High ammonia storage capacity SCR catalysts

Abstract

The present invention relates to a catalyst for the selective catalytic reduction of nitrogen oxide, the catalyst comprising a first coating comprising a 12-membered ring pore zeolitic material comprising a first metal which is one or more of copper and iron, and a second coating comprising an 8-membered ring pore zeolitic material comprising a second metal which is one or more of copper and iron.

Claims

1. A catalyst for selective catalytic reduction of nitrogen oxide comprising: (i) a 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 substrate extending therethrough; (ii) a first coating comprising a 12-membered ring pore zeolitic material comprising a first metal which is one or more of copper and iron, wherein the 12-membered ring pore zeolitic material comprises the first metal in an amount of z1 weight-%, calculated as the weight of the first metal, calculated as CuO and Fe.sub.2O.sub.3, divided by the weight of the 12-membered ring pore zeolitic material comprising the first metal; (iii) a second coating comprising an 8-membered ring pore zeolitic material comprising a second metal which is one or more of copper and iron, wherein the 8-membered ring pore zeolitic material comprises the second metal in an amount of z2 weight-%, calculated as the weight of the second metal, calculated as CuO and Fe.sub.2O.sub.3, divided by the weight of the 8-membered ring pore zeolitic material comprising the second metal; wherein the first coating is disposed on the surface of the internal walls of the substrate, which surface defines the interface between the internal walls and the passages, and extends over x % of the substrate axial length from the inlet end toward the outlet end of the substrate, wherein x ranges from 10 to 75; wherein the second coating extends over y % of the substrate axial length from the outlet end toward the inlet end of the substrate, wherein y ranges from 25 to 90; wherein the ratio z1:z2 is in the range of from 0.5:1 to 0.95:1.

2. The catalyst of claim 1, wherein y is 100x.

3. The catalyst of claim 1, wherein the 12-membered ring pore zeolitic material contained in the first coating has a framework type selected from the group consisting of BEA, FAU, USY, GME, MOR, OFF, a mixture of two or more thereof, and a mixed type of two or more thereof.

4. The catalyst of claim 1, wherein the 12-membered ring pore zeolitic material contained in the first coating comprises a first metal which is iron, wherein the 12-membered ring pore zeolitic material comprises iron in an amount of z1 weight-%, calculated as the weight of the first metal, calculated as Fe.sub.2O.sub.3, divided by the weight of the 12-membered ring pore zeolitic material comprising the first metal, and wherein z1 ranges of from 1.0 to 10.

5. The catalyst of claim 1, wherein the first coating further comprises a 10-membered ring pore zeolitic material comprising a third metal which is one or more of copper and iron; and wherein the 10-membered ring pore zeolitic material contained in the first coating has a framework type selected from the group consisting of MFI, MWW, AEL, HEU, FER, AFO, a mixture of two or more thereof, and a mixed type of two or more thereof.

6. The catalyst of claim 1, wherein the first coating (ii) has an ammonia storage capacity A1.sub.(NH3) of at least 2.1 mmol/g, the ammonia storage capacity determined by Thermal Gravimetric Analysis.

7. The catalyst of claim 1, wherein the 8-membered ring pore zeolitic material contained in the second coating has a framework type selected from the group consisting of CHA, AEI, RTH, LEV, DDR, KFI, ERI, AFX, a mixture of two or more thereof, and a mixed type of two or more thereof.

8. The catalyst of claim 1, wherein the second coating (iii) has an ammonia storage capacity A2.sub.(NH3) of less than 2 mmol/g, the ammonia storage capacity determined by Thermal Gravimetric Analysis.

9. The catalyst of claim 1, wherein the first coating (ii) has an ammonia storage capacity A1.sub.(NH3) and the second coating (iii) has an ammonia storage capacity A2.sub.(NH3), wherein A1.sub.(NH3) is superior to A2.sub.(NH3), and the ammonia storage capacity determined by Thermal Gravimetric Analysis.

