EP2635372A1

EP2635372A1 - Ammonia oxidation catalyst having low n2o by-product formation

Info

Publication number: EP2635372A1
Application number: EP11779617.7A
Authority: EP
Inventors: Markus Reichinger; Gerd Maletz; Klaus Wanninger; Andreas Bentele; Martin Schneider
Original assignee: Clariant Produkte Deutschland GmbH
Current assignee: Clariant Produkte Deutschland GmbH
Priority date: 2010-11-03
Filing date: 2011-10-28
Publication date: 2013-09-11
Also published as: WO2012059423A1; US20130343975A1; CN103459013A; DE102010050312A1; US9573097B2; CN103459013B

Abstract

The invention relates to a catalytic composition comprising a noble metal on an acid tungsten-containing mixed oxide, to a method for producing the catalytic composition, and to the use of the catalytic composition as an oxidation catalyst. The invention further relates to a catalyst molding which comprises the catalytic composition on a substrate, to a wash coat containing the catalytic composition according to the invention, and to the use of the wash coat for producing a coated catalyst molding.

Description

Ainmoniak oxidation catalyst with low N ₂ O.

By-product formation

The present invention relates to a catalytic

A composition comprising a noble metal on an acidic tungsten-containing mixed oxide, a process for the preparation of the catalytic composition and the use of the

catalytic compound as oxidation catalyst. The invention is further based on a shaped catalyst body

directed, which has the catalytic composition on a support, a washcoat containing the

Inventive catalytic composition and the

Use of the washcoat for producing a coated catalyst shaped body. To a strong reduction of NO _x emissions of

To achieve diesel engines, a so-called selective catalytic reduction (SCR) has been in use for some years

Urea used for exhaust aftertreatment. Usually, a urea solution is atomized into the exhaust stream. The urea is hydrolyzed to ammonia and the

released ammonia reacts in a Komproport ionization with nitrogen oxides to nitrogen.

The use of SCR allows engines to be optimized for very lean combustion. This leads to high

Nitrogen emissions of the engine, which are then eliminated with the SCR, but allows a significant

Saving fuel. The disadvantage, however, is that ammonia already in a concentration of more than 20 ppm as an odor is perceptible. This means that ammonia emissions <20 ppm are required under all, even dynamic, operating conditions. For this the completeness of the

Ammonia sales in the SCR be very high. That can not always be guaranteed. So the so-called

To prevent ammonia slip, a so-called ammonia oxidation catalyst (= blocking catalyst = ASC = AMOX) can be installed after the SCR. The oxidation of the ammonia proceeds very rapidly and therefore this reaction can be used to prevent the residual emissions of ammonia after the SCR. However, it is important that the catalyst has a high selectivity to N ₂ and produces little NO, NO ₂ and N ₂ O (nitrous oxide). These relationships and requirements are described in an overview in SAE 2006-01-0640.

Although N ₂ O as a by-product is not legally limited, low N ₂ O emissions are still important, because N ₂ O is one of the greenhouse gases and thus has a climate-damaging effect. Nitrous oxide is classified as having a GWP (Global Warming Potential) of 310, which means that the amount of nitrous oxide in the atmosphere is 310 times greater in terms of global warming

more harmful than the same amount of CO ₂ . Unfortunately, it is precisely the platinum-containing catalysts which today have a high activity for the oxidation of ammonia that exhibit a high tendency toward N ₂ O formation.

The post-treatment of an exhaust gas after an SCR with the aid of an ammonia oxidation catalyst is also in the

Exhaust aftertreatment in stationary plants such as power plants has long been used. In these

Stationary plants are usually used V / W / Ti-based SCR catalysts. It was found early on that the selectivity of these ammonia oxidation catalysts can be increased by directly using an SCR catalyst. Catalyst can be combined. This means that the noble metal catalyst is not simply arranged only after the SCR catalyst, but that at the same time the SCR function is still present in the oxidation catalyst zone. NO and N ₂ O formed on the noble metal catalyst can then be selectively re-converted to N ₂ with ammonia still present.

