Double-layered three-way catalyst with significantly improved CO conversion

Abstract

The present invention relates to a catalyst comprising two layers on an inert catalyst support. Layer A contains at least palladium as a platinum group metal, aluminum oxide, and a first cerium / zirconium / lanthanum / yttrium mixed oxide. A layer B applied to layer A contains at least rhodium as a platinum group metal, aluminum oxide, and a second cerium / zirconium / lanthanum / yttrium mixed oxide.

Description

[0001] The present invention relates to a three-way catalyst which is composed of two superimposed, catalytically active layers and which is suitable for cleaning the exhaust gases of internal combustion engines.

[0002] Three-way catalytic converters are used to clean the exhaust gases of combustion engines that operate primarily in stoichiometric conditions. In stoichiometric operation, the amount of air supplied to the engine corresponds exactly to the amount required for complete combustion of the fuel. In this case, the air-fuel ratio (λ) is exactly 1. Three-way catalytic converters, operating near λ = 1, are able to simultaneously convert hydrocarbons, carbon monoxide, and nitrogen oxides into harmless components.

[0003] The catalytically active materials used are typically platinum group metals, especially platinum, palladium, and rhodium, which are present, for example, on γ-aluminum oxide as a support material. Three-way catalysts also contain oxygen storage materials, such as cerium / zirconium mixed oxides. In the latter, cerium oxide, a rare-earth metal oxide, is the key component for oxygen storage. Besides zirconium oxide and cerium oxide, these materials can contain additional components such as other rare-earth metal oxides or alkaline earth metal oxides. Oxygen storage materials are activated by the application of catalytically active materials such as platinum group metals and thus also serve as a support material for the platinum group metals.

[0004] The components of a three-way catalyst can be contained in a single coating layer on an inert catalyst support, see for example EP1541220B1.

[0005] However, double-layer catalysts are frequently used, which allow for the separation of different catalytic processes and thus an optimal coordination of the catalytic effects in the two layers. Catalysts of the latter type are disclosed, for example, in WO95 / 35152A1, WO2008 / 000449A2, EP0885650A2, EP1046423A2, EP1726359A1 and EP1974809B1.

[0006] EP1974809B1 discloses double-layer three-way catalysts containing cerium / zirconium mixed oxides in both layers, wherein the cerium / zirconium mixed oxide in the upper layer has a higher proportion of zirconium than that in the lower layer.

[0007] EP1900416B1 describes double-layer three-way catalysts containing mixed oxides of cerium, zirconium and neodymium in both layers and additionally cerium / zirconium / yttrium / lanthanum oxide-aluminum oxide particles in the lower layer.

[0008] EP1726359A1 describes double-layer three-way catalysts containing cerium / zirconium / lanthanum / neodymium mixed oxides with a zirconium content of more than 80 mol% in both layers, wherein the cerium / zirconium / lanthanum / neodymium mixed oxide in the upper layer may have a higher proportion of zirconium than that in the lower layer.

[0009] WO2008 / 000449A2 also discloses double-layer catalysts containing cerium / zirconium mixed oxides in both layers, with the mixed oxide in the upper layer having a higher zirconium content. In some cases, the cerium / zirconium mixed oxides can also be replaced by cerium / zirconium / lanthanum / neodymium or cerium / zirconium / lanthanum / yttrium mixed oxides.

[0010] WO2009 / 012348A1 even describes three-layer catalysts, where only the middle and top layers contain oxygen storage materials.

[0011] EP3045226A1 discloses double-layer three-way catalysts with improved aging stability, wherein a layer A lying directly on the catalyst support contains at least one platinum group metal, as well as a cerium / zirconium / SE mixed oxide, and a layer B applied to layer A and in direct contact with the exhaust gas stream contains at least one platinum group metal, as well as a cerium / zirconium / SE mixed oxide, wherein SE stands for a rare earth metal other than cerium, characterized in that the proportion of the SE oxide in the cerium / zirconium / SE mixed oxide of layer A is smaller than the proportion of the SE oxide in the cerium / zirconium / SE mixed oxide of layer B.

