System for the removal of particulate matter and noxious compounds from engine exhaust gas

1. A system for the removal of volatile organic com-pounds, particulate matter and nitrogen oxides from exhaust gas of a compression ignition engine comprising

an oxidation unit with a catalyst active in oxidation of volatile organic compounds and carbon monoxide to carbon dioxide and water, and nitrogen oxide to nitrogen dioxide; and

a catalysed wall flow particulate filter downstream of the oxidation unit, the catalysed wall flow particulate filter comprising a plurality of longitudinal inlet flow channels and outlet flow channels separated by gas permeable porous partition walls, each inlet flow channel having an open inlet end and a closed outlet end, and each outlet flow channel having a closed inlet end and an open outlet end;

wherein the system is adapted for the introduction of a urea solution into the exhaust gas downstream from the catalysed wall flow particulate filter;

wherein the system further comprises an SCR active catalyst downstream of the catalysed wall flow particulate filter to receive exhaust gas into which urea solution has been introduced, the SCR active catalyst comprising oxides of vanadium and tungsten;

wherein the system further comprises a three way catalyst unit downstream of the SCR active catalyst, the three way catalyst unit having an NH 3 -SCR activity, an ammonia oxidation activity, and an adsorption capacity for volatile vanadium and tungsten compounds volatilized off the SCR active catalyst, the three way catalyst unit comprising compounds selected from the group consisting of metal oxides, zeolites, silica, non-zeolite silica alumina, and mixtures thereof.

2. The system of claim 1 further comprising a second SCR active catalyst comprising oxides of vanadium, tungsten, and titanium, the second SCR catalyst being arranged between the catalyzed wall flow particulate filter and the three way catalyst unit.

3. The system according to claim 1 , wherein the three way catalyst unit comprises a bottom layer comprising platinum, alumina, and/or titania, and optionally palladium coated on a substrate, and a top layer comprising oxides of vanadium, tungsten, and titanium admixed with at least one of ceria, alumina, silica, zirconia, non-zeolite silica alumina, and zeolite.

4. The system according to claim 3 , wherein the top layer has layer thickness of between 40 μm and 250 μm.

5. The system according to claim 3 , wherein the bottom layer has a layer thickness of between 5 μm and 450 μm.

6. The system according to claim 3 , wherein the top layer has a porosity of between 20% and 80%.

7. The system according to claim 3 , wherein the bottom layer of the three way catalyst unit contains 0.0018 g-0.35 g platinum and/or palladium per liter of the substrate.

8. The system according to claim 3 , wherein top layer of the three way catalyst unit comprises, per liter of the substrate, 1.0 g-20 g vanadium pentoxide, 3 g-40 g tungsten oxide, 40 g-460 g titania, 0 g-86 g silica, 0 g-86 g ceria, 0 g-86 g alumina, 0 g-86 g non-zeolite silica alumina, and 0 g-86 g of a zeolite.

9. The system according to claim 1 , wherein the three way catalyst unit comprises a substrate with a flow through monolith shape.

10. The system according to claim 9 , wherein the amount of the top layer in the three way catalyst unit is between 50 to 500 g per liter of the substrate.

11. The system according to claim 9 , wherein the amount of the bottom layer in the three way catalyst unit is between 5 and 255 g per liter of the substrate.

12. The system according to claim 1 , wherein the SCR active catalyst further comprises titania.

13. The system according to claim 1 , wherein the catalyst in oxidation unit comprises platinum and/or palladium supported on silica-alumina and/or alumina and/or titania with a weight ratio of platinum to palladium of 1:0 to 1:1.

14. The system according to claim 13 , wherein the content of platinum and/or palladium is between 0.1 g and 2 g per liter of the catalyst.