EP 410440 Bl discloses a V / W / Ti solid extrudate SCR catalyst which is coated in its exit zone with a noble metal.

EP 544282 B1 discloses a catalyst comprising a combination of a V / W / Ti catalyst and platinum on a zeolite, or platinum on alumina or platinum on S1O ₂ . Here a very low platinum concentration is used. In this document it is shown that a pure Pt / mordenite catalyst without V / W / Ti component (see

Comparative Example 2) shows a negative NO _x conversion, ie ammonia partially oxidized back to NO _x , whereas a pure V / W / Ti catalyst has a high ammonia slip. Only the combination of both catalysts shows both a high selectivity to N ₂ and a low

Ammonia slip. EP 1 399 246 B1 discloses a monolith catalyst having a V / W / Ti based SCR component which is based on its

Outlet side is impregnated with platinum. The catalytically active material of the reduction catalyst (VWT-SCR) is the carrier for the platinum group metal. Here, the platinum is thus applied directly to the V / W / Ti catalyst and not mixed two catalysts as in EP 544282 Bl.

US 7,393,511 B2 discloses a method for removing

Ammonia from exhaust gases, wherein the gas is added ammonia and after an SCR the gas is passed over an oxidation catalyst comprising a noble metal and vanadium on a Ti0 ₂ or A1 ₂ 0 _3, Si0 _2, Zr0 _2, Al-Si oxide, MgO, Hf0 ₂ or La ₂ 0 ₃ support includes. This is therefore also a

Catalyst as described in EP 544282 or EP 1 399 246.

The basic idea behind all these catalysts is one

Combination of a first component with activity for the reduction of nitrogen oxides with ammonia and a second

Component with activity to form nitrogen oxides from ammonia (for example via a noble metal).

For the reduction of nitrogen oxides with ammonia also Cu and Fe zeolites are suitable. Therefore, it was also obvious in the prior art to combine these zeolites with noble metal catalysts to obtain a Ammoniaksperrkatalysator. These barrier catalysts are described, for example, in US Pat. No. 7,722,845 B2. Both the combination of a zeolite SCR catalyst with Pt / Siralox and Pt directly on an Fe-BEA zeolite are described.

More recently, it has also been reported that acidic tungsten-containing oxides are also suitable for the SCR reaction. Especially preferred are W / Ce / Zr mixed oxides (see SAE 2007-01-0238, WO 2009/001131 and US 2010/0034717).

EP 7597865 also discloses the use of similar acidic Ce / Zr / Ti sulfate catalysts and Ti / Zr / Ce / W-SCR catalysts, respectively, followed by a barrier catalyst comprising platinum on a Si / Al oxide.

All the above precious metal-based

Oxidation catalysts oxidize ammonia to both NO _x and N ₂ O. These oxidation reactions are very fast. The aim of ammonia oxidation in the barrier catalyst is the oxidation to nitrogen. The reaction of the over-oxidized

Components N0 _X and N ₂ O with the SCR catalyst is then slower and not uniform for N0 _X and N ₂ 0. It is therefore difficult to both the NO _x replica in this catalyst and the N ₂ 0 emissions low hold. To control this and especially here to ensure low N ₂ 0 emissions is not possible according to the current state of the art. The object of the present invention was therefore to provide a catalytic composition which is suitable as an ammonia oxidation catalyst and a particularly high selectivity for the ammonia oxidation

having .

The invention provides a catalytic composition comprising a noble metal on an acidic tungsten-containing mixed oxide. Surprisingly, it was found that a catalytic

Composition containing a noble metal on an acidic tungsten-containing mixed oxide, has a particularly high selectivity for the ammonia oxidation and only at higher

Temperature N ₂ 0 forms and generally less N ₂ 0 emissions

caused.

