VANADIUM-BASED SELECTIVE CATALYTIC REDUCTION CATALYST

Abstract

The present invention relates to a selective catalytic reduction catalyst for the treatment of an exhaust gas of a diesel engine comprising (i) a flow-through substrate comprising an inlet end, an outlet end, a substrate axial length extending from the inlet end to the outlet end and a plurality of passages defined by internal walls of the flow-through substrate extending therethrough; (II) a coating disposed on the surface of the internal walls of the substrate, where-in the surface defines the interface between the passages and the internal walls, wherein the coating comprises a vanadium oxide supported on an oxidic material comprising titania, and further comprises a mixed oxide of vanadium and one or more of iron, erbium, bismuth, cerium, europium, gadolinium, holmium, lanthanum, lutetium, neodymium, praseodymium, promethium, samarium, scandium, terbium, thulium, ytterbium, yttrium, molybdenum, tungsten, manganese, cobalt, nickel, copper, aluminum and antimony.

Claims

1. A selective catalytic reduction catalyst for the treatment of an exhaust gas of a diesel engine, the catalyst comprising: a flow-through substrate comprising an inlet end, an outlet end, a substrate axial length extending from the inlet end to the outlet end, and a plurality of passages defined by internal walls of the flow-through substrate extending therethrough; and a coating disposed on the surface of the internal walls of the substrate, wherein the surface defines the interface between the passages and the internal walls, wherein the coating comprises a vanadium oxide supported on an oxidic material comprising titania, and wherein the coating further comprises a mixed oxide of vanadium and at least one selected from the group consisting of iron, erbium, bismuth, cerium, europium, gadolinium, holmium, lanthanum, lutetium, neo-dymium, praseodymium, promethium, samarium, scandium, terbium, thulium, ytterbium, yttrium, molybdenum, tungsten, manganese, cobalt, nickel, copper, aluminum, and antimony.

2. The selective catalytic reduction catalyst of claim 1, wherein the oxidic material further comprises one or more oxides.

3. The selective catalytic reduction catalyst of claim 1, wherein the oxidic material comprises from 75 to 100 weight-% titania.

4. The selective catalytic reduction catalyst of claim 1, wherein the mixed oxide is a mixed oxide of vanadium and at least one selected from the group consisting of iron, erbium, bismuth, aluminum, and antimony.

5. The selective catalytic reduction catalyst of claim 1, wherein the vanadium oxide, calculated as V.sub.2O.sub.5, is present in the catalyst at a loading (Iv1)/(g/in.sup.3), wherein the vanadium of the mixed oxide, calculated as V.sub.2O.sub.5, is present in the catalyst at a loading (Iv2)/(g/in.sup.3), and wherein the ratio (Iv1):(Iv2) is in the range of from 0.1:1 to 3:1.

6. The selective catalytic reduction catalyst of claim 1, wherein the amount of vanadium comprised in the coating, the vanadium being calculated as V.sub.2O.sub.5, is in the range of from 2.5 to 8 weight-%, based on the total weight of the coating.

7. The selective catalytic reduction catalyst of claim 1, wherein in the catalyst, the oxidic material is present at a loading in the range of from 1 to 8 g/in.sup.3.

8. The selective catalytic reduction catalyst of claim 1, wherein the coating further comprises an oxidic binder.

9. The selective catalytic reduction catalyst of claim 1, wherein in the catalyst, the coating is present at a loading in the range of from 1.5 to 10 g/in.sup.3.

