Oxidation catalyst with phosphorus trap

Abstract

The present invention relates to a catalyst comprising: a support substrate having a first end a and a second end b and a length L; a material region A containing a platinum group metal on a support material at a loading of 40 g / l to 150 g / l relative to the volume of the support substrate; and a material region B containing a platinum group metal on a support material at a loading of 75 g / l to 200 g / l relative to the volume of the support substrate, wherein the material region B has a greater platinum group metal content than the material region A, calculated in g / l, relative to the volume of the support substrate, and wherein the proportion of pores having a diameter of 0.5 μm to 50 μm in the material region B is 20% to 30%.

Description

[0001] The present invention relates to an oxidation catalyst for purifying exhaust gas from a diesel engine, the oxidation catalyst comprising a phosphorus trap.

[0002] In addition to carbon monoxide CO, hydrocarbons HC and nitrogen oxides NO x In addition, the raw exhaust gas of diesel fuels contains a relatively high oxygen content of up to 15% by volume. In addition, particulate emissions are present, which mainly consist of soot residues and, in some cases, organic agglomerates and are caused by partially incomplete combustion of the fuel in the cylinder.

[0003] While diesel particulate filters with and without catalytically active coatings are suitable for removing particulate emissions, and nitrogen oxides can be converted into nitrogen, for example, by selective catalytic reduction (SCR) in so-called SCR catalysts, carbon monoxide and hydrocarbons are rendered harmless by oxidation in suitable oxidation catalysts.

[0004] Oxidation catalysts are widely described in the literature. These are, for example, so-called flow-through substrates made of ceramic or metal, which support noble metals such as platinum and palladium as the main catalytically active components on a large-surface, porous, refractory oxide such as alumina.

[0005] Also described are zoned oxidation catalysts which, in the flow direction of the exhaust gas, have material zones of different composition with which the exhaust gas comes into contact successively.

[0006] For example, US 2010 / 257843, US 2011 / 099975, and WO 2012 / 079598 A1 describe zoned oxidation catalysts comprising platinum and palladium.

[0007] In addition to the components already mentioned, diesel engine exhaust often contains phosphorus compounds originating from oil additives. These deposit on the diesel oxidation catalyst and impair its oxidation capacity over its service life.

[0008] This problem and possibilities for solving it have been described in the literature.

[0009] For example, JP 2015066516A discloses a three-way catalyst containing a composite oxide of magnesium and cerium as a phosphorus trap in its uppermost layer.

[0010] JP2013146706A proposes a double-layer catalyst, a lower layer containing a precious metal and an upper layer containing no precious metal or containing less precious metal than the lower layer, wherein the upper layer has a higher porosity than the lower layer.

[0011] There remains a need for oxidation catalysts that are effective in preventing phosphorus contamination throughout their service life.

[0012] Therefore, the present invention relates to a catalyst comprising

[0013] a carrier substrate having a first end a and a second end b and a length L,

[0014] a material region A containing a platinum group metal on a support material in an amount of 40 g / l to 150 g / l relative to the volume of the support substrate, and

[0015] a material region B having a loading of 75 g / l to 200 g / l relative to the volume of the carrier substrate,

[0016] The material region B contains a platinum group metal on a support material,

[0017] Material region B has a greater content of platinum group metal than material region A, relative to the volume of the carrier substrate and calculated in g / l, and material region B has a proportion of pores with a diameter of 0.5 μm to 50 μm of 20% to 30%.

[0018] Material region B contains, for example, only platinum, only palladium, or both platinum and palladium as platinum group metals. Platinum and palladium are preferred, specifically in a weight ratio of 20:1 to 1:5, preferably in a weight ratio of 10:1 to 1:3.

[0019] In material region B, the platinum group metal is specifically present in an amount of 0.53 g / l to 2.5 g / l, preferably 0.60 g / l to 1.4 g / l, relative to the volume of the support substrate.

[0020] Material region B has pores with diameters of 0.5 μm to 50 μm in a proportion of 20% to 30%.

