SCR catalyst device containing vanadium oxide and molecular sieve containing iron

Abstract

The invention relates to a catalyst device for purifying exhaust gases containing nitrogen oxide by means of selective catalytic reduction (SCR), comprising at least two catalytic regions, the first region containing vanadium oxide and cerium oxide, and the second region containing a molecular sieve containing iron. The invention also relates to uses, the catalyst device and methods for purifying exhaust gases.

Claims

1. A catalyst device for purifying exhaust gases containing nitrogen oxide by means of selective catalytic reduction (SCR), comprising at least two catalytic regions, the first region comprises the following components: (a) from 0.5 to 10% by weight vanadium oxide, calculated as V.sub.2O.sub.5, (b) from 0.2 to 10% by weight cerium oxide, calculated as CeO.sub.2, (c) from 0 to 17% by weight tungsten oxide, calculated as WO.sub.3, (d) from 25 to 98% by weight titanium oxide, calculated as TiO.sub.2, (e) from 0 to 15% by weight silicon oxide, calculated as SiO.sub.2, (f) from 0 to 15% by weight aluminum oxide, calculated as Al.sub.2O.sub.3 and in each case based on the weight of the first region, and the second region containing a molecular sieve containing iron.

2. The catalyst device according to claim 1, wherein the first region contains, in addition to said components (a), (b) and (d), a quantity of at least one of (c), (e) and (f).

3. The catalyst device according to claim 1, wherein the first region comprises from 0.5 to 5% by weight of cerium oxide, calculated as CeO.sub.2, and 1 to 5% by weight vanadium oxide, calculated as V.sub.2O.sub.5, each based on the weight of the first region.

4. The catalyst device according to claim 1, wherein the first region comprises from 0.5 to 7% by weight of cerium oxide, calculated as CeO.sub.2 and based on the weight of the first region.

5. The catalyst device according to claim 1, wherein the first region comprises from 0.5 to 3% by weight of cerium oxide, calculated as CeO.sub.2 and based on the weight of the first region.

6. The catalyst device according to claim 1, wherein the molecular sieve is a zeolite.

7. The catalyst device according to claim 6, wherein the zeolite is an iron-exchanged zeolite that preferably has a structure whose maximum ring size has more than 8 tetrahedra.

8. The catalyst device of claim 6, wherein the zeolite has a structure selected from AEL, AFI, AFO, AFR, ATO, BEA, GME, HEU, MFI, MWW, EUO, FAU, FER, LTL, MAZ, MOR, MEL, MTW, OFF and TON.

9. The catalyst device according to claim 1, which is designed in such a manner that the exhaust gases initially contact the first region.

10. The catalyst device according to claim 9, wherein the first and second region form adjacent zones, wherein the first region is a first zone and the second region is a second zone, wherein the first zone is upstream of the second zone in the direction of flow of the exhaust gas.

11. The catalyst device according to claim 10, wherein the first zone comprises a first pair of superimposed layers and the second zone comprises a second pair of superimposed layers.

12. The catalyst device according to claim 9, wherein the first and second region form superimposed layers, wherein the first region is the top layer.

13. The catalyst device according to claim 1, wherein the first and second region are applied onto an inert substrate that is preferably a ceramic monolith.

14. A device, comprising: the catalyst device according to claim 1 and an upstream device releasing exhaust gases.

15. The catalyst device according to claim 1, wherein the first region and the second region form adjacent zones, wherein the first region is a first zone and the second region is a second zone, wherein the first zone is downstream of the second zone in the direction of flow of the exhaust gas.

16. The catalyst device according to claim 1, wherein the first region and the second region form superimposed layers, wherein the second region is the top layer relative to the first region.

17. A method of selective catalytic reduction (SCR), comprising utilizing catalyst device according to claim 1 for purifying exhaust gases containing nitrogen oxide by means of selective catalytic reduction (SCR).

18. The method according to claim 17 wherein the method includes avoiding the formation of nitrous oxide.

19. The method according to claim 17, wherein the exhaust gases have an NO.sub.2/NO.sub.x ratio of >0.5 and/or a temperature of 180° C. to 450° C.

20. A method for purifying exhaust gases, comprising the steps of: (i) providing a catalyst device or a system according to claim 1, (ii) introducing exhaust gases containing nitrogen oxides into the catalyst device, (iii) introducing a reducing agent containing nitrogen into the catalyst device, and (iv) reducing nitrogen oxides in the catalyst device by means of selective catalytic reduction (SCR).