10. The selective catalytic reduction catalyst of claim 1, wherein the coating comprises two or more coats.

11. A process for preparing a selective catalytic reduction catalyst, the process comprising: (a) providing a flow-through substrate comprising an inlet end, an outlet end, a substrate axial length extending from the inlet end to the outlet end and a plurality of passages defined by internal walls of the flow-through substrate extending therethrough; (b) providing a slurry comprising: a solution of vanadium oxide, a powder of an oxidic material comprising titania, water, and a powder of a mixed oxide of vanadium and a least one selected from the group consisting of iron, erbium, bismuth, cerium, europium, gadolinium, holmium, lanthanum, lutetium, neodymium, praseodymium, promethium, samarium, scandium, terbium, thulium, ytterbium, yttrium, molybdenum, tungsten, manganese, cobalt, nickel, copper, aluminum, and antimony; (c) disposing the slurry obtained in (b) on the surface of the internal walls of the flowthrough substrate according to (a), wherein the surface defines the interface between the passages and the internal walls, obtaining a slurry-treated substrate; (d) optionally drying the slurry-treated substrate obtained in (c); (e) calcining the slurry-treated substrate obtained in (c), or the dried slurry-treated substrate obtained in (d), obtaining a coated substrate; and optionally (c′) disposing the slurry obtained in (b) on the surface of the coating disposed on the substrate as obtained in (e); (d′) optionally drying the slurry-treated substrate obtained in (c′); (e′) calcining the slurry-treated substrate obtained in (c′), or the dried slurry-treated substrate obtained in (d′); wherein from (e) or (e′), the selective catalytic reduction catalyst is obtained.

12. The process of claim 11, wherein (b) comprises; (b.1) mixing the solution of vanadium oxide, water, and the powder of an oxidic material comprising titania, obtaining a slurry; (b.2) adjusting the pH of the aqueous phase of the slurry obtained in (b.1) to a value in the range of from 6 to 8; and (b.3) adding the powder of a mixed oxide of vanadium and at least one selected from the group consisting of iron, erbium, bismuth, cerium, europium, gadolinium, holmium, lanthanum, lutetium, neodymium, praseodymium, promethium, samarium, scandium, terbium, thulium, ytterbium, yttrium, molybdenum, tungsten, manganese, cobalt, nickel, copper, aluminum, and antimony, to the slurry obtained in (b.2).

13. A selective catalytic reduction catalyst obtained by the process of claim 11.

14. An exhaust gas treatment system for treating an exhaust gas stream exiting from a diesel engine, comprising: a first selective catalytic reduction catalyst according to claim 1, and at least one selected from the group consisting of a diesel oxidation catalyst, an ammonia oxidation catalyst, a second selective catalytic reduction catalyst, a filter, and a catalyzed soot filter.

15. A method for the selective catalytic reduction of nitrogen oxides in an exhaust gas stream, the method comprising passing the exhaust gas stream through a selective catalytic reduction catalyst according to claim 1.

Description

EXAMPLES

Reference Example 1: Determination of Dv90 Values

[0208] The particle size distributions were determined by a static light scattering method using Sympatec HELOS equipment, wherein the optical concentration of the sample was in the range of from 5 to 10%.

Reference Example 2: Measurement of the BET Specific Surface Area

[0209] The BET specific surface area was determined according to DIN 66131 or DIN ISO 9277 using liquid nitrogen.

Reference Example 3: General Coating Method

[0210] In order to coat a flow-through substrate with one or more coatings, the flow-through substrate was immersed vertically in a portion of a given slurry for a specific length of the substrate. In this manner, the washcoat contacted the walls of the substrate. The sample was left in the slurry for a specific period of time, usually for 1-10 seconds. Vacuum was applied to draw the slurry into the substrate. The substrate was then removed from the slurry, and was inverted and excess slurry was removed from the substrate by allowing it to drain from the substrate, then by blowing with compressed air (against the direction of slurry penetration).

Example 1: Vanadium-Based SCR Catalyst (Dual Vanadium Source)

[0211] An aqueous vanadium oxalate mixture with a solid content of 11 weight-% was added to distilled water such that the final solid content of the mixture was of 3 weight-%. The amount of vanadium oxalate mixture used was calculated so that the vanadium oxide (from vanadium oxalate), calculated as V.sub.2O.sub.5, was present at a loading of 1% of the final loading of the coating in the catalyst after calcination. To this diluted vanadium oxalate mixture, a titania powder (TiO.sub.2 90 weight-% and 10 weight-% of WO.sub.3 with a BET specific surface area of 90 m.sup.2/g, a Dv90 of between 2 and 8.6 micrometers) was added, such that the final loading of titania+tungsten oxide in the catalyst after calcination was 3.88 g/in.sup.3.