[0021] In the context of this application, the pore fraction is understood to mean the pore volume relative to the total volume. In the present case, therefore, for example, a pore fraction of 20% of the material region B means that 20% of the total volume of the material region B is formed by pores having a diameter of 0.5 μm to 50 μm.

[0022] The pore proportion of the material zone or catalyst zone can be determined by means of a method comprising the following steps:

[0023] Select the catalyst area to be inspected,

[0024] creating a cross-section of the catalyst area to be examined, wherein the pores are filled with an organic resin,

[0025] creating an image of said cross section by means of a scanning electron microscope (SEM),

[0026] Select the area of ​​the SEM image to be inspected,

[0027] converting the selected partial area of ​​the SEM image into a binary image with the aid of an image analysis program, and

[0028] • Using the binary image for porosity distribution calculation and subsequent determination of the pore fraction with the aid of an image analysis program.

[0029] To produce a cross-section of the catalyst region in which the pores are filled with the organic resin, cross-sectioning methods known to those skilled in the art are available.

[0030] The generation of cross-sectional images using a scanning electron microscope (SEM) is also carried out according to known and fully described methods. The specific measurement conditions are not critical, i.e., they have no influence on the final result. In this case, the detection of backscattered primary electrons is used as the imaging method. These backscattered primary electrons provide a signal intensity that depends on the average atomic number of the material being examined. Therefore, heavier elements appear lighter because they scatter more, while lighter elements appear darker. As a result, in the obtained SEM images, the pores appear dark gray to black, while the washcoat material appears lighter. Figure 1 The following SEM image of the catalyst of Example 1, created in this manner, is shown, in which the support and the two material zones A and B are easily distinguishable from one another. In the next step, a partial region of the SEM image to be examined, or multiple partial regions to be examined, such as the region of the material zone, is selected, and a binary image is created using a thresholding method using image analysis software. In the context of the present invention, the image analysis software was programmed using Matlab Version 2019 and the associated image processing toolbox. Figure 2 The image analysis software is shown by Figure 1 A binary image is created from the SEM image. In the image, pores appear black, and the washcoat material appears white. Gray areas represent non-selected regions of no interest. The pore ratio is then calculated using image analysis software using morphological image processing methods. In addition to the aforementioned Matlab-programmed image analysis software, other programs known to those skilled in the art, such as ImageJ, can also be used, which can be adapted, for example, as needed for programming plugins, to perform pore analysis.

[0031] The support material in the material region B may be aluminum oxide, doped aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, or a mixture, mixed oxide, or composite oxide of at least two of these materials. However, it is specifically aluminum oxide.

[0032] Material region A contains, for example, only platinum, only palladium, or both platinum and palladium as platinum group metals. Platinum and palladium are preferred, specifically in a weight ratio of 20:1 to 1:5, preferably 15:1 to 1:1.

[0033] In material region A, the platinum group metal is specifically present in an amount of 0.18 g / l to 1.4 g / l, preferably 0.25 g / l to 0.7 g / l, relative to the volume of the support substrate.

[0034] The platinum group metals in material zone A are present on a support material. All materials familiar to those skilled in the art for this purpose are considered support materials. Specifically, it has a 30m 2 / g to 250m 2 / g, preferably 100m 2 / g to 200m 2 / g of BET surface area (determined according to DIN 66132) and is preferably selected from the group consisting of aluminum oxide, doped aluminum oxide, silicon oxide, magnesium oxide, titanium oxide and mixtures or mixed oxides or composite oxides of at least two of these materials.

[0035] Preference is given to aluminum oxide, magnesium / aluminum mixed oxides, and aluminum / silicon mixed oxides.

[0036] Furthermore, aluminum oxide doped with, for example, 1 to 10% by weight, specifically 3 to 6% by weight, of lanthanum oxide may also be used.

[0037] The pore proportion of the material region A is not critical. The material region A preferably has pores with a diameter of 0.5 μm to 50 μm with a proportion of 10% to 30%.

[0038] In the oxidation catalyst according to the invention, the loading with material region B is preferably higher than the loading with material region A, in each case in g / l and relative to the volume of the support substrate.

