WO1989003366A1

WO1989003366A1 - Vanadium/tungsten catalyst and process

Info

Publication number: WO1989003366A1
Application number: PCT/US1988/003439
Authority: WO
Inventors: Hyun Jong Jung; Roman Edward Lis; Eckhard Robert Becker
Original assignee: Johnson Matthey Inc.
Priority date: 1987-10-06
Filing date: 1988-10-06
Publication date: 1989-04-20

Abstract

A supported vanadium oxides/tungsten oxides catalyst, a process for using the same for the reduction of nitrogen oxide, and a process for preparing the same by impregnating a support with a tungsten solution followed by impregnating with a vanadium solution, calcining to convert tungsten and vanadium to oxides.

Description

VANADIUM/TUNGSTEN CATALYST AND PROCESS

This invention relates to an improved catalyst and a process for the selective catalytic reduction of nitrogen oxides.

BACKGROUND OF THE INVENTION

It is well known to remove nitrogen oxides (NO_χ) from gas exhaust from stationary emission sources such as boilers, internal combustion engines or gas turbines by adding ammonia (NH3) to the exhaust gas, in appropriate proportion to the NO_χ present, and passing the mixture over a catalyst whereby non-polluting nitrogen (N2) and water (H2O) are formed. -A wide variety of catalysts have been proposed for this purpose but there is still considerable room from improvement. For example, metal oxide catalysts containing vanadium oxide and at least one of molybdenum oxide and tungsten oxide have been described as active selective catalytic reduction (SCR) catalysts (U.S. patents 4,071,601 and 4,075,283). These catalysts are prepared from a mixed solution of the metal salts. It has also been proposed that such catalysts have a vanadium/tungsten mole ratio greater than or equal to 1.4. It has been found, however, that a vanadium oxide/tungsten oxide catalyst prepared from mixed solution has an activity which is too low to be effectively useful commercially.

The invention avoids this problem by preparing a vanadium oxide/tungsten oxide catalyst in two steps, (1) by impregnating a suitable support material first with a tungsten solution, followed by (2) impregnating the support with a vanadium solution and calcining. It has been found that the resultant mixed oxide catalyst has an activity significantly higher than a catalyst prepared using a mixed solution containing both tungsten and vanadium. In addition, a vanadium oxide, tungsten oxide catalyst prepared in two steps as described above has a high activity when the vanadium/tungsten mole ratio is less than 1.4.

Accordingly an object of this invention is to provide a vanadium oxide/tungsten oxide catalyst having high activity.

Another object of this invention is to provide a method for preparing a vanadium oxide/- tungsten oxide catalyst.

A further object of the invention is to provide a catalyst exhibiting high activity for selective reduction of nitrogen oxides. Other objects, aspects, as well as the several advantages of the invention, will be apparent to those skilled in the art upon reading the specification and the appended claims.

SUMMARY OF THE INVENTION According to the invention, a supported vanadium oxide/tungsten oxide catalyst having high activity for the reduction of nitrogen oxide is provided which catalyst is preparing by impregnat¬ ing a support with a tungsten solution followed by impregnating with a vanadium solution and calcining to convert tungsten and vanadium to oxides. In another embodiment of the invention, a process for the selective reduction of nitrogen oxides is provided which comprises contacting a gas steam containing nitrogen oxide with ammonia and an oxygen-containing gas under reducing conditions in the presence of a supported vanadium oxide/tungsten oxide catalyst prepared as set forth above.

In accordance with a further embodiment of the invention, a washcoated monolith support is provided which comprises preparing the support by dispersing a support material in water to form a slurry, coating monolith with the slurry, and calcining the washcoated monolith prior to impregnation with a tungsten solution. In one preferred embodiment of the invention, a washcoated monolith support is impregnated first with a tungsten solution, calcined, impregnated with a vanadium solution and calcined to form a vanadium oxide/tungsten oxide catalyst having high activity for the selective reduction of nitrogen oxides.