[0212] Further, an organic dispersant (acrylic polymer) mixture with a solid content of 39 weight-% and a pH of 8 was added to the mixture. The amount of dispersant mixture was calculated as 5% by weight of the total weight of the coating in the catalyst after calcination. The resulting mixture was stirred for several minutes and the pH was adjusted to about 7.0 by adding an ammonium-hydroxide solution. Further, distilled water was added in order to obtain a slurry with a solid content to 45.3 weight-%.

[0213] After mixing the slurry for 5 minutes, iron vanadate (FeVO.sub.4 having a molar ratio of Fe:V of 1:1) powder was added to the slurry. The amount of iron vanadate used was calculated such that the vanadium (from the iron vanadate), calculated as V.sub.2O.sub.5, was present at a loading of 4% of the final loading of the coating in the catalyst after calcination (the loading of FeVO.sub.4 was 8.39% of the final loading of the coating in the catalyst after calcination). Afterwards, an aqueous colloidal silica (a solid content of 40 weight-%) was added to the slurry, in an amount such that the final SiO.sub.2 loading in the catalyst after calcination was 0.19 g/in.sup.3, along with additional distilled water to obtain a final slurry solid content of 43 weight-%.

[0214] A portion of the final slurry was disposed over the full length of an uncoated honeycomb cordierite monolith substrate according to the method described in Reference Example 3 (diameter: 26.67 cm (10.5 inches)×length: 15.24 cm (6 inches) cylindrically shaped substrate with 400/(2.54).sup.2 cells per square centimeter and 0.10 millimeter (4 mil) wall thickness). The coated substrate was dried in stages between 110° C. and 130° C. for 30 minutes in total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C. The remaining portion of the final slurry was disposed over the full length of the coated substrate, dried in stages between 110° C. and 130° C. for 30 minutes in total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C., to obtain a final loading of the coating in the catalyst of 4.5 g/in.sup.3, including 3.88 g/in.sup.3 of titania+tungsten oxide, 0.045 g/in.sup.3 of vanadium (calculated as V.sub.2O.sub.5—from vanadium oxalate), 0.377 g/in.sup.3 of FeVO.sub.4 (including 0.18 g/in.sup.3 of vanadium calculated as V.sub.2O.sub.5), 0.19 g/in.sup.3 of SiO.sub.2.

Comparative Example 1: Vanadium-Based SCR Catalyst (Single Vanadium Source)

[0215] An organic dispersant (acrylic polymer) mixture with a solid content of 39 weight-% and a pH of 8, was added to distilled water and mixed for 5 minutes. The amount of dispersant mixture was calculated as 5 weight-% of the total weight of the coating in the catalyst after calcination. Afterwards, a titania powder (TiO.sub.290 weight-% and 10 weight-% of WO.sub.3 with a BET specific surface area of 90 m.sup.2/g, a Dv90 of between 2 and 8.6 micrometers) was added to the mixture, such that the final loading of titania+tungsten oxide in the catalyst after calcination was 3.84 g/in.sup.3, to form a slurry. The slurry was stirred for several more minutes and the pH was adjusted to about 7 by adding an ammonium-hydroxide solution. Once the pH was of about 7, iron vanadate (FeVO.sub.4 having a molar ratio of Fe:V of 1:1) powder was added to the slurry. The amount of iron vanadate used was calculated such that the vanadium oxide (from iron vanadate), calculated as V.sub.2O.sub.5, was present at a loading of 5% of the final loading of the coating in the catalyst after calcination (the loading of FeVO.sub.4 was 10.48% of the final loading of the coating in the catalyst after calcination).

[0216] Afterwards, an aqueous colloidal silica (a solid content of 40 weight-%) was added to the slurry, such that the final SiO.sub.2 loading was 5% of the final titania+tungsten oxide loading in the catalyst after calcination, along with additional distilled water to obtain a final slurry solid content of 43 weight-%. The pH was checked and adjusted again to 7.0 by adding an ammonium hydroxide solution.