According to one embodiment of the invention, this is acidic

Tungsten-containing mixed oxide preferably a Ce / Zr / W mixed oxide, a Y-stabilized Zr / W mixed oxide (Y / Zr / W mixed oxide) or a Ce / Zr / Ti / W mixed oxide, particularly preferably a Ce / Zr / W

Mixed oxide or consists only of the said mixed oxides. Combinations of the mentioned mixed oxides are possible. The mixed oxide preferably has a tungsten content of 5 to 15 wt .-%, more preferably 5.5 to 10 wt .-% and most preferably 6 to 7 wt .-% tungsten, based on the total

Mixed oxide, on.

According to another embodiment of the present invention

Invention, the noble metal is preferably platinum. The

Mixed oxide preferably has 0.05 to 1 wt .-% platinum, more preferably 0.06 to 0.75 wt .-% platinum and most preferably 0.07 to 0.5 wt .-% platinum, based on the total Mixed oxide, on. Preferably, no other precious metal is present in the catalytic composition besides platinum.

The ratio Ce / Zr in the acidic tungsten-containing mixed oxide is preferably in the range of 80:20 to 50:50, more preferably 60:40 to 70:30, still more preferably 65:35. As a result, a precisely matched acidic character of the mixed oxide is obtained, which contributes to the selectivity increase.

The invention also provides a method for

Preparation of the above-described catalytic

A composition comprising the steps of: a) impregnating an acidic mixed oxide with a

Tungsten precursor compound;

b) calcining the impregnated mixed oxide;

c) impregnating the calcined mixed oxide from step b) with a noble metal precursor compound;

d) calcining the with the noble metal precursor compound

impregnated mixed oxide from step c). The mixed oxide used, for example a Ce / Zr mixed oxide used according to the invention, and also the W mixed oxide produced have preferably a BET surface area of 20-250 m ² / g, preferably 40-100 m ² / g, most preferably SOSO m ² / g. The BET surface area is determined according to DIN 66132 (according to the method of Brunauer, Emmet and Teller).

As tungsten precursor compound is in principle any tungsten compound, which converts under the action of temperature in the calcination in the corresponding oxide, preferably ammonium metatungstate (NH ₄ ) ₆ H ₂ Wi ₂ 0 ₄ o, ammonium tungstate (NH ₄ ) i ₉ H ₂ (W ₂ 0 ₇ ) 6 and tungstic acid H ₂ W0 ₄ . Particularly preferred as the tungsten precursor compound is ammonium metatungstate

used.

After impregnating the acidic mixed oxide with the

Tungsten precursor compound is a whisker, preferably at a temperature of 400 to 800 ° C, more preferably 500 to 600 ° C, most preferably at about

550 ° C. As a result, the acidic tungsten-containing forms

Mixed oxide, which is preferably a ternary mixed oxide.

Subsequently, the calcined mixed oxide with a

Precious metal precursor compound impregnated. Impregnation is preferably by the incipient wetness method, but other methods, e.g. Dipping,

Spray method and the like can be used.

Suitable noble metal precursor compounds are, for example, ethanolammonium hexahydroxoplatinate, hexachloroplatinic acid, tetrachloroplatinic acid, diaminedinitroplatinate (II),

Tetraammineplatinum (II) chloride, ammonium tetrachloroplatinate (II), ammonium hexachloroplatinate (IV), platinum ethylenediamine dichloride, tetraammineplatinum (I) nitrate, tetraammineplatinum (II) hydroxide,

Methylethanolamine platinum (I) hydroxide and platinum nitrate, with ethanol ammonium hexahydroxoplatinate being most preferred.

After the impregnation of the calcined mixed oxide with the noble metal precursor compound takes place again

Calcining step, preferably at a temperature between 400 and 800 ° C, more preferably between 500 and 600 ° C, most preferably at about 550 ° C, whereby preferably finely divided noble metal is deposited on the mixed oxide. The particle size of the deposited noble metal is preferred

(preferably platinum) in the range of 1 to 20 nm, particularly preferably in the range of 3 to 17 nm. The

Determination of particle size can be determined by transmission electron microscopy (TEM). Before the

Calcination may optionally also take place a drying step, for example at temperatures of 50 to 200 ° C, preferably 80 to 150 ° C.

