A SELECTIVE CATALYTIC REDUCTION SUSPENSION

Abstract

The present invention relates to an aqueous suspension comprising water, a source of one or more of a vanadium oxide and a tungsten oxide, and particles of an oxidic support; wherein the particles of the aqueous suspension exhibit a polymodal particle size distribution characterized by a particle size distribution curve comprising a first peak with a maximum M(I) in the range of from 0.5 to 15 micrometers and a second peak with a maximum M(II) in the range of from 1 to 40 micrometers, wherein (M(I)/μm):(M(II)/μm)<1:1.

Claims

1-15. (canceled)

16. An aqueous suspension comprising: water, a source of one or more of a vanadium oxide and a tungsten oxide, and particles of an oxidic support; wherein the particles of the aqueous suspension exhibit a polymodal particle size distribution characterized by a particle size distribution curve comprising a first peak with a maximum, M(I), ranging from 0.5 micrometers to 15 micrometers and a second peak with a maximum, M(II), ranging from 1 micrometers to 40 micrometers, wherein M(I):M(II) is less than 1:1.

17. The suspension of claim 16, wherein the particles of the aqueous suspension exhibit a bi-modal particle size distribution.

18. The suspension of claim 16, further comprising particles of a mixed oxide comprising cerium.

19. The suspension of claim 18, wherein the particles of the mixed oxide exhibit a monomodal particle size distribution or a polymodal particle size distribution.

20. The suspension of claim 16, wherein M(II)−M(I) is greater than or equal to 0.5.

21. The suspension of claim 16, wherein M(I) ranges from 0.5 micrometers to 5 micrometers; wherein M(II) ranges from 5.5 micrometers to 40 micrometers.

22. The suspension of claim 16, wherein the particles of the aqueous suspension have a Dv50 ranging from 0.2 micrometers to 10 micrometers.

23. The suspension of claim 16, wherein the aqueous suspension comprises a source of a vanadium oxide at an amount, calculated as V.sub.2O.sub.5, ranging from 1.5 weight-% to 8 weight-%, based on the weight of the oxidic support.

24. The suspension of claim 16, wherein the particles of the oxidic support exhibit a monomodal particle size distribution; or wherein the particles of the oxidic support exhibit a polymodal particle size distribution.

25. The suspension of claim 16, wherein the oxidic support comprises one or more of titanium, silicon, zirconium, and tungsten.

26. The suspension of claim 16, wherein the aqueous suspension further comprises a source of an oxidic binder.

27. A process for preparing the aqueous suspension according to claim 16, the process comprising: (i) optionally, preparing an aqueous suspension comprising water and particles of a mixed oxide comprising cerium wherein the particles of the mixed oxide comprised in the aqueous suspension exhibit a monomodal particle size distribution; or optionally, preparing an aqueous suspension comprising water and particles of a mixed oxide comprising cerium wherein the particles of the mixed oxide comprised in the aqueous suspension exhibit a polymodal particle size distribution characterized by a particle size distribution curve comprising a first peak with a maximum M1 ranging from 0.5 micrometers to 20 micrometers and a second peak with a maximum M2 ranging from 1 micrometers to 50 micrometers, wherein M1:M2 is less than 1:1; (ii) preparing an aqueous suspension comprising water, a source of one or more of a vanadium oxide and a tungsten oxide and further comprising particles of an oxidic support, wherein the particles of the oxidic support comprised in the aqueous suspension exhibit a monomodal particle size distribution, characterized by a particle size distribution curve comprising a peak with a maximum ranging from 0.5 micrometers to 5 micrometers, or wherein the particles of the oxidic support comprised in the aqueous suspension exhibit a polymodal particle size distribution; (iii) mixing the suspension obtained from (ii), optionally with the suspension obtained from (i), to obtain an aqueous suspension comprising water, a source of one or more of a vanadium oxide and a tungsten oxide, particles of an oxidic support, and optionally particles of a mixed oxide comprising cerium, wherein the particles of the aqueous suspension exhibit a polymodal particle size distribution, characterized by a particle size distribution curve comprising a first peak with a maximum M(I) ranging from 0.5 micrometers to 15 micrometers and a second peak with a maximum M(II) ranging from 1 micrometers to 40 micrometers, wherein M(I):M(II) is less than 1:1.

