SCR catalyst

Abstract

The invention relates to a catalyst comprising at least two catalytically active layers, A and B, wherein A contains a carrier oxide and components A1 and A2, and B contains a carrier oxide and components B1, B2, and B3, wherein A1, A2, and B1 to B3 are defined as disclosed in claim 1. The proportion of component A1 in layer A is thereby greater than the proportion of component B1 in layer B, wherein the proportion of layer A with respect to the total weight of layers A and B, is greater than the proportion of layer B. The invention further relates to a method for reducing nitrogen oxides in exhaust gases of lean-burn internal combustion engines and to an exhaust gas cleaning system.

Claims

1. A catalyst for the selective reduction of nitrogen oxides comprising two catalytically active layers A and B, wherein A contains a carrier oxide and the components A1 and A2, and B contains a carrier oxide and the components B1, B2, and B3, wherein A1 and B1 stand for at least one oxide of vanadium, A2 and B2 for at least one oxide of tungsten, and B3 for at least one oxide of silicon, characterized in that the proportion of component A1 in layer A in wt % with respect to the total weight of layer A is greater than the proportion of component B1 in layer B in wt % with respect to the total weight of layer B, and the proportion of layer A in wt % with respect to the total weight of layers A and B is greater than the proportion of layer B, and wherein layers A and B are applied to a catalytically inert supporting body made from ceramic or metallic material having a first end a, a second end b, and a length L, which extends between the ends a and b, and such that layer B covers over layer A such that layer B is exposed to exhaust gas before layer A.

2. The catalyst according to claim 1, characterized in that layer A, in addition to components A1 and A2, contains a component A3, wherein A3 stands for at least one oxide of silicon, and wherein the proportion of component A3 in layer A in wt % with respect to the total weight of layer A is smaller than the proportion of component B3 in layer B in wt % with respect to the total weight of layer B.

3. The catalyst according to claim 1, characterized in that it comprises at least two catalytically active layers A and B, wherein A contains a carrier oxide, vanadium pentoxide as component A1, and tungsten trioxide as component A2, and B contains a carrier oxide, vanadium pentoxide as component B1, tungsten trioxide as component B2, and silicon dioxide as component B3, characterized in that the proportion of vanadium pentoxide in layer A in wt % with respect to the total weight of layer A is greater than the proportion of vanadium pentoxide in layer B in wt % with respect to the total weight of layer B, and the proportion of layer A in wt % with respect to the total weight of layers A and B is greater than the proportion of layer B.

4. The catalyst according to claim 1, characterized in that the proportion of component A2 in layer A and component B2 in layer B in wt % respectively with respect to the total weight of layer A or B is equal, or the proportion of component A2 in layer A in wt % with respect to the total weight of layer A is smaller than the proportion of component B2 in layer B in wt % with respect to the total weight of layer B.

5. The catalyst according to claim 1, characterized in that the proportion of component A1 calculated as vanadium pentoxide amounts to 1.5 to 5 wt % with respect to the total weight of layer A.

6. The catalyst according to claim 1, characterized in that the proportion of component B1 calculated as vanadium pentoxide amounts to 1 to 4 wt % with respect to the total weight of layer B.

7. The catalyst according to claim 1, characterized in that the proportion of component A2 with respect to the total weight of layer A and the proportion of component B2 with respect to the total weight of layer B are equal and, calculated as tungsten trioxide, amounts to 3 to 12 wt %.

8. The catalyst according to claim 1, characterized in that the proportion of component A2 with respect to the total weight of layer A is smaller than the proportion of component B2 with respect to the total weight of layer B and, calculated as tungsten trioxide, amounts to 3 to 5.5 wt %, wherein the proportion of component B2 with respect to the total weight of layer B amounts to 4.5 to 12 wt %.

9. The catalyst according to claim 1, characterized in that the proportion of component B3 with respect to the total weight of layer B and calculated as silicon dioxide amounts to 3 to 12 wt %.

10. The catalyst according to claim 1, characterized in that the proportion of component A3 with respect to the total weight of layer A and calculated as silicon dioxide amounts to 0 to 5 wt %.

