Vanadium trapping SCR system

Abstract

The present invention is directed towards the use of an ion-exchanged zeolite containing ASC as a trap for volatile vanadium compounds in a downstream position of a vanadium containing SCR-catalyst.

Claims

1. An NH.sub.3-SCR-system comprising a vanadium based upstream SCR-catalyst and a downstream ASC, with the ASC comprising a Cu- or Fe-exchanged BEA zeolite as a trap for volatile vanadium compounds, and wherein the ASC comprises platinum group metal and is designed such that the Cu- or Fe-exchanged BEA zeolite is positioned, at least to some extent, for contact with the exhaust gas with volatile vanadium compounds prior to exhaust contact with the platinum group metal of the ASC.

2. The NH.sub.3-SCR-system according to claim 1, wherein the ASC is located in the same housing as the SCR-catalyst.

3. The NH.sub.3-SCR-system according to claim 1, wherein the Cu- or Fe-exchanged BEA zeolite contains ion-exchanged and non-ion exchanged Cu or Fe-ions in a molar ratio of greater than 90:10.

4. The NH.sub.3-SCR-system according to claim 1, wherein the Cu- or Fe-exchanged BEA zeolite is provided in a coating coated over an entire length of the ASC.

5. The NH.sub.3-SCR-system according to claim 4, wherein the platinum group metal is provided in a noble metal layer and wherein the coating of Cu- or Fe-exchanged BEA zeolite extends over the noble metal layer as to provide for the prior contact with volatile vanadium compounds.

6. The NH.sub.3-SCR-system according to claim 5 wherein the platinum group metal in the noble metal layer comprises Pt.

7. The NH.sub.3-SCR-system according to claim 6 wherein the platinum group metal in the noble metal layer comprises only Pt.

8. The NH.sub.3-SCR-system according to claim 7, wherein the noble metal layer is entirely covered over by the coating of Cu- or Fe-exchanged BEA zeolite.

9. The NH.sub.3-SCR-system according to claim 1 wherein the ASC is a coating of Fe-exchanged BEA zeolite.

10. The NH.sub.3-SCR-system according to claim 1 wherein the ASC is a coating of Cu-exchanged BEA zeolite.

11. A method of catalytic treatment of exhaust gas comprising passing exhaust gas through the NH.sub.3-SCR-system of claim 1.

12. The method of claim 11 wherein the exhaust gas is diesel exhaust gas and the diesel exhaust gas at an outlet of the NH.sub.3-SCR-system has a measured zero ppm Vanadium concentration at 475° C. exhaust gas temperature level as measured at an exit point of the vanadium based upstream SCR-catalyst.

13. A method of assembling the NH.sub.3-SCR-system of claim 1 comprising positioning each of the vanadium based upstream SCR-catalyst and the downstream ASC within an exhaust passageway.

14. An NH.sub.3-SCR-system comprising a first and a second vanadium based SCR-catalyst and an ASC positioned downstream of both the first and second vanadium based SCR-catalysts, with the ASC comprising a Cu- or Fe-exchanged BEA zeolite as a trap for volatile vanadium compounds, and wherein the ASC comprises platinum group metal and is designed such that the Cu- or Fe-exchanged BEA zeolite is positioned, at least to some extent, for contact with exhaust gas with volatile vanadium compounds prior to exhaust gas contact with the platinum group metal of the ASC.

15. The NH.sub.3-SCR-system according to claim 1, wherein the Cu- or Fe-exchanged BEA zeolite is positioned so as to entirely come in first contact with the exhaust gas with volatile vanadium prior to exhaust gas contact with the platinum group metal.

16. The NH.sub.3-SCR-system according to claim 1 wherein the platinum group metal is in an amount of 3 to 7 g/ft.sup.3.

17. The NH.sub.3-SCR-system according to claim 1 wherein the Cu- or Fe-exchanged BEA zeolite is in a continuous layer above a layer containing the platinum group metal such that the Cu- or Fe-exchanged BEA zeolite entirely comes in first contact with the exhaust gas with volatile vanadium prior to exhaust gas contact with the platinum group metal.

18. The NH.sub.3-SCR-system according to claim 1 wherein the vanadium based upstream SCR-catalyst and the downstream ASC are provided on a common brick.

19. An NH.sub.3-SCR-system comprising a vanadium based upstream SCR-catalyst and a downstream ASC, with the ASC comprising a Cu- or Fe-exchanged large pore zeolite as a trap for volatile vanadium compounds, and wherein the ASC comprises platinum group metal and is designed such that the Cu- or Fe-exchanged large pore zeolite is provided in a layer overlying a layer containing the platinum group metal.

