Dual-layer three-way catalyst with improved stability over time

Abstract

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

Description

Technical Field

[0001] The present invention relates to a three-way catalyst composed of two catalytically active layers arranged overlapping each other and suitable for purifying exhaust gas from a combustion engine.

[0002] The three-way catalyst is used for purifying exhaust gas from a combustion engine that is operated substantially stoichiometrically. In stoichiometric operation, the amount of air supplied to the combustion engine exactly corresponds to the amount required for complete combustion of the fuel. In this case, the air-fuel ratio λ (also called the air ratio) is exactly 1. Around λ = 1, the three-way catalyst can simultaneously convert hydrocarbons, carbon monoxide, and nitrogen oxides into harmless components.

[0003] Generally, platinum group metals are used as catalytically active materials. In particular, platinum, palladium, and rhodium are present, for example, on γ-aluminum oxide as a carrier material. In addition, the three-way catalyst contains an oxygen storage material, for example, a cerium / zirconium mixed oxide. In the latter case, cerium oxide (cerium dioxide) is a rare earth metal oxide and constitutes an essential component for oxygen storage. Together with zirconium dioxide and cerium oxide, these materials may contain additional components such as further rare earth metal oxides or alkaline earth metal oxides. The oxygen storage material is activated by applying a catalytically active material such as a platinum group metal and thus also serves as a carrier material for the platinum group metal.

[0004] The components of the three-way catalyst may be present in a single coating layer on an inert catalyst carrier. See, for example, European Patent No. 1541220 (B1).

[0005] However, bilayer catalysts are used in most cases, as these catalysts facilitate the separation of different catalytic processes and thus allow for optimal adjustment of the catalytic effects in the two layers. The latter type of catalyst is disclosed, for example, in International Publication Nos. 95 / 35152(A1), 2008 / 000449(A2), European Patent Nos. 0885650(A2), 1046423(A2), 1726359(A1), and 1974809(B1).

[0006] European Patent No. 1974809(B1) discloses a bilayer ternary catalyst (or more) containing a cerium / zirconium mixed oxide 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] European Patent No. 1900416(B1) describes a bilayer ternary catalyst (or multiple catalysts) comprising a mixed oxide of cerium, zirconium, and neodymium in both layers, and additionally, cerium / zirconium / yttrium / lanthanum oxide-aluminum oxide particles in the lower layer.

[0008] European Patent No. 1726359(A1) describes a bilayer ternary catalyst (or multiple catalysts) containing a cerium / zirconium / lanthanum / neodymium mixed oxide 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 each contain a higher proportion of zirconium than that in the lower layer.

[0009] International Publication No. 2008 / 000449(A2) also discloses a bilayer ternary catalyst (or more) containing a cerium / zirconium mixed oxide in both layers, wherein the mixed oxide in the upper layer has a higher proportion of zirconium. The cerium / zirconium mixed oxide can be replaced to some extent with a cerium / zirconium / lanthanum / neodymium mixed oxide or a cerium / zirconium / lanthanum / yttrium mixed oxide.

[0010] International Publication No. 2009 / 012348(A1) further describes three-layer catalysts in which only the intermediate and upper layers contain oxygen storage material.

[0011] European Patent No. 3045226(A1) discloses a bilayer ternary catalyst with improved aging stability, wherein layer A, directly provided on a catalyst support, comprises at least one platinum group metal and one cerium / zirconium / RE metal mixed oxide, and layer B, applied to layer A and in direct contact with the exhaust gas flow, comprises at least one platinum group metal and a cerium / zirconium / RE metal mixed oxide (where RE represents a rare earth metal other than cerium), and the ratio of RE metal oxide in the cerium / zirconium / RE metal mixed oxide of layer A is smaller than the ratio of RE metal oxide in the cerium / zirconium / RE metal mixed oxide of layer B.