11. The catalyst according to claim 1, characterized in that the proportion of component A1 calculated as vanadium pentoxide amounts to 1.5 to 5 wt % with respect to the total weight of layer A, and the proportion of component B1 calculated as vanadium pentoxide amounts to 1 to 4 wt % with respect to the total weight of layer B, and the proportion of component A2 with respect to the total weight of layer A and the proportion of component B2 with respect to the total weight of layer B are equal and, calculated as tungsten trioxide, amount to 3 to 12 wt %, or the proportion of component A2 with respect to the total weight of layer A is smaller than the proportion of component B2 with respect to the total weight of layer B and, calculated as tungsten trioxide, amounts to 3 to 5.5 wt %, wherein the proportion of component B2 with respect to the total weight of layer B amounts to 4.5 to 12 wt %, and the proportion of component B3 with respect to the total weight of layer B and calculated as silicon dioxide amounts to 3 to 12 wt %, wherein component A3 is not present, or its proportion with respect to the total weight of layer A and calculated as silicon dioxide amounts to 1 to 5 wt %.

12. The catalyst according to claim 1, characterized in that the proportion of component A1 calculated as vanadium pentoxide amounts to 2 to 4 wt % with respect to the total weight of layer A, and the proportion of component B1 calculated as vanadium pentoxide amounts to 1.5 to 3.5 wt % with respect to the total weight of layer B, and the proportion of component A2 with respect to the total weight of layer A and the proportion of component B2 with respect to the total weight of layer B are equal and, calculated as tungsten trioxide, amount to 4.5 to 10 wt %, or the proportion of component A2 with respect to the total weight of layer A is smaller than the proportion of component B2 with respect to the total weight of layer B and, calculated as tungsten trioxide, amounts to 4.5 to 5 wt %, wherein the proportion of component B2, with respect to the total weight of layer B, amounts to 5 to 10 wt %, and the proportion of component B3 with respect to the total weight of layer B and calculated as silicon dioxide amounts to 3.5 to 10 wt %, wherein component A3 is not present, or its proportion with respect to the total weight of layer A and calculated as silicon dioxide amounts to 1 to 5 wt %.

13. The catalyst according to claim 1, characterized in that layer A further comprises component A4, or layer B further comprises component B4, or each of layers A and B further comprise A4 and B4, respectively, wherein A4 stands for one or more metal oxides which are selected from the series consisting of oxides of copper, iron, manganese, molybdenum, antimony, niobium, silver, palladium, platinum, and rare earth elements, and B4, independently of A4, stands for one or more metal oxides which are selected from the series consisting of oxides of copper, iron, manganese, molybdenum, antimony, niobium, silver, and rare earth elements.

14. The catalyst according to claim 13, characterized in that the proportion of component A4 with respect to the total weight of layer A amounts to 0.1 to 15 wt %, wherein, in the case of silver, platinum and palladium, the proportion is calculated as metal in each case, and in the case of the remaining components, the proportion is calculated as oxides in each case, namely, as CuO, Fe.sub.2O.sub.3, MnO, MoO.sub.3, Sb.sub.2O.sub.5, Nb.sub.2O.sub.5, CeO.sub.2, or Er.sub.2O.sub.3.

15. The catalyst according to claim 1, characterized in that the carrier oxide in layer A and/or B contains titanium dioxide, zirconium dioxide, aluminum oxide, or mixtures thereof.

16. The catalyst according to claim 1, characterized in that the carrier oxide in layer A and B is titanium dioxide.

17. The catalyst according to claim 1, characterized in that the catalytically inert supporting body is a flow honeycomb body or a wall flow filter.

18. The catalyst according to claim 1, wherein the catalytically inert supporting body is a wall flow filter.

19. The catalyst according to claim 1, wherein the catalytically inert supporting body is a flow honeycomb body or a wall flow filter, and layer A is applied directly to the flow honeycomb body or the wall flow filter along its entire length, and layer B is applied to layer A and completely covers it on the exhaust gas side.

20. A method for the reduction of nitrogen oxides in exhaust gases of lean-burn internal combustion engines comprising the method steps of adding a reducing agent to the exhaust gas containing nitrogen oxides, and passing the resulting mixture of exhaust gas containing nitrogen oxides and reducing agent over a catalyst according to claim 1, wherein the catalyst is arranged such that the mixture of exhaust gas containing nitrogen oxides and reducing agent comes into contact with layer B first.