20. The NH.sub.3-SCR-system according to claim 19 wherein the large pore zeolite is BEA.

21. The NH.sub.3-SCR-system according to claim 1 wherein Cu- or Fe-exchanged BEA zeolite is the sole zeolite of the ASC.

22. The NH.sub.3-SCR-system according to claim 1 wherein the platinum group metal is in an amount of 2 to 10 g/ft.sup.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the set-up of the aging equipment;

(2) FIG. 2 shows the catalyst set-up for the long time aging;

(3) FIG. 3 shows the ASC located in the same canning as the SCR-catalyst; and

(4) FIG. 4 shows the vanadium concentration in the samples tested.

EXAMPLE

(5) To evaluate the Vanadium release properties of V-SCR catalysts technologies the sampling and analytical method for the analysis of sublimation based vanadium from SCR catalysts proposed by US-EPA is applied (Recommendation of Sampling and Analytical Method for the Analysis of Sublimation Based Vanadium from SCR Catalysts; http://www.epa.gov/otaq/cert/documents/nrciscr-web-conf.2011-07-25.pdf).

(6) Vanadium Release Measurement

(7) In principle, the Vanadium catalyst is mounted into a quartz glass tube and exposed to a gas mixture at a constant temperature. Behind the catalyst an adsorber bed is installed which quantitatively adsorbs all vanadium. Chapman et al. showed that high surface area γ-alumina yields quantitative adsorption of released vanadium compounds (Behavior of Titania-supported Vanadia and Tungsta SCR Catalysts at High Temperatures in Reactant Streams: Tungsten and Vanadium Oxide and Hydroxide Vapor Pressure Reduction by Surficial Stabilization. Applied Catalysis A: General, 2011, 392, 143-150). After a given measurement time the adsorber bed is analyzed for trapped vanadium. The measurement is repeated at different temperatures. For each temperature a new V-SCR sample and adsorber bed is installed into the quartz glass tube.

(8) Evaluation of Vanadium Release in a Given after Treatment System Configuration

(9) In a commercial after treatment system, the V-SCR system is the first component of the system. Downstream of the V-SCR catalyst positioned in a flow-through catalyst is the ASC. To assess the release of vanadium out of the after treatment system long time aging in a burner setup is conducted. In principle, a V-SCR is positioned upstream of an ASC in an exhaust stream of a Diesel burner (FIG. 2). The aging is done at constant temperature for at least 100 h. Afterwards the ASC is analyzed for vanadium. The experiment is repeated at different temperatures with a further V-SCR and ASC.

(10) The catalyst sample is aged within a synthetic gas bench (FIG. 1). For this a drill core (size Ø 1″, length 1.6″) of catalyst is mounted within a quartz tube reactor. Directly behind the catalyst a quartz glass tube is positioned filled with γ-alumina. Through the tube a constant gas flow of exhaust gas stream is pumped. After condensation of water the flow is measured by means of a gas meter. With this setup of the experiment the gas flow of exhaust over the adsorption bed is measured precisely. FIG. 1 shows the set-up of the aging equipment.

(11) After installation, the reactor is purged with N.sub.2 and heated up to the desired measurement temperature. Then the aging gas mixture is applied (Table 1). The aging time is 24 h. During aging the gas flow is measured with a gas meter. After finishing the aging the adsorber material is analyzed for vanadium.

(12) TABLE-US-00001 TABLE 1 Gas Mixture during aging Gas Value NO 500 pm NH3 500 ppm O2 10 vol.-% H2O 5 vol.-% N2 Balance GHSV 30,000 1/h

(13) Measurement of the next temperature point is done with new catalyst sample and new adsorber material. As adsorber material, a lab grade γ-alumina is used. Temperatures measured are:

(14) The reference is a commercial V-SCR coated catalyst. Other samples including the V-SCR-catalyst are equipped with a coated substrate with a potential V-SCR scavenger function. Table 2 displays the measured samples.

(15) TABLE-US-00002 TABLE 2 Measured sample and aging Sample Aging V-SCRa 580° C., 610° C. V-SCR + γ-alumina 580° C., 610° C. V-SCR + FeBEA-ASC 580° C., 610° C. V-SCR + white zeolite 580° C., 610° C.