[0012] The demand for reducing emissions from combustion engines has consistently increased, necessitating further and continuous development of catalysts. In Europe, the durability requirement has been raised to 160,000 km under Euro 5 emissions regulations. Even in the United States, the durability requirement is 150,000 miles. [Overview of the Initiative] [Problems that the invention aims to solve]

[0013] Therefore, the aging stability of catalysts is becoming even more important. Important indicators of activity after aging degradation are, on the one hand, the starting temperature for the catalyst's conversion of pollutants, and on the other hand, its dynamic conversion capacity. The starting temperature for pollutants refers to the temperature at which, for example, more than 50% of the pollutants are converted. The lower this temperature, the more quickly the pollutants can be converted after a cold start. Under full load, exhaust gas temperatures of up to 1050°C can be directly generated by the motor output. The better the temperature stability of the catalyst, the closer the catalyst can be placed to the combustion engine. This also improves exhaust gas purification after a cold start.

[0014] The Euro 6c regulations, which came into effect in September 2017, specify exhaust gas measurements under actual driving conditions (RDE) as required by European emissions regulations. Depending on the driving conditions, this may mean that catalysts must meet much stricter requirements, particularly regarding the dynamic conversion of carbon monoxide and nitrogen oxides.

[0015] The goal is to maintain stoichiometric exhaust emissions under all operating conditions, especially at high speeds. The previous practice of enriching the fuel mixture to lower exhaust gas temperature should be avoided, as it increases CO emissions and fuel consumption. However, avoiding enrichment means that the exhaust gas temperature to which the catalyst is exposed at high speeds continues to rise. These further increased demands must be addressed by the catalyst. For the same reason, the aging degradation stability of the three-way catalyst needs to be further improved.

[0016] The conventional catalysts mentioned above already possess very good characteristics in terms of starting temperature and dynamic conversion capability after degradation over time. However, due to increasingly stringent legal requirements, there is a need to consider developing even better catalysts.

[0017] For this reason, the present invention aims to provide a catalyst that has an even lower starting temperature compared to conventional catalysts due to further increased temperature stability, and that has improved dynamic conversion capability after degradation over time. [Means for solving the problem]

[0018] Surprisingly, as the requirements for degradation over time increase, it has been found that this problem can be solved if rare earth elements, and optionally platinum group metals present as components of the oxygen storage material, are distributed in a specific manner on the two layers of the bilayer ternary catalyst.

[0019] Therefore, the subject of the present invention is a catalyst comprising two layers on an inert catalyst support, wherein layer A comprises at least palladium as a platinum group metal and a cerium / zirconium / lanthanum / yttrium mixed oxide, and layer B, applied to layer A, comprises at least rhodium as a platinum group metal and a cerium / zirconium / lanthanum / yttrium mixed oxide. In both layer A and layer B, the lanthanum oxide content is 1% to 5% by weight based on the cerium / zirconium / lanthanum / yttrium mixed oxide, and the yttrium oxide content is 8% to 20% by weight based on the cerium / zirconium / lanthanum / yttrium mixed oxide.

[0020] A preferred embodiment is characterized in that the yttrium oxide content in both layers A and B is 10% to 15% by weight based on a cerium / zirconium / lanthanum / yttrium mixed oxide. A yttrium oxide content of 12% to 13% by weight is particularly preferred. As shown in the examples, lower starting temperatures can be achieved despite concentrated degradation over time, and ultimately emissions during dynamic operation are reduced.

[0021] According to the present 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 independently contain platinum as a further platinum group metal. Layer A preferably contains palladium and platinum, and layer B preferably contains rhodium and platinum or rhodium, palladium, and platinum. In further embodiments of the present invention, the catalyst according to the present invention does not contain platinum. Particularly preferably, layer A contains only palladium, layer B contains only rhodium, or layer B contains only palladium and rhodium.

[0022] A mixed oxide of cerium / zirconium / lanthanum / yttrium can function as a support material for platinum group metals in layer A and / or layer B. However, furthermore, all or part of the platinum group metals in layer A and / or layer B may also be supported by activated aluminum oxide.

[0023] Therefore, in a preferred embodiment of the present invention, layers A and B contain activated aluminum oxide. It is particularly preferred that the activated aluminum oxide is stabilized by doping with lanthanum oxide. The preferred activated aluminum oxide contains 0.5 to 6% by weight, particularly 3 to 5% by weight, of lanthanum oxide (La2O3).

[0024] The term "activated aluminum oxide" is known to those skilled in the art. This term is particularly known as 100m 2 / g~200m 2 Describe γ-type aluminum oxide having a specific surface area of ​​1 / g. Activated aluminum oxide is frequently described in the literature and is commercially available.