Alternatively to the above-mentioned method can after

Preparation of the mixed oxide first applied to a suitable support, for example in the form of a

Washcoats, and only then impregnated with the platinum compound. If the impregnation only

partially at one end of the carrier to obtain a combined SCR catalyst with Ammoniaksperrwirkung.

Another object of the invention is the use of the catalytic composition, as described above or as prepared by the method described above, as

Oxidation catalyst, in particular as ammonia oxidation catalyst.

The catalytic composition is preferably used together with an SCR catalyst. The SCR catalyst is preferably arranged upstream of the catalytic composition according to the invention in the flow direction. In this case, the catalytic composition (oxidation catalyst) and the SCR catalyst in a single catalyst component

be arranged, for example, in the flow direction first an SCR-active component and then the catalytic composition of the invention on a

suitable carriers are applied, or SCR-active

The component and the composition according to the invention can be accommodated on separate carriers in two separate components.

The SCR catalyst may be, for example, an SCR active

Component based on a Ce / Zr / W mixed oxide. However, any other SCR-active component can be used in the context of this invention, for example the above

mentioned and known in the art systems.

The catalytic composition can become a washcoat

usually a solvent, for example water and preferably also a binder are added. The washcoat is suitable for producing a coated catalyst shaped body. Another object is accordingly a catalyst shaped body containing a carrier and the catalytic composition as described above. Preferably, the catalytic composition is at least partially applied as a coating on the carrier, more preferably, the coating is applied to the entire carrier surface. The necessary coating techniques are known to the person skilled in the art. For example, the catalytic composition is processed into an aqueous coating dispersion. This dispersion may be added as a binder, for example, silica sol. The viscosity of the dispersion can be adjusted by suitable additives, so that it is possible to apply the required coating amount, for example, in a single operation on the walls of flow channels. If this is not possible, then the coating be repeated several times, the freshly applied coating is fixed in each case by an intermediate drying. The finished coating is then dried at elevated temperature and calcined for a period of 1 to 4 hours at temperatures between 300 and 600 ° C.

The catalytic composition can also be added with the addition of suitable auxiliaries such as inorganic binders (e.g.

Silicasol), pore formers, plasticizers and

Moistening in a known manner to moldings such as

Tablets and extrudates are processed. However, the catalytic composition is preferably in the form of a

Coating (as washcoat) on the inner walls of the

Flow channels of metallic or ceramic honeycomb bodies (monoliths) applied.

Preferably, the carrier is thus a metallic or

ceramic support, more preferably a so-called

monolithic carrier, which has a plurality of parallel passage openings and thus a large surface area. The support material is preferably either a ceramic such as cordierite or a metal or alloy (Emitec). In addition, for example, metallic foams (Inco) can be used as the carrier.

Usually, the shaped catalyst body is installed in a housing having an inlet and outlet opening for an exhaust gas to be cleaned. The invention is therefore also a

Catalyst component containing the inventive

Catalyst bodies. The catalyst component may be part of an exhaust gas purification system, preferably further

Components, such as a particulate filter, a upstream oxidation catalyst, a

Urea hydrolysis catalyst, an SCR catalyst and the like. The invention will now be explained in more detail with reference to some embodiments, but their content is not as

is to be understood as limiting the scope of the present invention. In this case, reference is additionally made to FIGS. 1 to 3.

