Process for the oxidation of ammonia contained in an exhaust gas stream

Description

[0001] The present invention relates to a process for the oxidation of ammonia contained in an exhaust gas stream using a platinum and zinc-containing zeolite, in particular for purifying the exhaust gases of lean-burn internal combustion engines, in particular diesel engines.

[0002] In addition to carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx), the raw exhaust gas of diesel engines contains a relatively high oxygen content of up to 15 vol. It also contains particulate emissions, which consist predominantly of soot residues and possibly organic agglomerates and result from partially incomplete fuel combustion in the cylinder.

[0003] While carbon monoxide and hydrocarbons can be rendered harmless using diesel oxidation catalysts, diesel particulate filters with and without catalytically active coating are suitable for removing particulate emissions.

[0004] Nitrogen oxides can be converted to nitrogen, for example, by selective catalytic reduction (SCR) on a so-called SCR catalyst using ammonia as the reducing agent. Ammonia can be made available by thermolysis and hydrolysis of an ammonia precursor compound fed into the exhaust gas. Examples of such precursor compounds are ammonium carbamate, ammonium formate, and preferably urea. Alternatively, the ammonia can be formed by catalytic reactions within the exhaust gas.

[0005] To achieve the most complete conversion of nitrogen oxides in the SCR catalyst, it may be necessary to feed ammonia in a superstoichiometric amount, typically 10 to 20% above the stoichiometric amount. This, in turn, leads to unconverted ammonia in the exhaust gas, which is undesirable given its toxic effects and greenhouse gas properties. Consequently, ammonia emissions are increasingly being restricted in emissions legislation.

[0006] To prevent ammonia emissions, so-called ammonia slip catalysts (ASCs) have already been developed. These catalysts typically include an oxidation catalyst for oxidizing ammonia at the lowest possible temperatures.

[0007] Such oxidation catalysts typically comprise a noble metal, such as palladium and especially platinum, on a support oxide. However, they have the disadvantage of oxidizing ammonia not only to nitrogen, water, and oxygen, but also to harmful species such as nitrous oxide (N 2 O), nitrogen monoxide (NO), and nitrogen dioxide (NO 2 ).

[0008] It is known that the selectivity of ammonia oxidation to nitrogen can be improved by combining an oxidation catalyst with an SCR catalyst. In this case, the components are typically arranged in a layered arrangement, with the SCR layer normally forming the upper layer and positioned on top of the underlying oxidation layer. ASC catalysts are typically applied to a monolithic support substrate, such as a flow-through substrate or a wall-flow filter.

[0009] ASC catalysts of this type are known, for example, from EP410440A1, WO02 / 100520A1, EP2117702A2 and WO2010 / 062730A2.

[0010] However, there is still a need for ASC catalysts that exhibit good thermal stability with good selectivity to nitrogen and, in particular, form only little N 2 O. Surprisingly, it has now been found that zeolites comprising platinum and zinc exhibit the required properties.

[0011] Platinum-impregnated Zn-ZSM-5 nanocatalysts are described in Catalysis Letters 148(2), June 2018, for xylene isomerization reactions.

[0012] Platinum and zinc-containing zeolites of the LTA structure type for the oxidation of ethane are disclosed in Journal of Energy Chemistry, Volume 30, March 2019, pages 42-48.

[0013] US2018 / 280945 discloses ammonia slip catalysts comprising a zeolite containing zinc incorporated into the zeolite framework. Zeolites containing zinc incorporated into the zeolite framework are also disclosed in GB2552262A. These products are used as oxidation catalysts for stoichiometrically operated natural gas engines.

[0014] DE1545293 discloses a process for the treatment of previously hydroformed hydrocarbons. Platinum and zinc-containing zeolites of the "zeolite A" structure type and erionite can be used as catalysts in this process. However, the document does not provide any information on the SAR value of these zeolites.

[0015] EP2604590A1 relates to a process for producing unsaturated hydrocarbons and discloses, among other catalysts, a zeolite of the MFI structure type, on which platinum and zinc are present. The zeolites used have very high SAR values.

[0016] JP2013-163647A relates to a similar subject matter to EP2604590A1, using zeolites of the structure type MFI, FER and BEA.

[0017] The present invention relates to a process for the oxidation of ammonia contained in an exhaust gas stream with a zeolite which comprises zinc and platinum and which is selected from the group consisting of zeolites of the structural types AEI, AFX, BEA, CHA, ERI, FER, KFI, LEV and MFI, wherein the zinc is present (i) as a zinc cation in ion-exchanged form in the zeolite structure and (ii) as zinc oxide in the zeolite structure and / or on the surface of the zeolite structure and wherein the zeolite has a SAR (silica-to-alumina ratio) value of 2 to 1000.