[0012] EP4096811A1 describes a catalyst which, due to its further increased temperature stability compared to prior art catalysts, exhibits further reduced start-up temperatures and improved dynamic conversion capacity after aging. It comprises two layers on an inert catalyst support, wherein layer A contains at least palladium as a platinum group metal and a cerium / zirconium / lanthanum / yttrium mixed oxide, and layer B applied to layer A contains at least rhodium as a platinum group metal and a cerium / zirconium / lanthanum / yttrium mixed oxide. In both layers A and B, the lanthanum oxide content is between 1 wt% and 5 wt% based on the cerium / zirconium / lanthanum / yttrium mixed oxide, and the yttrium oxide content is between 8 wt% and 20 wt% based on the cerium / zirconium / lanthanum / yttrium mixed oxide.

[0013] The ever-increasing demands on emission reduction from combustion engines necessitate the continuous development of catalytic converters. Since July 2023, the CN6b legislation has been in effect in China. This stipulates a carbon monoxide (CO) limit of 500 mg / km for passenger vehicles. The CN6b limits must be met over a mileage of 200,000 km. Furthermore, CN6b mandates exhaust gas measurements under real-world driving conditions (Real Driving Emissions, RDE). Depending on driving conditions, this can result in significantly higher demands on the catalytic converter, particularly with regard to the dynamic conversion of carbon monoxide (CO) and nitrogen oxides (NOx).

[0014] The aim is to maintain stoichiometric exhaust gas values ​​under all driving conditions, especially at high speeds. The previously common practice of enriching the fuel mixture to lower exhaust gas temperature must be avoided, as this leads to high CO emissions and increases fuel consumption. However, avoiding enrichment further increases the exhaust gas temperature to which the catalytic converter is exposed at high speeds. The catalytic converter must cope with these increased demands. This is another reason why further improvements in the aging stability of three-way catalytic converters are necessary.

[0015] The catalysts based on the aforementioned state of the art already exhibit very good properties regarding dynamic conversion capacity after aging. However, the increased legal requirements necessitate the search for even better catalysts. In particular, compliance with the low CO limit for CN6b is very challenging and requires targeted improvements in catalyst performance.

[0016] The objective of this invention was therefore to provide a catalyst that exhibits a significantly improved conversion of CO under dynamic conditions.

[0017] The present invention thus relates to a catalyst comprising two layers on an inert catalyst support, wherein a layer A contains at least palladium as a platinum group metal, aluminium oxide and a first cerium / zirconium / lanthanum / yttrium mixed oxide, and a layer B applied to layer A contains at least rhodium as a platinum group metal, aluminium oxide and a second cerium / zirconium / lanthanum / yttrium mixed oxide.

[0018] In layer A, the cerium oxide content is between 40 wt% and 50 wt%, based on the first cerium / zirconium / lanthanum / yttrium mixed oxide; the lanthanum oxide content is between 2 wt% and 10 wt%, based on the first cerium / zirconium / lanthanum / yttrium mixed oxide; and the yttrium oxide content is between 2 wt% and 8 wt%, based on the first cerium / zirconium / lanthanum / yttrium mixed oxide. The mass ratio of mixed oxide to aluminum oxide is at least 1.5:1 and at most 1.75:1.

[0019] In layer B, the cerium oxide content is between 20 wt.% and 30 wt.%, based on the second cerium / zirconium / lanthanum / yttrium mixed oxide, the lanthanum oxide content is between 1 wt.% and 5 wt.%, based on the second cerium / zirconium / lanthanum / yttrium mixed oxide, and the yttrium oxide content is between 8 wt.% and 20 wt.%, based on the second cerium / zirconium / lanthanum / yttrium mixed oxide.

[0020] By adjusting the composition of the mixed oxide and the ratio of the components of the lower layer, a solution to the problem has now been surprisingly found. If the lower layer of the catalyst according to the invention is provided with a mass ratio of mixed oxide to aluminum oxide of at least 1.5:1 and at most 1.75:1, preferably between 1.6:1 and 1.7:1, three-way catalysts are obtained that exhibit a significantly improved catalytic conversion of CO under dynamic conditions than known three-way catalysts.

