VANADIUM-FREE TITANIA-BASED SCR CATALYST ARTICLE

Abstract

The present invention relates to a titania-based selective catalytic reduction (SCR) catalyst article which shows comparable or better performance to those which contain vanadium. In particular, the invention relates to the provision of a titania-based SCR catalyst article comprising ceria and niobia and to methods of making these catalysts.

Claims

1. A selective catalytic reduction (SCR) catalyst article comprising a vanadium-free extruded titania substrate comprising: a titania-based portion, a filler portion and, optionally, a zeolitic portion, wherein: (a) the titania-based portion comprises the following elements on an oxide basis, based on the weight of the substrate: (i) Nb in an amount of 1 to 10 wt %; (ii) Ce in an amount of 5 to 15 wt %; (iii) W in an amount of 0 to 10 wt %, preferably from 1 to 10 wt %; and, optionally, (iv) Fe in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (v) Zr in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (vi) Si in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (vii) La in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (viii) Er in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (ix) Nd in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; wherein the balance is at least 60 wt % Ti; (b) the filler portion is in an amount of 1 to 20 wt %, based on the weight of the substrate, and (c) the optional zeolitic portion comprises one or more zeolitic SCR components in a total amount of up to 20 wt %, based on the weight of the substrate.

2. The SCR catalyst article according to claim 1, wherein the filler portion comprise glass fibers and/or clay.

3. The SCR catalyst article according to claim 1, wherein the filler portion is in an amount of 5 to 15 wt %.

4. The SCR catalyst article according to claim 1, consisting of the titania-based portion and the filler portion.

5. The SCR catalyst article according to claim 1, wherein the titania-based portion consists of oxides of Nb, Ce, W, Ti and, optionally Si.

6. The SCR catalyst article according to claim 1, wherein the ratio of Nb to Ce is from 1:1 to 1:5 by weight on an oxide basis, preferably 1:1.5 to 1:4.

7. The SCR catalyst article according to claim 1, wherein the ratio of Nb to Ce is from 1:2 to 1:3.

8. A method for the manufacture of the SCR catalyst article according claim 1, wherein the method comprises: forming a mixture comprising a paste of optionally-doped titania, and one or more salts of at least Nb and Ce, together with fillers; extruding the mixture to form a catalyst article precursor; calcining the catalyst article precursor to form the SCR catalyst article.

9. The method according to claim 8, wherein the titania is doped with silica.

10. A selective catalytic reduction (SCR) catalyst article comprising a substrate having a vanadium-free titania-based washcoat layer thereon, the washcoat layer comprising: a titania-based portion, a binder portion and, optionally, a zeolitic portion, wherein: (a) the titania-based portion comprises the following elements on an oxide basis, based on the weight of the washcoat: (i) Nb in an amount of 1 to 10 wt %; (ii) Ce in an amount of 5 to 15 wt %; (iii) W in an amount of 0 to 10 wt %, preferably from 1 to 10 wt %; and, optionally, (iv) Fe in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (v) Zr in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (vi) Si in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (vii) La in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (viii) Er in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; (ix) Nd in an amount of 0 to 5 wt %, preferably from 1 to 5 wt %; wherein the balance is at least 60 wt % Ti; (b) the binder portion is in an amount of 1 to 20 wt %, based on the weight of the washcoat, and (c) the optional zeolitic portion comprises one or more zeolitic SCR components in a total amount of up to 20 wt %, based on the weight of the washcoat.

11. The SCR catalyst article according to claim 10, consisting of the titania-based portion and the binder portion.

12. The SCR catalyst article according to claim 10, wherein the titania-based portion consists of oxides of Nb, Ce, W, Ti and, optionally Si.

13. The SCR catalyst article according to claim 10, wherein the ratio of Nb to Ce is from 1:1 to 1:5 by weight on an oxide basis, preferably 1:1.5 to 1:4.

14. The SCR catalyst article according to claim 10, wherein the ratio of Nb to Ce is from 1:2 to 1:3.

15. A method for the manufacture of the SCR catalyst article according to claim 10, wherein the method comprises: providing a substrate for a catalyst article; forming a washcoat composition comprising optionally-doped titania, and one or more salts of at least Nb and Ce; washcoating the composition onto the substrate to form a catalyst article precursor; calcining the catalyst article precursor to form the SCR catalyst article.

