeolite and a pentasil-type zeolite are selected as the zeolite having the pores of the small diameter equal to or larger than 3.5 Å and smaller than 6 Å and no pore of the small diameter equal to or larger than 6 Å and smaller than 7 Å, and a faujasite-type zeolite is selected as the zeolite having the pores of the small diameter equal to or larger than 7 Å and smaller than 10 Å and no pore of the small diameter equal to or larger than 6 Å and smaller than 7 Å, and in a three-component composition diagram in which each of ratios is indicated by one point within an equilateral triangle having three apexes each indicating the case where the mixture includes only one of zeolite components, the weight ratio of the ferrierite-type, pentasil-type and faujasite-type zeolites is set to lie in a region surrounded by a line segment connecting a point of (0:0.2:0.8), a point of (0.08:0.12:0.8), a point of (0.4:0.08:0.52), a point of (0.67:0.1:0.23), a point of (0.67:0.23:0.1), a point of (0.4:0.51:0.09), a point of (0.09:0.81:0.1) and a point of (0:0.81:0.19) to one another.