[0021] A preferred embodiment is characterized in that in layer A the cerium oxide content is between 43 wt.% and 46 wt.%, based on the first cerium / zirconium / lanthanum / yttrium mixed oxide, the lanthanum oxide content is between 5 wt.% and 7 wt.%, based on the first cerium / zirconium / lanthanum / yttrium mixed oxide, and the yttrium oxide content is between 4 wt.% and 6 wt.%, based on the first cerium / zirconium / lanthanum / yttrium mixed oxide. In this preferred embodiment, the cerium oxide content in layer B is between 23 wt.% and 25 wt.%, based on the second cerium / zirconium / lanthanum / yttrium mixed oxide, the lanthanum oxide content is between 2 wt.% and 4 wt.%, based on the second cerium / zirconium / lanthanum / yttrium mixed oxide, and the yttrium oxide content is between 12 wt.% and 13 wt.%, based on the second cerium / zirconium / lanthanum / yttrium mixed oxide.

[0022] In a further preferred embodiment, the catalyst according to the invention has in layer B a mass ratio of second cerium / zirconium / lanthanum / yttrium mixed oxide to aluminium oxide of at least 1:1 and at most 1.5:1, preferably 1.2:1 to 1.35:1.

[0023] As shown in the examples, this allows for very good CO conversion despite intensive aging, ultimately resulting in lower emissions during dynamic driving.

[0024] According to the invention, layer A contains at least palladium as a platinum group metal, and layer B contains at least rhodium as a platinum group metal. In embodiments of the present invention, layer A and / or layer B additionally contain platinum as a further platinum group metal, independently of one another. Preferably, layer A contains palladium and platinum, and layer B contains rhodium and platinum, or rhodium, palladium, and platinum. In further embodiments of the present invention, the catalyst according to the invention is free of platinum. Particularly preferably, layer A contains only palladium and layer B only rhodium, or layer B contains only palladium and rhodium.

[0025] Cerium / zirconium / lanthanum / yttrium mixed oxides can serve as support materials for the platinum group metals in layer A and / or layer B. Alternatively, the platinum group metals in layer A and / or layer B can also be fully or partially supported on active aluminum oxide.

[0026] In a preferred embodiment of the present invention, layers A and B therefore contain active aluminum oxide. It is particularly preferred if the active aluminum oxide is stabilized by doping, especially with lanthanum oxide. Preferred active aluminum oxides contain 0.5 to 6 wt.%, particularly 3 to 5 wt.%, lanthanum oxide (La₂O₃).

[0027] The term "active aluminum oxide" is familiar to those skilled in the art. It refers in particular to γ-aluminum oxide with a specific surface area of ​​100 m². 2 / g up to 200 m 2 / g. Active aluminum oxide is widely described in the literature and available on the market.

[0028] The term "cerium / zirconium / lanthanum / yttrium mixed oxide" as used in the present invention excludes physical mixtures of cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. Rather, "cerium / zirconium / lanthanum / yttrium mixed oxides" are characterized by a largely homogeneous, three-dimensional crystal structure that is ideally free of phases of pure cerium oxide, zirconium oxide or lanthanum oxide, and yttrium oxide. Depending on the manufacturing process, however, products that are not completely homogeneous may also be formed, which can generally be used without disadvantage.

[0029] The cerium / zirconium / lanthanum / yttrium mixed oxides of the present invention, in particular, do not contain aluminum oxide in their crystal structure.

[0030] In embodiments of the present invention, one or both layers contain alkaline earth compounds such as barium oxide or barium sulfate. Preferred embodiments contain barium sulfate in layer A. The amount of barium sulfate is particularly 5 g / l to 20 g / l of the volume of the inert catalyst support.

