Abstract: In the distribution of surface vacancies which open on the surface of the cellular walls of pores, more than 8% of total opening area of all surface vacancies that the pores are open on the surface of the cellular walls is occupied by total opening area of surface vacancies having maximum diameter of from 10 to 50 ?m, in the distribution of inner pores, more than 20% of total cross-sectional area of all pores is occupied by total cross-sectional area of pores having cross-sectional area equivalent to that of a circle having diameter more than 300 ?m.

Abstract: In the distribution of surface vacancies which open on the surface of the cellular walls of pores, more than 8% of total opening area of all surface vacancies that the pores are open on the surface of the cellular walls is occupied by total opening area of surface vacancies having maximum diameter of from 10 to 50 ?m measured by a direct observation method and, in the distribution of inner pores, more than 20% of total cross-sectional area of all pores is occupied by total cross-sectional area of pores having cross-sectional area equivalent to that of a circle having diameter more than 300 ?m measured by cross-sectional observation using CT scan. Even when a large amount of PMs are emitted densely at a short time, since they are dispersed and collected by each pore, PM deposition is controlled and the increase of the pressure loss is suppressed, as well as collecting efficiency is improved.