10. A process for preparing a catalyst for selective catalytic reduction of nitrogen oxide comprising: (1) providing a 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 substrate extending therethrough; (2) preparing a first mixture comprising water and a 12-membered ring pore zeolitic material comprising a first metal which is one or more of copper and iron, wherein the 12-membered ring pore zeolitic material comprises the first metal in an amount of z1 weight-%, calculated as the weight of the first metal, calculated as CuO and Fe.sub.2O.sub.3, divided by the weight of the 12-membered ring pore zeolitic material comprising the first metal; (3) disposing the first mixture obtained in (2) on the surface of the internal walls of the substrate provided in (1), over x % of the substrate axial length from the inlet end toward the outlet end of the substrate, wherein x ranges from 10 to 75; (4) drying the mixture-treated substrate obtained in (3), obtaining the substrate having a first coating disposed thereon; optionally calcining; (5) preparing a second mixture comprising water and a 8-membered ring pore zeolitic material comprising a second metal which is one or more of copper and iron, wherein the 8-membered ring pore zeolitic material comprises the second metal in an amount of z2 weight-%, calculated as the weight of the second metal, calculated as CuO and Fe.sub.2O.sub.3, divided by the weight of the 8-membered ring pore zeolitic material comprising the second metal; (6) disposing the second mixture obtained in (5) on the substrate having a first coating disposed thereon obtained in (4) over y % of the substrate axial length from the outlet end toward the inlet end of the substrate, wherein y ranges from 25 to 90; (7) drying the mixture-treated substrate obtained in (6), obtaining the substrate having the first coating and a second coating disposed thereon; and (8) calcining the substrate having the first coating and the second coating disposed thereon obtained in (7), obtaining the catalyst; wherein the ratio z1:z2 ranges from 0.5:1 to 0.95:1.

11. The process of claim 10, wherein (2) further comprises: (2.1) preparing a 12-membered ring pore zeolitic material; (2.2) mixing a source of a first metal with the 12-membered ring pore zeolitic material obtained in (2.1); (2.3) calcining the mixture obtained in (2.2), obtaining the 12-membered ring pore zeolitic material comprising the first metal; (2.4) admixing water and the 12-membered ring pore zeolitic material comprising B the first metal.

12. An exhaust gas treatment system for treating an exhaust gas exiting from a combustion engine, wherein the system comprising one or more catalysts for the selective catalytic reduction of nitrogen oxide according to claim 1, and one or more of a diesel oxidation catalyst, a catalyzed soot filter, and an ammonia oxidation catalyst.

13. A catalyst for selective catalytic reduction of nitrogen oxide comprising: a 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 substrate extending therethrough, and a coating disposed on the surface of the internal walls of the substrate, which surface defines the interface between the internal walls and the passages, wherein the coating comprising a 12-membered ring pore zeolitic material comprising a first metal which is one or more of copper and iron, and the 12-membered ring pore zeolitic material comprises the first metal in an amount of z weight-%, calculated as the weight of the first metal, calculated as CuO and Fe.sub.2O.sub.3, divided by the weight of the 12-membered ring pore zeolitic material comprising the first metal, wherein the coating further comprises a 10-membered ring pore zeolitic material comprising a second metal which is one or more of copper and iron, wherein the 10-membered ring pore zeolitic material comprises the second metal in an amount of y weight-%, calculated as the weight of the second metal, calculated as CuO and Fe.sub.2O.sub.3, divided by the weight of the 10-membered ring pore zeolitic material comprising the second metal; and wherein y ranges from 0.5 to 9 and y<z.

14. The catalyst of claim 13, wherein from 95 weight-% to 100 weight % of the framework structure of the 12-membered ring pore zeolitic material consist of Si, Al, O, and optionally H, wherein in the framework structure, the molar ratio of Si to Al, calculated as molar SiO.sub.2:Al.sub.2O.sub.3, is ranges from 4:1 to 20:1.

15. The catalyst of claim 13, wherein the coating comprises the 10-membered ring pore zeolitic material comprising the second metal in an amount in the range of from 1 weight-% to 8 weight-%, based on the weight of the coating.

