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A filter for filtering particulate matter (PM) from exhaust gas emitted from a positive ignition engine or a compression ignition engine, which filter comprising a porous substrate having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porou

A filter for filtering particulate matter (PM) from exhaust gas emitted from a positive ignition engine or a compression ignition engine, which filter comprising a porous substrate having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a washcoat comprising a plurality of solid particles 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.

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1. A system for filtering particulate matter from exhaust gas, said system comprising: a filter having inlet surfaces, outlet surfaces, and a porous structure between the inlet surfaces and the outlet surfaces;a selective catalytic reduction (SCR) washcoat comprising a small pore zeolite promoted wi

1. A system for filtering particulate matter from exhaust gas, said system comprising: a filter having inlet surfaces, outlet surfaces, and a porous structure between the inlet surfaces and the outlet surfaces;a selective catalytic reduction (SCR) washcoat comprising a small pore zeolite promoted with at least one metal selected from the group consisting of Cr, Co, Cu, Fe, Hf, La, Ce, In, V, Mn, Ni, Zn, Ga, Ag, Au, Pt, Pd, and Rh, wherein the washcoat is disposed: on the inlet surfaces, on the outlet surfaces, or within the porous structure and provides a washcoated filter for filtering the particulate matter; and wherein the porous structure has substrate pores of a first mean pore size; the washcoat defines pores of a second mean pore size; and the second mean pore size is less than the first mean pore size, and;a NOx absorber catalyst disposed upstream of the washcoat. 2. A system according to claim 1, wherein the washcoat is disposed within the porous structure. 3. A system according to claim 1, wherein the transition metal is selected from the group consisting of Cu, Fe, and Ce. 4. A system according to claim 1, wherein the zeolite has a CHA framework structure. 5. A system according to claim 4, wherein the transition metal consists of Cu. 6. A system according to claim 1, wherein: the washcoat is present on both the inlet surfaces and the outlet surfaces and defines pores; the mean pore size of the washcoat on the inlet surfaces is less than the mean pore size of the washcoat on the outlet surfaces; and the NOx absorber catalyst is disposed upstream of the filter. 7. A system according to claim 1, wherein the washcoat is present on the outlet surfaces and the NOx absorber catalyst is present on the inlet surfaces. 8. A system according to claim 1, wherein: the porous structure has substrate pores of a first mean pore size; the SCR washcoat is present on the inlet surfaces and defines pores of a second mean pore size; and the second mean pore size is less than the first mean pore size. 9. A system according to claim 8, further comprising a second SCR washcoat present on the outlet surfaces and defining pores having a third mean pore size greater than the first mean pore size. 10. A system according to claim 1, the filter is a ceramic wallflow filter. 11. A system according to claim 1, wherein: the porous structure has substrate pores; and the SCR washcoat is one or more layers substantially covering the substrate pores at the inlet surfaces and/or the outlet surfaces. 12. A system according to claim 1, wherein: the washcoat is present on the inlet surfaces; and the NOx absorber catalyst is disposed upstream of the filter. 13. A system according to claim 1, wherein the zeolite has an AEI framework structure and the metal is selected from Cu and Fe. 14. A method for trapping particulate matter from exhaust gas emitted from an engine, said method comprising: contacting the exhaust gas containing the particulate matter with a NOx absorber catalyst, then directing the exhaust gas across a filter having inlet surfaces, outlet surfaces, and a porous structure between the inlet surfaces and the outlet surfaces, wherein a selective catalytic reduction (SCR) washcoat comprising a small pore zeolite promoted with at least one transition metal selected from the group consisting of Cr, Co, Cu, Fe, Hf, La, Ce, In, V, Mn, Ni, Zn, Ga, Ag, Au, Pt, Pd, and Rh is disposed: on the inlet surfaces, on the outlet surfaces, or within the porous structure and provides a washcoated filter for filtering the particulate matter and wherein the porous structure has substrate pores of a first mean pore size; the washcoat defines pores of a second mean pore size; and the second mean pore size is less than the first mean pore size. 15. A method according to claim 14, wherein the zeolite has a CHA framework structure. 16. A method according to claim 15, wherein the transition metal consists of Cu. 17. A method according to claim 14, wherein the washcoat is disposed within the porous structure; the porous structure has substrate pores of a first mean pore size; the washcoat defines pores of a second mean pore size; and the second mean pore size is less than the first mean pore size. 18. A method according to claim 14, wherein the exhaust gas is a lean exhaust gas comprising NOx and the NOx absorber catalyst absorbs NOx from the lean exhaust gas, and the method further comprises at least one of: (1) periodically decreasing the oxygen concentration in the exhaust gas; or (2) elevating the temperature of the NOx absorber catalyst, to cause the NOx absorber catalyst to desorb NOx. 19. A method according to claim 14, wherein the washcoat is present on the outlet surfaces and the NOx absorber catalyst is present on the inlet surfaces. 20. The method according to claim 14, wherein the zeolite has an AEI framework structure and the metal is selected from Cu and Fe. 21. A filter for filtering particulate matter (PM) from an exhaust gas, the filter comprising: a filter substrate having inlet surfaces, outlet surfaces, and a porous structure between the inlet surfaces and the outlet surfaces, wherein the porous structure has substrate pores of a first mean pore size; anda selective catalytic reduction (SCR) washcoat comprising a small pore zeolite promoted with at least one metal selected from the group consisting of Cr, Co, Cu, Fe, Hf, La, Ce, In, V, Mn, Ni, Zn, Ga, Ag, Au, Pt, Pd, and Rh, wherein (a) the washcoat is disposed within the porous structure and optionally on at least one of the inlet surfaces and the outlet surfaces, and provides a washcoated filter for filtering the particulate matter, (b) the washcoat defines pores of a second mean pore size, and the second mean pore size is less than the first mean pore size, and (c) the washcoat has a mean particle size of about 1 to 18 μm and/or a D90 particle size of about 1 to 18 μm. 22. The filter of claim 21, wherein the washcoat is a layer coated on an inlet surface and/or outlet surface and within the substrate pores, and wherein second mean pore size is a narrowing of the substrate pores caused by the washcoat layer. 23. The filter of claim 21, wherein the filter is a wall-flow filter. 24. The filter of claim 23, wherein washcoat is a layer coated on an outlet surface and additionally within the substrate pores, and wherein second mean pore size is a narrowing of the substrate pores caused by the washcoat layer. 25. The filter of claim 21, wherein the washcoat is loaded at a concentration of at least 0.25 g/in3. 26. The filter of claim 21, wherein the washcoat is loaded at a concentration of 0.80 to 3.00 g/in3. 27. The filter of claim 21, wherein the washcoat has a mean particle size of about 3 to 12 μm and/or a D90 particle size of about 3 to 12 μm. 28. The filter of claim 21, wherein the small pore zeolite has a CHA or AEI framework. 29. The filter of claim 21, wherein the small pore zeolite has a CHA framework and the metal is Cu or Fe. 30. The filter of claim 21, wherein the small pore zeolite has an AEI framework and the metal is Cu or Fe. 31. The filter of claim 21, wherein the filter comprises aluminium titanate. 32. The filter of claim 21, wherein the first mean pore size is 8-45 μm and a porosity of 45-75%. 33. The filter of claim 21, wherein the first mean pore size is 8-25 μm. 34. The filter of claim 21, wherein the exhaust gas is generated by a diesel engine.