[0039] The loading of material zone B is preferably 75 g / l to 150 g / l. The loading of material zone A is preferably 50 g / l to 75 g / l.

[0040] The material areas A and B can be arranged in different ways on the carrier substrate.

[0041] In a preferred embodiment, material zone A is coated over the entire length L of the carrier substrate, while material zone B extends from the first end a of the carrier substrate over 20% to 80%, preferably 30% to 70%, of the length L and is located on material zone A.

[0042] In another preferred embodiment, the material region A extends from the second end of the carrier substrate to 40% to 60% of the length L, and the material region B extends from the first end of the carrier substrate to 40% to 60% of the length L, wherein

[0043] L=L A +L B or

[0044] L <L A +L B or

[0045] L>L A +L B

[0046] Among them L A is the length of the material area A, and L B is the length of material area B.

[0047] Situation L>L A +L B This means that a portion of the carrier body remains uncoated. Specifically, in this case, a gap of at least 0.5 cm in length (ie, for example, 0.5 cm to 1 cm) remains between the material areas A and B.

[0048] The catalyst according to the invention comprises a support body. This is in particular a flow-through substrate, but can also be a wall-flow filter.

[0049] A wall-flow filter is a support body comprising channels of length L extending parallel to and between a first end and a second end of the wall-flow filter, the channels being alternately closed at either the first end or the second end and separated by porous walls. Specifically, a flow-through substrate differs from a wall-flow filter in that the channels of length L are open at both ends.

[0050] In the uncoated state, the wall-flow filters have a porosity of, for example, 30% to 80%, in particular 50% to 75%. In the uncoated state, their average pore size is, for example, 5 μm to 30 μm.

[0051] Generally speaking, the pores of wall-flow filters are so-called open pores, meaning they are connected to channels. Furthermore, the pores are often interconnected. This facilitates coating the inner pore surfaces and allows exhaust gases to pass easily through the porous walls of the wall-flow filter.

[0052] Like wall-flow filters, flow-through substrates are known to those skilled in the art and are commercially available. They consist, for example, of silicon carbide, aluminum titanate or cordierite.

[0053] The catalyst according to the invention can be produced by methods familiar to those skilled in the art, such as by conventional dip coating methods or pump and suction coating methods with the aid of a washcoat, i.e. an aqueous suspension of the components forming the corresponding material zones. A thermal aftertreatment (calcination) may follow.

[0054] Those skilled in the art will recognize that, in the case of wall-flow filters, their average pore size and the average particle size of the material to be coated can be matched to one another so that they settle on the porous walls forming the channels of the wall-flow filter (wall coating). It is also possible to select the average particle size of the material to be coated so that they are located in the porous walls forming the channels of the wall-flow filter; that is, to coat the inner pore surfaces (wall coating). In this case, the average particle size of the coating material must be small enough to penetrate into the pores of the wall-flow filter.

[0055] In another embodiment of the present invention, a carrier substrate consisting of a corrugated sheet of an inert material is used. Such carrier substrates are referred to by those skilled in the art as "corrugated substrates." Suitable inert materials are, for example, fibrous materials having an average fiber diameter of 50 μm to 250 μm and an average fiber length of 2 mm to 30 mm. Preferably, the fibrous material is heat-resistant and consists of silicon dioxide, in particular glass fibers.

[0056] To produce such a carrier substrate, a sheet of the aforementioned fibrous material is corrugated, for example, in a known manner, and the individual corrugated sheets are formed into a cylindrical, integrally constructed body with channels extending through the body. Preferably, the integrally constructed body having a transverse corrugated structure is formed by stacking a plurality of corrugated sheets in parallel layers, wherein the orientation of the corrugations differs between the layers. In one embodiment, non-corrugated (i.e., flat) sheets may be arranged between the corrugated sheets.

[0057] Substrates made of corrugated sheets can be coated directly with the material zones A and B, but they are preferably first coated with an inert material, such as titanium dioxide, and only then with the catalytic material.

