Catalyst Ceramic Candle Filter for Combined Particulate Removal and 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 cornprising: —a ceramic candle filter substrate and—a coating 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 0.03 to 0.27, —wherein the mass ratio is calculated based on the mass of vanadium metal and titanium metal, and—wherein the catalyst comprises from about 1 to about 10% by weight of the catalytically active material, and—wherein the catalytic metal oxide is adsorbed onto the surface of the oxidic metal carrier.

Claims

1. A catalyst for use in the selective catalytic reduction (SCR) of nitrogen oxides comprising a ceramic candle filter substrate and a coating 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 0.03 to 0.27, wherein the mass ratio is calculated based on the mass of vanadium metal and titanium metal, and wherein the catalyst comprises from 1 to 20% by weight of the catalytically active material, and wherein the catalytic metal oxide is adsorbed onto the surface of the oxidic metal carrier.

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

3. The catalyst according to claim 2, wherein the catalytic metal oxide of the coating which comprises an oxide of vanadium comprises in addition an oxide of tungsten and/or molybdenum and/or antimony.

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

5. The catalyst according to claim 4, wherein the oxidic metal carrier of the coating which comprises an oxide of titanium further comprises an oxide of aluminum, cerium, zirconium or a mixture or mixed oxide comprising at least one oxide of aluminum, cerium, or zirconium.

6. 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 1 and 1000 nm.

7. The catalyst according to claim 1, wherein the mass ratio vanadium/titanium is 0.03 to 0.27, and wherein the mass ratio is calculated based on the mass of vanadium metal and titanium metal.

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

9. A process for preparing a catalyst according to claim 1, comprising the steps of a) providing a ceramic candle filter substrate b) providing an aqueous impregnation liquid comprising one or more catalyst metal precursor compounds comprising a vanadium compound dispersed on particles of an oxidic metal carrier comprising titanium oxide; wherein the impregnation liquid is prepared by the steps of adding the one or more catalyst metal precursor compounds and the oxidic metal carrier to water and continuously adding an acid to the thus obtained mixture to maintain the pH of the mixture at a value where the surface charge of the one or more catalyst precursor metal compound is negative and the Zeta potential of the oxidic metal carrier is positive; adsorbing the one or more catalyst metal precursor compound onto the surface of the oxidic metal carrier; and optionally adding the dispersing agent to the thus prepared mixture in an amount to obtain a pH value above 7 of the thus prepared washcoat slurry; iv) optionally milling the mixture; c) impregnating the ceramic candle filter substrate with the resulting impregnation liquid; and d) drying and thermally activating the impregnated ceramic candle filter substrate at a temperature 150 to 600° C. to convert the one or more metal precursor compounds to their catalytically active form.

10. The process according to claim 9, wherein the vanadium compound is ammonium metavanadate.

11. The process according to claim 9, wherein the aqueous impregnation liquid comprises a primary amine soluble in that liquid.

12. The process according to claim 11, wherein the primary amine is mono-ethyl amine.

13. 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

[0080] a) Preparation of an ammonium vanadate/titania containing impregnation liquid

[0081] 1. 160 kg of demineralized water is mixed with 70.4 kg TiO.sub.2.

[0082] 2. 14.5 kg of ammonia metavanadate (AMV) is added under continuous stirring. The NH.sub.4VO.sub.3/TiO.sub.2-ratio is 0.23.

[0083] 3. The pH is monitored and increases continuously.

[0084] 4. The pH is adjusted with concentrated nitric acid in the interval of 4.0-4.5.

[0085] 5. After a few hours the pH of the liquid remains constant and the slurry is left under constant stirring for at least 5-10 hours. However, the pH needs adjustment every 3 hours.

[0086] The resulting slurry has a yellow color and 5.58 kg ethylamine (68% in water) is added (or until pH ˜9.2-9.5).

[0087] The slurry becomes pale yellow and is subsequently milled down to a particle size of 90-110 nm (D50) where it obtains liquid behavior.

[0088] 6. The liquid is diluted with 575 L of a 0,12% Ethylamine solution to a dry matter of approximately 9 wt %.

[0089] 7. Optionally adding a rheology modifier and/or surface tension modifier to alter liquid properties

[0090] Suitable rheology modifiers are non-ionic or ionic dispersion agents or thickeners. The dispersion agents can be selected from polymers and polyelectrolytes with molecular weights from 2,000 up to 100,000. Typical functional groups for these dispersants include hydroxyl (—OH), carboxyl (—COOH), sulfonate (—SO.sub.3H), sulfate (—OSO.sub.3—), ammonium (—NH.sub.4+), amino (—NH.sub.2), imino (—NH—) and polyoxyethylene (—CH.sub.2CH.sub.2O—) groups.

[0091] Suitable surfactants can also be non-ionic or ionic. Anionic surfactants are, for instance, sulfates, sulfonates, carboxylates and phosphate esters. Cationic surfactants are primary, secondary, or tertiary amines, depending on the pH value, and quaternary organic ammonium salts. Non-ionic surfactants can be chosen from: [0092] Ethoxylates:fatty alcohol ethoxylates, alkylphenol ethoxylates (APEs), fatty acid ethoxylates, ethoxylated fatty esters and oils, ethoxylated amines and/or fatty acid amides, terminally blocked ethoxylates [0093] Fatty acid esters of polyhydroxy compounds:fatty acid esters of glycerol, fatty acid esters of sorbitol, fatty acid esters of sucrose, alkyl polyglucosides, [0094] Amine oxides [0095] Sulfoxides [0096] Phosphine oxides It is obvious from the above list that some substances listed above can act both as a rheology modifier and as a surface tension modifier. This is well known to the skilled person.

[0097] b) A ceramic candle filter substrate is impregnated with the aqueous impregnation liquid. The V/Ti ratio of the catalyst thus obtained is 0.17.

EXAMPLE 2

[0098] The method of Example 1 was repeated with the exception that the catalyst obtained has a V/Ti ratio of 0.12

EXAMPLE 3

[0099] The method of Example 1 was repeated with the exception that the catalyst obtained has a V/Ti ratio of 0.21

EXAMPLE 4

[0100] The method of Example 1 was repeated with the exception that the catalyst obtained has a V/Ti ratio of 0.24

EXAMPLE 5

[0101] The method of Example 1 was repeated with the exception that the catalyst obtained has a V/Ti ratio of 0.27

[0102] Tests

[0103] A part of each of the aqueous slurries obtained according to Examples 2 to 5 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 Table 1 below and shown in FIG. 1.

TABLE-US-00001 TABLE 1 NOx conversion of Examples 2 to 5 at various temperatures X NO [%] T [° C.] Example 2 Example 3 Example 4 Example 5 350 57 69 75 74 325 50 63 66 67 300 34 55 56 57 280 26 48 47 49 250 16 28 26 28 230 10 20 18 19 220 8 16 14 15 210 6 13 11 12 200 5 10 9 9 190 4 8 6 7 180 3 6 4 5 170 2 4 3 3 160 2 3 2 2 X NO [%] = NOx conversion in per cent

[0104] As is apparent from FIG. 1, the powders corresponding to the catalysts of Examples 2 to 5 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.

[0105] 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).

[0106] Accordingly, the present invention allows to gain an optimal NO conversion while restricting the unwanted SO.sub.2 oxidation to a minimum.

[0107] Particle Size Distribution

[0108] The particles obtained in Examples 2 to 5 were beadmilled for 8.75 h. Afterwards, the particles size (D50) was determined using laser diffraction analysis.

[0109] The results are shown in FIG. 2