16. A catalyst for selective catalytic reduction of nitrogen oxide comprising: a first catalyst for selective catalytic reduction of nitrogen oxide according to claim 13; and a second catalyst for selective catalytic reduction of nitrogen oxide comprising: a 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; a coating disposed on the surface of the internal walls of the substrate, wherein the coating comprises an 8-membered ring pore zeolitic material comprising a second metal which is one or more of copper and iron, wherein the 8-membered ring pore zeolitic material comprises the second metal in an amount of z2 weight %, calculated as the weight of the second metal, calculated as CuO and Fe.sub.2O.sub.3, divided by the weight of the 8-membered ring pore zeolitic material comprising the second metal, and wherein the second coating extends over y % of the substrate axial length from the outlet end toward the inlet end of the substrate, wherein y ranges from 25 to 90; wherein the first catalyst is disposed upstream of the second catalyst, and wherein there is less than 0.2 inch between the substrate of the first catalyst and the substrate of the second catalyst which are juxtaposed.

Description

EXAMPLES

Reference Example 1: Ammonia Storage Capacity Measurement

(1) The ammonia storage capacity that a given catalyst may have was measured by Thermal Gravimetric Analysis (TGA) as defined in the U.S. Pat. No. 9,597,639 B2.

Reference Example 2: Determination of the Volume-Based Particle Size Distributions (Dv90)

(2) The particle size distributions were determined by a static light scattering method using Sympatec HELOS (3200) & QUIXEL equipment, wherein the optical concentration of the sample was below 10%.

Reference Example 3: BET Specific Surface Area Measurement

(3) The BET specific surface area was determined according to DIN 66131 or DIN ISO 9277 using liquid nitrogen.

Reference Example 4: Coating Method

(4) In order to coat the flow-through substrate with one or more coatings, the flow-through substrate was suitably immersed vertically in a portion of a given slurry for a specific length of the substrate which was equal to the targeted length of the coating to be applied. In this manner, the slurry contacted the walls of the substrate.

Reference Example 6: Preparation of a Selective Catalytic Reduction (SCR) Catalyst (Fe-BEA Containing)

(5) Slurry Preparation:

(6) A Fe-BEA with a Fe content of 1.55 weight-%, calculated as Fe.sub.2O.sub.3, based on the weight of the Fe-BEA (Dv90 of 5.15 micrometers, a SiO.sub.2:Al.sub.2O.sub.3 molar ratio of 39 and a BET specific surface area of about 700 m.sup.2/g) was dispersed in a solution of water and tartaric acid (1.3 weight-% of tartaric based on the weight of Fe-BEA) forming a slurry. The BEA zeolite was prepared by a synthesis route using a template. The resulting slurry was milled using a continuous milling apparatus so that the Dv90 value of the particles was of about 5 micrometers.

(7) A Fe-MFI with a Fe content of 3.5 weight-%, calculated as Fe.sub.2O.sub.3, based on the weight of the Fe-MFI (Dv90 of 17.5 micrometers, a SiO.sub.2:Al.sub.2O.sub.3 molar ratio of 27.5 and a BET specific surface area of about 385 m.sup.2/g) was added to the Fe-BEA containing slurry. The amount of Fe-MFI was calculated such that it was about 5.5 weight-% based on the weight of the Fe-BEA. Further, a binder, colloidal silica, was added to the mixture. The amount of colloidal silica was calculated such that it was about 5.4 weight-% based on the weight of Fe-BEA. Finally, a viscosity agent (0.13 weight-% based on the weight of Fe-BEA) was added. The resulting slurry was mixed and milled using a continuous milling apparatus so that the Dv90 value of the particles was of about 5 micrometers. The pH of the aqueous phase of the slurry was measured and adjusted to about 3-3.5 and the solid content of said slurry was of 44 weight-%.

(8) An uncoated honeycomb flow-through monolith cordierite substrate (diameter: 1 inch x length: 2 inches, cylindrically shaped with 300 cells per square inch and 5 mill wall thickness) was coated with the obtained slurry over 100% of the substrate length according to the method described in Reference Example 4. The coated substrate was dried at 140 C. for 30 minutes and calcined in air at 590 C. for about 30 minutes (coating/drying and calcining were repeated once). The final loading of the coating in the catalyst after calcination is of about 3.2 g/in.sup.3, including 2.88 g/in.sup.3 of Fe-BEA, 0.16 g/in.sup.3 of Fe-MFI and 0.1562 g/in.sup.3 of silica. The ammonia storage capacity of the coating measured according to Reference Example 1 was 1.2 mmol/g.