Description

EXEMPLARY EMBODIMENTS

(1) Pre-Tests

(2) FIGS. 1 and 2 show the dependence of the conversion of nitrogen monoxide, see FIG. 1, or nitrogen dioxide (NO.sub.2/NO.sub.x=75%), see FIG. 2, on the cerium content of the SCR catalyst.

(3) In FIGS. 1 and 2:

(4) Reference=SCR catalyst made of 3% V.sub.2O.sub.5, 4.3% WO.sub.3 and remainder TiO.sub.2 with 5% SiO.sub.2

(5) 1% cerium oxide=as reference, but TiO.sub.2/SiO.sub.2 replaced by cerium oxide up to a cerium oxide content of the catalyst of 1%

(6) 5% cerium oxide=as reference, but TiO.sub.2/SiO.sub.2 replaced by cerium oxide up to a cerium oxide content of the catalyst of 5%

(7) 10% cerium oxide=as reference, but TiO.sub.2/SiO.sub.2 replaced by cerium oxide up to a cerium oxide content of the catalyst of 10%

(8) The catalysts were coated in the usual way on commercial flow-through substrates with a wash coat load of 160 g/l and with a GHSV=60000 1/h of NO.sub.x conversion measured with the following test gas composition: NO.sub.x: 1000 ppm

(9) NO.sub.2/NO.sub.x: 0% (FIG. 1) or 75% (FIG. 2)

(10) NH.sub.3: 1100 ppm (FIG. 1) or 1350 ppm (FIG. 2)

(11) O.sub.2: 10%

(12) H.sub.2O: 5%

(13) N.sub.2: Remainder

(14) As shown in FIG. 1, the conversion of NO deteriorates with increasing cerium oxide content, while as shown in FIG. 2, the conversion at NO.sub.2/NO.sub.x=75% improves with increasing cerium oxide content.

Example 1: Preparation of Coating Suspensions (Wash Coats)

(15) Preparation of Coating Suspension a (Vanadium SCR)

(16) A commercially available titanium dioxide in anatase form doped with 5% by weight silicon dioxide was dispersed in water. Subsequently, an aqueous solution of ammonium metatungstate and ammonium metavanadate dissolved in oxalic acid were added as a tungsten or vanadium precursor in an amount to result in a catalyst of composition 87.4% by weight TiO.sub.2. 4.6% by weight SiO.sub.2, 5.0% by weight WO.sub.3 and 3.0% by weight V.sub.2O.sub.5. The mixture was stirred intensively and finally homogenized in a commercial agitator ball mill and ground to d90<2 μm.

(17) Preparation of Coating Suspension B (Vanadium SCR with 1% Cerium Oxide)

(18) A commercially available titanium dioxide in anatase form doped with 5% by weight silicon dioxide was dispersed in water. Subsequently, an aqueous solution of ammonium metatungstate as tungsten precursor, ammonium metavanadate dissolved in oxalic acid as vanadium precursor and an aqueous solution of cerium acetate as cerium precursor were added in an amount to result in a catalyst of a composition calculated as 86.4% by weight TiO.sub.2, 4.6% by weight SiO.sub.2, 5.0% by weight WO.sub.3, 3.0% by weight VO.sub.2O.sub.5 and 1% CeO.sub.2. The mixture was stirred intensively and finally homogenized in a commercial agitator ball mill and ground to d90<2 μm.

(19) Preparation of Coating Suspension C (Fe—SCR, SAR=25)

(20) A coating suspension for a commercially available SCR catalyst based on an iron-exchanged beta zeolite was prepared. For this purpose, a commercial SiO.sub.2 binder, a commercial boehmite binder (as coating aids), iron(III) nitrate nonahydrate and commercially available beta zeolite with a molar SiO.sub.2/Al.sub.2O.sub.3 ratio (SAR) of 25 were suspended in water so that a catalyst of composition 90% by weight of β zeolite and an iron content, calculated as Fe.sub.2O.sub.3, of 4.5% by weight results.

(21) Preparation of Coating Suspension D (Fe—SCR, SAR=10)

(22) A coating suspension for a commercially available SCR catalyst based on an iron-exchanged beta zeolite was prepared. For this purpose, a commercial SiO.sub.2 binder, a commercial boehmite binder (as coating aids), iron (III) nitrate nonahydrate and commercially available beta zeolite with a molar SiO.sub.2/Al.sub.2O.sub.3 ratio (SAR) of 10 were suspended in water so that a catalyst of composition 90% by weight of β zeolite and an iron content, calculated as Fe.sub.2O.sub.3, of 4.5% by weight results.