[0217] A portion of the final slurry was disposed over the full length of an uncoated honeycomb cordierite monolith substrate according to the method in Reference Example 3 (diameter: 26.67 cm (10.5 inches)×length: 15.24 cm (6 inches) cylindrically shaped substrate with 400/(2.54).sup.2 cells per square centimeter and 0.10 millimeter (4 mil) wall thickness). The coated substrate was dried in stages between 110° C. and 130° C. for 30 minutes total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C. The remaining portion of the final slurry was disposed over the full length of the coated substrate, dried in stages between 110° C. and 130° C. for 30 minutes in total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C., to obtain a final coating loading of 4.5 g/in.sup.3, including 3.84 g/in.sup.3 of titania+tungsten oxide, 0.47 g/in.sup.3 of FeVO.sub.4, 0.19 g/in.sup.3 of S102.

Comparative Example 2: Vanadium-Based SCR Catalyst (Single Vanadium Source)

[0218] The catalyst of Comparative Example 2 was prepared as the catalyst of Comparative Example 1 except that the amount of iron vanadate was calculated such that the loading of FeVO.sub.4 was 8.38% of the final loading of the coating in the catalyst after calcination (including a vanadium loading (from iron vanadate), calculated as V.sub.2O.sub.5, of 4% of the final loading of the coating in the catalyst after calcination). Thus, the final loading of the coating in the catalyst after calcination was of 4.5 g/in.sup.3, including 3.93 g/in.sup.3 of titania+tungsten oxide, 0.38 g/in.sup.3 of FeVO.sub.4, 0.196 g/in.sup.3 of SiO.sub.2.

Comparative Example 3: Vanadium-Based SCR Catalyst (Single Vanadium Source)

[0219] The catalyst of Comparative Example 3 was prepared as the catalyst of Comparative Example 1 except that the amount of iron vanadate was calculated such that the loading of FeVO.sub.4 was 12.58% of the final loading of the coating in the catalyst after calcination (including a vanadium loading (from iron vanadate), calculated as V.sub.2O.sub.5, of 6% of the final loading of the coating in the catalyst after calcination) and that the aqueous colloidal silica used, was added such that the final SiO.sub.2 loading represented 7.5% of the final titania+tungsten oxide loading in the catalyst after calcination. Thus, the final coating loading in the catalyst after calcination was of 4.5 g/in.sup.3, including 3.66 g/in.sup.3 of titania+tungsten oxide, 0.57 g/in.sup.3 of FeVO.sub.4, 0.27 g/in.sup.3 of SiO.sub.2.

Example 2: Use of the Catalysts of Example 1 and of Comparative Examples 1 to 3 DeNOx

[0220] The performance of the catalysts of Example 1 and of Comparative Examples 1 to 3 in NOx conversion was measured at different temperatures, namely at 200, 240, 375, 450 and 500° C., (Gas Hourly Space Velocity (GHSV): 40 000 h.sup.−1 at 200, 240, 375 and 450° C. and GHSV: 80 000 h.sup.−1 at 500° C.) under fresh and aged conditions (ageing at 550° C. for 50 hours in an oven). During testing ad-blue solution (a mixture of 32.5% urea and 67.5% de-ionized water as described in ISO 22241) was dosed into the exhaust gas, sufficiently upstream of the SCR catalyst to ensure complete mixing and a homogeneous distribution throughout the flow. The amount of ad-blue dosed was calculated based on the normalized stoichiometric ratio (NSR) of the resulting NH.sub.3 molar flow versus the calculated NOx molar flow. The ad-blue dosing was adjusted starting from 0.0 step-wise based on the SCR catalyst inlet temperature. At 500° C., the NSR was adjusted from 0.0 to 1.0 to 1.1. At 450° C., 375° C., and 240° C., the NSR was adjusted from 0.0 to 0.4 to 0.6 to 0.8 to 1.0 to 1.1. At 200° C., the NSR was adjusted from 0.0 to 0.8 to 1.0 to 1.1. In all cases, the catalyst was purged of NH.sub.3 before being brought to the next load point/temperature. Only maximum deNOx values are reported below in Table 1.