[0018] Zeolites are two- or three-dimensional structures, the smallest of which can be considered to be SiO4 and AlO4 tetrahedra. These tetrahedra combine to form larger structures, with two of them being connected by a common oxygen atom. Rings of various sizes can be formed, for example rings of four, six, or even nine tetrahedrally coordinated silicon or aluminum atoms. The different zeolite types are often defined by their largest ring size, because this size determines which guest molecules can penetrate the zeolite structure and which cannot. It is common to distinguish between large-pore zeolites with a maximum ring size of 12, medium-pore zeolites with a maximum ring size of 10, and small-pore zeolites with a maximum ring size of 8.

[0019] Zeolites are further divided into structural types by the Structural Commission of the International Zeolite Association, each assigned a three-letter code, see, for example, Atlas of Zeolite Framework Types, Elsevier, 5th edition, 2001.

[0020] Preferred zeolites are selected from the group consisting of zeolites of the structural types AEI, AFX, CHA and FER.

[0021] A very preferred zeolite belongs to the structural type AEI.

[0022] Another highly preferred zeolite belongs to the AFX structure type. Another highly preferred zeolite belongs to the CHA structure type. Another highly preferred zeolite belongs to the FER structure type.

[0023] The zeolite used according to the invention has in particular a SAR (silica-to-alumina ratio) value of 2 to 500, preferably of 2 to 100 and particularly preferably of 5 to 50.

[0024] In the context of the present invention, the term "zeolite" expressly does not include mixed oxides comprising aluminum oxide and silicon oxide, as described in the literature, for example, as "SiO 2 / Al 2 O 3 ".

[0025] In the zeolite used according to the invention, the platinum is preferably present as a platinum cation in the zeolite structure, i.e., in ion-exchanged form. However, it can also be present wholly or partially as platinum metal and / or platinum oxide in the zeolite structure and / or on the surface of the zeolite structure.

[0026] The platinum may be present in amounts of 0.01 to 20 wt.%, based on the sum of the weights of zeolite, zinc and platinum and calculated as zinc metal and platinum metal.

[0027] The platinum is preferably present in amounts of 0.5 to 10, particularly preferably 0.5 to 6 wt.% and most particularly preferably 0.5 to 5 wt.%, based on the sum of the weights of zeolite, zinc and platinum and calculated as zinc metal and platinum metal.

[0028] The zinc in the zeolite used according to the invention is present (i) as zinc cation in ion-exchanged form in the zeolite structure and (ii) as zinc oxide

[0029] in the zeolite structure and / or on the surface of the zeolite structure. This means, in particular, that the zinc according to the present invention is not a component or building block of the zeolite framework. The zinc can be present in amounts of 0.01 to 20 wt. %, based on the sum of the weights of zeolite, zinc, and platinum, and calculated as zinc metal and platinum metal.

[0030] The zinc is preferably present in amounts of 0.5 to 10, particularly preferably 0.5 to 6 wt.% and most particularly preferably 0.5 to 5 wt.%, based on the sum of the weights of zeolite, zinc and platinum and calculated as zinc metal and platinum metal.

[0031] The zeolite of material A preferably has a platinum : zinc mass ratio of 10:1 to 1:17, particularly preferably 6:1 to 1:10 and very particularly preferably 1:2 to 1:7, where platinum is calculated as platinum metal and zinc as zinc metal.

[0032] The zeolite used according to the invention can be produced by methods known per se. For example, it is obtained by impregnating a zeolite, which is present, for example, in the H or NH 4 form, with an aqueous solution of a mixture of platinum and zinc salts in the appropriate amounts using the incipient wetness method, followed by drying and calcination. Platinum nitrate (Pt[NO 3 ] 2 ) is particularly suitable as a water-soluble platinum salt, and zinc acetate (Zn[Ac] 2 ) is particularly suitable as a water-soluble zinc salt.

[0033] The calcination of the impregnated zeolite takes place primarily at temperatures of 250 to 550°C.

[0034] In an alternative process, an aqueous suspension of a zeolite at alkaline pH values is mixed with a platinum salt, such as Pt-TEAH (tetraethylammonium), so that the platinum can be adsorbed onto the zeolite. An acidic pH is then adjusted and zinc is added, for example, in the form of zinc acetate.

[0035] In a preferred embodiment of the present invention, the zeolite used according to the invention is present on a carrier substrate.

