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

Abstract

System for the removal of noxious compounds and particulate matter from exhaust gas of a compression ignition engine comprising a three way catalyst unit having an NH.sub.3-SCR activity, an ammonia oxidation activity and an adsorption activity of volatile vanadium and tungsten compounds volatilized off an upstream SCR active catalyst.

Claims

1. A system for the removal of volatile organic compounds, 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; wherein the system is adapted for the introduction of a urea solution into the exhaust gas downstream from the oxidation unit; wherein the system further comprises an SCR active catalyst downstream of the oxidation unit for receiving the exhaust gas into which the urea solution has been introduced, the SCR active catalyst comprising oxides of vanadium and tungsten; wherein the system further comprises a catalysed wall flow particulate filter downstream of the SCR active catalyst, 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 further comprises a three way catalyst unit downstream of the catalysed wall flow particulate filter, the three way catalyst unit having an NH.sub.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 active 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 m and 250 m.

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

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 the 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.

Description

(1) FIG. 1 displays the NOx conversion, together with the outlet concentrations of NOx, N.sub.2O, and N.sub.2. The performance under these conditions in NH.sub.3-SCR is documented by a conversion of about 50-60% in the temperature range of interest (250-400 C.) with a low yield of N.sub.2O and a high yield of N.sub.2. FIG. 1 shows NOx conversion for NH3-SCR and outlet concentrations of NOx, N2, and N2O for a Pt/V-W-oxide based monolith three way catalyst, using a feed of 250 ppm NOx, 300 ppm NR3, 12% O2, and 4% water in nitrogen at a space velocity of 100000 h.sup.1.

(2) FIG. 2 shows the conversion of ammonia, and selectivities to N2, NOx, N2O in the selective oxidation to ammonia. In the temperature range of interest (250-400 C), the ammonia is almost completely converted and the reaction product consists mainly of nitrogen. FIG. 2 shows NH.sub.3 conversion for selective oxidation of ammonia and selectivities to NOx, N.sub.2, and N.sub.2O for a Pt/V-W-oxide based monolith three way catalyst, using a feed of 200 ppm NH.sub.3, 12% O.sub.2, and 4% water in nitrogen at a space velocity of 100000 h.sup.1.

EXAMPLE 1

(3) This example demonstrates the performance in NH.sub.3-SCR of a three way catalyst. The catalyst consists of Pt impregnated on a glass fiber paper based monolith that is reinforced with TiO.sub.2, on top of which a washcoat layer, containing vanadium and tungsten, titanium dioxide and silica, having NH.sub.3-SCR activity, is applied. The Pt content in the catalyst was 88 mg/l. The content of the SCR active washcoat layer was 197 g/l, of which 5% was silica. The catalyst was degreened at 550 C. for 1 hour prior to the performance test. The reactor feed gas consisted of 250 ppm NOx, of which less than 5% is present as NO.sub.2, 300 ppm NH.sub.3, 12% O.sub.2, and 4% water in nitrogen. The flow rate was adjusted to reach a space velocity of 100000 h.sup.1, based on the monolith volume.

EXAMPLE 2

(4) This example shows the performance of the three way catalyst, as characterized in Example 1, for selective oxidation of ammonia to reduce ammonia slip. The catalyst was degreened for 1 h at 550 C. The feed gas used in this measurement was 200 ppm NH3, 12% O2 and 4% water in nitrogen. The flow was adjusted to reach a space velocity of 100000 h-1 based on the monolith volume.