Novel features of the invention include: (i) Vanadium oxide/tungsten oxide catalysts when prepared by impregnating a support such as titania first with a tungsten solution followed by impregnation with a vanadium solution have higher activity than a catalyst prepared by impregnation with a mixture of vanadium and tungsten compounds This is illustrated in Table I . (ii) Vanadium oxide/tungsten oxide catalysts when prepared by impregnating a support such as titania first with a tungsten solution followed by impregnation with a vanadium solution have higher activity than a catalyst prepared by impregnating first with a vanadium solution followed by impregnation with a tungsten solution (Also illustrated in Table I) . (iii) Vanadium oxide/tungsten oxide catalysts when prepared by impregnating a support such as titania first with a tungsten solution followed by impregnation with a vanadium solution have high activity even when the vanadium/tungsten mole ratio is less than 1.4.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS As indicated hereinbefore the instant vanadium oxide/tungsten oxide catalyst is prepared by impregnating a support with a tungsten solution followed by impregnation with a vanadium solution. Impregnation of the support with the metal solutions is followed by calcination to convert the metal compounds to the oxides.

Suitable vanadium and tungsten compounds that can be used for impregnation include those that are preferably water soluble and are conver¬ tible to the metal oxides upon calcination. Representative examples of suitable vanadium and/or tungsten compounds that can be used include metal oxides, e.g. divanadium pentaoxide, divanadium tetraoxide, tungsten trioxide, tungsten dioxide, ad the like, metal complexes, such as vanadyl oxalate, vanadyl chloride, or a chelate compound with an organic amine or salts, e.g. ammonium metavanadate, ammonium tungstate or acids, e.g. silicotungstic acid, and the like. The most preferred starting materials are water soluble salts of vanadium and acids of tungsten for the purpose of mixing the catalyst components homogeneously. Various ethanolamines, amines, and oxalic acid can be used for forming complex compounds of vanadium and tungsten and thereby increase the solubility thereof.

The catalyst used in the present invention can be prepared, for example, by mixing a solution of silicotungstic acid with a suitable support material, drying the impregnated support and then calcining, mixing the calcined support with a solution of ammonium vanadate in water, drying the impregnated support, and then calcining under conditions which convert the tungsten and vanadium compounds to the oxides and form a catalyst active for the selective reduction of nitrogen oxides.

The drying is generally carried out at 60 to 250 C until the impregnated support becomes substantively anhydrous. After drying, the catalyst can be calcined at 300° to 700°C, preferably 350° to 500°C.

Any suitable support material can be used in the preparation of the instant catalyst. Preferably the inert carriers are high surface areas support material. By the term "high surface area" it is intended that the support have a surface area of at least 5m²/g. Representative examples of suitable support materials that can be used include alumina, titania, silica, magnesia, zirconia, silica-alumina, titania-silica, titania- alumina, and the like, and mixtures thereof. The impregnated support is subjected to calcination by heating at an elevated temperature sufficient to convert the metal compounds to the metal oxides as described above. In one embodiment of the invention a washcoated ceramic or metallic monolith is used as the support material. The metal monolith consists of alternate layers of flat and corrugated foil strips stacked to form the honeycomb structure. The foil is typically 0.002 thick and can be any of several ferritiσ stainless steels or other alloys. Cell densities of 100, 200, and 400 cell/sq in are obtained by varying the spacing and depth of the corrugations. This particular support is prepared by dispersing a support material in water to form a slurry, coating the monolith with the slurry and calcining the washcoated monolith prior to impregnation with the tungsten solution.

One preferred embodiment of the invention is set forth below.

A washcoat slurry is prepared by dispers¬ ing a high surface area oxide material in water and a ceramic or metallic monolith is then coated with this slurry. The washcoated monolith is impreg- nated with an aqueous solution of a tungsten compound such a silicotungstic acid at a desired proportion, dried, and calcined at 400°C to 500°C. The washcoated monolith, now containing tungsten oxide, is impregnated with an aqueous solution of a vanadium compound such as ammonium metavanadate at a desired vanadium/tungsten ratio, dried, and calcined at 350°to 450°C. The monolith catalyst is installed at a place in the waste gas where the temperature is in the range of 300-400°C. NH3 is injected into the gas stream upstream of the catalyst. The catalyst achieves NO removal in excess of 85% in the temperature range 300-400°C with an NH3/NO ratio of 1.0 and a gas hourly space velocity of 20000hr-¹.