TABLE-US-00001 TABLE 1 NOx conversion Comp. Comp. Comp. Example 1 Example 1 Example 2 Example 3 NOx NOx NOx NOx Temp. conv. (%) conv. (%) conv. (%) conv. (%) (° C.) Fresh Aged Fresh Aged Fresh Aged Fresh Aged 200 68 51 48 45 60 41 53 38 240 96 89 85 85 92 79 88 79 375 98 98 98 99 98 98 98 98 450 96 96 97 96 96 96 98 97 500 89 82 93 89 90 87 82 82

[0221] As may be taken from Table 1, the catalyst of Example 1 (dual-source—5% vanadium) exhibits improved NOx conversions at low temperatures, at 200 and 240° C., under fresh and aged conditions compared to the catalyst of Comparative Example 1 (single source—4% vanadium), to the catalyst of Comparative Example 2 (single source—5% vanadium) and to the catalyst of Comparative Example 3 (single source—6% vanadium). Further, the catalyst of Example 1 exhibits good NOx conversions (from 82 to 98%) under fresh and aged conditions at higher temperatures, said performance being comparable to those of the catalysts of the comparative examples representative of the prior art. Thus, this example demonstrates that the use of vanadium containing selective catalytic reduction catalyst having a dual source of vanadium permits to increase the low temperature deNOx while maintaining great performance at higher temperatures, namely up to 500° C. The example further demonstrates that the catalysts of the invention have an improved thermal stability compared to the catalysts representative of the prior art (Comp. Examples 1-3).

Example 3: Vanadium-Based SCR Catalyst (Dual Vanadium Source)

[0222] An aqueous vanadium oxalate mixture with a solid content of 11 weight-% was added to distilled water such that the final solid content of the mixture was of 3 weight-%. The amount of vanadium oxalate used was calculated such that the vanadium oxide (from the vanadium oxalate), calculated as V.sub.2O.sub.5, was present at a loading of 2% of the final loading of the coating in the catalyst after calcination. To this vanadium oxalate mixture, a titania powder (TiO.sub.290 weight-% and 10 weight-% of WO.sub.3 with a BET specific surface area of 90 m.sup.2/g, a Dv90 of between 2 and 8.6 micrometers) was added, such that the final loading of titania+tungsten oxide in the catalyst after calcination was 3.84 g/in.sup.3.

[0223] Further, an organic dispersant (acrylic polymer) mixture with a solid content of 39 weight-% and a pH of 8 was added to the mixture. The amount of dispersant mixture was calculated as 5% by weight of the total weight of the coating in the catalyst after calcination. The resulting mixture was stirred for several minutes and the pH was adjusted to about 7.0 by adding an ammonium-hydroxide solution. Further, distilled water was added in order to obtain a slurry with a solid content to 45.3 weight-%.

[0224] After mixing the slurry for 5 minutes, an iron vanadate (FeVO.sub.4 having a molar ratio of Fe:V of 1:1) powder was added to the slurry. The amount of iron vanadate used was calculated such that the vanadium (from the iron vanadate), calculated as V.sub.2O.sub.5, was present at a loading of 4% of the final loading of the coating in the catalyst after calcination (the loading of FeVO.sub.4 was 8.39% of the final loading of the coating in the catalyst after calcination). Afterwards, an aqueous colloidal silica (a solid content of 40%) was added to the slurry, such that the final SiO.sub.2 loading after calcination in the catalyst was 0.19 g/in.sup.3, along with additional distilled water to obtain a final slurry solid content of 43 weight-%.

[0225] A portion of the final slurry was disposed over the full length of an uncoated honeycomb cordierite monolith substrate according to the method in Reference Example 3 (diameter: 26.67 cm (10.5 inches)×length: 15.24 cm (6 inches) cylindrically shaped substrate with 400/(2.54).sup.2 cells per square centimeter and 0.10 millimeter (4 mil) wall thickness). The coated substrate was dried in stages between 110° C. and 130° C. for 30 minutes total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C. The remaining portion of the final slurry was disposed over the full length of the coated substrate, dried in stages between 110° C. and 130° C. for 30 minutes in total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C., to obtain a final loading of the coating in the catalyst of 4.5 g/in.sup.3, including 3.84 g/in.sup.3 of titania+tungsten oxide, 0.09 g/in.sup.3 of vanadium (calculated as V.sub.2O.sub.5—from vanadium oxalate), 0.38 g/in.sup.3 of FeVO.sub.4 (including 0.18 g/in.sup.3 of vanadium calculated as V.sub.2O.sub.5), 0.19 g/in.sup.3 of SiO.sub.2.

