EP3419752A1

EP3419752A1 - Catalyst for reduction of nitrogen oxides

Info

Publication number: EP3419752A1
Application number: EP17706452.4A
Authority: EP
Inventors: Thomas UTSCHIG; Ruediger Hoyer; Naohiro Kato
Original assignee: Umicore AG and Co KG
Current assignee: Umicore AG and Co KG
Priority date: 2016-02-22
Filing date: 2017-02-21
Publication date: 2019-01-02
Also published as: US20200030745A1; CN108778490A; WO2017144426A1; KR20180116396A

Abstract

The invention relates to a nitrogen oxide storage catalyst composed of at least two catalytically active washcoat layers on a support body, wherein a lower washcoat layer A comprises cerium oxide, an alkaline earth metal compound and/or an alkali compound, platinum and palladium, and an upper washcoat layer B located above washcoat layer A comprises cerium oxide, platinum and palladium, does not contain any alkali and alkaline-earth compounds, and has macropores. Also disclosed is a method for converting NO_x in exhaust gases from motor vehicles operated with lean-burn engines.

Description

Catalyst for the reduction of nitrogen oxides

The present invention relates to a catalyst for the reduction of nitrogen oxides, which is contained in the exhaust lean burned internal combustion engines.

The exhaust of motor vehicles which are operated with lean-burn internal combustion engines, for example with diesel engines, in addition to carbon monoxide (CO) and nitrogen oxides (NO _x ) also contains components resulting from the incomplete combustion of the fuel in the combustion chamber of the cylinder. In addition to residual hydrocarbons (HC), which are also mostly present in gaseous form, these include particulate emissions, also referred to as "diesel soot" or "soot particles". These are complex agglomerates of predominantly carbon-containing solid particles and an adhering liquid phase, which mostly consists of relatively long-chain hydrocarbon condensates. The liquid phase adhering to the solid components is also referred to as "Soluble Organic Fraction SOF" or "Volatile Organic Fraction VOF". To purify these exhaust gases, the constituents mentioned must be converted as completely as possible into harmless compounds, which is possible only with the use of suitable catalysts.

To remove the nitrogen oxides, so-called nitrogen oxide storage catalysts are known, for which the term "lean NOx trap" or "LNT" is also common Are stored in the form of nitrates and this decomposed in a subsequent rich operating phase of the engine again and the thus released nitrogen oxides with the reducing

Exhaust gas components are converted to the storage catalyst to nitrogen, carbon dioxide and water. This procedure is described for example in SAE SAE 950809. Suitable storage materials are, in particular, oxides, carbonates or hydroxides of magnesium, calcium, strontium, barium, the alkali metals, the rare earth metals or mixtures thereof. Due to their basic properties, these compounds are capable of forming nitrates with the acidic nitrogen oxides of the exhaust gas and of storing them in this way. They are suitable for generating a large interaction area with the exhaust gas in the highest possible dispersion

Carrier materials deposited. In addition, nitrogen oxide storage catalysts generally contain noble metals such as platinum, palladium and / or rhodium as catalytically active components. Their task is, on the one hand, to oxidise NO to NO 2, CO and HC to CO 2 under lean conditions and, on the other hand, to reduce released NO 2 to nitrogen during the rich operating phases in which the nitrogen oxide storage catalyst is regenerated.

With the change of the exhaust gas legislation after Euro 6 become

Future exhaust systems must have sufficient NOx conversion both at cold city cycle temperatures and at high temperatures such as those encountered at high loads. However, known nitrogen oxide storage catalysts show either a pronounced NOx

Storage at low or high temperatures. There is a need for catalysts that provide good NOx conversion over a wide temperature range of 200 to 450 ° C. EP 0 885 650 A2 describes an exhaust gas purification catalyst for internal combustion engines with two catalytically active layers on a support body. The layer directly on the support body comprises one or more highly dispersed alkaline earth oxides, at least one

Platinum group metal, as well as at least one finely divided, oxygen-storing material. The platinum group metals are in close contact with all components of the first layer. The second layer is in direct contact with the exhaust gas and contains at least one platinum group metal and at least one finely divided oxygen storing material. Only part of the finely divided solids of the second layer serve as a carrier for the platinum group metals. The catalyst is a three-way catalytic converter that controls the harmful components of the exhaust gas

Essentially at stoichiometric conditions, i. at the air ratio λ of 1.

