Catalyst article for use in an emission treatment system

Abstract

A catalyst article for treating a flow of a combustion exhaust gas comprises: a catalytically active substrate comprising one or more channels extending along an axial length thereof through which, in use, a combustion exhaust gas flows, the one or more channels having a first surface for contacting a flow of combustion exhaust gas; wherein the substrate is formed of an extruded vanadium-containing SCR catalyst material, wherein a first layer is provided on at least a portion of said first surface, wherein the first layer comprises an ammonia slip catalyst composition comprising one or more platinum group metals supported on titania, a silica-titania mixed oxide, a CeZr mixed oxide, or a mixture thereof, and a second layer is provided on at least a portion of the first layer and comprises an SCR catalyst composition.

Claims

1. A catalyst article for treating a flow of a combustion exhaust gas, the article comprising: a catalytically active substrate comprising one or more channels extending along an axial length thereof through which, in use, a combustion exhaust gas flows, the one or more channels having a first surface for contacting a flow of combustion exhaust gas; wherein the substrate is formed of an extruded vanadium-containing SCR catalyst material, wherein a first layer is provided on at least a portion of said first surface, wherein the first layer comprises an ammonia slip catalyst composition comprising one or more platinum group metals supported on titania, a silica-titania mixed oxide, a CeZr mixed oxide, or a mixture thereof, and a second layer is provided on at least a portion of the first layer and comprises an SCR catalyst composition.

2. The catalyst article according to claim 1, wherein the substrate is a honeycomb flow-through monolith substrate.

3. The catalyst article according to claim 1, wherein the substrate comprises from 1 to 3 wt % vanadium oxide.

4. The catalyst article according to claim 1, wherein the first layer comprises from 0.05 to 0.5 wt % of the platinum group metal.

5. The catalyst article according to claim 1, wherein the platinum group metal is Pt.

6. The catalyst article according to claim 1, wherein the first layer covers up to 50% of the axial length of the substrate, and extends from an end of the article.

7. The catalyst article according to claim 1, wherein the first layer covers at least 50% of the axial length of the substrate.

8. The catalyst article according to claim 1, wherein the SCR catalyst composition of the second layer comprises a copper-promoted zeolite, an iron-promoted zeolite or a combination thereof.

9. The catalyst article according to claim 1, wherein the second layer has an ammonia storage capacity of up to 0.1 g per g of the second layer when measured at 200 C.

10. The catalyst article according to claim 1, wherein the second layer covers 100% of the first layer by area.

11. An emission treatment system for treating a flow of a combustion exhaust gas, the system comprising a source of combustion exhaust gas in fluid communication with the catalyst article of claim 1, and a source of nitrogenous reductant arranged upstream of said article.

12. The emission treatment system according to claim 11, wherein the first layer covers up to 50% of the axial length of the substrate and is provided extending from a downstream end of the article.

13. The emission treatment system according to claim 11, wherein the source of combustion exhaust gas is a diesel engine.

14. A method for treating a flow of a combustion exhaust gas, the method comprising: contacting a flow of combustion exhaust gas with the catalyst article according to claim 1 in the presence of a nitrogenous reductant.

Description

(1) The present disclosure will now be described in relation to the following non-limiting figure, in which:

(2) FIG. 1 shows a cross-section of a flow-through monolith substrate.

(3) FIG. 2 shows a graph showing activity of fresh and aged catalysts as described herein and in the prior art.

(4) FIG. 1 shows a single channel 5 within a flow-through substrate 2 according to the catalyst article 1 of the present disclosure. The substrate 1 has walls 10 formed from an extruded vanadium-containing SCR material. The walls 10 define the channel 5 through which an exhaust gas 15 flows. The walls 10 each have a surface 20 for contacting the exhaust gas 15.

(5) Provided on a downstream end 25 of the surfaces 20 is a first layer 30. The first layer 30 comprises an ammonia slip catalyst material comprising Pt on a silica-titania mixed oxide.

(6) The first layer 30 is entirely coated with a second layer 35 comprising an SCR catalyst composition.

(7) In use, the exhaust gas 15 contacts the SCR material in the substrate 1 in the presence of ammonia. This converts the NO.sub.x in the exhaust gas 15 into nitrogen and water.

(8) Excess ammonia in the exhaust gas 15 then contacts the ASC in the first layer 30 and is converted into nitrogen. This conversion can also produce addition NO.sub.x which then contacts the SCR in the second layer 35 and is converted back into nitrogen.

(9) In a preferred embodiment, the flow-through substrate 2 is an extruded blend of vanadium/tungsten/titania and an iron-promoted ZSM-5 zeolite. This is provided with a first layer 30 applied as a washcoat containing about 0.15 wt % Pt, an iron-promoted zeolite and less than 10 wt % of a silica sol binder. The binder helps to make the layer adhere to the substrate but is preferably present in a minimal amount to avoid an increase in back pressure.

(10) A second layer 35 is then applied of an SCR composition comprising an iron-promoted zeolite. This is applied with an alumina binder. The first and second layers 30, 35 may be dried after application and then calcined in air at about 500 C. to fix them.

(11) The completed article 1 is then canned for installation in an exhaust system.

(12) The present disclosure will now be described in relation to the following non-limiting examples.