[0025] The term "cerium / zirconium / lanthanum / yttrium mixed oxide," within the scope of this invention, excludes physical mixtures of cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide. Rather, "cerium / zirconium / lanthanum / yttrium mixed oxide" is characterized by a substantially homogeneous, three-dimensional crystalline structure and ideally does not contain pure cerium oxide, zirconium oxide, or lanthanum oxide and yttrium oxide phases. Depending on the manufacturing process, products that are not perfectly homogeneous may be obtained, but generally they can be used without any problems.

[0026] According to the present invention, the ratio of cerium oxide to zirconium oxide in the cerium / zirconium / lanthanum / yttrium mixed oxide can be widely varied. In layer A, the ratio is, for example, from 0.1 to 1.0, preferably from 0.2 to 0.7, and even more preferably from 0.3 to 0.5. In layer B, the ratio is, for example, from 0.1 to 1.0, preferably from 0.2 to 0.7, and even more preferably from 0.3 to 0.5. The cerium / zirconium / lanthanum / yttrium mixed oxide of the present invention does not contain aluminum oxide, particularly in its crystal structure.

[0027] In a plurality of embodiments of the present invention, one or both of the layers contain an alkaline earth metal compound such as barium oxide or barium sulfate. A preferred embodiment is that barium sulfate is contained in layer A. The amount of barium sulfate is particularly from 5 g / L to 20 g / L based on the volume of the inert catalyst support.

[0028] In a further embodiment of the present invention, one or both of the layers additionally contain additives such as rare earth compounds, for example lanthanum oxide, and / or binders, for example aluminum compounds. These additives are used in amounts that can be varied within wide limits and, in certain cases, amounts that can be determined by those skilled in the art in a simple manner.

[0029] In a further embodiment of the present invention, layer A is provided directly on the inert catalyst support, that is, there is no additional 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, that is, there is no additional layer or overcoat on layer B.

[0030] In a further embodiment of the present invention, the catalyst according to the present invention is composed of layer A and layer B on an inert catalyst support. This means that layer A is provided directly on the inert catalyst support, layer B is in direct contact with the exhaust gas stream, and there are no other layers.

[0031] A honeycomb structure made of ceramic or metal and having a volume V has parallel flow paths for exhaust gases of a combustion engine and is particularly suitable as a catalyst-inert catalyst support. These may be either a so-called flow-through honeycomb structure or a wall-flow filter. In particular, in the case of a wall-flow filter, the catalyst coating according to the present invention may be placed entirely on the wall of the wall-flow filter, partially or entirely within the wall.

[0032] According to the present invention, the wall region of the flow channel is coated with two catalyst layers A and B. Layer A coats the catalyst support, so the solids supplied to this layer are suspended in water, and the catalyst support is optionally coated on and / or inside the wall using the coating suspension of the catalyst support thus obtained. This process is repeated using the coating suspension, and the solids supplied to layer B are suspended in water.

[0033] Preferably, both layer A and layer B are coated along the entire length of the inert catalyst support. This means that layer B completely covers layer A, and as a result only layer B comes into direct contact with the exhaust gas flow. However, a variation of the zoned coating is also possible, in which layer A is at least partially covered by layer B. [Modes for carrying out the invention]

[0034] [Examples] In the following Example 1 and Comparative Example 1, the number of cells was 93 / cm². 2 A double-layer catalyst was fabricated by twice coating a flow-through honeycomb body made of ceramic with a wall thickness of 0.11 mm and dimensions of 10.6 cm in diameter and 11.4 cm in length. For this purpose, two different suspensions were prepared for layers A and B, respectively. The support was first coated with the suspension of layer A and then calcined in air at 550°C for 4 hours. Subsequently, the support coated with layer A was coated with the suspension of layer B and then calcined under the same conditions as layer A.

[0035] Example 1 The bilayer catalyst was first prepared by manufacturing two suspensions. The composition of the first suspension of layer A (based on the volume of catalyst support) was 66 g / L of stabilized activated aluminum oxide containing 4 wt% La2O3, 24 wt% CeO2, 60 wt% ZrO2, 66 g / L of cerium / zirconium / lanthanum / yttrium mixed oxide containing 3.5 wt% La2O3 and 12.5 wt% Y2O3, 16 g / L of BaSO4, and 1.413 g / L of Pd.

