Filtering particulate matter from exhaust gas

摘要

A filter for filtering particulate matter (PM) from exhaust gas emitted from a positive ignition engine comprises a porous substrate 10 having inlet and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores 12 of a first mean pore size. The porous substrate is coated with a catalytic surface washcoat layer 14 comprising a plurality of solid particles, wherein the washcoat layer covers surface pores of the porous substrate and the pores of the washcoated porous substrate are defined in part by spaces between the particles in the washcoat. The porous structure of the washcoated porous substrate contains pores 16 of a second mean pore size and the second mean pore size is less than the first mean pore size. The catalytic washcoat is a selective catalytic reduction (SCR) catalyst. The washcoat may comprise at least one molecular sieve. The molecular sieve can be metallised. An exhaust system comprises the filter and may have a NOx absorber catalyst disposed upstream of the filter. A method of trapping particulate matter is also claimed.