EXAMPLES

Example 1: Preparation of Extruded Honeycomb Substrate

(13) An extruded honeycomb substrate catalyst according to WO 2014/027207 A1 was prepared by firstly mixing a MFI aluminosilicate zeolite that has been ion-exchanged with >1 wt. % iron with 2 wt. % V.sub.2O.sub.5WO.sub.3/TiO.sub.2 balance components with inorganic auxiliaries to improve rheology for extrusion and increase mechanical strength of the extrudate. Suitable organic auxiliaries such as extrusion lubricants and plasticisers can be added to facilitate mixing to form an homogeneous extrudable mass. The organic auxiliaries may include cellulose, water soluble resins such as polyethylene glycol and are burnt out from the final substrate during calcination. The appropriate proportions of the zeolite, V.sub.2O.sub.5WO.sub.3/TiO.sub.2, inorganic auxiliaries were selected so thatfollowing removal of the organic auxiliariesthe substrate comprised 16 wt. % of the Fe/zeolite component, 72 wt. % of the V.sub.2O.sub.5WO.sub.3/TiO.sub.2 component, 12 wt. % of the inorganic auxiliaries. The extrudable mass was extruded to form 10.5 inch diameter7.0 inch long and 400 cells per square inch honeycomb bodies in the flow-through configuration (i.e. cells open at both ends) having honeycomb cell wall thicknesses of 11 thousandths of an inch (mil). The extruded honeycomb substrate is then dried and calcined to form the finished product.

Example 2: Preparation of Ion-Exchanged Copper Zeolite SCR Catalyst Washcoat (Second Layer)

(14) Commercially available synthetic aluminosilicate zeolite CHA was NH.sub.4.sup.+ ion exchanged in a solution of NH.sub.4NO.sub.3, then filtered. The resulting materials were added to an aqueous solution of Cu(NO.sub.3).sub.2 with stirring. The slurry was filtered, then washed and dried. The procedure can be repeated to achieve a 3 wt % metal loading. The final product was calcined.

Comparative Example 3: Preparation of Pt Silica-Doped Alumina Ammonia Slip Catalyst Washcoat (First Layer)

(15) A solution of platinum nitrate was impregnated onto a commercially available silica-doped alumina to form a slurry in which the target Pt content of the silica-doped alumina was 0.2 wt. % and a Pt nominal loading of 3 g/ft.sup.3.

Example 4: Preparation of Pt/Silica-Titania Mixed Oxide Ammonia Slip Catalyst Washcoat (First Layer)

(16) A solution of platinum nitrate was impregnated onto a commercially available silica-titania mixed oxide having a silica content of about 10 wt. % to form a slurry in which the Pt content of the silica-titania mixed oxide was 0.2 wt. % and a Pt nominal loading of 3 g/ft.sup.3.

Example 5: Preparation of Pt/CeZr Mixed Oxide Ammonia Slip Catalyst Washcoat (First Layer)

(17) A solution of platinum nitrate was impregnated onto a commercially available ceria-zirconia mixed oxide having a ceria content of 50 wt. % to form a slurry in which the Pt content of the ceria-zirconia mixed oxide was 0.2 wt. % and a Pt nominal loading of 3 g/ft.sup.3.

Example 6: Coating of Substrates with First Layer Catalyst Compositions

(18) Three honeycomb substrates prepared according to Example 1 were each coated from one end of the honeycomb substrate with a washcoat of the first layer catalyst compositions described in Examples 3 (Comparative), 4 and 5 using the process described in WO 99/47260 A1 to a depth of 2 inches, i.e. a method comprising the steps of (a) locating a containment means on top of the substrate, (b) dosing a pre-determined quantity of a liquid component into said containment means, either in the order (a) then (b) or (b) then (a), and (c) by applying vacuum, drawing the entirety of said quantity of liquid component into at least a portion of the substrate, and retaining substantially all of said quantity within the support, without recycle. The coated substrates were then dried and calcined.

Example 7: Coating of Substrates with Second Layer Catalyst Compositions

(19) The three honeycomb substrates obtained from Example 6 were each further coated with the Cu/zeolite catalyst composition of Example 2 using the same methodology described in Example 6, i.e. WO 99/47260 A1. The second layer coatings were applied via the same end of the honeycomb substrate so that the second layer was coated over the first layer to the extent that none of the first layer was exposed, i.e. the second layer was applied as a slight overlap over the first layer at the end of the first layer distal to the end of the substrate from which the coating was applied, to avoid exposed first layer oxidising ammonia in incoming gas. The coated substrates were then dried and calcined.

Example 8: Ageing Conditions

(20) The extruded catalyst honeycomb substrates resulting from Example 7 were aged thermally (no water present) in an accelerated ageing step either by heating them in an oven at above 600 C. for 2 hours (referred to herein as fresh) or at 650 C. for 100 hours (referred to herein as aged) to simulate the expected exposure of the honeycomb substrates to automotive vehicular exhaust gases over a vehicle end-of-life, according to European emission standard legislation.

Example 9: Testing Conditions

(21) 1 inch diameter cores were cut from the fresh and aged substrates from Example 7 and were each loaded into a synthetic catalytic activity test (SCAT) laboratory apparatus to test each sample's ability to oxidize NH.sub.3 in a simulated exhaust gas containing 500 ppm NH.sub.3, 4.5 wt. % CO, 5 wt. % H.sub.2O, 200 ppm CO.sub.2, 12 wt. % O.sub.2, and the balance N.sub.2. The test was conducted at an exhaust gas space velocity of 150,000 hr.sup.1. The results for % NH.sub.3 conversion vs. temperature is shown in FIG. 2.

(22) As shown in FIG. 2, the silica-alumina supported catalyst provides good fresh activity, but this falls away on aging. The silica-titania and ceria-zirconia-supported catalysts, have comparable fresh activity which declines much less on aging than the silica-alumina sample.

(23) The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

(24) For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.