[0036] The present disclosure also relates to a catalyst comprising a support substrate of length L and a zeolite which comprises zinc and platinum and which is selected from the group consisting of zeolites of the structure types AEI, AFX, BEA, CHA, ERI, FER, KFI, LEV and MFI, wherein the zinc is present (i) as a zinc cation in ion-exchanged form in the zeolite structure and / or (ii) as zinc oxide in the zeolite structure and / or on the surface of the zeolite structure and wherein the zeolite has a SAR (silica-to-alumina ratio) value of 2 to 1000.

[0037] The carrier substrate can be a flow-through substrate or a wall-flow filter.

[0038] A wall-flow filter is a support substrate comprising channels of length L extending parallel between a first and a second end of the wall-flow filter, which are alternately closed at either the first or second end, and which are separated by porous walls. A flow-through substrate differs from a wall-flow filter in particular in that the channels of length L are open at both ends.

[0039] Wall-flow filters, for example, have porosities of 30 to 80%, particularly 50 to 75%, in their uncoated state. Their average pore diameter in their uncoated state is, for example, 5 to 30 micrometers.

[0040] Typically, the pores of the wall-flow filter are so-called open pores, meaning they are connected to the channels. Furthermore, the pores are usually interconnected. This allows for easy coating of the inner pore surfaces and, at the same time, for the exhaust gas to pass easily through the porous walls of the wall-flow filter.

[0041] Flow-through substrates are familiar to experts, just like wall-flow filters, and are available on the market. They consist of materials such as silicon carbide, aluminum titanate, or cordierite.

[0042] In one embodiment of the catalyst used according to the invention, the zeolite, which comprises zinc and platinum, is present in the form of a coating on the support substrate. The coating can extend over the entire length L of the support substrate or only over a portion of it. In both cases, the support substrate can also support one or more additional catalytically active coatings.

[0043] In the case of a wall-flow filter, the coating can be located on the surfaces of the inlet channels, on the surfaces of the outlet channels and / or in the porous wall between the inlet and outlet channels.

[0044] Catalysts used according to the invention, in which the zeolite comprising zinc and platinum is present in the form of a coating on the carrier substrate, can be produced by methods familiar to those skilled in the art, such as conventional dip coating processes or pump and suction coating processes with subsequent thermal post-treatment (calcination). Those skilled in the art are aware that, in the case of wall-flow filters, their average pore size and the average particle size of the materials to be coated can be matched to one another such that they lie on the porous walls that form the channels of the wall-flow filter (on-wall coating). However, the average particle size of the materials to be coated can also be selected such that they are located in the porous walls that form the channels of the wall-flow filter, thus resulting in a coating of the inner pore surfaces (in-wall coating).In this case, the average particle size of the materials to be coated must be small enough to penetrate the pores of the wall flow filter.

[0045] In another embodiment of the present disclosure, the support substrate is formed from the zeolite comprising zinc and platinum and a matrix component.

[0046] Support substrates, flow-through substrates, and wall-flow filters that consist not only of inert material, such as cordierite, but also contain a catalytically active material are known to those skilled in the art. To produce them, a mixture of, for example, 10 to 95 wt.% inert matrix component and 5 to 90 wt.% catalytically active material is extruded using conventional methods. Any inert materials otherwise used to produce catalyst substrates can be used as matrix components. These include, for example, silicates, oxides, nitrides, or carbides, with magnesium aluminum silicates being particularly preferred.

[0047] The extruded support substrate comprising the zeolite containing zinc and platinum may, in embodiments of the present invention, be coated with one or more catalytically active coatings.

[0048] In a further embodiment of the present disclosure, a carrier substrate composed of corrugated sheets of inert materials is used. Such carrier substrates are known to those skilled in the art as "corrugated substrates." Suitable inert materials include, for example, fibrous materials with an average fiber diameter of 50 to 250 µm and an average fiber length of 2 to 30 mm. Preferably, fibrous materials are heat-resistant and consist of silicon dioxide, in particular glass fibers.

[0049] To produce such carrier substrates, for example, sheets of the aforementioned fiber materials are corrugated in a known manner, and the individual corrugated sheets are formed into a cylindrical, monolithically structured body with channels extending through the body. Preferably, a monolithically structured body with a crosswise corrugation structure is formed by stacking a number of the corrugated sheets into parallel layers with different corrugation orientations between the layers. In one embodiment, non-corrugated, i.e., flat, sheets can be arranged between the corrugated sheets.