[0031] In further embodiments of the present invention, one or both layers additionally contain additives such as rare earth compounds, e.g., lanthanum oxide, and / or binders, e.g., aluminum compounds. These additives are used in quantities that can vary within wide limits and which a person skilled in the art can determine in a specific case using simple means.

[0032] In a further embodiment of the present invention, layer A lies directly on the inert catalyst support, i.e., there is no further layer or "undercoat" between the inert catalyst support and layer A. In a further embodiment of the present invention, layer B is in direct contact with the exhaust gas stream, i.e., there is no further layer or "overcoat" on layer B.

[0033] In a further embodiment of the present invention, the catalyst according to the invention consists of layers A and B on an inert catalyst support. This means that layer A lies directly on the inert catalyst support, layer B is in direct contact with the exhaust gas stream, and that no further layers are present.

[0034] Suitable catalytically inert catalyst carriers are ceramic or metal honeycomb bodies with a volume V, featuring parallel flow channels for the exhaust gases of the combustion engine. These can be either flow-through honeycomb bodies or wall-flow filters. Particularly in the case of a wall-flow filter, the catalytic coating according to the invention can be located completely on, partially in, or completely within the wall of the wall-flow filter.

[0035] According to the invention, the wall surfaces of the flow channels are coated with the two catalyst layers A and B. To coat the catalyst support with layer A, the solids intended for this layer are suspended in water, and the catalyst support is optionally coated on and / or in the wall with the coating suspension thus obtained. The process is repeated with a coating suspension containing the solids intended for layer B suspended in water.

[0036] Preferably, both layer A and layer B are coated over the entire length of the inert catalyst support. This means that layer B completely covers layer A, and consequently only layer B comes into direct contact with the exhaust gas flow. However, a zoned coating variant is also possible, in which layer A is at least partially covered by layer B.

[0037] The present invention also relates to an exhaust system for reducing the harmful components in the exhaust gas of a combustion engine, particularly one operated predominantly stoichiometrically, comprising a catalyst according to the invention. Furthermore, the exhaust system may include other exhaust gas purification components known to those skilled in the art for this purpose. Preferably, the exhaust system also includes a particulate filter. Advantageous exhaust systems in which at least one of the three-way catalysts therein can be replaced by the one according to the invention are described, for example, in WO2020079131A1.

[0038] The present invention also relates to the use of a catalyst or exhaust system according to the invention for reducing the harmful components in the exhaust gas of a combustion engine, particularly one operated predominantly stoichiometrically. However, a use in which the combustion engine is in a state where it does not burn a rich exhaust gas mixture for more than 90% of its operating time, preferably 100% of its operating time, is preferred. In this case, there is a latent risk that the catalyst according to the invention will be exposed to high exhaust gas temperatures. For conventional catalysts, this would lead to a rapid decrease in cleaning efficiency. The result is that more exhaust gases can reach the atmosphere unimpeded. However, the three-way catalyst according to the invention can evidently maintain its cleaning effect to a particularly impressive degree under these exhaust gas temperatures.This was more than surprising given the state of the art. Examples:

[0039] In the following example 1 and the comparative examples 1 to 3, two-layer catalysts were produced by coating ceramic flow honeycomb supports with 93 cells per cm² twice. 2 and with a wall thickness of 0.11 mm and dimensions of 11.8 cm in diameter and 11.4 cm in length. Two different suspensions were prepared for layers A and B. The substrate was first coated with the suspension for layer A and then calcined in air at 550°C for 4 hours. Afterward, the substrate coated with layer A was coated with the suspension for layer B and then calcined under the same conditions as for layer A.

[0040] Example 1 contains, according to the invention, a higher proportion of mixed oxide in layer A than comparative examples 1-3. The mass ratio of mixed oxide to aluminum oxide in Example 1 is 1.66:1, while the comparative examples each have a ratio of mixed oxide to aluminum oxide of 1:1. Example 1 (according to the invention)

[0041] A two-layer catalyst was prepared by first generating two suspensions. The composition of the first suspension for layer A was (based on the volume of the catalyst support) 49.4 g / L activated aluminum oxide stabilized with 4 wt% La₂O₃, 82.3 g / L cerium / zirconium / lanthanum / yttrium mixed oxide with 44.5 wt% CeO₂, 44.5 wt% ZrO₂, 6 wt% La₂O₃ and 5 wt% Y₂O₃, 16 g / L BaSO₄, 0.954 g / L Pd.