21. An exhaust gas cleaning system for the reduction of nitrogen oxides in exhaust gases of lean-burn internal combustion engines which, in the direction of flow of the exhaust gas, comprises an oxidation catalyst, a diesel particle filter, and a catalyst for the selective reduction of nitrogen oxides (SCR catalyst) according to claim 1, wherein the SCR catalyst is arranged such that the exhaust gas containing nitrogen oxides comes into contact with layer B first.

22. A method for the reduction of nitrogen oxides in exhaust gases of lean-burn internal combustion engines, characterized in that the exhaust gas is channeled through an exhaust gas cleaning system according to claim 21.

23. An exhaust gas cleaning system for the reduction of nitrogen oxides in exhaust gases of lean-burn internal combustion engines which, in the direction of flow of the exhaust gas, comprises an oxidation catalyst and a diesel particle filter on which an SCR catalyst according to claim 1 is present as a coating, wherein the SCR catalyst coating is arranged such that the exhaust gas containing nitrogen oxides comes into contact with layer B first.

24. A method for the reduction of nitrogen oxides in exhaust gases of lean-burn internal combustion engines, characterized in that the exhaust gas is channeled through an exhaust gas cleaning system according to claim 23.

25. A catalyst for the selective reduction of nitrogen oxides comprising two catalytically active layers A and B, wherein A contains a carrier oxide and the components A1 and A2, and B contains a carrier oxide and the components B1, B2, and B3, wherein A1 and B1 stand for at least one oxide of vanadium, A2 and B2 for at least one oxide of tungsten, and B3 for at least one oxide of silicon, characterized in that the proportion of component A1 in layer A in wt % with respect to the total weight of layer A is greater than the proportion of component B1 in layer B in wt % with respect to the total weight of layer B, and the proportion of layer A in wt % with respect to the total weight of layers A and B is greater than the proportion of layer B, and wherein layers A and B are applied to a catalytically inert supporting body made from ceramic or metallic material having a first end a, a second end b, and a length L, which extends between the ends a and b, and characterized in that layer A is applied directly to the inert supporting body along its entire length, and layer B is applied to layer A and completely covers it on the exhaust gas side.

26. The catalyst according to claim 25, characterized in that layer A, in addition to components A1 and A2, contains a component A3, wherein A3 stands for at least one oxide of silicon, and wherein the proportion of component A3 in layer A in wt % with respect to the total weight of layer A is smaller than the proportion of component B3 in layer B in wt % with respect to the total weight of layer B.

27. A catalyst for the selective reduction of nitrogen oxides comprising two catalytically active layers A and B, wherein A contains a carrier oxide and the components A1 and A2, and B contains a carrier oxide and the components B1, B2, and B3, wherein A1 and B1 stand for at least one oxide of vanadium, A2 and B2 for at least one oxide of tungsten, and B3 for at least one oxide of silicon, characterized in that the proportion of component A1 in layer A in wt % with respect to the total weight of layer A is greater than the proportion of component B1 in layer B in wt % with respect to the total weight of layer B, and the proportion of layer A in wt % with respect to the total weight of layers A and B is greater than the proportion of layer B, and wherein layer A is formed as an extruded carrier to which layer B is applied in the form of a coating.

Description

EXAMPLES 1 TO 8

(1) Production of Catalyst Powders:

(2) A) Catalyst Powder A of the Composition

(3) 87.8 wt % TiO.sub.2 as carrier, 2.2 wt % V.sub.2O.sub.5, and 10 wt % WO.sub.3; was produced as follows: Commercially available titanium dioxide (anatase) in powder form was placed in a container. Then, an aqueous solution of ammonium metatungstate and ammonium metavanadate dissolved in oxalic acid was slowly added in the appropriate quantity under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(4) B) Catalyst Powder B of the Composition

(5) 87.8 wt % TiO.sub.2 as carrier, 2.2 wt % V.sub.2O.sub.5, and 10 wt % WO.sub.3; was produced as follows: Commercially available titanium dioxide (anatase) in powder form doped with 10 wt % tungsten oxide was placed in a container. Then, ammonium metavanadate dissolved in oxalic acid was slowly added in the appropriate quantity under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(6) C) Catalyst Powder C of the Composition