From US2009 / 320457 a nitrogen oxide storage catalyst is known which comprises two superimposed catalyst layers on a carrier substrate. The lower layer lying directly on the carrier substrate comprises one or more noble metals, as well as one or more nitrogen oxide storage components. The upper layer comprises one or more

Precious metals, as well as cerium oxide and is free of alkali or alkaline earth components. Catalyst substrates containing nitrogen oxide storage materials and having two or more layers are also described in WO 2012/029050. The first layer is directly on the carrier substrate and comprises platinum and / or palladium, while the second layer is on the first and comprises platinum. Both layers also contain one or more oxygen storage materials and one or more nitric oxide storage materials comprising one or more alkali and / or alkaline earth metals. The total amount of alkali and alkaline earth metal in the nitrogen oxide storage materials is 0.18 to 2.5 g / in ³ calculated as alkali metal oxide M ₂ O and alkaline earth metal oxide MO.

Catalyst coatings are already known which, owing to a relatively large porosity, have improved flow with exhaust gas and thus improved contact of the exhaust gas constituents with the catalytically active centers. Such catalyst coatings can be obtained, for example, by coating an inert carrier body with an aqueous coating suspension (washcoat) which contains a so-called pore-forming agent. Become a pore builder

Materials used, which adjoin the coating Calcining the catalyst burn out without residue and thus

Leave voids in the coating.

For example, US 2015/273462 describes the use of resin particles and EP 2 050 495 A1 describes synthetic resins, such as polyurethane, polystyrene, polyethylene, polyester or acrylic ester resins, as pore formers. In addition, EP 1 832 344 A1 mentions activated carbon, graphite powder, cellulose powder, organic fibers and synthetic fibers as suitable for this purpose.

According to WO 2014/137827 A1, the porosity of a catalytically active coating is increased by means of an aqueous oil-in-water macroemulsion.

The present invention relates to a nitrogen oxide storage catalyst comprising at least two catalytically active washcoat layers on one

Supporting body, wherein

a lower washcoat A ceria, an alkaline earth compound

and / or an alkali compound, as well as platinum and palladium; and

an upper washcoat layer B over the washcoat layer A

contains ceria, as well as platinum and palladium and is free of alkali and alkaline earth compounds,

characterized in that the upper washcoat layer B has macropores having an average pore size of less than 15 m, the macropores forming a pore volume in the upper washcoat layer B of 5 to 25% by volume.

The cerium oxide used in washcoat layers A and B can

commercial quality, ie. have a cerium oxide content of 90 to 100 wt .-%.

In embodiments of the present invention, cerium oxide is used in the washcoat layer A in an amount of from 110 to 160 g / l, for example from 125 to 145 g / l. In the washcoat layer B, cerium oxide is used in amounts of 22 to 120 g / l, for example 40 to 100 g / l or 45 to 65 g / l. In particular, oxides, carbonates and / or hydroxides of magnesium, strontium and / or barium are suitable as the alkaline earth compound in the washcoat layer A, in particular magnesium oxide, barium oxide and / or

Strontium oxide, especially barium oxide, strontium oxide or barium oxide and strontium oxide.

