Method of Preparation of a Monolithic Catalyst for Selective Catalytic Reduction of Nitrogen Oxides

Abstract

Method of preparing a monolithic SCR catalyst with a plurality of gas flow channels comprising the steps of (a) providing a monolithic shaped substrate with a plurality of parallel gas flow channels; (b) coating the substrate with a washcoat slurry comprising titania; (c) drying and calcining the washcoat slurry; (d) impregnating the dried and calcined washcoat with an 10 aqueous impregnation solution comprising a precursor of a vanadium oxide; (e) drying the thus coated and impregnated washcoat at a drying rate of 5 mm/min or less along flow direction through the gas flow channels; and 15 (f) activating the dried, coated and impregnated washcoat by calcining.

Claims

1. A method of preparing a monolithic SCR catalyst with a plurality of gas flow channels comprising the steps of (a) providing a monolithic shaped substrate with a plurality of parallel gas flow channels; (b) coating the substrate with a washcoat slurry comprising titania; (c) drying and calcining the washcoated substrate; (d) impregnating the dried and calcined washcoated substrate with an aqueous impregnation solution comprising a precursor of a vanadium oxide; (e) drying the thus coated and impregnated washcoated substrate at a drying rate of 5 mm/min or less; and (f) activating the dried washcoated and impregnated substrate by calcining.

2. The method of claim 1, wherein the drying rate is controlled by controlling a drying air flow rate to 0-3 m/s and a drying air temperature of below 70? C.

3. The method of claim 1, wherein the drying of the washcoated and impregnated substrate is performed by means of micro wave or long wave heating.

4. The method of claim 1, wherein the drying of the washcoated and impregnated substrate is performed by placing the substrate in a position horizontally relative to gravity.

5. The method of claim 1, wherein the aqueous impregnation solution further comprises a precursor compound a tungsten oxide.

6. The method of claim 1, wherein the monolithic shaped substrate is composed of a number of corrugated sheets stacked upon each other.

7. The method of claim 6, wherein each of the corrugated sheets are provided with a flat sheet prior to be stacked.

8. The method of any claim 1, wherein the corrugated shaped substrate is formed by rolling up a single corrugated sheet.

9. The method of claim 8, wherein the single corrugated sheet is provided with a flat sheet prior to rolling up.

10. The method of claim 6, wherein the corrugated sheet(s) comprise(s) fiberglass.

11. The method of claim 1, wherein the monolithic shaped substrate is obtained by extrusion of ceramic material.

12. The method of claim 1, wherein the activated substrate of step (d) comprises vanadium pentoxide.

13. The method of claim 12, wherein the activated substrate further comprises tungsten trioxide.

14. The method of claim 1, wherein the monolithic SCR catalyst is in form of a wall flow filter.

15. The method of claim 2, wherein the drying air rate is 0 m/s and the drying is performed at room temperature.

Description

EXAMPLES

Example 1

[0050] A honeycomb structured substrate was washcoated with a titania slurry to a wash coat layer thickness of 0.3 to 0.5 mm. The washcoated substrate was dried and calcined at 550? C. The dried and calcined substrate was subsequently impregnated with an aqueous impregnation solution containing 1.95 wt % ammonium meta-vanadate and 9.66 wt % ammonium meta-tungstate. The impregnated substrate was then dried with warm air at 50? C. and an air flow rate of 2 m/sec resulting in a drying rate of 0.8-1.2 mm/min. After drying the impregnated substrate was calcined at 450? C. for 2 hours. The distribution profile of vanadium and tungsten over wall thickness in the calcined substrate is shown in FIG. 3. As seen in FIG. 3, the slow dried substrate has almost no concentration gradients in the distribution of tungsten across the thickness of the washcoat, and the concentration gradient of vanadium is very much reduced compared to the gradients in the fast dried substrate prepared in the following comparison example (see FIG. 2)

Comparison Example

[0051] A washcoated and impregnated honeycomb structured substrate was prepared as in Example 1. The impregnated substrate was dried with warm air at 250? C. and an air flow rate of 2 m/sec resulting in a drying rate of 6-8 mm/min and calcined at 450? C. for 2 hours.

Example 2

[0052] The SCR activity of the honeycomb catalysts prepared in Example 1 and in the Comparison Example was tested at temperatures between 200 and 550? C. at a NO/NH.sub.3 molar ratio of 1.2. The test results are shown in FIG. 1. As apparent from FIG. 1, the honeycomb catalyst dried at a rate of 0.8-1.2 mm/min has an improved SCR activity at temperatures above 350? C. compared to the catalyst dried at a higher drying rate of 6-8 mm/min.