Reference Example 7: Preparation of a SCR Catalyst (Cu-CHA Containing)

(9) An aqueous zirconium acetate solution was added to water. The amount of zirconium acetate was calculated such that the amount of zirconia in the coating, calculated as ZrO.sub.2, was 5 weight-% based on the weight of the Cu-Chabazite. Further, a Cu-Chabazite with a Cu content of 5.1 weight-%, calculated as CuO, based on the weight of the Chabazite (a SiO.sub.2:Al.sub.2O.sub.3 molar ratio of 18 and a BET specific surface area of about 565 m.sup.2/g) was added to the solution with zirconium acetate to form a mixture having a solid content of 46 weight-%. Water was added and the resulting slurry was milled using a continuous milling apparatus so that the Dv90 value of the particles was of about 7 micrometers. Further, a dispersant was added as well as an acid to obtain a pH of the aqueous phase of the obtained slurry of 4. The solid content of said slurry was adjusted by adding water to a value of 40 weight-%.

(10) An uncoated honeycomb flow-through monolith cordierite substrate (diameter: 1 inch x length: 1 inch, cylindrically shaped with 600 cells per square inch and 3 mil wall thickness) was coated with the obtained slurry over 100% of the substrate length according to the method described in Reference Example 4. The coated substrate was dried at 140 C. for 30 minutes and calcined in air at 450 C. for 30 minutes. The final loading of the coating in the catalyst after calcination is of about 2.75 g/in.sup.3, including 2.62 g/in.sup.3 of Cu-CHA, 0.13 g/in.sup.3 of ZrO.sub.2. The ammonia storage capacity of the coating measured according to Reference Example 1 was 1.9 mmol/g.

Example 1: Preparation of a SCR Catalyst

(11) Slurry Preparation:

(12) A Fe-BEA with a Fe content of 4.6 weight-%, calculated as Fe.sub.2O.sub.3, based on the weight of the Fe-BEA (Dv90 of about 13.5 micrometers, a SiO.sub.2:Al.sub.2O.sub.3 molar ratio of 9.75 and a BET specific surface area of about 612.5 m.sup.2/g) was dispersed in a solution of water and tartaric acid (1.3 weight-% based on the weight of Fe-BEA) forming a slurry. The BEA zeolite was prepared by a template-free synthesis. The resulting slurry was milled using a continuous milling apparatus so that the Dv90 value of the particles was of about 5 micrometers.

(13) A Fe-MFI with a Fe content of 3.5 weight-%, calculated as Fe.sub.2O.sub.3, based on the weight of the Fe-MFI (Dv90 of 17.5 micrometers, a SiO.sub.2:Al.sub.2O.sub.3 molar ratio of 27.5 and a BET specific surface area of about 385 m.sup.2/g) was added to the Fe-BEA containing slurry. The amount of Fe-MFI was calculated such that it was about 5.5 weight-% based on the weight of the Fe-BEA. Further, a binder, colloidal silica, was added to the mixture. The amount of colloidal silica was calculated such that it was about 5.4 weight-% based on the weight of Fe-BEA. Finally, a viscosity agent (0.13 weight-% based on the weight of Fe-BEA) was added. The resulting slurry was mixed and milled using a continuous milling apparatus so that the Dv90 value of the particles was of about 5 micrometers. The pH of the aqueous phase of the slurry was measured and adjusted to about 3-3.5 and the solid content of said slurry was of 39 weight-%. a) An uncoated honeycomb flow-through monolith cordierite substrate (diameter: 1 inch x length: 2 inches, cylindrically shaped with 600 cells per square inch and 3 mil wall thickness) was coated with the obtained slurry over 100% of the substrate length according to the method described in Reference Example 4. The coated substrate was dried at 140 C. for 30 minutes and calcined in air at 590 C. for 30 minutes (coating/drying and calcining were repeated once or maximum twice to attain the targeted loading below).