Show it:

Fig. 1 shows the ammonia conversion of the compound according to the invention in comparison with not inventive

Links. Based on this diagram, the

Ammonia light-off temperature can be determined. the ΝΟχ-conversion of the compound according to the invention in comparison with compounds not according to the invention. the N ₂ O exit concentration in ppm of the compound according to the invention in comparison with compounds not according to the invention.

embodiments

The invention is based on the example 1 and the

Comparative examples shown. All of the following

Catalysts have 5 g / ft ³ platinum (0.1766 g platinum / IL honeycomb = 0.1766, g platinum / 250 g Ce / Zr / W oxide = 0.0706%) on the respective honeycomb. The honeycombs are all 1 "in diameter and 2" in length. In the example of the catalyst according to the invention by

Coating a honeycomb with a W / Ce / Zr oxide and

Subsequent Pt impregnation produced. In

Comparative Example 1 becomes a catalyst based on

Produced Pt / titanium dioxide, a catalyst without SCR

Properties . In Comparative Example 2, a catalyst with Pt on an Fe-BEA zeolite is prepared and in

Comparative Example 3, a catalyst based on Pt is prepared on a V / W / Ti-SCR catalyst. Example 4 describes the comparison test.

Example 1 :

381 g of a Ce / Zr mixed oxide (Ce / Zr = 63: 36) is impregnated with a solution of 37.5 g of ammonium metatungstate in 190 mL of deionized water to 6.1% W content and calcined at 550 ° C. From this powder, a washcoat is produced and a

Ceramic honeycomb coated on 250 g / 1 Washcoatbeladung (dry) in several steps and calcined at 550 ° C. After each coating step, it is dried and calcined. (Honeycomb 1 "in diameter and 2" in length were coated

V = 25.3 ml coated with 6.425 g washcoat.)

Thereafter, the water absorption of the honeycomb is determined and this dried again.

It is a platinum in ethanolamine solution (ethanol ammonium hexahydroxoplatate) diluted with 13.1% platinum content. It should be 5 g of platinum / ft ³ honeycomb volume of precious metal on the honeycomb. (5 g / ft ³ = 0.1766 g / 1) This means that a platinum solution must be prepared in which 0.00447 g of platinum in the

Water amount of water absorption are included. The honeycomb is then completely immersed in a platinum inethanolamine solution of this concentration for 10 seconds, in this way via "incipient wetness". impregnated. The honeycomb is dried and calcined at 550 ° C.

Comparative Example 1

A washcoat of 20 kg DT 51D titanium dioxide from Milenium and 26 kg titanium dioxide sol (12%, nitric acid stabilized) from Sachtleben as binder are prepared. A honeycomb is coated to 32 g / 1 washcoat and calcined at 450 ° C.

A platinum ethanolamine solution (ethanolammonium hexahydroxoplatinate) with a platinum content of 13.1% is diluted. It should be 5 g of platinum / ft ³ honeycomb volume of precious metal on the honeycomb. (5 g / ft ³ = 0.1766 g / 1) This means that a platinum solution must be prepared in which 0.00447 g of platinum in the

Water amount of water absorption are included. The honeycomb is then completely immersed in a platinum ethanolamine solution of this concentration for 10 seconds and impregnated in this way by means of incipient wetness, and the honeycomb is dried at 550.degree

kal ziert. Comparative Example 2:

A washcoat of Fe-BEA zeolite is made to which a colloidal silica sol is added as a binder. A

Ceramic honeycomb is coated to 250 g / 1 washcoat and calcined at 550 ° C.

Thereafter, the water absorption of the honeycomb is determined and this dried again. A platinum ethanolamine solution (ethanolammonium hexahydroxoplatinate) with a platinum content of 13.1% is diluted. It should be 5 g of platinum / ft ³ honeycomb volume of precious metal on the honeycomb. (5 g / ft ³ = 0.1766 g / 1) This means that a platinum solution must be prepared in which 0.00447 g of platinum in the

calcined.

Comparative Example 3

A washcoat of W stabilized titania DT 52 and 2.5% V ₂ O _{5 is} prepared. A ceramic honeycomb is coated to 200 g / 1 washcoat and calcined at 450 ° C.

