US Patent 7972400 Honeycomb filter system

A honeycomb filter system comprising a first honeycomb filter disposed upstream: first filter having partition wall matrix of a mean pore diameter of 25 μm or more and below 70 μm and a porosity of 40% or more and below 70%, and carrying an oxidation catalyst containing at least one material selected from the group consisting of platinum (Pt), palladium, ceria, and alumina in at least a part of inner surfaces of pores of the matrix; and a second honeycomb filter disposed downstream; having a surface layer of a peak pore diameter of 0.3 μm or more and below 20 μm which is equivalent to or smaller than that of its matrix, and a porosity of 60% or more and below 95% which is higher than that of the matrix; and having the other specified relations.