Suitable alkali compounds in the washcoat A are, in particular, oxides, carbonates and / or hydroxides of lithium, potassium and / or sodium. In embodiments of the present invention, the alkaline earth or alkali compound in washcoat A is present in amounts of 10 to 50 g / l, especially 15 to 20 g / l, calculated as alkaline earth oxide and based on the volume of the support body. In embodiments of the present invention, washcoat layer A may include manganese oxide. In washcoat layer A, this is present in particular in amounts of 1 to 10% by weight, preferably 2.5 to 7.5% by weight, based on the sum of the washcoat layers A and B, in each case calculated as MnO. In further embodiments, the washcoat layer B also contains

Manganese oxide. In these cases, the amount of manganese oxide is in

Washcoat layer B at up to 2.5 wt .-%, preferably 0.5 to 2.5 wt .-%, based on the sum of Washcoatschichten A and B.

Manganese oxide can serve as a carrier material for the noble metals platinum, palladium and optionally rhodium. However, in preferred embodiments of the present invention, manganese oxide is not used as a carrier material, neither for the noble metals platinum, palladium and optionally rhodium, nor for another component of washcoat A and optionally washcoat B.

The term manganese oxide in the context of the present invention means in particular MnO, MnO ₂ or Mn ₂ O ₃ or combinations of MnO ₂ , MnO and / or Mn ₂ O ₃ . In embodiments of the present invention, manganese oxide is not in the form of mixed oxides with other oxides of washcoat A and B layers. In particular, manganese oxide is not present in the form of a mixed oxide with cerium oxide, for example not in the form of MnOx-CeCh, MnO-ZrCte and

The ratio platinum to palladium in the washcoat A in embodiments of the present invention is for example 4: 1 to 18: 1 or 6: 1 to 16: 1, for example 8: 1, 10: 1, 12: 1 or 14: 1.

The ratio platinum to palladium in the washcoat layer B is also in embodiments of the present invention, for example 4: 1 to 18: 1 or 6: 1 to 16: 1, for example 8: 1, 10: 1, 12: 1 or 14: 1 but independent of the ratio in washcoat A In embodiments of the present invention, washcoat B contains rhodium as another precious metal. Rhodium is in this case

especially in amounts of 0.003 to 0.35 g / l (0.1 to 10 g / ft ³ ),

in particular 0.18 to 0.26 g / l (5 to 7.5 g / ft ³ ), in each case based on the volume of the supporting body before.

The total amount of noble metal, i. of platinum, palladium and

optionally rhodium, in the nitrogen oxide storage catalyst according to the invention is in embodiments of the present invention, 2.12 to 7.1 g / l (60 to 200 g / ft ³ ), based on the volume of the support body.

Both in the washcoat layer A and in the washcoat layer B, the noble metals are platinum and palladium and optionally rhodium

usually on suitable carrier materials. As such, in particular oxides having a BET surface area of 30 to 250 m ² / g, preferably from 100 to 200 m ² / g (determined according to DIN 66132)

used, for example, alumina, silica, titanium dioxide, but also mixed oxides such as aluminum-silicon mixed oxides and cerium-zirconium mixed oxides. In embodiments of the present invention, platinum and palladium and, optionally, rhodium are used as support material for the noble metals

Alumina is used, in particular those which is stabilized by 1 to 6 wt .-%, in particular 4 wt .-%, lanthanum oxide.

It is preferred if the precious metals platinum, palladium and optionally rhodium only on one or more of the above

Carrier materials are supported and thus are not in close contact with all components of the respective Washcoatschicht. In particular, manganese oxide preferably does not serve as a support for platinum and palladium and optionally rhodium.

The total washcoat load of the support body is in

Embodiments of the present invention 300 to 600 g / l, based on the volume of the support body.

In embodiments of the present invention, the macropores of the upper washcoat layer B have an average pore size of 2 to 12 μm, preferably 4 to 7 μm.

In further embodiments of the present invention form the

Macropores a pore volume in the upper Washcoatschicht B from 5 to 20 vol .-%, for example 5 to 10 vol .-% or 10 to 15 vol .-%.

The average pore size of the macropores in Washcoatschicht B is usually identical to the average particle size of the pore-forming agent used, because each particle of the used

Pore forming a macro pore in the calcined catalyst corresponds.

