Catalyst for Use in the Selective Catalytic Reduction (SCR) of Nitrogen Oxides

Abstract

The present invention pertains to a catalyst for use in the selective catalytic reduction (SCR) of nitrogen oxides comprising: • a monolithic substrate and • a coating A which comprises an oxidic metal carrier comprising an oxide of titanium and a catalytic metal oxide which comprises an oxide of vanadium wherein the mass ratio vanadium/titanium is 0.07 to 0.26.

Claims

1. A catalyst for use in the selective catalytic reduction (SCR) of nitrogen oxides comprising a monolithic substrate and a coating A which comprises an oxidic metal carrier comprising an oxide of titanium and a catalytic metal oxide which comprises an oxide of vanadium wherein the mass ratio vanadium/titanium is 0.07 to 0.26.

2. The catalyst according to claim 1, wherein the monolithic substrate comprises corrugated sheets of glass fiber.

3. The catalyst according to claim 1, wherein the catalytic metal oxide of coating A which comprises an oxide of vanadium comprises vanadium pentoxide (V.sub.2O.sub.5).

4. The catalyst according to claim 3, wherein the catalytic metal oxide of coating A which comprises an oxide of vanadium comprises in addition an oxide of tungsten and/or molybdenum.

5. The catalyst according to claim 1, wherein the oxidic metal carrier of coating A which comprises an oxide of titanium comprises titanium dioxide.

6. The catalyst according to claim 5, wherein the oxidic metal carrier of coating A which comprises an oxide of titanium comprises in addition an oxide of aluminum, cerium, zirconium or mixtures, mixed oxides or compounds comprising at least one of these oxides.

7. The catalyst according to claim 1, wherein the oxidic metal carrier consists of either single or agglomerated nanoparticles of titanium dioxide with a primary particle size of between 10 and 150 nm.

8. The catalyst according to claim 1, wherein the mass ratio vanadium/titanium is 0.1 to 0.21.

9. The catalyst according to claim 1, which is free of platinum group metals.

10. The catalyst according to claim 1, which comprises an additional coating B directly on the monolithic substrate and below coating A.

11. The catalyst according to claim 10, wherein coating B comprises an oxide of titanium and optionally of aluminum, cerium, zirconium or mixtures, mixed oxides or compounds comprising at least one of these oxides.

12. A process for preparing a catalyst according to claim 1, comprising the steps of a) providing a monolithic substrate b) providing an aqueous washcoat slurry comprising one or more catalyst metal precursor compounds comprising a vanadium compound dispersed on particles of an oxidic metal carrier comprising titanium oxide; c) impregnating the monolithic substrate with the washcoat slurry; and d) drying and thermally activating the impregnated monolithic substrate at a temperature 150 to 600° C. to convert the one or more metal precursor compounds to their catalytically active form.

13. The process according to claim 12, wherein the catalyst metal precursor compound comprising a vanadium compound is ammonium metavanadate.

14. The process according to claim 12, wherein the aqueous washcoat slurry comprises a primary amine soluble in that liquid.

15. The process according to claim 14, wherein the primary amine is mono-ethyl amine.

16. The process according to claim 12, wherein the one or more catalyst metal precursor compounds comprising a vanadium compound dispersed on particles of an oxidic metal carrier comprising titanium oxide have a particle size D50 of 200 to 750 nm.

17. A method for treating the off-gas from automotive and stationary sources and selectively reduce nitrogen oxides contained in said off-gas, wherein the off-gas is passed over a catalyst according to claim 1.

Description

EXAMPLE 1

[0056] a) Preparation of an ammonium vanadate/titania containing washcoat slurry [0057] 1. 2250 g of demineralized water is mixed with 2630 g TiO2. [0058] 2. 381 g of ammonia meta vanadate (AMV) is added under continuous stirring. The NH.sub.4VO.sub.3/TiO.sub.2—ratio is 0.16. [0059] 3. The pH is monitored and increases continuously. [0060] 4. The pH is adjusted with concentrated nitric acid in the interval of 4.0-4.5. [0061] 5. After a few hours the pH of the liquid remains constant and the slurry is left under constant stirring for at least 24 hours. However, the pH needs adjustment every 3 hours. [0062] The resulting liquid has a red color and 100 g ethylamine (70% in water) is added (or until pH˜9.2-9.5). [0063] The liquid becomes ivory white and is subsequently milled down to a particle size of 250-400 nm (D50). [0064] Optionally a 5.6 g of surfactant is added to increase wash-coat uptake on the corrugated substrate

[0065] b) A monolithic substrate consisting of glass fibers is washcoated with the aqueous slurry. The V/Ti ratio of the catalyst thus obtained is 0.12.

