FILTER FOR FILTERING PARTICULATE MATTER FROM EXHAUST GAS EMITTED FROM A POSITIVE IGNITION ENGINE

A filter for filtering particulate matter (PM) from exhaust gas emitted from a compression ignition engine, which filter comprising a porous substrate (10) having inlet surfaces (16) and outlet surfaces, wherein the inlet surfaces (16) are separated from the outlet surfaces by a porous structure containing pores (12) of a first mean pore size, wherein the porous substrate (10) is coated with a washcoat (14) coated on inlet and/or outlet surfaces and also within the porous structure of the porous substrate (10), wherein pores at a surface of the porous structure comprise a pore opening and the washcoat (14) causes a narrowing of substantially all the surface pore openings, which washcoat (14) comprising a plurality of solid particles comprising a molecular sieve promoted with at least one transition metal wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, and wherein the second mean pore size is less than the first mean pore size