[0058] The catalyst according to the invention is particularly suitable for oxidizing carbon monoxide, hydrocarbons and nitrogen monoxide in the exhaust gases of diesel engines. In said process, it is less susceptible to contamination with phosphorus compounds than catalysts known hitherto.

[0059] The present invention also relates to a method for purifying exhaust gases of a motor vehicle operated with a lean-burn engine, such as a diesel engine, characterized in that the exhaust gases pass through a catalyst according to the invention, wherein the exhaust gases enter the catalyst at the first end a of the carrier substrate and leave it again at the second end b.

[0060] Example 1

[0061] a) A commercially available circular flow-through substrate of cordierite having a pore density of 400 cpsi and a wall thickness of 4 mils, 5.66" x 4.00" in size, was coated over its entire length with a washcoat comprising 0.353 g / l of platinum and palladium in a 12:1 mass ratio supported on commercially available alumina doped with 5% silica. The washcoat loading was 60 g / l. The pore ratio was determined by the method described above (see also Figure 1 and Figure 2 ) is 15%.

[0062] b) In a second step, a washcoat layer was applied to the catalyst obtained in step a) starting from one end over 75% of its length, comprising 0.79 g / l of platinum and palladium in a mass ratio of 1:1 on highly porous alumina. The washcoat layer loading was 110 g / l. The pore proportion of the layer applied in the second step (pores with a diameter of 0.5 μm to 50 μm) was determined by the method described above (see also Figure 1 and Figure 2 ) is 24%.

[0063] The catalyst obtained is referred to hereinafter as K1.

[0064] Example 2

[0065] a) A commercially available circular flow-through substrate of cordierite having a pore density of 400 cpsi and a wall thickness of 4 mils, measuring 5.66" x 4.00", was coated over 50% of its length with a washcoat comprising 0.353 g / l of platinum and palladium supported on commercially available alumina doped with 5% silica in a 12:1 mass ratio. The washcoat loading was 60 g / l. The pore fraction, determined by the method described above, was 15%.

[0066] b) In a second step, a washcoat layer was applied to the catalyst obtained in step a) over 50% of its length, starting from the uncoated end, comprising 1.4 g / l of platinum and palladium in a 1:1 mass ratio on highly porous alumina. The washcoat layer loading was 75 g / l. The pore fraction (pores having a diameter of 0.5 μm to 50 μm) of the layer applied in the second step, determined by the method described above, was 24%.

[0067] The catalyst obtained is referred to hereinafter as K2.

[0068] Comparative Example 1

[0069] Example 1 was repeated, except that in step b) the washcoat loading was only 60 g / l. The catalyst obtained is referred to below as VK1.

[0070] Comparative Example 2

[0071] a) A commercially available circular flow-through substrate of cordierite having a pore density of 400 cpsi and a wall thickness of 4 mils, measuring 5.66" x 4.00", was coated over 50% of its length with a washcoat comprising 0.353 g / l of platinum and palladium in a 3:1 mass ratio supported on commercially available alumina doped with 5% silica. The washcoat loading was 100 g / l. The pore proportion (pores having a diameter of 0.5 μm to 50 μm), as determined by the method described above, was 15%.

[0072] b) In a second step, a washcoat layer was applied to the catalyst obtained in step a) over 50% of its length, starting from the uncoated end, comprising 1.4 g / l of platinum and palladium in a mass ratio of 3:1 on highly porous alumina. The washcoat layer loading was 100 g / l. The pore fraction (pores having a diameter of 0.5 μm to 50 μm) of the layer applied in the second step, determined by the method described above, was 15%.

[0073] The catalyst obtained is referred to hereinafter as VK2.

[0074] Comparison Test

[0075] a) Catalysts K1, K2, VK1 and VK2 were aged at 650°C under 7% humidity.

[0076] 50 hours.

[0077] b) The aged catalyst was aged on an engine test bench for 10 hours, 20 hours, 30 hours and 40 hours by adding a phosphorus-containing oil additive to the fuel. The addition of 0.6 g / l P of exposed oil additive was calculated per hour.