Likewise, the pore volume of the washcoat layer A results as the sum of the volumes of the particles of the pore-forming agent used. Average pore size and pore volume thus result from the size and amount of pore-forming agent used and can be easily determined. Alternatively, average pore size and pore volume, of course, by the usual and known in the art methods, eg

Mercury porosimetry can be determined. In a preferred embodiment, the present invention relates to a nitrogen oxide storage catalyst comprising at least two catalytically active washcoat layers on a support body,

in which

a lower washcoat layer A

o cerium oxide in an amount of 100 to 160 g / l,

o platinum and palladium in a mass ratio of 10: 1, and

o magnesium oxide and / or barium oxide; and

an upper washcoat layer B is disposed over the lower washcoat layer A and

o no alkaline earth compound and no alkali compound,

o platinum and palladium in a mass ratio of 10: 1 and

o Cerium oxide in an amount of 45 to 65 g / l

contains, wherein the amount g / l each refers to the volume of the support body and wherein the upper Washcoatschicht B macropores having an average pore size of 2 to 12 μηι and wherein the macropores a pore volume in the upper Washcoatschicht B a pore volume of 5 to 20 Vol .-% form.

In a particular embodiment of this type washcoat layer A contains manganese oxide in an amount of 5 to 15 g / l.

In a further particular embodiment of this type

Washcoat A in amounts of 250 to 350 g / l and washcoat B in amounts of 80 to 130 g / l before. The application of the catalytically active Washcoatschichten A and B on the support body by means of a coating suspension according to the usual dip coating method or pump and suction coating method with subsequent thermal treatment (calcination and optionally reduction with forming gas or hydrogen). These methods are well known in the art.

Thus, in a first step, the coating suspension for washcoat A in the appropriate amount is applied to the support body and dried. In a second step, the coating suspension for Washcoatschicht B in the appropriate amount on the already with

Washcoat layer A coated support applied and also dried. Then we calcined the finished coated support body. The necessary coating suspensions can be obtained by methods known to those skilled in the art. Thus, the ingredients, such as ceria, alkaline earth and / or alkali compound, on appropriate

Supported materials supported noble metals, and optionally manganese oxide or another manganese compound in the appropriate amounts in water and milled in a suitable mill, in particular a ball mill, to a particle size of dso = 3 to 5 m. It is preferred to use manganese in the form of manganese carbonate of the coating suspension in the last step, i. E. just before grinding, to admit. To prepare the macropores, the coating suspension for washcoat B is added to pore formers. Preferably, this addition is carried out after the grinding of the coating suspension to a particle size of dso = 3 to 5 μηι.

The pore formers are made of materials that completely and in the calcination of the finished coated support body from about 350 ° C

burn out without residue leaving the macropores behind.

Suitable pore formers consist in particular of synthetic resins, such as polyurethane, polystyrene, polyethylene, polyester, polyacrylonitrile or polyacrylic ester resins. Pore formers are particularly preferred

Polymethylmethacrylate or polyacrylonitrile. To achieve macropores with the claimed pore size, the pore formers must have an average particle size of less than 15 μm, for example 2 to 12 μm, preferably 4 to 7 μm.

In order to obtain the claimed pore volume formed by the macropores, the coating suspension for producing the washcoat layer B pore-forming agent must be added in an appropriate amount. This can be determined from the average particle size of the pore formers in a simple manner. Suitable pore formers are known and can be purchased on the market.

The nitrogen oxide storage catalysts according to the invention are outstandingly suitable for the conversion of NO _x in exhaust gases from

Motor vehicles operated by lean-burn engines, such as diesel engines. They achieve good NOx conversion

Temperatures of about 200 to 450 ° C, without the NOx conversion is adversely affected at high temperatures. The nitrogen oxide storage catalysts according to the invention are thus suitable for Euro 6 applications.