[0050] Corrugated sheet substrates can be coated directly with the zeolite comprising zinc and platinum, but preferably they are first coated with an inert material, for example titanium dioxide, and only then with the catalytic material.

[0051] The following examples 1-4 are not according to the invention. Example 1

[0052] First, a mixed platinum nitrate / zinc acetate solution is prepared, the volume of which corresponds to the 50 percent water absorption of the zeolite (a commercially available CHA zeolite). Based on the final composition of the platinum and zinc-containing zeolite, 0.42 wt% platinum and 0.07 wt% zinc (Pt:Zn mass ratio = 6:1) are applied to the zeolite in a mechanical mixer. The subsequent thermal treatment includes drying at 120 °C, calcination at 350 °C, and annealing at 550 °C in air.

[0053] In the subsequent washcoat preparation, 10% of a commercially available alumina sol (based on the total loading) is added and coated onto a commercially available ceramic support substrate with a washcoat loading of 25 g / l. The substrate is then dried in air at 120 °C, calcined at 350 °C, and annealed at 550 °C. The resulting catalyst is referred to below as K1. Example 2

[0054] Example 1 is repeated with the difference that the amount of zinc is 0.2 wt% (mass ratio Pt:Zn = 2:1).

[0055] The catalyst obtained is referred to as K2 below. Example 3

[0056] Example 1 is repeated with the difference that the amount of zinc is 0.6 wt% (mass ratio Pt:Zn = 1:1.5).

[0057] The catalyst obtained is referred to as K3 below. Example 4

[0058] Example 1 is repeated with the difference that the amount of zinc is 2.64 wt% (mass ratio Pt:Zn = 1:6.6).

[0059] The catalyst obtained is referred to as K4 below. Comparison example 1

[0060] Example 1 is repeated except that no zinc is used. The resulting catalyst is referred to below as VK1. Example 5

[0061] A commercially available CHA zeolite is placed in water and the pH is adjusted to 10. Pt-TEAH is then added, and the suspension is stirred for 24 h. The pH is then adjusted to 6, and zinc acetate and 10% alumina sol are added. The Pt:Zn mass ratio is 1:1.7.

[0062] Following linear milling, a commercially available ceramic support substrate is coated with a washcoat quantity of 25 g / l. The final thermal treatment in air includes drying at 120 °C, calcination, and annealing at 350 and 550 °C. The total precious metal concentration on the final catalyst (hereinafter referred to as K5) is 0.42 wt.%. Comparison example 2

[0063] Example 5 is repeated, except that after adding Pt-TEAH, the mixture is stirred for 20 h and no zinc is used. The resulting catalyst is referred to below as VK2. Determination of NH 3 light off and N 2 O formation a) Aging

[0064] Four core samples were cut from each of the catalysts K1 to K5 and VK1 and VK2, two of which were measured in the fresh state and two after hydrothermal aging (10% H 2 O, 10% O 2 , balance N 2 ) in a furnace for 16 hours at 800°C (hereinafter 16H800). b) Test conditions in the laboratory reactor

[0065] A synthetic test exhaust gas consisting of 300 ppm NH 3 , 5% O 2 , 5% H 2 O, balance N 2 (Test A or B) or a test exhaust gas consisting of 300 ppm NH 3 , 200 ppm NO, 5% O 2 , 5% H 2 O, balance N 2 (Test C or D) was passed through the drill cores obtained according to a) in a laboratory reactor at 1950 L / hour. After a conditioning phase (~ 30 K / min from 150 to 600 °C in 5% O 2 , balance N 2 ), the temperature of the test exhaust gas was increased at 10 K / min from 150 to 600 °C and the NH 3 conversion was determined using a conventional method. c) Results