(14) The final loading of the coating in the catalyst after calcination is of about 3.2 g/in.sup.3, including 2.88 g/in.sup.3 of Fe-BEA, 0.16 g/in.sup.3 of Fe-MFI and 0.1562 g/in.sup.3 of silica. The ammonia storage capacity of the coating measured according to Reference Example 1 was 2.8-3.0 mmol/g. b) An uncoated honeycomb flow-through monolith cordierite substrate (diameter: 1 inch x length: 2 inches, cylindrically shaped with 300 cells per square inch and 5 mil wall thickness) was coated with the obtained slurry over 100% of the substrate length according to the method described in Reference Example 4. The coated substrate was dried at 140 C. for 30 minutes and calcined in air at 590 C. for 30 minutes (coating/drying and calcining were repeated once or maximum twice to attain the targeted loading below).

(15) The final loading of the coating in the catalyst after calcination is of about 3.2 g/in.sup.3, including 2.88 g/in.sup.3 of Fe-BEA, 0.16 g/in.sup.3 of Fe-MFI and 0.1562 g/in.sup.3 of silica. The ammonia storage capacity of the coating measured according to Reference Example 1 was 2.8-3.0 mmol/g.

Example 2: Testing of the Catalysts of Example 1 a) and Reference Example 6DeNOx Performance and N.SUB.2.O Formation

(16) The Catalysts of Example 1 a) and of Reference Example 6 were hydrothermally aged in an oven at 650 C. for 25 hours. The deNOx (%) and the N.sub.2O (nitrous oxide) formation (in ppm) were measured when using the catalysts of Example 1 a) and of Reference Example 6E-lab evaluation. Space Velocity: 60000 h.sup.1 NOx=NO inlet concentration: 500 ppm/NH.sub.3 inlet concentration: 550 ppm/H.sub.2O: 10 vol.-%/O.sub.2: 10 vol.-% Test Temperatures: 450, 400, 350, 300, 250, 225, 200, 180 and 150 C.
General Test Procedure for Lab Reactor Evaluation: 1. Adjust maximum test temperature and feed gas composition 2. Allow to stabilize the concentrations measured behind the catalyst under investigation 3. Determine the NOx, NH.sub.3 and N.sub.2O concentrations at the catalyst outlet and use the inlet concentrations either from Bypass measurements or from the reactor setup measurements 4. Go to the next lower temperature and repeat step 2 and 3.
Calculations: DeNOx: (NOxInNOxOut)/NOxIn*100 Unit: Percent N.sub.2O make: N.sub.2OOutN.sub.2OIn Unit: ppm

(17) The results are illustrated on FIGS. 1 and 2.

(18) As may be taken from FIGS. 1 and 2, the catalyst of Example 1 a) exhibits a T.sub.50 (deNOx) of about 250 C. and a NOx conversion of about 98% at 450 C. while presenting very low nitrous oxide formation, or only minor nitrous oxide formation of less than 0.5 ppm N.sub.2O, in a temperature of from 150 to 450 C. In contrast thereto, the catalyst of Reference Example 6 discloses a lower NOx conversion under the same conditions. Thus, said example shows that the catalyst of the present invention permits to improve the NOx conversion while exhibiting a low N.sub.2O formation.

Example 3: Measurement of the Ammonia Storage Capacity for Example 1 a), of Reference Examples 6 and 7

(19) The ammonia storage capacity of the coatings of these catalyst were measured as defined in Reference Example 1 under fresh conditions. In particular, the measurements were made on the powders comprising the components of each coating. The results are displayed in Table 1 below.

(20) TABLE-US-00001 TABLE 1 Ammonia storage capacity (fresh conditions) Ammonia storage capacity (mmol/g) Example 1 2.8-3.0 Ref. Example 6 1.2 Ref. Example 7 1.9

(21) As may be taken from Table 1, the Fe-BEA catalyst of Example 1 a) shows a much higher ammonia storage capacity compared to the Fe-BEA catalyst of Reference Example 6 (prior art). Thus, without wanting to be bound to any theory it is believed that the silica to alumina ratio of a BEA zeolitic material and the amount of Fe have an impact on the ammonia storage capacity of a Fe-BEA catalyst. Further, without wanting to be bound to any theory it is believed that the process for preparing a BEA zeolitic material, namely a template-free process for preparing a BEA zeolitic material, also has an influence on the ammonia storage capacity of the final catalyst. Further, the Fe-BEA catalyst of Example 1 a) also exhibits a much higher ammonia storage capacity compared to the Cu-CHA catalyst of Reference Example 7.