The catalyst of the invention is effective for the removal of nitrogen oxides from gas streams containing same. Typically a gas stream containing nitrogen oxide is contacted with ammonia in the presence of an oxygen-containing gas and contacted with the vanadium oxide/tungsten oxide catalyst of the invention under conditions which reduce the nitrogen oxides to innocuous materials. The reaction temperature suitable for the present selective removal of nitrogen oxides can vary depending upon the components of the catalyst and the space velocity of the gases to be treated, but usually is in the range of 150° to 700°C, preferably 250° to 500°C.

The amount of ammonia to be supplied to the reaction can be 0.3 to 4 mol, preferably 0.4 to 2 mol to 1 mol nitrogen oxides. The ammonia can be used in various forms, such as gaseous ammonia, aqueous ammonia or an aqueous solution of an ammonium salt which can be easily decomposed to ammonia at the reaction temperature.

The amount of oxygen-containing gas, e.g. air, present during the reaction is preferably sufficient to provide an excess of O2.

The gases to be treated can be contacted with the catalyst at a space velocity of 500 to 100,000 hr^-1, more often 1,000 to 30,000 hr^-1. A presently preferred embodiment of the invention is set forth below.

The preferred embodiment of the invention is: A catalyst for the selective reduction of

NO_χ with NH3 in the presence of excess O2 in a process which comprises mixing NH3 in the gas stream at a molar ratio with the NO_x of from 0.4 to 2.0, and contacting said mixture with a catalyst at a temperature of 250°to 500°C at a gas hourly

_ι space velocity of lOOOhr , the active components of said catalyst consisting of vanadium and tungsten oxides in the mole ratio of 0.2 to 4.8 vanadium/tungsten, prepared by impregnating first a solution containing tungsten onto a high surface area support followed by impregnation of a solution containing vanadium.

The following examples are representative of the invention. However, it will be understood that these are only examples and in no way limit the present invention.

EXAMPLE 1 A metal honeycomb monolith with 100 cells per square inch was washcoated with 9:1 anatase/silica at a loading of 2600g per cubic ft. volume of monolith. The washcoated monolith was dryed and calcined at 500°C for 30 minutes. Comparative Example Catalyst A was prepared as follows: A washcoated monolith was impregnated with an aqueous solution of silicotungstic acid so as to obtain a W loading of 120g/ft³, dried at 250°C for 30 minutes, then fired at 450°C for 30 minutes. The monolith was then impregnated with an aqueous solution of ammonium metavanadate/oxalic acid so as to obtain a V loading of 160g/ft³ volume of monolith, dried at 250°C for 30 minutes, and fired at 450°C for 30 minutes. The V/ mole ratio was 4.8.

Comparative Example B was prepared as follows: A washcoated monolith was impregnated with an aqueous solution of ammonium metavanadate/oxalic acid so as to obtain a V loading of 160g/ft³ volume of monolith, dried at 250°C for 30 minutes, and fired at 450° C for 30 minutes. The washcoated monolith was then impregnated with an aqueous solution of silicotungstic acid so as to obtain a W loading of 120 g/ft³ volume of monolith, dried at 250°C for 30 minutes, and fired at 450°C for 30 minutes. The V/W mole ratio was 4.8. A continuous flow reactor was loaded with a 2.75 cubic inch monolith catalyst sample. A simulated turbine exhaust gas consisting of 42 ppm NO, 15% 0₂, 4.5% C0₂, 10% H₂0, and the balance N₂, was introduced after being admixed with 42 ppm NH₃, into the reactor at a flow rate of 45.05 standard liters perminute (GHSV=60000hr^_1) at a temperature of 300-450^QC. The outlet gas was measured for NO concentration to determine the relationship between the catalyst temperature and percent NO reduction. Reactor test results are given in Table I for Catalysts A and B. It is evident that Catalyst A has significantly higher activity than Catalyst B. Consequently, a two-step impregnation with tungsten impregnated first in superior to a two- step impregnation where vanadium is impregnated first.