(7) 79.4 wt % TiO.sub.2 as carrier, 1.8 wt % V.sub.2O.sub.5, 10 wt % WO.sub.3, and 8.8 wt % SiO.sub.2 was produced as follows:

(8) Commercially available titanium dioxide (anatase) in powder form doped with 10 wt % SiO.sub.2 was placed in a container. Then, an aqueous solution of ammonium metatungstate and ammonium metavanadate dissolved in oxalic acid was slowly added in the appropriate quantity under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(9) D) Catalyst Powder D of the Composition

(10) 78.5 wt % TiO.sub.2 as carrier, 1.8 wt % V.sub.2O.sub.5, 10 wt % WO.sub.3, and 9.7 wt % SiO.sub.2 was produced as follows:

(11) Commercially available titanium dioxide (anatase) in powder form doped with 10 wt % SiO.sub.2 and 9 wt % tungsten oxide was placed in a container. Then, ammonium metavanadate dissolved in oxalic acid was slowly added in the appropriate quantity under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

EXAMPLE 1

(12) a) To produce a catalyst according to the invention, catalyst powder A was slurried in water and coated in the usual way along the entire length of a commercial flow substrate. It was then dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load amounted to 210 g/l

(13) b) Then, catalyst powder C was slurried in water and coated in the usual way along the entire length of the flow substrate obtained in accordance with the above step a) and coated with catalyst powder A. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer amounted to 70 g/l

(14) The catalyst according to the invention thus obtained is referred to below as K1.

EXAMPLE 2

(15) Catalyst powder D was slurried in water and coated in the usual way along the entire length of the flow substrate obtained in accordance with Example 1a) and coated with catalyst powder A. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer amounted to 70 g/l

(16) The catalyst according to the invention thus obtained is referred to below as K2.

EXAMPLE 3

(17) a) To produce a catalyst according to the invention, catalyst powder B was slurried in water and coated in the usual way along the entire length of a commercial flow substrate. It was then dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load amounted to 210 g/l.

(18) b) Then, catalyst powder C was slurried in water and coated in the usual way along the entire length of the flow substrate obtained in accordance with the above step a) and coated with catalyst powder B. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer amounted to 70 g/l.

(19) The catalyst according to the invention thus obtained is referred to below as K3.

EXAMPLE 4

(20) Catalyst powder D was slurried in water and coated in the usual way along the entire length of a commercial flow substrate obtained in accordance with Example 3a) and coated with catalyst powder B. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer amounted to 70 g/l.

(21) The catalyst according to the invention thus obtained is referred to below as K4.

EXAMPLE 5

(22) a) To produce a catalyst according to the invention, catalyst powder A was slurried in water and coated in the usual way along 75% of the total length of a commercial flow substrate starting from one side. It was then dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load amounted to 280 g/l.

(23) b) Then, catalyst powder C was slurried in water and coated in the usual way along the remaining 25% of the total length of a commercial flow substrate obtained in accordance with the above step a) and coated with catalyst powder A. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer amounted to 280 g/l.

(24) The catalyst according to the invention thus obtained is referred to below as K5.

EXAMPLE 6

(25) Catalyst powder D was slurried in water and coated in the usual way along the remaining 25% of the total length of a commercial flow substrate obtained in accordance with Example 5a) and coated along 75% of its length with catalyst powder A. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer amounted to 280 g/l.

(26) The catalyst according to the invention thus obtained is referred to below as K6.

EXAMPLE 7

(27) a) To produce a catalyst according to the invention, catalyst powder B was slurried in water and coated in the usual way along 75% of the total length of a commercial flow substrate starting from one side. It was then dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load amounted to 280 g/l.

(28) b) Then, catalyst powder C was slurried in water and coated in the usual way along the remaining 25% of the total length of a commercial flow substrate obtained in accordance with the above step a) and coated with catalyst powder A. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer amounted to 280 g/l.

(29) The catalyst according to the invention thus obtained is referred to below as K7.

EXAMPLE 8

(30) Catalyst powder D was slurried in water and coated in the usual way along the remaining 25% of the total length of a commercial flow substrate obtained in accordance with Example 7a) and coated along 75% of its length with catalyst powder B. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer amounted to 280 g/l.