[0042] The composition of the second suspension for layer B was (based on the volume of the catalyst support) 47 g / L activated aluminum oxide stabilized with 4 wt% La2O3, 60 g / L cerium / zirconium / lanthanum / yttrium mixed oxide with 24 wt% CeO2, 60 wt% ZrO2, 3.5 wt% La2O3 and 12.5 wt% Y2O3, 0.106 g / L Rh. Comparative example 1 (according to EP3045226A1)

[0043] A two-layer catalyst was prepared analogously to Example 1. The composition of the first suspension for layer A was 66 g / L activated aluminum oxide stabilized with 4 wt% La₂O₃, 66 g / L cerium / zirconium / lanthanum / yttrium mixed oxide with 25 wt% CeO₂, 67.5 wt% ZrO₂, 3.5 wt% La₂O₃ and 4 wt% Y₂O₃, 16 g / L BaSO₄, 0.954 g / L Pd.

[0044] The composition of the second suspension for layer B was the same as in example 1. Comparative example 2 (according to EP4096811A1)

[0045] A two-layer catalyst was prepared analogously to Example 1. The composition of the first suspension for layer A was (based on the volume of the catalyst support) 66 g / L activated aluminum oxide stabilized with 4 wt% La₂O₃, 66 g / L cerium / zirconium / lanthanum / yttrium mixed oxide with 24 wt% CeO₂, 60 wt% ZrO₂, 3.5 wt% La₂O₃ and 12.5 wt% Y₂O₃, 16 g / L BaSO₄, 0.954 g / L Pd.

[0046] The composition of the second suspension for layer B was the same as in example 1. Comparative example 3 (according to EP1974809B1)

[0047] A two-layer catalyst was prepared analogously to Example 1. The composition of the first suspension for layer A was (based on the volume of the catalyst support) 66 g / L activated aluminum oxide stabilized with 4 wt% La₂O₃, 66 g / L cerium / zirconium / lanthanum / yttrium mixed oxide with 44.5 wt% CeO₂, 44.5 wt% ZrO₂, 6 wt% La₂O₃ and 5 wt% Y₂O₃, 16 g / L BaSO₄, 0.954 g / L Pd.

[0048] The composition of the second suspension for layer B was the same as in example 1.

[0049] Example 1 and comparative examples 1-3 were aged in an engine test bench. The aging process consisted of overrun fuel cut-off aging with an exhaust gas temperature of 950°C before the catalyst inlet. This resulted in a maximum bed temperature of 1020°C in the catalyst. The aging time was 38 hours.

[0050] Subsequently, the dynamic conversion was determined on an engine test bench within a range of λ from 0.99 to 1.01 at a constant temperature of 510°C. The amplitude of λ was ±6.8%, and the exhaust gas mass flow rate was 190 kg / h. Table 2 shows the conversion at the intersection of the CO and NOx conversion curves, as well as the corresponding HC conversion.

[0051] Table 1 contains the revenue at the intersection of the CO and NOx revenue curves, as well as the corresponding HC revenue. Table 1: Dynamic Implementation CO / NOx turnover at the intersection HC turnover at λ of the CO / NOx intersection point Example 1 87% 91% Comparative example 1 83,5% 91% Comparative example 2 85,5% 91 Comparative example 3 84% 90,5

[0052] Example 1 according to the invention shows a significant improvement in the dynamic CO / NOx conversion after aging compared to all comparative examples.