28. The process of claim 27, wherein preparing the aqueous suspension according to (i) comprises: (i.1) providing particles of the mixed oxide comprising cerium, wherein the particles of the mixed oxide exhibit a polymodal particle size distribution characterized by a particle size distribution curve comprising a first peak with a maximum M1′ ranging from 0.5 micrometers to 30 micrometers and a second peak with a maximum M2′ ranging from 1 micrometers to 60 micrometers, wherein M1′:M2′ is less than 1:1; (i.2) preparing an aqueous suspension comprising suspending the particles provided in (i.1) in water; (i.3) optionally, milling the aqueous suspension prepared in (i.2) until the particles of the aqueous suspension exhibit a polymodal particle size distribution having a particle size distribution curve comprising a first peak with a maximum M1 and a second peak with a maximum M2, wherein M1:M2 is less than 1:1 and wherein M2 is less than M2′ and/or M1 is less than M1′.

29. The process of claim 28, wherein the particles of the mixed oxide provided in (i.1) have a Dv50 ranging from 1 micrometers to 30 micrometers.

30. A process for preparing a selective catalytic reduction catalyst, the process comprising: (a) preparing an aqueous suspension according to claim 16; (b) disposing the suspension obtained in (a) on the surface of the internal walls of a substrate comprising an inlet end, an outlet end, a substrate axial length extending between the inlet end and the outlet end, and a plurality of passages defined by internal walls of the substrate extending therethrough, wherein the interface between the passages and the internal walls is defined by the surface of the internal walls; and optionally drying the substrate comprising the suspension disposed thereon; and (c) calcining the substrate obtained in (b).

Description

EXAMPLES

Reference Example 1 Determination of the Particle Size Distribution, Dv10, Dv50, Dv90 Values

[0286] 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

[0287] The BET specific surface area was determined according to DIN 66131 or DIN ISO 9277 using liquid nitrogen.

Reference Example 3 General Coating Method

[0288] 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).

Reference Example 4 Determination of the Average Porosity of an Uncoated Substrate

[0289] The average porosity of the porous wall-flow substrate was determined by mercury intrusion using mercury porosimetry according to DIN 66133 and ISO 15901-1.

Example 1.1 Vanadium-Based SCR Catalyst not According to the Present Invention (with a Bi-Modal Particle Size Distribution Oxidic Support)

[0290] An aqueous vanadium oxalate solution (which upon removal of all solvents leads to 11 weight-% of vanadium oxide in water based on the weight of the solution) was diluted in distilled water such that the final solute mass percentage of the solution was 3 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 2.5% of the final coating loading 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 about 90 m.sup.2/g, a Dv10 of about 0.4 micrometers, a Dv50 of about 1 micrometer, a Dv90 of about 2.5 micrometers and a Dv99 of about 15.3 micrometers, and a bi-modal particle size distribution characterized by a particle size distribution curve determined according to Reference Example 1 herein, said particle size distribution curve comprising a first peak with a maximum at about 1.25 micrometers and a second peak with a maximum at about 14 micrometers) was added. Further, an organic dispersant (acrylic polymer) solution—with a pH of 8—whereby 39 weight-% of the weight of the solution is the acrylic polymer was added to the suspension. The amount of the dispersant mixture was calculated such that it was 5% by weight of the total weight of the titania+tungsten oxide in the catalyst after calcination. The resulting suspension was stirred for several minutes and the pH was adjusted to about 5.5 by adding an ammonium-hydroxide solution. Further, distilled water was added in order to obtain a suspension with a solid content to 45.3 weight-%.

[0291] Afterwards, an aqueous colloidal silica (with a solid content of 40 weight-%) was added to the obtained suspension, in an amount such that the final SiO.sub.2 loading in the catalyst after calcination was 0.21 g/in.sup.3, along with additional distilled water to obtain a final suspension solid content of 45 weight-%. The particles of the suspension exhibited a bi-modal particle size distribution characterized by a particle size distribution curve determined according to Reference Example 1 herein, said particle size distribution curve comprising a first peak with a maximum at about 1.25 micrometers and a second peak with a maximum at about 14 micrometers. The particle size distribution curve of the particles of said suspension is displayed in FIG. 1.