(31) The catalyst according to the invention thus obtained is referred to below as K8.

COMPARATIVE EXAMPLE 1

(32) By analogy with example 1a, a commercial flow substrate is coated along its entire length with catalyst powder A in a quantity of 280 g/l.

(33) The catalyst obtained is referred to below as VK1.

COMPARATIVE EXAMPLE 2

(34) By analogy with example 3a, a commercial flow substrate is coated along its entire length with catalyst powder B in a quantity of 280 g/l.

(35) The catalyst obtained is referred to below as VK2.

COMPARATIVE EXAMPLE 3

(36) To produce a comparative catalyst, catalyst powder C was slurried in water and coated in the usual way along the entire length of a commercial flow substrate. It was then dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load amounted to 280 g/l. The catalyst thus obtained is referred to below as VK3.

COMPARATIVE EXAMPLE 4

(37) To produce a comparative catalyst, catalyst powder D was slurried in water and coated in the usual way along the entire length of a commercial flow substrate. It was then dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load amounted to 280 g/l. The catalyst thus obtained is referred to below as VK4.

(38) Prior to the catalysts being tested in accordance with examples 1 to 8 and comparative examples 1 to 4, they were first hydrothermally aged for 100 hours at 580? C. in a gas atmosphere (10% O.sub.2, 10% H.sub.2O, remainder N.sub.2).

(39) In the case of the layer catalysts from examples 1 to 4, in order to determine the NO-rates of the aged catalyst, drill cores with L=3 and D=1% were tested in a quartz glass reactor between 150 and 540? C. under stationary conditions (GHSV=30000 1/h, synthesis gas composition: 500 ppm NO, 450 ppm NH.sub.3 (?=xNH.sub.3/xNOx=0.9; xNOx=xNO+xNO.sub.2, where x denotes concentration in each case), 5% 0.sub.2, 5% H.sub.2O, remainder N.sub.2.

(40) The NO rates of the zoned catalysts from examples 5 to 8 were determined analogously, wherein drill cores were used, which exhibit the two zones in the same length ratio as in the originally coated substrate.

(41) The following NO rates in %, standardized to a, were obtained:

(42) TABLE-US-00001 Temperature [? C.] 150 175 200 250 300 350 400 450 500 550 K1 7 18 42 97 100 100 100 99 93 73 K2 6 15 34 90 100 100 100 99 92 64 K3 9 21 44 95 99 100 100 99 90 63 K4 8 19 41 92 98 99 99 97 86 50 K5 7 17 37 90 100 100 100 100 90 64 K6 7 17 37 89 100 100 100 99 89 61 K7 8 18 39 91 100 100 100 99 85 54 K8 7 19 40 92 100 100 100 99 85 52 VK1 8 21 43 92 99 100 100 96 73 18 VK2 8 21 44 95 100 100 100 95 66 ?3 VK3 4 11 27 78 97 99 99 98 95 84 VK4 4 9 24 70 93 96 96 96 93 82

EXAMPLES 9 TO 20

(43) To produce additional catalysts according to the invention, the following catalyst powders were used:

(44) E) Catalyst Powder E of the Composition

(45) 87.8 wt % TiO.sub.2 as carrier, 3.5 wt % V.sub.2O.sub.5, 4.5 wt % WO.sub.3, and 4.6 wt % SiO.sub.2 was produced as follows:

(46) Commercially available titanium dioxide (anatase) in powder form doped with 5 wt % SiO.sub.2 was placed in a container. Then, an aqueous solution of ammonium metatungstate and ammonium metavanadate dissolved in oxalic acid was slowly added in the appropriate quantity under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(47) F) Catalyst Powder F of the Composition

(48) 92.0 wt % TiO.sub.2 as carrier, 3.0 wt % V.sub.2O.sub.5, and 5 wt % WO.sub.3 was produced as follows: Commercially available titanium dioxide (anatase) in powder form was placed in a container. Then, an aqueous solution of ammonium metatungstate and ammonium metavanadate dissolved in oxalic acid was slowly added in the appropriate quantity under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(49) G) Catalyst Powder G of the Composition