Example 4: Vanadium-Based SCR Catalyst (Dual Vanadium Source)

[0226] An aqueous vanadium oxalate mixture with a solid content of 11 weight-% was added to distilled water such that the final solid content of the mixture was of 3.0 weight-%. The amount of vanadium oxalate used was calculated such that the vanadium oxide (from the vanadium oxalate), calculated as V.sub.2O.sub.5, was present at a loading of 2.5% of the final loading of the coating in the catalyst after calcination. To this diluted vanadium oxalate mixture, a titania powder (TiO.sub.2 90 weight-% and 10 weight-% of WO.sub.3 with a BET specific surface area of 90 m.sup.2/g, a Dv90 of between 2 and 8.6 micrometers) was added, such that the final loading of titania+tungsten oxide in the catalyst after calcination was 4.0 g/in.sup.3.

[0227] Further, an acrylic based organic dispersant (acrylic polymer) mixture with a solid content of 39 weight-% and a pH of 8 was added to the mixture. The amount of dispersant mixture was calculated as 5% by weight of the final loading of the coating in the catalyst after calcination. The resulting mixture was stirred for several minutes and the pH was adjusted to about 7.0 by adding an ammonium-hydroxide solution. Further, distilled water was added in order to obtain a slurry with a solid content to 45 weight-%.

[0228] After mixing the slurry for 5 minutes, iron vanadate (FeVO.sub.4 having a molar ratio of Fe:V of 1:1) powder was added to the slurry. The amount of iron vanadate used was calculated such that the vanadium (from the iron vanadate), calculated as V.sub.2O.sub.5, was present at a loading of 2% of the final loading of the coating in the catalyst after calcination (the loading of FeVO.sub.4 was 4.19% of the final loading of the coating in the catalyst after calcination). Afterwards, an aqueous colloidal silica (a solid content of 40 weight-%) was added to the slurry, such that the final SiO.sub.2 loading in the catalyst after calcination was 0.2 g/in.sup.3, along with additional distilled water to obtain a final slurry solid content of 43 weight-%.

[0229] A portion of the final slurry was disposed over the full length of an uncoated honeycomb cordierite monolith substrate according to the method in Reference Example 3 (diameter: 26.67 cm (10.5 inches)×length: 15.24 cm (6 inches) cylindrically shaped substrate with 400/(2.54).sup.2 cells per square centimeter and 0.10 millimeter (4 mil) wall thickness). The coated substrate was dried in stages between 110° C. and 130° C. for 30 minutes total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C. The remaining portion of the final slurry was disposed over the full length of the coated substrate, dried in stages between 110° C. and 130° C. for 30 minutes total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C., to obtain a final coating loading in the catalyst of 4.5 g/in.sup.3, including 4.00 g/in.sup.3 of titania+tungsten oxide, 0.11 g/in.sup.3 of vanadium (calculated as V.sub.2O.sub.5—from vanadium oxalate), 0.19 g/in.sup.3 of FeVO.sub.4 (including 0.09 g/in.sup.3 of vanadium calculated as V.sub.2O.sub.5), 0.2 g/in.sup.3 of SiO.sub.2.

Comparative Example 4: Vanadium-Based SCR Catalyst not According to the Present Invention (Single Vanadium Source)

[0230] An aqueous vanadium oxalate mixture with a solid content of 11 weight-% was added to distilled water such that the final solid content of the mixture was 3.1 weight-%. The amount of vanadium oxalate used was calculated such that the vanadium oxide, calculated as V.sub.2O.sub.5, was present at a loading of 4% of the final coating loading in the catalyst after calcination. An acrylic based organic dispersant (acrylic polymer) mixture with a solid content of 39 weight-% and a pH of 8 was added forming a mixture. The amount of dispersant mixture was calculated as 5 weight-% of the total weight of the coating in the catalyst after calcination.