[0078] c) For the catalysts treated with phosphorus according to step b), the time at which the supported hydrocarbons are no longer oxidized and break through (THC slip) is determined. To this end, the same amount of diesel fuel is added to all catalysts to generate exotherm, and the HC slip is measured during the process. Achieving the target temperature of 575° C. is also measured.

[0079] d) Results

[0080]

[0081] K1 was the only catalyst to reach the target temperature of 575°C with an exposure of 24 g / L. VK1, with a lower washcoat loading in material zone B, only reached 525°C in the test, already at 18 g / L.

[0082] Compared to VK2, K2 is tolerant to phosphorus and extinguishes at only 12 g / LP at 540°C with an HC breakthrough of 5000 ppm, whereas VK2 extinguishes at 490°C with an HC breakthrough of 8000 ppm upon this exposure.

Claims

1. Catalyst, containing a carrier substrate having a first end a and a second end b and a length L, A material region A containing a platinum group metal on a support material having a loading amount of 40 g / L to 150 g / L relative to the volume of the support substrate, and The material region B has a loading amount of 75 g / L to 200 g / L relative to the volume of the carrier substrate, wherein the material region B contains a platinum group metal on a supporting material. in, Material region B has a greater content of platinum group metal than material region A, relative to the volume of the carrier substrate and calculated in g / L, and wherein material region B has pores with a diameter of 0.5 μm to 50 μm in a proportion of 20% to 30%, and material region A is coated over the entire length L of the carrier substrate, while material region B extends from the first end a of the carrier substrate over 20% to 80% of the length L and is located on material region A, or The material region A extends from the second end of the carrier substrate to 40% to 60% of the length L, and the material region B extends from the first end of the carrier substrate to 40% to 60% of the length L, wherein L = L A + L B or L < L A + L B or L > L A + L B Among them L A is the length of the material area A, and L B is the length of material area B.

2. The catalyst according to claim 1, characterized in that The material region A is coated over the entire length L of the carrier substrate, while the material region B extends from the first end a of the carrier substrate over 30% to 70% of the length L and is located on the material region A.

3. The catalyst according to claim 1, characterized in that Material region A contains palladium and platinum as platinum group metals.

4. The catalyst according to claim 3, characterized in that The weight ratio of platinum to palladium in material region A is 20:1 to 1:

5.

5. The catalyst according to claim 1, characterized in that The platinum group metal in material zone A is present in an amount of 0.18 g / L to 0.53 g / L relative to the volume of the support substrate.

6. The catalyst according to claim 1, characterized in that Material region B contains platinum and palladium as platinum group metals.

7. The catalyst according to claim 6, characterized in that The weight ratio of platinum to palladium in material region B is 20:1 to 1:

5.

8. The catalyst according to claim 1, characterized in that The platinum group metal in material zone B is present in an amount of 0.53 g / L to 1.06 g / L relative to the volume of the support substrate.

9. The catalyst according to claim 1, characterized in that The material region A is applied over the entire length L of the carrier substrate, while the material region B is located on the material region A and extends from 40% to 80% of the length L starting from the first end a of the carrier substrate.

10. The catalyst according to claim 1, characterized in that The carrier substrate is a flow-through substrate.

11. The catalyst according to any one of claims 1 to 10, characterized in that The pore ratio is determined by a method comprising the following steps: Select the catalyst area to be inspected, creating a cross section of the catalyst region to be examined, wherein the pores are filled with an organic resin, An image of the cross section is created with the aid of a scanning electron microscope (SEM), Select a portion of the SEM image to be inspected, converting the selected partial area of ​​the SEM image into a binary image with the aid of an image analysis program, and The binary image is used by means of an image analysis program to perform a porosity distribution calculation and subsequently determine the pore fraction.

12. Method for purifying exhaust gases of a motor vehicle operated with a lean-burn engine, characterized in that The exhaust gas passes through the catalyst according to any one of claims 1 to 11, wherein the exhaust gas enters the catalyst at the first end a of the carrier substrate and leaves again at the second end b.

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