Example 4: A SCR Catalyst Comprising the Catalysts of Example 1 b) and of Reference Example 7

(22) A SCR catalyst was prepared by combining the catalyst of Example 1 b) and the catalyst of Reference Example 7, in a manner that the catalyst of Example 1 b) is upstream of the catalyst of Reference Example 7 and the catalyst of Reference Example 7 is downstream of the catalyst of Example 1 b) and wherein there is no gap between the two catalysts. The formed catalyst had a length of 3 inches (2 inchesExample 1b)and 1 inchReference Example 7).

(23) Thus, the upstream portion of the catalyst of Example 4 made of the catalyst of Example 1 b) had an ammonia storage capacity of 2.8-3.0 mmol/g and the downstream portion of the catalyst of Example 4 made of the catalyst of Reference Example 7 had an ammonia storage capacity of 1.9 mmol/g, the ammonia storage capacity being determined as defined in Reference Example 1. The ammonia storage capacity in the upstream zone of the catalyst of Example 4 was higher than in the downstream zone of said catalyst.

Comparative Example 1: A SCR Catalyst Comprising the Catalysts of Reference Example 6 and of Reference Example 7

(24) A SCR catalyst was prepared by combining the catalyst of Reference Example 6 and the catalyst of Reference Example 7, in a manner that the catalyst of Reference Example 6 is upstream of the catalyst of Reference Example 7 and the catalyst of Reference Example 7 is downstream of the catalyst of Reference Example 6 and wherein there is no gap between the two catalysts. The formed catalyst had a length of 3 inches (2 inchesReference Example 6and 1 inchReference Example 7).

(25) Thus, the upstream portion of the catalyst of Comparative Example 1 made of the catalyst of Reference Example 6 had an ammonia storage capacity of 1.2 mmol/g and the downstream portion of the catalyst of Comparative Example 1 made of the catalyst of Reference Example 7 had an ammonia storage capacity of 1.9 mmol/g, the ammonia storage capacity being determined as defined in Reference Example 1. The ammonia storage capacity of the catalyst of Comparative Example 1 is higher in the downstream zone than in the upstream zone. This example is representative of the prior art U.S. Pat. No. 9,597,636 B2, wherein it is disclosed that the ammonia storage capacity has to be higher in the downstream portion of the catalyst.

Example 5: Testing of the Catalysts of Example 4 and of Comparative Example 1NOx Emissions

(26) A selective catalytic reduction (SCR) steady state was performed with ramps

(27) $(C_{NOx}=C_{NO}\left(\mathrm{feed}\right)=750\mathrm{ppm},$
NSR (Normalized Stoichiometric Ratio) of ammonia to NOx=1.2, space velocity for the 3 inches total length of 80 000 h.sup.1, C.sub.O2-Feed=10 wt.-%, C.sub.H2O-Feed=5 wt.-%, C.sub.CO2-Feed=5 wt. %). The NOx concentration was measured at different inlet temperatures (T=180 C. to 450 C.FIG. 3/T=250 to 450 C.FIG. 4) of the tested catalysts in function of time during urea dosing and after switch off of the urea dosing. The test was conducted on a lab reactor. The test was designed to simulate a sudden Temperature change in combination with no ammonia addition into the feed. Therefore, the NH.sub.3 supply was switched-off at the same time when the temperature was increased. The catalyst now has to operate with the NH.sub.3 stored at the time when the ammonia supply was switched-off and the dynamic behavior of the catalyst can be studied. The test shall mimic typical engine acceleration conditions, where the ammonia, or in this case urea, supply cannot adapted fast enough according to the dynamics of the engine exhaust composition and temperature change. The results are displayed in FIGS. 3 and 4.