[0231] A titania powder (TiO.sub.2 87 weight-% with 8 weight-% of WO.sub.3 and 5 weight-% of SiO.sub.2 with a BET specific surface area of 85 m.sup.2/g, a Dv90 of 2.5 micrometers) was added to the obtained mixture, forming a slurry. The final loading of titania+tungsten oxide+silica in the catalyst after calcination was 4.11 g/in.sup.3. The pH of the slurry was adjusted to about 7.0 using ammonium hydroxide solution. Additional distilled water was added to bring the solid content of the slurry to 40.6 weight-%. A second organic-based dispersant (organic polymer) mixture was added to the slurry, this one having a solid content of 31 weight-%. This second dispersant mixture was added in the amount of 3.22 weight-% of the total final solid loading of the catalyst. Lastly, an aqueous colloidal silica (a solid content of 40 weight-%) was added to the slurry along with additional distilled water to obtain a final slurry with a solid content of 39 weight-%. The amount of colloidal silica used was calculated such that the final SiO.sub.2 loading (from the colloidal silica) was 5% of the final loading of titania+tungsten oxide+silica in the catalyst after calcination. The pH of the aqueous phase of the resulting slurry was checked and adjusted to a pH of about 7 by the addition of ammonium hydroxide.

[0232] A portion of the final slurry was disposed over the full length of an uncoated honeycomb cordierite monolith substrate according to the method in Reference Example 3 (diameter: 26.67 cm (10.5 inches)×length: 15.24 cm (6 inches) cylindrically shaped substrate with 400/(2.54).sup.2 cells per square centimeter and 0.10 millimeter (4 mil) wall thickness). The coated substrate was dried in stages between 110° C. and 130° C. for 30 minutes total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C. The remaining portion of the final slurry was disposed over the full length of the coated substrate, dried in stages between 110° C. and 130° C. for 30 minutes total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C., to obtain a final loading in the catalyst of 4.5 g/in.sup.3, including 4.11 g/in.sup.3 of titania supporting WO.sub.3 and SiO.sub.2, 0.18 g/in.sup.3 of vanadium (calculated as V.sub.2O.sub.5), 0.21 g/in.sup.3 of SiO.sub.2 (from the colloidal silica).

Example 5: Vanadium-Based SCR Catalyst (Dual Vanadium Source)

[0233] An aqueous vanadium oxalate solution with a solid content of 11 weight-% was added to distilled water such that the final solid content of the mixture was of 3 weight-%. The amount of vanadium oxalate used was calculated such that the vanadium oxide (from vanadium oxalate), calculated as V.sub.2O.sub.5, was present at a loading of 2.5% of the final loading of the coating in the catalyst after calcination. To this diluted vanadium oxalate mixture, a titania powder (TiO.sub.2 87 weight-% with 8 weight-% of WO.sub.3 and 5 weight-% of SiO.sub.2 with a BET specific surface area of 85 m.sup.2/g, a Dv90 of 2.5 micrometers) was added, such that the final loading of titania+tungsten oxide+silica in the catalyst after calcination was 3.82 g/in.sup.3.

[0234] Further, an acrylic based organic dispersant (acrylic polymer) mixture with a solid content of 39 weight-% was added to the obtained mixture. The amount of dispersant mixture was calculated as 5% by weight of the total weight of the coating in the catalyst after calcination. The resulting mixture was stirred for several minutes and the pH was adjusted to about 7.0 by adding an ammonium-hydroxide solution. Further, distilled water was added in order to obtain a slurry with a solid content to 40.4 weight-%.

[0235] After mixing the slurry for 5 minutes, iron vanadate (FeVO.sub.4 having a molar ratio of Fe:V of 1:1) powder was added to the slurry. The amount of iron vanadate used was calculated such that the vanadium (from the iron vanadate), calculated as V.sub.2O.sub.5, was present at a loading of 4% of the final loading of the coating in the catalyst after calcination (the loading of FeVO.sub.4, calculated as FeVO.sub.4, was 8.39% of the final loading of the coating in the catalyst after calcination). Afterwards, an aqueous colloidal silica (a solid content of 40 weight-%) was added to the slurry, such that the final SiO.sub.2 loading (from the colloidal silica) in the catalyst after calcination was 0.19 g/in.sup.3, along with additional distilled water to obtain a final slurry solid content of 40 weight-%.