16. The method according to claim 15, wherein the substrate is a porous honeycomb substrate.

17. An exhaust system for the treatment of an exhaust gas, the system comprising the SCR catalyst article according to claim 1 and, optionally, means for injecting a nitrogenous reductant arranged upstream of said article.

18. An exhaust system for the treatment of an exhaust gas, the system comprising the SCR catalyst article according to claim 10, and, optionally, means for injecting a nitrogenous reductant arranged upstream of said article.

Description

FIGURES

[0110] The invention will now be described further in relation to the following non-limiting figures, in which:

[0111] FIG. 1 shows an exhaust system comprising a catalyst article as described herein.

[0112] FIG. 1 shows a diesel combustion and exhaust gas treatment system 32. The system 32 comprises a diesel engine 30 having a manifold 40 for passing exhaust gas from the diesel engine 30 into an exhaust gas duct 34.

[0113] The exhaust gas duct 34 conveys the exhaust gas firstly to a DOC 42, secondly to an SCR catalyst article 50 as described herein, and finally the treated exhaust gas is released to the atmosphere 36. Before the exhaust gas enters the SCR catalyst article 50 it is dosed with ammonia 48 from an ammonia dosing spray 46.

[0114] The SCR catalyst article 50 may be an extruded catalyst article or a washcoated catalyst article as described herein.

EXAMPLES

[0115] The invention will now be described further in relation to the following non-limiting examples.

[0116] 1. Making an extruded catalyst

[0117] Extruded catalyst samples were prepared according to the following process. Powdered titania, niobium oxalate, cerium acetate, and, where employed, tungsten oxide were combined with clay minerals and glass fibres and then cellulose, a plasticizer/extrusion aid (for example, Zusoplast, a mixture of oleic acid, glycols, acids and alcohols-a brand name of Zschimmer & Schwarz GmbH & Co KG) and a polyethylene oxide (Alkox@ PEO) at room temperature to form a mouldable paste. The solids content of the paste was set such that its rheological properties were suitable for extrusion. The quantitative proportions of the starting materials were selected such that the final solid catalyst body contained the weight percentages of titania, ceria, niobia, and tungsten oxide indicated in the Tables below.

[0118] The mouldable paste was extruded at 20° C. into a flow-through honeycomb having a circular cross-section of 1 inch diameter and a cell density of 400 cpsi (cells per square inch). The extruded honeycomb was freeze dried for several hours at 2 mbar according to the method described in WO 2009/080155 and then calcined at a temperature of 500-650° C. in a lab scale muffle oven to form a solid catalyst body.

[0119] 2. Investigating Performance

[0120] A number of extruded SCR catalyst article samples were produced according to the method described above. These were then tested for NOx conversion and N.sub.2O production under different temperatures and against a standard vanadium-containing extruded article (VTiW) in a synthetic catalytic activity test (SCAT) apparatus. The tests were performed both on fresh and aged catalyst samples.

[0121] Testing for the NOx performance was performed with a standardised gas mixture containing 300 ppm NO, 300 ppm NH.sub.3, 9.3% O.sub.2, 7.0% H.sub.2O and the balance nitrogen at a space velocity of 120000 h.sup.−1.

[0122] The aging conditions were 100 hrs at 580° C. in air.

[0123] In the below tables, the numbers preceding each element represent the quantities (wt %) of that element, on an oxide basis, based on the total weight of the extruded catalyst sample (the balance being filler components). The parentheses in the examples linking the titania to certain elements denotes that these were pre-doped onto the titania.

TABLE-US-00001 TABLE 1 NOx conversion (%) fresh/aged Sample 180° C. 250° C. 400° C. 500° C. VTiW 14/13 55/54 84/86 80/74 82.0Ti/2.75Nb/5.0Ce 2/3 10/9  75/55 82/68 (73.8Ti/8.2W)/2.75Nb/5.0Ce 2/3 14/17 78/72 83/76 (73.8Ti/4.1W/4.1Si)//2.75Nb/5.0Ce 2/4 13/16 74/64 75/68

[0124] This data demonstrates that the catalysts disclosed herein can have similar NO.sub.x performance at high temperatures, compared to conventional V-containing catalysts. The data further confirms that the presence of W stabilises the catalyst for high temperature performance after aging.

[0125] Surprisingly, since Si is often added as a stabiliser for titania, it was found that the addition of silica reduced stability and lessened performance. Preferably the titania-based portion does not contain silica.