[0292] An uncoated honeycomb flow-through cordierite monolith substrate (an average porosity of 35%, 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) was coated with the obtained suspension from the inlet end toward the outlet end of the substrate, over 100% of the length of the substrate according to the method described in Reference Example 3. 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 coated substrate was coated a second time with said suspension from the outlet end toward the inlet end of the substrate over the 100% of the length of the substrate, then 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 final loading of the coating in the catalyst after calcination was 4.5 g/in.sup.3, including 4.18 g/in.sup.3 of titania+WO.sub.3, 0.11 g/in.sup.3 of vanadium (calculated as V.sub.2O.sub.5) and 0.21 g/in.sup.3 of SiO.sub.2.

Comparative Example 1 Vanadium-Based SCR Catalyst not According to the Present Invention (with No Bi-Modal Particle Size Distribution Component)

[0293] An aqueous vanadium oxalate solution (which upon removal of all solvents leads to 11 weight-% of vanadium oxide in water based on the weight of the solution) was diluted in distilled water such that such that the final solution upon calcination leads to 3.1 weight-% of vanadium oxide in water based on the weight of the solution. 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) solution, having a pH of 8, whereby 39 weight-% of the weight of the solution is the acrylic polymer was added to form a mixture. The amount of dispersant mixture was calculated such that it was 5 weight-% of the total weight of the coating in the catalyst after calcination.

[0294] 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 about 85 m.sup.2/g, a Dv10 of 0.7 micrometers, a Dv50 of 1.3 micrometers, a Dv90 of 2.5 micrometers and a Dv99 of about 5 micrometers) was added to the obtained mixture, forming a suspension. The final loading of titania+tungsten oxide+silica in the catalyst after calcination was 4.11 g/in.sup.3. The pH of the suspension was adjusted to about 7.0. Additional distilled water was added to bring the solid content of the suspension to 40.6 weight-%. A second organic-based dispersant (organic polymer) mixture was added to the suspension, this one having a solid content of 31 weight-%. This second dispersant mixture was added in the amount of 1 weight-% of the final coating loading in the catalyst.

[0295] Lastly, an aqueous colloidal silica (a solid content of 40 weight-%) was added to the suspension along with additional distilled water to obtain a final suspension 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 particles of the suspension exhibited a monomodal particle size distribution characterized by a particle size distribution curve determined according to Reference Example 1 herein, said particle size distribution curve comprising a peak with a maximum at about 1.7 micrometers. The particle size distribution of the particles of said final suspension is displayed in FIG. 2. The pH of the suspension was measured and adjusted to about 7.

[0296] An uncoated honeycomb flow-through cordierite monolith substrate (an average porosity of 35%, 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) was coated with the final suspension from the inlet end toward the outlet end of the substrate, over 100% of the length of the substrate according to the method described in Reference Example 3. 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 coated substrate was coated a second time with said suspension from the outlet end toward the inlet end of the substrate over the 100% of the length of the substrate, then 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 final loading of the coating in the catalyst after calcination was 4.5 g/in.sup.3, including 4.11 g/in.sup.3 of titania+tungsten oxide+silica, 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 1.2 Vanadium-Based SCR Catalyst (with a Bi-Modal Particle Size Distribution Mixed Oxide)

[0297] A powder of a mixed oxide of Ce and Zr (40 weight-% of Ce, calculated as CeO.sub.2, 50 weight-% of Zr (˜49 weight-%) and Hf, calculated as ZrO.sub.2 and HfO.sub.2 respectively, 5 weight-% of La.sub.2O.sub.3, and 5 weight-% of Pr.sub.6O.sub.11, having a BET specific area of about 80 m.sup.2/g, a Dv10 of about 1 micrometers, a Dv50 of about 4 micrometers, a Dv90 of about 21 micrometers and a Dv99 of about 37 micrometers and a bi-modal particle size distribution characterized by a particle size distribution curve determined according to Reference Example 1 herein, said particle size distribution curve comprising a first peak with a maximum at about 2.25 micrometers and a second peak with a maximum at 20 micrometers as displayed in FIG. 3) is added to distilled water and tartaric acid (1 g of tartaric acid every 4000 g of the mixed oxide). The resulting suspension was milled using a continuous milling apparatus so that the Dv10 of the particles of said suspension was of 0.98 micrometer, the Dv50 of the particles of said suspension was of 3.06 micrometers, the Dv90 of the particles of said suspension was of 13.61 micrometers, and the Dv99 of the particles of said suspension was of 27.34 micrometers. The resulting suspension exhibited a bi-modal particle size distribution characterized by a particle size distribution curve determined according to Reference Example 1 herein, said particle size distribution curve comprising a first peak with a maximum at about 2.25 micrometers and a second peak with a maximum at about 15 micrometers as displayed in FIG. 4. The amount of the mixed oxide of Ce and Zr was calculated such that its loading in the catalyst was of 1 g/in.sup.3. The solid content of the obtained suspension was of 35 weight-%.