[0036] The composition of the second suspension for layer B (based on the volume of catalyst support) was 60 g / L of stabilized activated aluminum oxide containing 4 wt% La2O3, 24 wt% CeO2, 60 wt% ZrO2, 47 g / L of cerium / zirconium / lanthanum / yttrium mixed oxide containing 3.5 wt% La2O3 and 12.5 wt% Y2O3, and 0.177 g / L of Rh.

[0037] Comparative Example 1 (according to European Patent No. 3045226(A1)) A bilayer catalyst was prepared in the same manner as in Example 1. The composition of the first suspension for layer A was 66 g / L of stabilized activated aluminum oxide containing 4 wt% La2O3, 25 wt% CeO2, 67.5 wt% ZrO2, 66 g / L of cerium / zirconium / lanthanum / yttrium mixed oxide containing 3.5 wt% La2O3 and 4 wt% Y2O3, 16 g / L of BaSO4, and 1.143 g / L of Pd.

[0038] The composition of the second suspension for layer B was 60 g / L of stabilized activated aluminum oxide containing 4 wt% La2O3, 47 g / L of cerium / zirconium / lanthanum / yttrium mixed oxide containing 24 wt% CeO2, 60 wt% ZrO2, 3.5 wt% La2O3 and 12.5 wt% Y2O3, and 0.177 g / L of Rh.

[0039] Example 1 and Comparative Example 1 were subjected to an aging process on an engine test bench. In each case, two similar catalysts were sequentially exposed to hot exhaust gas. This aging process consisted of an overrun cutoff aging process with an exhaust gas temperature of 950°C before catalyst introduction. As a result, the first catalyst (CC1) achieved a maximum floor temperature of 1100°C and the second catalyst (CC2) achieved a maximum floor temperature of 1040°C. The aging period was set to 100 hours.

[0040] Next, using an engine test bench, the starting performance was tested at a constant average air ratio λ, and the dynamic conversion when λ changed was also tested.

[0041] Table 1 shows the temperature T at which 50% of the component in question is converted in each case. 50 This includes the following. In this way, the starting performance was determined based on the stoichiometric exhaust gas composition (λ=0.999, ±3.4% amplitude).

[0042] [Table 1]

[0043] The dynamic conversion performance was determined at a constant temperature of 510°C for λ in the range of 0.99 to 1.01. The amplitude of λ in this case was ±3.4%. Table 2 shows the conversion rates at the intersection of the CO and NOx conversion curves, as well as the corresponding HC conversion rates.

[0044] [Table 2]

[0045] Example 1 of the present invention shows significant improvements in starting performance and CO / NOx dynamic conversion after degradation over time.

Claims

1. A catalyst comprising two layers arranged one above the other on an inert catalyst support, layer A comprises at least palladium as platinum group metal and a cerium / zirconium / lanthanum / yttrium mixed oxide, layer B applied to layer A comprises at least rhodium as platinum group metal and a cerium / zirconium / lanthanum / yttrium mixed oxide, Catalyst characterized in that in both layers A and B, the lanthanum oxide content is between 3.5% and 5% by weight, based on the cerium / zirconium / lanthanum / yttrium mixed oxide, and the yttrium oxide content is between 10% and 15% by weight, based on the cerium / zirconium / lanthanum / yttrium mixed oxide.

2. Catalyst according to claim 1, characterised in that layer A and / or layer B, independently of one another, additionally contain platinum as the further platinum group metal.

3. Catalyst according to claim 1, characterised in that, as platinum group metals, layer A contains only palladium and layer B contains only rhodium or layer B contains only palladium and rhodium.

4. Catalyst according to any one or more of claims 1 to 3, characterized in that layer A and layer B comprise active aluminium oxide.

5. 5. A catalyst according to claim 4, characterised in that all or part of the platinum group metals in layer A and / or in layer B are supported on activated aluminium oxide.

6. Catalyst according to any one or more of the preceding claims, characterized in that the weight ratio of cerium oxide to zirconium oxide in the cerium / zirconium / RE metal mixed oxide in layers A and B is between 0.1 and 1.

0.

7. Catalyst according to any one or more of the preceding claims, characterized in that layer A is applied directly on said inert catalyst support.