Water amount of water absorption are included. The honeycomb is then completely immersed for 10 s in a platinum ethanolamine solution of this concentration and impregnated in this way via "incipient wetness". The honeycomb is dried and at 450 ° C

kal ziert. Example 4:

The catalysts were tested in a glass tube reactor (internal diameter 27 mm) under the following conditions:

250 ppm NO

50 ppm NH ₃

5% 0 ₂

5% C0 ₂

5% H ₂ 0

Remainder N ₂

Space velocity = 84,000 hT ¹

Measurement cooling from 420 ° C to 100 ° C In Fig. 1, the ammonia conversion is against the temperature

shown. This can be used to determine the ammonia light-off temperature (temperature at 50% conversion).

It can be seen that although the Pt catalyst on the Fe-BEA zeolite (Comparative Example 2) is best, but the other two (VWT and the catalyst of the invention) are not significantly worse. Considering the fact that at least 200 ° C is required to produce ammonia from the urea solution, one sees here that all

Catalysts other than the catalyst containing no SCR component (TiO ₂ , Comparative Example 1) meet this requirement.

2, the NO _x conversion of this test is shown. A negative NO _x conversion means that additional NO _x from the

Ammonia is formed by ammonia oxidation. At low temperature, the reaction is selective, as it dominates the SCR function. As the temperature increases, the oxidation becomes so fast that more ammonia is readily oxidized before it enters the SCR can selectively react with the NO _x . The higher this temperature, at which this negative NO _x conversion, ie more ammonia oxidation to NO _x , takes place, the better is the

Catalyst. Here it is clear that the

Inventive catalyst (squares) and the VWT

Catalyst (Comparative Example 3, stars) are best.

The decisive advantage of the catalyst according to the invention over the entire prior art, however, can be clearly seen in FIG. In Fig. 3, the N ₂ 0-

Outlet concentration in ppm shown. N ₂ O is an undesirable by-product. It can be clearly seen that the catalyst according to the invention (squares) only exceeds the 5 ppm N ₂ O at 250 ° C., ie N ₂ O forms only at a higher temperature and with a maximum of 20 ppm at 300 ° C. the lowest N ₂ 0 Having formation.

Claims

claims

A catalytic composition comprising a noble metal on an acidic tungsten-containing mixed oxide.

The catalytic composition according to claim 1, wherein the acid tungsten-containing mixed oxide is a Ce / Zr / W mixed oxide.

A catalytic composition according to claim 1 or 2, wherein the mixed oxide has a tungsten content of 5 to 15% by weight, based on the total mixed oxide.

A catalytic composition according to any one of claims 1 to 3, wherein the noble metal is platinum.

The catalytic composition according to any one of claims 1 to 4, wherein the composite oxide has 0.05 to 1.0% by weight of platinum based on the total composite oxide.

A catalytic composition according to any one of claims 1 to 5, wherein the ratio Ce / Zr is 80:20 to 50:50.

Process for the preparation of a catalytic composition according to one of Claims 1 to 6, comprising the steps:

a) impregnating an acidic mixed oxide with a tungsten precursor compound;

b) calcining the impregnated mixed oxide;

c) impregnating the calcined mixed oxide from step b) with a noble metal precursor compound; d) calcining the mixed oxide impregnated with the noble metal precursor compound from step c).

8. Use of the catalytic composition according to any one of claims 1 to 6 as an oxidation catalyst, in particular as an ammonia oxidation catalyst.

9. Use according to claim 8, wherein the oxidation catalyst is preceded by an SCR catalyst.

10. Use according to claim 9, wherein the SCR catalyst and the oxidation catalyst are present on a common carrier.

11. Catalyst shaped body comprising a support and the catalytic composition according to one of claims 1 to 6.

12. Catalyst molding according to claim 11, wherein the catalytic composition is at least partially present as a coating on the support.

13. Catalyst molding according to claim 11 or 12, comprising an SCR-active component.

14. A shaped catalyst body according to any one of claims 11 to 13, wherein the carrier is a metallic or ceramic carrier.

A washcoat comprising the catalytic composition of any one of claims 1 to 6, a binder and a solvent. Use of the washcoat according to claim 15 Preparation of a coated catalyst tablet