EXAMPLE 2

[0066] The method of Example 1 was repeated with the exception that the catalyst obtained has a V/Ti ratio of 0.21

EXAMPLE 3

[0067] The method of Example 1 was repeated with the exception that the catalyst obtained has a V/Ti ratio of 0.24

EXAMPLE 4

[0068] The method of Example 1 was repeated with the exception that the catalyst obtained has a V/Ti ratio of 0.25

EXAMPLE 5

[0069] a) Preparation of an ammonium vanadate/titania containing washcoat slurry [0070] 1. 2250 g of demineralized water is mixed with 2630 g TiO2. [0071] 2. 381 g of ammonia meta vanadate (AMV) is added under continuous stirring. The NH.sub.4VO.sub.3/TiO.sub.2—ratio is 0.16. [0072] 3. The pH is monitored and increases continuously. [0073] 4. The pH is adjusted with concentrated nitric acid in the interval of 4.0-4.5. [0074] 5. After a few hours the pH of the liquid remains constant and the slurry is left under constant stirring for at least 24 hours. However, the pH needs adjustment every 3 hours. [0075] The resulting liquid has a yellow color and 100 g ethylamine (70% in [0076] water) is added (or until pH˜9.2-9.5). [0077] The liquid becomes pale yellow and is subsequently milled down to a particle size of 600-700 nm (D50).

[0078] Optionally a 5.6 g of surfactant is added to increase wash-coat uptake on the corrugated substrate

[0079] b) A monolithic substrate consisting of glass fibers is washcoated with the aqueous slurry. The V/Ti ratio of the catalyst thus obtained is 0.12.

Tests

[0080] A part of each of the aqueous slurries obtained according to Examples 1 to 4 was dried and the powders thus obtained were tested in the ammonia SCR of NO with a gas composition containing 500 ppm NO, 533 ppm NH.sub.3, 10 vol % O.sub.2, 4% H.sub.2O and balance N.sub.2. The results of the tests are summarized in FIG. 1.

[0081] As is apparent from FIG. 1, the powders corresponding to the catalysts of Examples 1 to 4 according to the invention possess very good NO conversion activity at higher temperatures. It is also apparent that increasing the Ti/V ratio above a value of 0.20, in particular above 0.24 does not result in a higher activity, the activity is levelled out.

[0082] On the other hand, it is the general knowledge of a person skilled in the field of selective catalytic reduction of nitrogen oxides that the (unwanted) oxidation of SO.sub.2 increases with an increasing Ti/V ratio (see, for example, Applied Catalysis b: Environmental 19 (1998) 103-117).

[0083] Accordingly, the present invention allows to gain an optimal NO conversion while restricting the unwanted SO.sub.2 oxidation to a minimum.

[0084] FIG. 2 shows the particle distribution of Example 5.

[0085] It has to be noted that the milling can also be performed after step a)1. as described in Examples 1 and 2. This means that the slurry of TiO.sub.2 in demineralized water can be milled prior to the addition of ammonium metavanadate.

[0086] The skilled person knows how to adjust milling conditions in order to obtain a desired particle size. Suitable particle sizes of the one or more catalyst metal precursor compounds comprising a vanadium compound dispersed on particles of an oxidic metal carrier comprising titanium oxide (D50) are 200 to 750 nm, preferably 250 to 600 nm, more preferably 300 to 500 nm. As described above, the milling can take place prior to the addition of ammonium metavanadate, i.e. after step a)1., or after the adjustment of the pH with an amine, i.e. during step a)5. as described above in Examples 1 to 5.