[0298] Separately, an aqueous vanadium oxalate solution (which upon removal of all solvents leads to 11 weight-% of vanadium oxide in water based on the weight of the solution) was diluted in distilled water such that the final solution upon calcination leads to 3.3 weight-% of vanadium oxide in water based on the weight of the solution. 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 4.5% of the loading of titania+tungsten oxide+silica in the catalyst after calcination. An acrylic based organic dispersant (acrylic polymer) solution, having a pH of 8, whereby 39 weight-% of the weight of the solution is the acrylic polymer was added to the vanadium-containing solution. The amount of dispersant mixture was calculated such that it was 5 weight-% of the total weight of the coating in the catalyst after calcination. 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 about 85 m.sup.2/g, a Dv10 of about 0.7 micrometers, a Dv50 of about 1.3 micrometers, a Dv90 of about 2.5 micrometers and a Dv99 of about 4.8 micrometers) was added to the obtained mixture, forming a suspension. The final loading of titania+tungsten oxide+silica in the catalyst after calcination was 3.2 g/in.sup.3. The obtained suspension had a solid content of about 42.3 weight-%.

[0299] The Ce—Zr containing suspension and the vanadium and titania containing suspension were admixed and stirred with a stirring impeller. The pH of the obtained suspension was measured and adjusted to about 6.5. Additional distilled water was added to bring the solid content of the suspension to 40 weight-%. The viscosity of the suspension was between 50 and 100 mPa*s at 300 rotations per second. Lastly, an aqueous colloidal silica (a solid content of 40 weight-%) was added to the suspension along with additional distilled water to obtain a final suspension with a solid content of 40 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 particle size distribution of the particles of said suspension is displayed in FIG. 5. The particles of the suspension exhibited a bi-modal particle size distribution characterized by a particle size distribution curve determined according to Reference Example 1 herein, said particle size distribution curve comprising a first peak with a maximum at about 1.75 micrometers and a second peak with a maximum at about 8 micrometers. The pH of the suspension was measured and adjusted to about 7.

[0300] An uncoated honeycomb flow-through cordierite monolith substrate (an average porosity of 35%, 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) was coated with the final suspension from the inlet end toward the outlet end of the substrate, over 100% of the length of the substrate according to the method described in Reference Example 3. 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 coated substrate was coated a second time with said suspension from the outlet end toward the inlet end of the substrate over the 100% of the length of the substrate, then 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 final loading of the coating in the catalyst after calcination was 4.5 g/in.sup.3, including 3.2 g/in.sup.3 of titania+WO.sub.3+SiO.sub.2, 0.14 g/in.sup.3 of vanadium (calculated as V.sub.2O.sub.5), 1 g/in.sup.3 of Ce/Zr mixed oxide, 0.16 g/in.sup.3 of SiO.sub.2 (from the colloidal silica).

Example 2 Washcoat Adhesion Test

[0301] A core was drilled-out from each of the catalysts (coated substrates) obtained in Comparative Example 1, Examples 1.1 and 1.2. Each of the cores had a length of 3 inches and a diameter of 1 inch. The three obtained cores were weighed. The core of each samples of Comparative Example 1, Examples 1.1 and 1.2 was exposed to a high pressure air treatment (2 bar), such that air was uniformly forced through the core. After the air stream was stopped the cores were again weighed. The weight was compared before and after the same high pressure air treatment. The results for each three cores are displayed in FIG. 6

[0302] As may be taken from FIG. 6, the coating of the catalyst of Example 1.1 and the coating of the catalyst of Example 1.2 according to the present invention exhibit a washcoat loss of less than 1.5% which shows a good washcoat adhesion while the coating of the catalyst of Comparative Example 1 exhibits an increased washcoat loss of about 18%. Without wanting to be bound to any theory, it is believed that it is due to the presence of a bi-modal particle size distribution in the suspension of Examples 1.1 and 1.2 of the present invention used for coating. The difference between the coating of the catalyst of Example 1.2 and the coating of the catalyst of Comparative Example 1 is the presence of a Ce/Zr mixed oxide having a bi-modal particle size distribution.