(50) 92.5 wt % TiO.sub.2 as carrier, 2.5 wt % V.sub.2O.sub.5, and 5 wt % WO.sub.3 was produced as follows: Commercially available titanium dioxide (anatase) in powder form was placed in a container. Then, an aqueous solution of ammonium metatungstate and ammonium metavanadate dissolved in oxalic acid was slowly added in the appropriate quantity under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(51) H) Catalyst Powder H of the Composition

(52) 91.5 wt % TiO.sub.2 as carrier, 2.5 wt % V.sub.2O.sub.5, 5 wt % WO.sub.3, and 1 wt % silver was produced as follows:

(53) Commercially available titanium dioxide (anatase) in powder form was placed in a container. Then, aqueous solutions of ammonium metatungstate and silver acetate and ammonium metavanadate dissolved in oxalic acid were slowly added in the appropriate quantities under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(54) I) Catalyst Powder I of the Composition

(55) 92.0 wt % TiO.sub.2 as carrier, 2.5 wt % V.sub.2O.sub.5, 5 wt % WO.sub.3, and 0.5 wt % MnO was produced as follows:

(56) Commercially available titanium dioxide (anatase) in powder form was placed in a container. Then, aqueous solutions of manganese acetate, ammonium metatungstate, and ammonium metavanadate dissolved in oxalic acid were slowly added in the appropriate quantities under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(57) J) Catalyst Powder J of the Composition

(58) 74.9 wt % TiO.sub.2 as carrier, 4.0 wt % V.sub.2O.sub.5, 8.3 wt % WO.sub.3, 9.3 wt % SiO.sub.2, and 3.5 wt % Fe.sub.2O.sub.3was produced as follows:

(59) Commercially available titanium dioxide (anatase) in powder form doped with 5 wt % SiO.sub.2 was placed in a container and intensively mixed with the appropriate quantity of iron vanadate. Then, an aqueous solution of ammonium metatungstate in the appropriate quantity was slowly added under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(60) K) Catalyst Powder K of the Composition

(61) 88.6 wt % TiO.sub.2 as carrier, 1.6 wt % V.sub.2O.sub.5, 5.0 wt % WO.sub.3, 4.3 wt % SiO.sub.2, and 0.5 wt % CuO was produced as follows:

(62) Commercially available titanium dioxide (anatase) in powder form doped with 5 wt % SiO.sub.2 was placed in a container. Then, aqueous solutions of ammonium metatungstate and copper acetate, and ammonium metavanadate dissolved in oxalic acid were slowly added in the appropriate quantities under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(63) L) Catalyst Powder L of the Composition

(64) 87.2 wt % TiO.sub.2 as carrier, 1.6 wt % V.sub.2O.sub.5, 5.0 wt % WO.sub.3, 4.2 wt % SiO.sub.2, and 2.0 wt % Nb.sub.2O.sub.5 was produced as follows:

(65) Commercially available titanium dioxide (anatase) in powder form doped with 5 wt % SiO.sub.2 was placed in a container. Then, aqueous solutions of ammonium metatungstate and ammonium niobium oxalate, and ammonium metavanadate dissolved in oxalic acid were slowly added in the appropriate quantities under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(66) M) Catalyst Powder M of the Composition

(67) 87.2 wt % TiO.sub.2 as carrier, 1.6 wt % V.sub.2O.sub.5, 5.0 wt % WO.sub.3, 4.2 wt % SiO.sub.2, and 2.0 wt % MoO.sub.3 was produced as follows:

(68) Commercially available titanium dioxide (anatase) in powder form doped with 5 wt % SiO.sub.2 was placed in a container. Then, aqueous solutions of ammonium metatungstate and ammonium molybdate, and ammonium metavanadate dissolved in oxalic acid were slowly added in the appropriate quantities under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(69) N) Catalyst Powder N of the Composition

(70) 88.8 wt % TiO.sub.2 as carrier, 1.5 wt % V.sub.2O.sub.5, 5.0 wt % WO.sub.3, and 4.7 wt % SiO.sub.2 was produced as follows:

(71) Commercially available titanium dioxide (anatase) in powder form doped with 5 wt % SiO.sub.2 was placed in a container. Then, an aqueous solution of ammonium metatungstate and ammonium metavanadate dissolved in oxalic acid was slowly added in the appropriate quantity under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(72) O) Catalyst Powder O of the Composition