[0236] A portion of the final slurry was disposed over the full length of an uncoated honeycomb cordierite monolith substrate according to the method in Reference Example 3 (diameter: 26.67 cm (10.5 inches)×length: 15.24 cm (6 inches) cylindrically shaped substrate with 400/(2.54).sup.2 cells per square centimeter and 0.10 millimeter (4 mil) wall thickness). The coated substrate was dried in stages between 110° C. and 130° C. for 30 minutes total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C. The remaining portion of the final slurry was disposed over the full length of the coated substrate, dried in stages between 110° C. and 130° C. for 30 minutes total (to remove up to 80% of the water) and calcined in air for 4 hours, of which approximately 30 minutes were at 450° C., to obtain a final coating loading in the catalyst of 4.5 g/in.sup.3, including 3.82 g/in.sup.3 of titania+tungsten oxide+silica, 0.11 g/in.sup.3 of vanadium (calculated as V.sub.2O.sub.5—from vanadium oxalate), 0.38 g/in.sup.3 of FeVO.sub.4 (including 0.18 g/in.sup.3 of vanadium calculated as V.sub.2O.sub.5), 0.19 g/in.sup.3 of SiO.sub.2 (from the colloidal silica).

Example 6: Use of the Catalysts of Examples 3 and 4 and of Comparative Examples 2, 3 and 4—DeNOx

[0237] The performance of the fresh catalysts of Examples 3 and 4 and of Comparative Examples 2, 3 and 4 in NOx conversion was measured at different temperatures, namely at 200, 240, 375, 450 and 500° C., (Gas Hourly Space Velocity (GHSV): 40 000 h.sup.−1 at 200, 240, 375 and 450° C. and GHSV: 80 000 h.sup.−1 at 500° C.). During testing ad-blue solution (a mixture of 32.5% urea and 67.5% de-ionized water as described in ISO 22241) was dosed into the exhaust gas, sufficiently upstream of the SCR catalyst to ensure complete mixing and a homogeneous distribution throughout the flow. The amount of ad-blue dosed was calculated based on the normalized stoichiometric ratio (NSR) of the resulting NH.sub.3 molar flow versus the calculated NOx molar flow. The ad-blue dosing was adjusted starting from 0.0 step-wise based on the SCR catalyst inlet temperature. At 500° C., the NSR was adjusted from 0.0 to 1.0 to 1.1. At 450° C., 375° C., and 240° C., the NSR was adjusted from 0.0 to 0.4 to 0.6 to 0.8 to 1.0 to 1.1. At 200° C., the NSR was adjusted from 0.0 to 0.8 to 1.0 to 1.1. In all cases, the catalyst was purged of NH.sub.3 before being brought to the next load point/temperature. Only maximum deNOx values are reported below in Table 2.

TABLE-US-00002 TABLE 2 NOx conversion Comp. Comp. Comp. Ex. 2 Ex. 3 Ex. 4 Example 3 Example 4 Temp. NOx NOx NOx NOx NOx (° C.) conv. (%) conv. (%) conv. (%) conv. (%) conv. (%) 200 60 53 56 63 64 240 92 88 91 93 95 375 98 98 95 98 98 450 96 98 85 96 97 500 90 82 65 87 89

[0238] As may be taken from Table 1, the catalysts of Example 3 (dual vanadium source—6% vanadium oxide) and of Example 4 (dual vanadium source—4.5% vanadium oxide) exhibit improved NOx conversions at low temperatures, at 200 and 240° C., compared to the catalyst of Comparative Example 2 (single vanadium source—5% vanadium oxide), to the catalyst of Comparative Example 3 (single vanadium source—6% vanadium oxide) and to the catalyst of Comparative Example 4 (single vanadium source—4% vanadium oxide). Further, the catalysts of Examples 3 and 4 exhibit good NOx conversions (from 89 to 98%), said performance being comparable or better to those of the catalysts of the comparative examples representative of the prior art. Thus, this example demonstrates that the use of vanadium-containing selective catalytic reduction catalyst having a dual source of vanadium permits to increase the low temperature deNOx while maintaining great performance at higher temperatures, namely up to 500° C.).