Example 2: Production of Catalyst Devices

(23) Various catalyst devices were produced by coating ceramic substrates with the coating suspensions A to D. Customary ceramic monoliths with parallel flow channels open on both sides were used as substrates. In this case, a first and a second layer (S1, S2) were applied to each substrate, each layer being divided into two adjacent zones (Z1, Z2). The exhaust gases to be purified flow in the direction of flow into the catalyst device, that is over the upper layer 2 and from zone 1 to zone 2. Scheme 1 shows the structure of the catalyst devices with four catalytic regions located in two layers and two zones.

(24) TABLE-US-00001 Scheme 1: Schematic structure of the catalyst devices produced in accordance exemplary embodiments Direction of flow .fwdarw. Layer 2 Zone 1 Layer 2 Zone 2 (S2Z1) (S2Z2) Layer 1 Zone 1 Layer 1 Zone 2 (S1Z1) (S1Z2) Substrate

(25) The compositions and quantities used of coating suspensions A to D are summarized in Table 1 below. The table also shows which catalytic layers S1 and S2 and zones Z1 and Z2 were applied. The catalysts E1 to E5 are according to the invention and V1 to V4 are reference catalysts.

(26) First, one of the dispersions A to D was applied using a conventional immersion process starting from the inlet side over the length of the region Z1S1 of a commercial flow-through substrate with 62 cells per square centimeters, a cell wall thickness of 0.17 millimeters and a length of 76.2 mm. The partially coated component was initially dried at 120° C. Subsequently, using the same method, one of the dispersions A to D was applied, starting from the outlet side, to the length of the region Z2S1. The coated component was then dried at 120° C., for 15 minutes at 350° C., then calcined at 600° C. for a duration of 3 hours. If the dispersion and wash coat loading were identical in the Z1S1 and Z2S1 regions, one of the dispersions A-D was applied to a commercial flow-through substrate with 62 cells per square centimeters and a cell wall thickness of 0.17 millimeters over its total length of 76.2 mm using a conventional immersion process. It was then dried at 120° C., for 15 minutes at 350° C., then calcined at 600° C. for a duration of 3 hours.

(27) The component calcined in this manner was subsequently coated with one of the suspensions A to D from the inlet side over the length of the region Z1 S2 using the process described above, and dried at 120° C. The previously described step was skipped if no coating was planned for the region Z1 S2. Subsequently, the coating was applied over the length of the region Z2S2 with one of the suspensions A to D starting from the outlet side. Drying then took place at 120° C. The previously described step was skipped if no coating was planned for the region Z2S2. Subsequently, calcination was carried out at 350° C. for 15 minutes, then at 600° C. for a duration of 3 hours. If the dispersion and wash coat loading were identical in the Z1S2 and Z2S2 regions, one of the dispersions A to D was applied to the entire length of the component of 76.2 mm using the process described above. It was then dried at 120° C., for 15 minutes at 350° C., then calcined at 600° C. for a duration of 3 hours.

(28) TABLE-US-00002 TABLE 1 Production of the catalyst devices with coating suspensions A to D in the first and second layer (S1, S2) and the first and second zone (Z1, Z2). Respectively indicated are the total quantity in each of the four regions (S1Z1 to S2Z2) in g/l after drying, calcination and tempering, and the length of the zones in % based on the total length of the catalyst device. The catalysts V1 to V4 are reference catalysts. Metal Coating suspension Z1S2 Z2S2 Z1S2 Z2S2 No. Z1S1 Z2S1 Z1S1 Z2S1 V1 Fe V  65 g/l D, L = 33% 140 g/l A, L = 67% Fe V  65 g/l D, L = 33% 140 g/l A, L = 67% E1 Fe V—Ce 110 g/l C, L = 33% 160 g/l B, L = 67% Fe V—Ce 110 g/l C, L = 33% 180 g/l B, L = 67% E2 V—Ce V—Ce 280 g/l B, L = 50% 280 g/l B, L = 50% Fe Fe  50 g/l C, L = 50%  50 g/l C, L = 50% V2 V V 280 g/l A, L = 50% 280 g/l A, L = 50% Fe Fe  50 g/l C, L = 50%  50 g/l C, L = 50% V3 V Fe 140 g/l A, L = 50% 110 g/l D, L = 33% V Fe 140 g/l A, L = 50% 110 g/l D, L = 33% V4 V Fe 140 g/l A, L = 50%  65 g/l D, L = 33% V Fe 140 g/l A, L = 50%  65 g/l D, L = 33% E3 V—Ce Fe 180 g/l B, L = 67% 110 g/l C, L = 33% V—Ce Fe 180 g/l B, L = 67% 110 g/l C, L = 33% E4 V—Ce Fe 160 g/l B, L = 67% 110 g/l C, L = 33% V Fe 200 g/l A, L = 67% 110 g/l C, L = 33% E5 V—Ce V 180 g/l B, L = 67% 280 g/l A, L = 33% V—Ce Fe 180 g/l B, L = 67% 100 g/l C, L = 33%