(28) As may be taken from FIG. 3, at t=4000 to 4121 seconds (inlet temperature of 180 C.), the NOx conversion of the catalyst of Example 4 is of about 16% while the NOx conversion of the catalyst of Comparative Example 1 is lower, namely of about 12%. Further, at t=4121 seconds, the NH.sub.3 dosing was switched off and the temperature was increased, the NOx conversions of the two catalysts increased drastically from t=4121 to 4150 seconds up to about 93%. However, at t=about 4160 to 4400 seconds the NOx conversion of the catalyst of Comparative Example 1 decreases. In contrast thereto, for the catalyst according to the present invention (Example 4), the NOx conversion continued to increase from t=4150 to 4200 seconds to about 99% of NOx conversion. Afterwards, at t=4200 seconds, the NOx conversion of the inventive catalyst started to decrease while still being superior to the NOx conversion of the catalyst of Comparative Example 1.

(29) As may be taken from FIG. 4, at t=13500 to 13821 seconds (inlet temperature of 250 C.), the NOx conversion of the catalyst of Example 4 is of about 91% while the NOx conversion of the catalyst of Comparative Example 1 is lower, namely of about 85%. Further, at t=13821 seconds, the NH.sub.3 dosing was switched off and the temperature was increased, the NOx conversions of the two catalysts both increased from t=13821 to 13850 seconds, the NOx conversion of the catalyst of Example 4 still being superior to those of the catalyst of Comparative Example 1. However, at t=about 1360 to 14000 seconds, the NOx conversion of the catalyst of Comparative Example 1 decreases. In contrast thereto, for the catalyst according to the present invention (Example 4), at t=13850 seconds, the NOx conversion of the catalyst of Example 4 was of 100% and maintained from 100 to about 98% to t=about 13890 seconds. Afterwards, at t=about 13890 seconds, the NOx conversion of the inventive catalyst started to decrease while still being superior to the NOx conversion of the catalyst of Comparative Example 1.

(30) Thus, FIGS. 3 and 4 illustrate that the catalyst according to the present invention permits to maintain for a longer period a great NOx conversion compared to the comparative catalyst not according to the present invention. Further, it is noted that it is believed to be due to the higher ammonia storage capacity in its upstream portion of the catalyst according to the present invention compared to the ammonia storage capacity in its downstream portion.

Reference Example 8: Preparation of a SCR Catalyst (Cu-CHA Containing)

(31) The catalyst of Reference Example 8 was prepared as the catalyst of Reference Example 7, except that a different substrate was used. In particular, an uncoated honeycomb flow-through monolith cordierite substrate (diameter: 1 inch x length: 6 inches (2 juxtaposed substrates of 3 inches longno gap between the substrates), cylindrically shaped with 600 cells per square inch and 3 mil wall thickness). The final loading of the coating in the catalyst after calcination is of about 2.75 g/in.sup.3, including 2.62 g/in.sup.3 of Cu-CHA, 0.13 g/in.sup.3 of ZrO.sub.2. The ammonia storage capacity of the coating measured according to Reference Example 1 was 1.9 mmol/g.

Example 6: A SCR Catalyst Comprising the Catalysts of Example 1 b) and of Reference Example 8

(32) A SCR catalyst was prepared by combining the catalyst of Example 1 b) and the catalyst of Reference Example 8, in a manner that the catalyst of Example 1 b) is upstream of the catalyst of Reference Example 8 and the catalyst of Reference Example 8 is downstream of the catalyst of Example 1 b) and wherein there is no gap between the two catalysts. The formed catalyst had a length of 8 inches (2 inchesExample 1b)and 6 inchesReference Example 8).

(33) Thus, the upstream portion of the catalyst of Example 6 made of the catalyst of Example 1 b) had an ammonia storage capacity of 2.8-3.0 mmol/g and the downstream portion of the catalyst of Example 4 made of the catalyst of Reference Example 8 had an ammonia storage capacity of 1.9 mmol/g, the ammonia storage capacity being determined as defined in Reference Example 1. The ammonia storage capacity in the upstream zone of the catalyst of Example 6 was higher than in the downstream zone of said catalyst.

Comparative Example 2: A SCR Catalyst Comprising the Catalysts of Reference Example 6 and of Reference Example 8

(34) A SCR catalyst was prepared by combining the catalyst of Reference Example 6 and the catalyst of Reference Example 8, in a manner that the catalyst of Reference Example 6 is upstream of the catalyst of Reference Example 8 and the catalyst of Reference Example 7 is downstream of the catalyst of Reference Example 8 and wherein there is no gap between the two catalysts.