[0303] Thus, the presence of this particular mixed oxide in the suspension for coating permits to greatly improve the washcoat adhesion on a substrate. Further, the difference between the coating of the catalyst of Example 1.2 and the coating of the catalyst of Example 1.1 is the type of titania support and the presence of a bi-modal Ce-containing mixed oxide. In particular, in the coating of Example 1.1, the titania support is a titania support with a bi-modal particle size distribution while the support used in Example 1.2 has a monomodal particle size distribution.

[0304] Thus, it is demonstrated that only the presence of a bi-modal particle size distribution Ce-containing mixed oxide (binder) can also permit to obtain similar washcoat adhesion compared to a coating comprising a bi-modal particle size distribution oxidic support which is present in much higher proportions. Thus, Example 2 demonstrates that the use of a bi-modal particle size distribution in a suspension prior to coating permits to greatly increase the washcoat adhesion.

Example 3.1 Vanadium-Based SCR Catalyst not According to the Present Invention (with a Bi-Modal Particle Size Distribution Oxidic Support)

[0305] The catalyst of Example 3.1 was prepared as the catalyst of Example 1.1 except that the monolith substrate was an uncoated honeycomb flow-through cordierite monolith substrate (diameter: 33.02 cm (13 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).

Example 3.2 Vanadium-Based SCR Catalyst (with a Bi-Modal Particle Size Distribution Ce-Containing Mixed Oxide)

[0306] The catalyst of Example 3.2 was prepared as the catalyst of Example 1.2 except that the monolith substrate was an uncoated honeycomb flow-through cordierite monolith substrate (diameter: 33.02 cm (13 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).

Example 4 Performance Evaluation—DeNOx

[0307] The SCR catalysts of Example 3.1 and 3.2 were tested on a 13 L motor. To do so, 3 diesel oxidation catalysts (DOC), each DOC was 12 inches×6 inches with 20 g/ft.sup.3 Pt, were mounted upstream of two SCR catalysts of Example 3.2 to achieve 80% NO.sub.2/NOx at the SCR inlet. Said two SCR catalysts were mounted in series downstream of the DOCs and a urea injector. An SCR-inlet temperature of 270° C. was achieved. Once the system reached steady-state, urea was dosed at increasing stoichiometric ratios until again steady-state was achieved. Same was done with SCR catalyst prepared according to Example 3.2. The results are displayed on FIG. 7.

[0308] As may be taken from FIG. 7, the results show that the reference plateaus in performance even before stoichiometric ratio of 100% is reached (although it does respond more quickly than the Ce/Zr variant). The advantage of using the Ce/Zr mixed is that, with heavy urea overdosing, ultimately a higher conversion than the reference at ultra-high NO.sub.2 concentrations is obtained.

BRIEF DESCRIPTION OF THE FIGURES

[0309] FIG. 1 shows the particle size distribution of the final suspension prepared in Example 1.1.

[0310] FIG. 2 shows the particle size distribution of the final suspension prepared in Comparative Example 1.

[0311] FIG. 3 shows the particle size distribution of the Ce/Zr mixed oxide used for preparing Example 1.2.

[0312] FIG. 4 shows the particle size distribution of the suspension comprising the Ce/Zr mixed oxide after milling in Example 1.2.

[0313] FIG. 5 shows the particle size distribution of the final suspension prepared in Example 1.2.

[0314] FIG. 6 shows the washcoat loss measured for the coatings of the catalysts of Comparative Example 1 and Examples 1.1 and 1.2.

[0315] FIG. 7 shows the DeNOx performance of the catalyst of Examples 3.1 and 3.2 measured at high NO.sub.2/NOx ratio at an SCR-inlet temperature of 270° C.

CITED LITERATURE

[0316] US 2015/0005158 A1