(16) According to the EPA proposal, the vanadium loss shall be given as gas phase concentration in μg/m.sup.3N. By measuring the gas flow and analyzing the Vanadium content in the adsorber this value is calculates as follows:

(17) ${\mathrm{Vanadia}}_{\mathrm{Gas}}\left[\mathrm{.Math.g}/m_N^3\right]=\frac{n_{\mathrm{Vanadia}}\left[\mathrm{.Math.g}\right]}{V_{\mathrm{Gas}}\left[m_N^3\right]}$

(18) To assess the risk of vanadium emission of an after treatment system configured as depicted in FIG. 2, a long time aging at a burner aging test rig was applied. In principle, a V-SCR catalyst is positioned downstream in the exhaust of a Diesel burner. Directly downstream of the V-SCR catalyst a FeZeo-ASC is placed. After exposure the FeZeo-ASC is analyzed for vanadium.

(19) Catalyst 1 and 2, respectively, are V-SCR catalysts and on position of Catalyst 3 a FeBEA containing ammonia slip catalyst (ASC) is used. FIG. 2 shows the catalyst set-up for the long time aging.

(20) The catalysts are mounted in a canning and installed in the exhaust stream of the burner. The burner is started and the exhaust temperature is regulated to the desired temperature. The temperature control is based on SCR outlet. After the aging, the ASC brick is dismounted and analyzed. To assess the axial gradient the catalyst is cut into three parts yielding inlet, middle and outlet of the ASC brick.

(21) The test matrix is as follows:

(22) TABLE-US-00003 Temperatur Duration [° C.] 100 h 200 h 300 h 475 X x x 550 X X x 600 X x x

(23) V-Analytics:

(24) The crucial point for assessment of vanadium release is the analytic measurement of this element. For both studies, inductively coupled plasma optical emission spectrometry (ICP-OES) method according to DIN EN ISO 11885 (2009) is used. The method has three general steps: Sample preparation: Samples are grinded to fine powder Sample dissolving: The powders are solved in mineral acids using pressure digestion until a clear solution is received

(25) Measurement with ICP-OES

(26) To ensure the quality of the measurement also the γ-alumina material is tested.

(27) Results:

(28) Catalyst samples were aged at different temperatures according method described above. Analytic values of the γ-alumina material are displayed in Table 3.

(29) TABLE-US-00004 TABLE 3 V-concentration in exhaust stream downstream SCR at different aging in μg/m.sup.3.sub.N V-concentration in Gas Phase [μg/Nm.sup.3] at Catalyst Sample 580 [° C.] 610 [° C.] V-SCR 4.3 8.3 V-SCR + γ-alumina <0.05 <0.05 V-SCR + FeZeo-ASC <0.05 <0.05 Detection limit 0.05

(30) Tests with the set-up according to FIG. 2 were conducted at different temperatures and times. The ASC sample is cut into three portions leading to inlet, middle and outlet part. The catalyst samples are grinded to fine powder and Vanadium content measured with ICP-OES. As vanadium can be a contaminant of the cordierite substrate, a reference part from same substrate lot is analyzed as well. The reference sample yielded a vanadium-content of 46 ppm. This is seen as an offset for all measured values. This value is subtracted from the measured vanadium concentration of the ASC sample given in Table 4. The detection limit is given in Table 3.

(31) TABLE-US-00005 TABLE 4 Vanadium concentration in ASC after burner aging Temperature Aging Time Vanadium concentration [ppm] [° C.] [h] Inlet Middle Outlet 475 100 50 4 0 550 100 129 14 0 600 100 307 38 2 475 200 64 6 0 550 200 186 24 2 600 200 614 116 13 475 300 118 12 0 550 300 261 35 3 600 300 866 222 43

(32) The results are displayed in FIG. 4. All samples exhibit a strong axial gradient of vanadium. With increasing temperature, the vanadium concentration at the Inlet of the ASC increases. Also longer aging duration leads to an increasing vanadium concentration at the inlet. The axial gradient at given temperature and aging time shows that almost all vanadium is adsorbed directly in the inlet of the ASC. Even at elevated temperature only a small amount of vanadium can be detected at the outlet of the ASC.

(33) Layout of the ASC-Catalyst:

(34) The ASC used is prepared by coating a Pt containing first layer (1.6″) onto the outlet of a 3″-flow-through substrate (400/4) with a Pt content of 5 g/ft.sup.3. In a second step a Fe-Beta-zeolith containing layer (3″) is coated on top of that.