EXAMPLE 2 Washcoated monoliths were prepared as in

Example I.

Comparative Catalyst C was prepared in the same way as Catalyst A except that the loading obtained was 40 g V/ft³ volume of monolith and 60 g W/ft³ volume monolith. The V/W mole ratio was 2.4.

Comparative Catalyst D was prepared by mixing an aqueous solution of ammonium metavanadate/oxalic acid with silicotungstic acid so as to obtain a 2.4 V/W mole ratio, impregnating a washcoated monolith, drying at 250°C for 30 minutes and firing at 450°C for 30 minutes to obtain a loading of 40 g V/ft³ volume monolith and 60 g W/ft³ volume of monolith.

Catalysts C and D were tested in a reactor as described in Example 1. As Table I illustrated, the activity of Catalyst C is significantly higher than the activity of Catalyst D. So a two-step impregnation with W impregnated first is superior to a co-impregnation of V and W.

EXAMPLE 3

A washcoated monolith was prepared as in Example 1.

Comparative Catalyst E was prepared in the same way as Catalyst A except that the loading obtained was 40 g V/ft³ volume monolith and 240 g W/ft³ volume monolith. The V/W mole ratio' was 0.6.

Catalyst E was tested in a reactor as described in Example 1. The test result in Table I shows the activity is comparable to catalyst compositions having a V/W mole ratio greater than 2. Thus, using the two-step impregnation method with W impregnated first, results in an active catalyst even when the ratio of V to W is low.

10 Table I Comparison of Activities for Catalyst Preparations

Claims

WHAT IS CLAIMED IS:

1. A method for preparing a supported vanadium oxide/tungsten oxide catalyst which comprises the steps of: 1) impregnating a support with a solution containing a tungsten compound convertible to the oxide,

2) impregnating the tungsten impregnated support of step 1 with a solution containing a vanadium compound convertible to the oxid , and

3) calcining the thus-impregnated support under conditions which convert the tungsten and vanadium compounds to the oxides and form an active vanadium oxide/tungsten oxide catalyst.

2. A method according to claim 1 wherein the metal impregnated support is calcined after each impregnation.

3. A method according to claim 1 wherein the amount of metal components present during each impregnation is sufficient to provide a mole ratio of vanadium to tungsten in the range of about 0.2 to 4.8.

4. A method according to claim 1 wherein said support is a washcoated monolith prepared by dispersing a support material in water to form a slurry, coating a monolith with the slurry, and calcining the washcoated monolith prior to impregnation with a tungsten solution.

5. A method according to claim 4 wherein said support is a metallic monolith washcoated with anatase/silica.

6. A method according to claim 4 wherein the washcoated monolith support is calcined after each impregnation.

7. A process for the selective reduction of nitrogen oxides which comprises contacting a gas stream containing N0_X with ammonia and an oxygen containing gas under reducing conditions in the presence of a supported vanadium oxide/tungsten oxide catalyst prepared according to claim 1.

8. A process according to claim 7 wherein the molar ratio of ammonia to NO_x ranges from 0.4 to 2 and the temperature during contacting ranges from about 250°to about 500°C.

9. A process according to claim 7 wherein said vanadium oxide/tungsten oxide catalyst is prepared by impregnating a support first with a tungsten solution followed by impregnation with a vanadium solution and calcining after each impregnation.

10. A process according to claim 9 wherein said support is a washcoated monolith prepared by dispersing a support material in water to form a slurry, coating a monolith with the slurry, and calcining the washcoated monolith with a tungsten solution.

11. A supported vanadium oxide/tungsten oxide catalyst prepared by impregnating a support first with a tungsten solution followed by impregnation with a vanadium solution and calcining to convert tungsten and vanadium to oxides.

12. A catalyst according to claim 11 wherein calcining is effected after each impregna¬ tion.

13. A catalyst according to claim 12 wherein the mole ratio of vanadium to tungsten ranges from about 0.2 to about 4.8.

14. A catalyst according to claim 13 wherein said support is a monolith prepared by dispersing a support material in water to form a slurry, coating a monolith with the slurry, and calcining the washcoated monolith prior to impregnation with a tungsten solution.