The present invention thus also relates to a method for

Conversion of NOx in exhaust gases of motor vehicles, which are operated with lean-burn engines, such as diesel engines, which is characterized in that the exhaust gas is passed through a nitrogen oxide storage catalyst of at least two catalytically active Washcoatschichten on a support body

a lower washcoat A ceria, an alkaline earth compound

and / or an alkali compound, as well as platinum and palladium; and

an upper washcoat layer B over the washcoat layer A

characterized in that the upper Washcoatschicht B macropores having an average pore size of less than 15 μηι, wherein the macropores form a pore volume in the upper washcoat B of 5 to 25% by volume.

Embodiments of the method according to the invention with regard to the nitrogen oxide storage catalyst correspond to the above

Descriptions.

The invention is explained in more detail in the following examples and figures.

Figure 1: NOx storage amount in g / l at 50% and at 75% of

Catalysts Kl, K2 and VK1

example 1

a) To prepare a catalyst according to the invention, a commercial honeycomb ceramic support is coated with a first coating suspension comprising Pt and Pd supported on alumina, ceria in an amount of 125 g / l, 21 g / l barium oxide, 15 g / l magnesium oxide and 7 , 5 g / l MnO in the form of manganese carbonate. The loading of Pt and Pd is 35 g / ft ³ (1.236 g / l) and 3.5 g / ft ³ (0.124 g / l) and the

Total loading of the washcoat layer about 293 g / l based on the volume of the ceramic carrier. After coating, the obtained

Washcoat layer A dried. b) A further washcoat layer B was applied to the first washcoat layer A. For this purpose, a coating suspension was also coated which also contained Pt and Pd supported on alumina and Rh supported on a lanthanum stabilized alumina. The loading of Pt, Pd and Rh in washcoat B was thus 35 g / ft ³ (1.236 g / l), 3.5 g / ft ³ (0.124 g / l) and 5 g / ft ³ (0.177 g / l). , The

Coating suspension also contained 55 g / l of ceria at a washcoat loading of layer B of about 81 g / l in the calcined catalyst. The coating suspension contained in addition to the above

Components also 5 g / l of a pore builder from a crosslinked

Polymethyl methacrylate resin having an average particle size of 5 to 7 μηι. The coating was dried and then calcined. After calcination, the pore volume in washcoat B was 6.5% by volume.

The catalyst thus obtained is hereinafter called Kl. Example 2

Example 1 was repeated with the difference that the coating suspension for washcoat B contained the pore former in an amount of 7.5 g / l pore former. After calcination, the pore volume in washcoat B was 9.7% by volume.

The catalyst thus obtained is hereinafter called K2.

Comparative Example 1

Example 1 was repeated with the difference that the coating suspension for washcoat layer B contained no pore-forming agent. The catalyst thus obtained is hereinafter called VK1.

comparison tests

a) The catalysts Kl, K2 and VK1 were hydrothermally aged at 800 ° C for 16 hours.

b) Subsequently, their nitrogen oxide storage capacity was determined as follows:

First, the sample was conditioned at 450 ° C. For this purpose, a lean gas composition corresponding to Table 1 and 10s a rich gas composition was passed alternately over the catalyst for a period of 15 minutes. lean fat adsorption

GHSV [1 / h] 50000 50000 50000

NO [ppm] 0 0 500

O ₂ [vol%] 8 0 8

CO [ppm] 0 40000 0

CO ₂ [vol%] 10 10 10

H ₂ O [vol%] 10 10 10

Table 1

Subsequently, the sample was cooled to measurement temperature (175 ° C. or 300 ° C.) under a nitrogen atmosphere, or kept at 450 ° C. At a constant measuring temperature then the NOx adsorption in the

The "Adsorption" gas composition is measured according to Table 1. The NOx storage capacity is calculated from the difference in the metered amount of NOx relative to the catalyst volume and the amount of NOx slip measured behind the catalyst sample relative to the catalyst volume at the time when the NOx -Umsatz over the sample 75%, or only 50% and is in Figure 1 as NOx

Memory amount shown. As a result, the NOx storage amount was expressed in g / l at 50% and at 75% conversion, with the storage amounts of VKl set at 100%, respectively, and the storage amounts of Kl and K2 related thereto. The results are shown in FIG. Example 3

Example 1 was repeated with the difference that the washcoat B coating suspension contained 5 g / l of a pore former of a crosslinked polymethyl methacrylate resin having an average particle size of 8 to 12 m. Example 4

Example 1 was repeated with the difference that the coating suspension for washcoat B 7.5 g / l of a pore former of a crosslinked polymethyl methacrylate resin with an average

Particle size of 4 to 5 μηι contained.