(29) As an alternative to the process described above, it would also be possible to produce two catalysts (Z1, Z2) corresponding to the zones Z1 and Z2 described above and to test both catalysts one after the other (Z1 before Z2).

(30) Catalyst Z1:

(31) Initially, apply one of the dispersions A to D over the entire length of the substrate with length Z1 (region Z1S1), dry at 120° C., then at 350° C. for 15 minutes, subsequently calcine at 600° C. for a duration of 3 hours. If intended, subsequently apply one of the dispersions A to D over the entire length of the component thus obtained (region Z1S2), then at 350° C. for 15 minutes, subsequently calcine at 600° C. for a duration of 3 hours.

(32) Catalyst Z2:

(33) Initially, apply one of the dispersions A to D over the entire length of the substrate with length Z2 (region Z2S1), dry at 120° C., then at 350° C. for 15 minutes, subsequently calcine at 600° C. for a duration of 3 hours. If intended, subsequently apply one of the dispersions A to D over the entire length of the component thus obtained (region Z2S2), then at 350° C. for 15 minutes, subsequently calcine at 600° C. for a duration of 3 hours.

Example 3: Reduction of Nitrogen Oxides by Means of SCR

(34) Measurement Method

(35) The catalyst devices produced according to Example 2 were tested for their activity and selectivity in the selective catalytic reduction of nitrogen oxides. In doing so, the nitrogen oxide conversion was measured at various defined temperatures (measured on the inlet side of the catalyst) as a measure of the SCR activity and the formation of nitrous oxide. On the inlet side, model exhaust gases containing preset proportions of NO, NH.sub.3, NO.sub.2 and O.sub.2, among other things, were introduced. The nitrogen oxide conversions were measured in a quartz glass reactor. For this purpose, drill cores with L=3″ and D=1″ were tested between 190 and 550° C. under stationary conditions. The measurements were carried out under the test conditions summarized below. GHSV is the gas hourly space velocity (gas flow rate: catalyst volume). The conditions of the measurement series TP1 to TP4 are summarized below:

(36) Test Parameter Set TP1:

(37) Gas hourly space velocity GHSV=60000 1/h with the following synthesis gas composition:

(38) 1000 vppm NO, 1100 vppm NH.sub.3, 0 vppm N.sub.2O

(39) a=xNH.sub.3/xNO.sub.x=1.1

(40) xNO.sub.x=xNO+xNO.sub.2+xN.sub.2O, where x in each case means concentration (vppm)

(41) 10% by vol. O.sub.2, 5% by vol. H.sub.2O, remainder N.sub.2.

(42) Test Parameter Set TP2

(43) GHSV=60000 1/h with the following synthesis gas composition:

(44) 250 vppm NO, 750 vppm NO.sub.2, 1350 vppm NH.sub.3, 0 vppm N.sub.2O

(45) a=xNH.sub.3/xNO.sub.x=1.35

(46) xNO.sub.x=xNO+xNO.sub.2+xN.sub.2O, where x in each case means concentration (vppm)

(47) 10% by vol. O.sub.2, 5% by vol. H.sub.2O, remainder N.sub.2.

(48) Test Parameter Set TP3

(49) GHSV=30000 1/h with the following synthesis gas composition:

(50) 500 vppm NO, 550 vppm NH.sub.3, 0 vppm N.sub.2O

(51) a=xNH.sub.3/xNO.sub.x=1.1

(52) xNO.sub.x=xNO+xNO.sub.2+xN.sub.2O, where x in each case means concentration (ppm)

(53) 10% by vol. O.sub.2, 5% by vol. H.sub.2O, remainder N.sub.2.