TABLE-US-00002 TABLE 2 NOx conversion (%) fresh/aged Sample 180° C. 250° C. 400° C. 500° C. VTiW 14/13 55/54 84/86 80/74 82.0Ti/2.75Nb/5.0Ce 2/3 10/9  75/55 82/68 77.0Ti/2.75Nb/10.0Ce 4/2 11/8  75/55 83/70 65.0Ti/8.6Nb/13.0Ce 2/3 10/14 58/57 65/67

[0126] This data demonstrates that the addition of Ceria is effective across a range from at least 5 to 15 wt %.

[0127] Nonetheless, addition of increasing amounts does not give significantly better performance, so lower levels may be more cost-effective. The data also suggests that high levels of Nb may be less desirable.

TABLE-US-00003 TABLE 3 NOx conversion (%) fresh/aged Sample 180° C. 250° C. 400° C. 500° C. VTiW 14/13 55/54 84/86 80/74 (73.8Ti/8.2W)/2.75Nb/5.0Ce 2/3 14/17 78/72 83/76 (69.3Ti/7.7W)/2.75Nb/10.0Ce 3/3 20/18 87/85 88/90 (66.6Ti/7.4W)/2.75Nb/13.0Ce 3/3 17/16 76/78 78/81

[0128] This data confirms a sweet-spot of Ce around 10 wt %, providing high temperature performance that exceeds that of the standard vanadium-containing alternatives.

TABLE-US-00004 TABLE 4 N.sub.2O production (%) fresh/aged Sample 180° C. 250° C. 400° C. 500° C. VTiW 0/0 0/0 0/2 14/54 (73.8Ti/8.2W)/2.75Nb/5.0Ce 2/4 3/4 2/0 31/8  (69.3Ti/7.7W)/2.75Nb/10.0Ce 2/1 2/1 3/1 28/29 (66.6Ti/7.4W)/2.75Nb/13.0Ce 2/4 2/4 2/2 23/10

[0129] This data confirms that the aged N.sub.2O production of all of the catalysts described herein is significantly better than that of the vanadium-containing standards.

TABLE-US-00005 TABLE 5 NOx conversion (%) fresh/aged Sample 180° C. 250° C. 400° C. 500° C. VTiW 14/13 55/54 84/86 80/74 (69.3Ti/7.7W)/2.75Nb/10.0Ce 3/3 20/18 87/85 88/90 (68.5Ti/7.6W)/3.3Nb/10.0Ce 3/3 19/18 86/79 87/81 (67.5Ti/7.5W)/4.0Nb/10.0Ce 3/3 25/21 87/82 90/83 (66.1Ti/7.3W)/6.0Nb/10.0Ce 3/4 26/26 86/83 90/86 (64.3Ti/7.1W)/8.0Nb/10.0Ce 4/4 28/19 86/77 87/82

[0130] This data looks at the effect of changing the Nb content when at the optimum 10 wt % Ce level. As can be seen, there is excellent performance across the range, including examples which exceed the performance of the standard. Again, since there is no improvement with increasing amounts of Nb, it may be more cost effective to work at the lower end. Indeed, the best performance is shown at 2.75 Nb.

TABLE-US-00006 TABLE 6 N.sub.2O production (%) fresh/aged Sample 180° C. 250° C. 400° C. 500° C. VTiW 0/0 0/0 0/2 14/54 (69.3Ti/7.7W)/2.75Nb/10.0Ce 2/1 2/1 3/1 28/29 (68.5Ti/7.6W)/3.3Nb/10.0Ce 1/3 2/3 3/2 37/12 (67.5Ti/7.5W)/4.0Nb/10.0Ce 1/3 2/3 2/3 34/10 (66.1Ti/7.3W)/6.0Nb/10.0Ce 1/3 2/3 2/2 26/13 (64.3Ti/7.1W)/8.0Nb/10.0Ce 1/2 2/3 3/1 32/5 

[0131] This data looks at the effect of changing the Nb content when at the optimum 10 wt % Ce level. As can be seen, there is excellent performance across the range when aged, including all examples having lower N.sub.2O production than the standard. Here, higher levels of Nb give lower N.sub.2O production suggesting that higher amounts may be desirable.

[0132] The term “comprising” as used herein can be exchanged for the definitions “consisting essentially of” or “consisting of”. The term “comprising” is intended to mean that the named elements are essential, but other elements may be added and still form a construct within the scope of the claim. The term “consisting essentially of” limits the scope of a claim to the specified materials or steps and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. The term “consisting of” closes the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith.

[0133] 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.

[0134] For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.