[0066] The following tables show the results obtained: Table 1: Test A: fresh, only NH 3 K1 K2 K3 K4 VK1 K5 VK2 NH 3 T50 / °C 222 221 220 218 228 208 220 NH 3 conversion (350 °C) / % 99 99 99 99 99 99 98 NH 3 conversion (550 °C) / % 99 99 99 100 100 100 99 NO formation (210 - 400 °C) / ppm 79 70 77 89 65 113 70 NO formation (550 °C) / ppm 206 200 206 235 196 225 202 N 2 O formation (210 - 400 °C) / ppm 44 48 48 48 47 35 44 N 2 O formation max. / ppm 81 86 85 86 87 66 79 Table 2: Test B: 16H800, only NH 3 K1 K2 K3 K4 VK1 K5 VK2 NH 3 T50 / °C 209 207 205 203 210 194 202 NH 3 conversion (350 °C) / % 97 97 97 97 97 95 97 NH 3 conversion (550 °C) / % 98 98 98 98 98 95 98 NO formation (210 - 400 °C) / ppm 124 125 128 133 125 155 135 NO formation (550 °C) / ppm 252 252 251 257 254 265 254 N 2 O formation (210 - 400 °C) / ppm 34 34 33 32 35 25 31 N 2 O formation max. / ppm 54 55 53 47 54 44 51 Table 3: Test C: fresh, NH 3 + NO K1 K2 K3 K4 VK1 K5 VK2 NH 3 T50 / °C 206 202 203 198 206 209 210 NH 3 conversion (350 °C) / % 99 99 99 99 99 99 99 NH 3 conversion (550 °C) / % 99 100 99 100 100 100 100 NO formation (210 - 400 °C) / ppm 192 175 186 200 168 238 195 NO formation (550 °C) / ppm 364 354 363 403 346 389 384 N 2 O formation (210 - 400 °C) / ppm 77 82 83 82 84 59 75 N 2 O formation max. / ppm 149 160 157 162 159 119 138 Table 4: Test D: 16H800, NH 3 + NO K1 K2 K3 K4 VK1 K5 VK2 NH 3 T50 / °C 214 213 214 215 217 212 214 NH 3 conversion (350 °C) / % 96 97 97 97 96 95 97 NH 3 conversion (550 °C) / % 98 98 98 98 97 96 98 NO formation (210 - 400 °C) / ppm 269 270 273 279 268 304 296 NO formation (550 °C) / ppm 434 433 433 440 432 453 461 N 2 O formation (210 - 400 °C) / ppm 60 60 57 55 60 42 49 N 2 O formation max. / ppm 105 105 102 94 102 83 91

[0067] The test results show that, depending on the zinc content of the non-inventive catalysts K1 to K4 and the inventive catalyst K5, the light-off temperatures for ammonia decrease compared to the comparative catalysts VK1 and VK2, which contain only platinum. Although this higher activity leads to higher NO formation, it results in lower, not higher, N2O formation. The addition of zinc therefore leads to higher NO selectivity and lower N2O selectivity. However, the higher NO selectivity is not disadvantageous, since NO can be converted into nitrogen and oxygen by an SCR layer.

Claims

1. Method for the oxidation of ammonia contained in an exhaust gas stream, characterized in that the exhaust gas stream is passed over a zeolite which comprises zinc and platinum and which is selected from the group consisting of zeolites of the structure types AEI, AFX, BEA, CHA, ERI, FER, KFI, LEV and MFI, wherein the zinc is present (i) as a zinc cation in ion-exchanged form in the zeolite structure and (ii) as zinc oxide in the zeolite structure and / or on the surface of the zeolite structure and wherein the zeolite has a SAR (silica-to-alumina ratio) value of 2 to 1000.

2. Method according to claim 1, characterized in that the zeolite is selected from the group consisting of zeolites of the structural types AEI, AFX, CHA, and FER.

3. Method according to claim 1 and / or 2, characterized in that the zeolite belongs to the AEI structural type.

4. Method according to claim 1 and / or 2, characterized in that the zeolite belongs to the AFX structural type.

5. Method according to claim 1 and / or 2, characterized in that the zeolite belongs to the CHA structural type.

6. Method according to claim 1 and / or 2, characterized in that the zeolite belongs to the FER structural type.

7. Method according to one or more of claims 1 to 6, characterized in that the zeolite has a SAR (silica-to-alumina ratio) value of 2 to 500.

8. Method according to one or more of claims 1 to 7, characterized in that the zeolite has a SAR (silica-to-alumina ratio) value of 2 to 100.

9. Method according to one or more of claims 1 to 8, characterized in that the zeolite has a SAR (silica-to-alumina ratio) value of 2 to 50.

10. Method according to one or more of claims 1 to 9, characterized in that the zeolite comprises platinum in amounts of 0.01 to 20% by weight, based on the sum of the weights of zeolite, zinc, and platinum and calculated as zinc metal and platinum metal.

11. Method according to one or more of claims 1 to 10, characterized in that the zeolite comprises zinc in amounts of 0.01 to 20 % by weight, based on the sum of the weights of zeolite, zinc, and platinum and calculated as zinc metal and platinum metal.

12. Method according to one or more of claims 1 to 11, characterized in that the mass ratio of platinum: Zinc of the zeolite is 6:1 to 1:7, wherein platinum is calculated as platinum metal and zinc is calculated as zinc metal.