[0053] Furthermore, emissions were determined on a highly dynamic engine test bench using a 1.2 L gasoline direct injection engine with exhaust gas turbocharger in a very dynamic driving cycle (RTS 95 or RTS Aggressive RTS 95 Cycle (dieselnet.com)). Table 2 shows the measured emissions. Table 2: Emissions in the RTS 95 cycle CO emission (mg / km) NOx emission (mg / km) HC emission (mg / km) Example 1 732 72 61 Comparative example 1 878 68 56 Comparative example 2 875 69 57 Comparative example 3 833 74 60

[0054] Example 1 according to the invention shows a very significant improvement of almost 17% in CO emissions compared to comparative example 1, and only minor disadvantages in NOx (<6%) and HC emissions (<9%). QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] EP 1541220B1

[0004] WO 95 / 35152A1

[0005] WO 2008 / 000449A2 [0005, 0009] EP 0885650A2

[0005] EP 1046423A2

[0005] EP 1726359A1 [0005, 0008] EP 1974809B1 [0005, 0006] EP 1900416B1

[0007] WO 2009 / 012348A1

[0010] EP 3045226A1

[0011] EP 4096811A1

[0012] WO 2020079131A1

[0037]

Claims

[1] Catalyst comprising two layers arranged one above the other on an inert catalyst support, wherein • a layer A contains at least palladium as a platinum group metal, aluminium oxide and a first cerium / zirconium / lanthanum / yttrium mixed oxide and • a layer B applied to layer A contains at least rhodium as a platinum group metal, aluminium oxide and a second cerium / zirconium / lanthanum / yttrium mixed oxide, characterized by , that • in layer A the cerium oxide content is between 40 wt.% and 50 wt.%, based on the first cerium / zirconium / lanthanum / yttrium mixed oxide, the lanthanum oxide content is between 2 wt.% and 10 wt.%, based on the first cerium / zirconium / lanthanum / yttrium mixed oxide, and the yttrium oxide content is between 2 wt.% and 8 wt.%, based on the first cerium / zirconium / lanthanum / yttrium mixed oxide, and the mass ratio of first cerium / zirconium / lanthanum / yttrium mixed oxide to aluminium oxide is at least 1.5:1 and at most 1.75:1, • and in layer B the cerium oxide content is between 20 wt.% and 30 wt.%, based on the second cerium / zirconium / lanthanum / yttrium mixed oxide, the lanthanum oxide content is between 1 wt.% and 5 wt.%, based on the second cerium / zirconium / lanthanum / yttrium mixed oxide, and the yttrium oxide content is between 8 wt.% and 20 wt.%, based on the second cerium / zirconium / lanthanum / yttrium mixed oxide. [2] Catalyst according to claim 1, characterized by that layer A and / or layer B additionally contain platinum as another platinum group metal independently of each other. [3] Catalyst according to claim 1, characterized by that layer A contains only palladium and layer B only rhodium, or layer B contains only palladium and rhodium as the platinum group metal. [4] Catalyst according to one or more of claims 1 to 3, characterized by that layer A and layer B contain active aluminum oxide. [5] Catalyst according to claim 4, characterized bythat the platinum group metal in layer A and / or in layer B is fully or partially supported on active aluminium oxide. [6] Catalyst according to one or more of claims 1 to 5, characterized by , that in layer B the mass ratio of second cerium / zirconium / lanthanum / yttrium mixed oxide to aluminium oxide is at least 1:1 and at most 1.5:

1. [7] Catalyst according to one or more of claims 1 to 6, characterized by that layer A lies directly on the inert catalyst support. Exhaust system for reducing the harmful components in the exhaust gas of a combustion engine, in particular one operated predominantly stoichiometrically, comprising a catalyst according to one of the preceding claims. [8] Exhaust system comprising a catalyst according to one or more of claims 1-7. [9] Exhaust system according to claim 8, characterized by that it still has a particulate filter. [10] Use of a catalyst according to one of claims 1-7 or of the exhaust system according to claim 8 or 9 for reducing the harmful components in the exhaust gas of an internal combustion engine, in particular one that is predominantly stoichiometrically operated. [11] Use according to claim 10, characterized by , that it is an internal combustion engine which, for more than 90% of its operating time, is in a state where it does not burn a rich exhaust mixture.