(54) Test Parameter Set TP4

(55) GHSV=30000 1/h with the following synthesis gas composition.

(56) 125 vppm NO, 375 vppm NO.sub.2, 675 vppm NH3, 0 vppm N.sub.2O

(57) a=xNH3/xNO=1.35

(58) xNO.sub.x=xNO+xNO.sub.2+xN.sub.2O, where x in each case means concentration (ppm) 10% by vol. O.sub.2, 5% by vol. H.sub.2O, remainder N.sub.2.

(59) The nitrogen oxide concentrations (nitrogen monoxide, nitrogen dioxide, nitrous oxide) after the catalyst device were measured. The nitrogen oxide conversion via the catalyst device for each temperature measuring point was calculated as follows (x is the concentration in vppm in each case) from the nitrogen oxide contents which were adjusted in the model exhaust gas and which were verified with a pre-catalyst exhaust gas analysis during conditioning at the start of the respective test run, and the measured nitrogen oxide contents after the catalyst device:
U.sub.NOx[%]=(1−x.sub.Output(NO.sub.x)/x.sub.Input(NO.sub.x))*100[%]
with
x.sub.Input (NO.sub.x)=x.sub.input(NO)+x.sub.Input(NO.sub.2)
x.sub.Output(NO.sub.x)=x.sub.Output(NO)+x.sub.Output(NO.sub.2)+2*x.sub.Output(N.sub.2O).

(60) x.sub.Output(N.sub.2O) was weighted with a factor of 2 in order to take stoichiometry into account.

(61) In order to determine the way in which the aging of the catalysts affects the result, the catalyst devices were subjected to hydrothermal aging for 100 hours at 580° C. in a gas atmosphere (10% O.sub.2, 10% H.sub.2O, remainder N.sub.2). Subsequently, the conversions of nitrogen oxides were determined according to the method described above.

(62) Results

(63) The results of the measurement series TP1 and TP3, with which the model exhaust gas contained exclusively NO as nitrogen oxide, are summarized in Table 2. The results of the measurement series TP2 and TP4, in which the model exhaust gas contained NO and NO.sub.2 as nitrogen oxides in a ratio of 1:3, are summarized in Table 3. The tables indicate which catalyst was used according to example 2 (Table 1). For each defined temperature value, the percentage of the initial concentration NO.sub.x removed is indicated. Table 3 also indicates, for each temperature value 2 to 7, the absolute quantity of N.sub.2O measured after the catalyst at each temperature value. Tables 4 and 5 summarize the conditions and results of the catalyst device tests after aging.

(64) The tests show that the SCR catalysts according to the invention with two catalytic regions, the first region containing a vanadium oxide and a cerium oxide and the second region containing a molecular sieve containing iron, achieve a high degree of efficiency in the reductive removal of nitrogen oxides (NO.sub.x) by means of SCR. They are suitable not only for the reaction with NO-rich exhaust gases but also for the treatment of NO.sub.2-rich exhaust gases. The reduction of nitrogen oxides from NO.sub.2-rich exhaust gases is especially efficient in the temperature range below 400° C. or below 350° C. In addition, the formation of nitrous oxide can be suppressed in the SCR with catalysts according to the invention. For example, the catalysts E3, E4 and E5 eliminate nitrogen oxides NO.sub.x almost completely both in NO-rich exhaust gases (Table 2) and in NO.sub.2-rich exhaust gases (Table 3), with the formation of nitrous oxide almost completely suppressed. Catalyst E2 also efficiently removes nitrogen oxides NO.sub.x from both NO- and NO.sub.2-rich exhaust gases and reduces the amount of nitrous oxide produced. The advantages of catalyst E2 after aging are particularly pronounced (Tables 4, 5), which is particularly important for practical applications in connection with combustion engines. The E1 catalyst also shows a high degree of efficiency with reduced nitrous oxide formation. The catalyst devices according to the invention thus combine several advantageous properties, specifically a high degree of efficiency with NO.sub.2-rich exhaust gases in the temperature range from approximately 180° C. to 500° C., and especially at low temperatures, a high degree of efficiency with NO-rich exhaust gases, and the avoidance of nitrous oxide formation. The effects can be seen with freshly produced catalysts and after an aging process.