(35) The formed catalyst had a length of 8 inches (2 inchesReference Example 6and 6 inchesReference Example 8).

(36) Thus, the upstream portion of the catalyst of Comparative Example 1 made of the catalyst of Reference Example 6 had an ammonia storage capacity of 1.2 mmol/g and the downstream portion of the catalyst of Comparative Example 1 made of the catalyst of Reference Example 8 had an ammonia storage capacity of 1.9 mmol/g, the ammonia storage capacity being determined as defined in Reference Example 1. The ammonia storage capacity in the downstream zone of the catalyst of Comparative Example 2 was higher than in the upstream zone of said catalyst. This example is representative of the prior art U.S. Pat. No. 9,597,636 B2, wherein it is disclosed that the ammonia storage capacity has to be higher in the downstream portion of the catalyst.

Example 7: Testing of the Catalysts of Example 6 and of Comparative Example 2NOx Emissions

(37) A selective catalytic reduction (SCR) steady state was performed with ramps (C.sub.NOxC.sub.NO(feed)=750 ppm, NSR (Normalized Stoichiometric Ratio) of ammonia to NOx=1.2, space velocity for the 8 inches total length of 30 000 h.sup.1, C.sub.O2-Feed=10 wt.-%, C.sub.H2O-Feed=5 wt.-%, C.sub.CO2-Feed=5 wt. %). The NOx concentration was measured at different inlet temperatures (T=180 C. to 450 C.FIG. 4) of the tested catalysts in function of time during urea dosing and after switch off of the urea dosing. The test was designed to simulate a sudden Temperature change in combination with no ammonia addition into the feed. Therefore, the NH3 supply was switched-off at the same time when the temperature was increased. The catalyst now has to operate with the NH3 stored at the time when the ammonia supply was switched-off and the dynamic behavior of the catalyst can be studied. The test shall mimic typical engine acceleration conditions, where the ammonia, or in this case urea, supply cannot adapted fast enough according to the dynamics of the engine exhaust composition and temperature change. The results are displayed in FIG. 5.

(38) As may be taken from FIG. 5, at t=4000 to 4122 seconds (inlet temperature of 180 C.), the NOx conversion of the catalyst of Example 6 is of about 56% while the NOx conversion of the catalyst of Comparative Example 2 is lower, namely of about 43%. Further, at t=4122 seconds, the NH.sub.3 dosing was switched off and the temperature was increased, the NOx conversions of the two catalysts increased drastically from t=4122 to 4170 seconds up to about 100%. However, at t=about 4300 to 4350 seconds the NOx conversion of the two catalysts decreases. However, for the catalyst according to the present invention (Example 6), the NOx conversion decreases in a slower manner compared to the NOx conversion of the catalyst of Comparative Example 2 and was thus still higher from t=4350 to 4550 seconds.

(39) Thus, FIG. 5 illustrates that the catalyst according to the present invention permits to maintain for a longer period a great NOx conversion compared to the comparative catalyst not according to the present invention. Further, it is noted that it is believed to be due to the higher ammonia storage capacity in its upstream portion of the catalyst according to the present invention compared to the ammonia storage capacity in its downstream portion.

BRIEF DESCRIPTION OF THE FIGURES

(40) FIG. 1 shows the NOx conversion performance of the catalyst of Example 1 a) after ageing.

(41) FIG. 2 shows the N.sub.2O formation obtained when using the catalyst of Example 1 a) after ageing.

(42) FIG. 3 shows the NOx emissions in function of time of the catalysts of Example 4 and Comparative Example 1 (ramp 180 to 450 C.).

(43) FIG. 4 shows the NOx emissions in function of time of the catalysts of Example 4 and Comparative Example 1 (ramp 250 to 450 C.).

(44) FIG. 5 shows the NOx emissions in function of time of the catalysts of Example 6 and Comparative Example 2 (ramp 180 to 450 C.).

CITED LITERATURE

(45) U.S. Pat. No. 9,352,307 B2 EP 2 520 365 A2 U.S. Pat. No. 9,597,636 B2