(73) 75.8 wt % TiO.sub.2 as carrier, 2.1 wt % V.sub.2O.sub.5, 8.4 wt % WO.sub.3, 9.4 wt % SiO.sub.2, and 4.3 wt % CeO.sub.2 was produced as follows:

(74) Commercially available titanium dioxide (anatase) in powder form doped with 10 wt % SiO.sub.2 and 9 wt % WO.sub.3 was placed in a container. Then, aqueous solutions of ammonium metatungstate and cerium acetate, and ammonium metavanadate dissolved in oxalic acid were slowly added in the appropriate quantities under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(75) P) Catalyst Powder P of the Composition

(76) 65.1 wt % TiO.sub.2 as carrier, 3.2 wt % V.sub.2O.sub.5, 7.2 wt % WO.sub.3, 8.0 wt % SiO.sub.2, 2.8 wt % Fe.sub.2O.sub.3, and 13.6 wt % Er.sub.2O.sub.3 was produced as follows:

(77) Commercially available titanium dioxide (anatase) in powder form doped with 10 wt % SiO.sub.2 and 9 wt % WO.sub.3 was placed in a container and mixed intensively with the appropriate quantity of iron erbium vanadate. Then, an aqueous solution of ammonium metatungstate in the appropriate quantity was slowly added under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(78) Q) Catalyst Powder Q of the Composition

(79) 87.7 wt % TiO.sub.2 as carrier, 1.6 wt % V.sub.2O.sub.5, 5.0 wt % WO.sub.3, 4.2 wt % SiO.sub.2, and 1.5 wt % Sb.sub.2O.sub.5 was produced as follows:

(80) Commercially available titanium dioxide (anatase) in powder form doped with 5 wt % SiO.sub.2 was placed in a container. Then, aqueous solutions of ammonium metatungstate and antimony acetate, and ammonium metavanadate dissolved in oxalic acid were slowly added in the appropriate quantities under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(81) Analogously, as described in examples 1 to 4, using catalyst powders E to Q, the layer catalysts according to the invention from examples 9 to 20 were obtained in accordance with the following table.

(82) TABLE-US-00002 Bottom layer Top layer Example (directly on the substrate) (on the bottom layer) 9 120 g/l catalyst powder E 40 g/l catalyst powder N 10 140 g/l catalyst powder E 20 g/l catalyst powder N 11 120 g/l catalyst powder F 40 g/l catalyst powder N 12 120 g/l catalyst powder H 40 g/l catalyst powder N 13 120 g/l catalyst powder E 40 g/l catalyst powder O 14 120 g/l catalyst powder L 40 g/l catalyst powder N 15 120 g/l catalyst powder J 40 g/l catalyst powder D 16 120 g/l catalyst powder E 40 g/l catalyst powder P 17 120 g/l catalyst powder I 40 g/l catalyst powder N 18 120 g/l catalyst powder K 40 g/l catalyst powder N 19 120 g/l catalyst powder M 40 g/l catalyst powder N 20 120 g/l catalyst powder Q 40 g/l catalyst powder N

(83) To produce comparative examples 5 to 12, a quantity of 160 g/l of catalyst powder E, N, F, H, O, P, I, or K was coated along the entire length of a commercially available flow substrate. The catalysts thus obtained are hereafter referred to as follows:

(84) VK5 (containing catalyst powder E),

(85) VK6 (containing catalyst powder N),

(86) VK7 (containing catalyst powder F) and

(87) VK8 (containing catalyst powder H)

(88) VK9 (containing catalyst powder O)

(89) VK10 (containing catalyst powder P)

(90) VK11 (containing catalyst powder I)

(91) VK12 (containing catalyst powder K)

(92) The NO rates of the fresh catalysts in accordance with examples 10, 11, 12, 13, 16, 17, and 18 (referred to below as K10, K11, K12, K13, K16, K17, and K18) and comparative catalysts VK5 to VK12 were determined as described above. The following NO rates in %, standardized to a, were obtained:

(93) TABLE-US-00003 Temperature [? C.] 150 175 200 250 300 350 400 450 500 540 K10 4 10 23 71 97 99 99 98 93 76 K11 4 11 26 79 100 100 100 100 95 75 K12 2 6 15 54 92 98 97 95 86 56 K13 5 14 32 78 89 90 90 89 84 65 K16 7 17 38 89 99 100 100 98 93 69 K17 3 8 20 68 98 100 100 99 95 75 K18 2 3 8 37 84 96 97 98 95 82 VK5 6 16 35 86 99 100 100 98 88 58 VK6 2 3 8 34 81 98 99 99 97 90 VK7 7 16 36 89 99 99 99 97 86 50 VK8 3 7 18 57 90 96 95 90 73 23 VK9 2 6 15 53 91 98 99 98 95 78 VK10 1 4 10 41 83 97 98 97 92 71 VK11 3 7 17 61 95 99 99 98 94 73 VK12 2 4 10 41 85 97 98 98 89 62

(94) R) Catalyst Powder R of the Composition

(95) 77.1 wt % TiO.sub.2 as carrier, 3.61 wt % V.sub.2O.sub.5, 11.17 wt % WO.sub.3, and 8.12 wt % SiO.sub.2 was produced as follows:

(96) A mixture of 11.29 wt % of a pure, commercially available titanium dioxide (anatase) and 81.23 wt % of a commercially available titanium dioxide (anatase) doped with 10 wt % SiO.sub.2 and 9 wt % WO.sub.3 was placed in a container. Then, aqueous solutions of ammonium metatungstate (3.86 wt % calculated as WO.sub.3) and ammonium metavanadate (3.61 wt % calculated as V.sub.2O.sub.5) were slowly added under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

(97) S) Catalyst Powder S of the Composition

(98) 73.02 wt % TiO.sub.2 as carrier, 3.42 wt % V.sub.2O.sub.5, 15.87 wt % WO.sub.3; and 7.69 wt % SiO.sub.2 was produced as follows:

(99) A mixture of 10.70 wt % of a pure, commercially available titanium dioxide (anatase) and 76.94 wt % of a commercially available titanium dioxide (anatase) doped with 10 wt % SiO.sub.2 and 9 wt % WO.sub.3 was placed in a container. Then, aqueous solutions of ammonium metatungstate (8.95 wt % calculated as WO.sub.3) and ammonium metavanadate (3.42 wt % calculated as V.sub.2O.sub.5) were slowly added under constant mixing. The powder thus obtained was dried at 110? C. and then calcined at 600? C. for 6 hours.

EXAMPLE 21

(100) a) To produce a catalyst according to the invention, catalyst powder S was slurried in water and coated in the usual way starting from one side along a length of 1.2 of a commercial flow substrate having a length of 3.0, i.e., 40% of its total length. It was then dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load amounted to 160 g/l.

(101) b) Then, catalyst powder R was slurried in water and coated in the usual way along the remaining 60% of the total length of the flow substrate obtained in accordance with the above step (a) and coated with catalyst powder S. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer likewise amounted to 160 g/l.

(102) The catalyst according to the invention thus obtained is referred to below as K13.

COMPARATIVE EXAMPLE 13

(103) a) To produce a comparative catalyst, catalyst powder R was slurried in water and coated in the usual way starting from one side along a length of 1.2 of a commercial flow substrate having a length of 3.0, i.e., 40% of its total length. It was then dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load amounted to 160 g/l.

(104) b) Then, catalyst powder S was slurried in water and coated in the usual way along the remaining 60% of the total length of the flow substrate obtained in accordance with the above step (a) and coated with catalyst powder R. It was again dried at 110? C. and calcined at 600? C. for 6 hours. The washcoat load of the second layer likewise amounted to 160 g/l.

(105) The catalyst thus obtained is referred to below as VK13.

(106) In VK13, the proportion of V.sub.2O.sub.5 in layer A (the layer produced in step (a)) in wt % with respect to the total weight of layer A is smaller than the proportion of V.sub.2O.sub.5 in layer B (the layer produced in step (b)) in wt % with respect to the total weight of layer B. In this respect, VK13 corresponds to example 2 of US 2013/205743.

(107) The NO rates of fresh catalysts K13 and VK13 were determined as described above. The following NO rates in %, standardized to a, were obtained:

(108) TABLE-US-00004 Temperature [? C.] 150 175 200 250 300 350 400 450 500 540 K13 6 16 38 94 99 99 99 98 90 62 VK13 6 15 35 93 100 100 100 98 85 44