(65) TABLE-US-00003 TABLE 2 Conditions and results of the reduction of NO with different catalyst devices (tests TP1 and TP3) at different actually measured temperatures 1 to 8. The depletion of NO.sub.x at the catalyst device outlet is indicated as a percentage of the initial amount used. Cat. Test Temperature [° C.] NO.sub.x [%] no. TP 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 V1 1 535 489 442 394 344 295 246 197 88% 95% 98%  98% 97% 93% 73% 31% E1 3 539 494 447 399 350 300 251 202 93% 97% 98%  97% 94% 92% 92% 74% V2 1 543 496 448 400 350 300 251 201 65% 92% 99% 100% 100%  98% 86% 42% E2 1 537 490 443 395 346 296 247 198 78% 94% 98%  99% 98% 94% 76% 34% V3 1 537 490 443 394 345 295 246 197 72% 96% 99% 100% 99% 96% 80% 40% V4 1 535 489 442 394 344 295 246 197 73% 96% 99% 100% 99% 95% 77% 35% E3 3 539 494 447 400 350 301 251 202 70% 92% 100%  100% 100%  100%  100%  74% E4 3 539 494 448 400 350 301 251 202 71% 92% 100%  100% 100%  100%  100%  73% E5 3 539 494 447 400 350 301 251 202 68% 92% 100%  100% 100%  100%  99% 76%

(66) TABLE-US-00004 TABLE 3 Test conditions and results of the reduction of NO.sub.2:NO in the ratio 3:1 (tests TP2 and TP4) at different actually measured temperatures. For measurements 2 to 7, the depletion of NO.sub.x at the catalyst device outlet as a percentage of the initial amount used and the measured values for N.sub.2O at the catalyst device outlet in ppm are indicated. Test Temperature [° C.] NO.sub.x [%] Quantity of N.sub.2O [ppm] Cat. TP 2 3 4 5 6 7 2 3 4 5 6 7 2 3 4 5 6 7 V1 2 489 441 392 343 293 244 97% 95% 89% 80% 65% 62% 12 27 56 83 98 56 E1 4 495 448 400 350 301 251 98% 98% 95% 92% 89% 83% 3 11 26 39 48 48 V2 2 496 447 399 349 300 250 97% 99% 97% 83% 65% 53% 14 7 13 50 96 78 E2 2 490 442 394 344 295 246 96% 97% 95% 90% 78% 62% 14 17 26 38 69 76 V3 2 490 442 393 343 294 245 97% 98% 98% 85% 68% 65% 14 9 10 48 97 72 V4 2 489 441 392 343 293 244 97% 98% 98% 83% 66% 65% 14 9 10 53 100 60 E3 4 495 448 400 351 301 251 96% 99% 99% 99%  8% 84% 15 7 6 4 9 37 E4 4 495 448 400 351 301 251 96% 99% 99% 99%  8% 84% 14 7 6 4 10 37 E5 4 495 448 400 351 301 261 94% 98% 99% 99%  5% 76% 19 11 7 4 8 22

(67) TABLE-US-00005 TABLE 4 Conditions and results of the reduction of NO with different catalyst devices after aging (tests TP1) at different actually measured temperatures 1 to 8. The depletion of NO.sub.x at the catalyst device outlet is indicated as a percentage of the initial amount used. Test Temperature [° C.] NO.sub.x No. TP 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 E2 1 543 495 447 398 348 299 249 199 70% 93% 97% 98% 97% 93% 71% 28% V2 1 542 496 448 400 350 300 250 201 68% 93% 99% 100%  100%  97% 77% 31%

(68) TABLE-US-00006 TABLE 5 Test conditions and results of the reduction of NO.sub.2:NO in the ratio 3:1 (test TP2) with different catalyst devices after aging at different actually measured temperatures. For measurements 2 to 7, the depletion of NO.sub.x at the catalyst device outlet as a percentage of the initial amount used and the measured values for N.sub.2O in ppm at the catalyst device outlet are indicated. Test Temperature [° C.] NO.sub.x [%] Quantity of N.sub.2O [ppm] No. TP 2 3 4 5 6 7 2 3 4 5 6 7 2 3 4 5 6 7 E2 2 495 447 398 348 298 249 92% 95% 95% 92% 77% 60% 28 20 19 28 60 74 V2 2 495 447 398 349 300 250 92% 96% 97% 81% 61% 56% 34 20 16 42 83 76