Further examples are listed in Table 2

In Table 2, mean:

a: pore builder of a crosslinked polymethyl methacrylate resin having an average particle size of 8 to 12 μηι.

b: pore former of a crosslinked polymethyl methacrylate resin having an average particle size of 5 to 7 μηι contained

c: Pore former of a polyacrylonitrile resin having an average particle size of 8 μηι.

Claims

claims

1. nitrogen oxide storage catalyst of at least two catalytically active Washcoatschichten on a support body, wherein

a lower washcoat A ceria, an alkaline earth compound

and / or an alkali compound, as well as platinum and palladium; and

an upper washcoat layer B over the washcoat layer A

characterized in that the upper Washcoatschicht B macro pores having an average pore size of less than 15 μηι, wherein the macropores form a pore volume in the upper washcoat B of 5 to 25 vol .-%.

2. nitrogen oxide storage catalyst according to claim 1, characterized in that the Washcoatschicht A contains ceria in an amount of 110 to 160 g / l.

3. nitrogen oxide storage catalyst according to claim 1 and / or 2, characterized in that the Washcoatschicht B contains ceria in an amount of 22 to 120 g / l.

4. nitrogen oxide storage catalyst according to one or more of claims 1 to 3, characterized in that in Washcoatschicht A the

Alkaline earth compound is an oxide, carbonate and / or hydroxide of magnesium, strontium and / or barium.

5. nitrogen oxide storage catalyst according to one or more of claims 1 to 4, characterized in that the alkaline earth compound in

Washcoat layer A is magnesium oxide, barium oxide and / or strontium oxide.

6. nitrogen oxide storage catalyst according to one or more of claims 1 to 5, characterized in that the alkaline earth or alkali compound in washcoat A in amounts of 10 to 50 g / l, calculated as alkaline earth or Alkali oxide and based on the volume of the support body, is present.

7. nitrogen oxide storage catalyst according to one or more of claims 1 to 6, characterized in that Washcoatschicht A contains manganese oxide.

8. nitrogen oxide storage catalyst according to claim 7, characterized in that manganese oxide in washcoat A in amounts of 1 to 10 wt .-%, based on the sum of the washcoat layers A and B and calculated as MnO, is present.

9. nitrogen oxide storage catalyst according to one or more of claims 1 to 8, characterized in that the ratio platinum to palladium in washcoat A and in washcoat B independently 4: 1 to 18: 1.

10. Nitrogen storage catalyst according to one or more of claims 1 to 9, characterized in that Washcoatschicht B contains rhodium.

11. Nitrogen storage catalyst according to claim 10, characterized

characterized in that rhodium in amounts of 0.003 to 0.35 g / l, based on the volume of the support body, is present.

12. Nitrogen storage catalyst according to one or more of claims 1 to 11, characterized in that the macropores of the upper

Washcoat layer B have an average pore size of 2 to 12 μηι.

13. Nitrogen storage catalyst according to one or more of claims 1 to 12, characterized in that the macropores form a pore volume in the upper washcoat B of 5 to 10 vol .-%.

14. Nitrogen storage catalyst according to one or more of claims 1 to 12, characterized in that the macropores form a pore volume in the upper washcoat B of 10 to 15 vol .-%.

15. A method for the conversion of NO _x in exhaust gases of motor vehicles, which are operated with lean-burn engines, characterized

characterized in that the exhaust gas is passed over a nitrogen oxide storage catalyst according to one or more of claims 1 to 14.