Catalyst for exhaust gas purification

Abstract

According to the technology disclosed in this specification, a catalyst for exhaust purification is provided that uses a rare earth-containing material and can exhibit high exhaust purification performance. The catalyst for exhaust purification of the present invention comprises a substrate and a catalyst layer formed on the surface of the substrate. The catalyst layer of the catalyst for exhaust purification comprises rare earth-supported alumina (50) on the surface of an alumina carrier (30) containing alumina, and primary particles of rare earth particles (40) containing at least one rare earth element are supported. The average particle size D of the rare earth particles (40) in the rare earth-supported alumina (50) obtained by TEM observation is 50 The particle size is 10 nm or less. This makes it possible to provide an exhaust gas purification catalyst having high NOx and CO adsorption performance.

Description

Technical Field

[0001] The present invention relates to an exhaust gas purification catalyst. Specifically, it relates to an exhaust gas purification catalyst that is disposed in the exhaust passage of an internal combustion engine and purifies exhaust gas discharged from the internal combustion engine. It should be noted that this international application claims priority based on Japanese Patent Application No. 2020-27925, filed on February 21, 2020, the entire contents of which are incorporated herein by reference. Background Art

[0002] The exhaust gas emitted from internal combustion engines such as automobile engines contains harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). In order to efficiently purify these harmful components, an exhaust gas purification catalyst is arranged in the exhaust passage of the internal combustion engine. Typically, the exhaust gas purification catalyst includes a substrate and a catalyst layer formed on the surface of the substrate. When the exhaust gas comes into contact with the catalyst layer of the exhaust gas purification catalyst, the harmful components such as CO, HC, and NOx are rendered harmless through chemical reactions such as oxidation-reduction. In addition, the catalyst layer also has a physical purification function of adsorbing and capturing harmful components in the exhaust gas.

[0003] In the field of exhaust gas purification catalysts, materials containing rare earth elements (rare earth-containing materials) can be added to the catalyst layer for various purposes. For example, in Patent Document 1, a Ce-containing composite oxide is added to the catalyst layer as an OSC material with oxygen storage capacity (OSC). In the exhaust gas purification catalyst disclosed in this document, the surface of the activated alumina powder supports a Ce-containing composite oxide powder composed of secondary particles formed by agglomeration of a large number of primary particles with an average particle size of 5 nm to 10 nm (secondary particles with a cumulative distribution of 50% by mass having a particle size of 150 nm to 210 nm).

[0004] Another example is the use of rare earth materials as NOx adsorbents. For example, in the exhaust gas purification catalyst described in Patent Document 2, a cerium-based material and a lanthanum-based material are added to the catalyst layer. The cerium-based material functions as a NOx adsorbent in low-temperature environments (150°C to 350°C) under lean-burn conditions, while the lanthanum-based material functions as a NOx adsorbent in high-temperature environments (350°C to 450°C) under lean-burn conditions. By adding two rare earth element compounds with different optimal temperatures as NOx adsorbents, high NOx purification performance can be achieved.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2011-36834

[0008] Patent Document 2: Japanese Patent Application Publication No. 2009-221913 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] Meanwhile, regulations on harmful components in exhaust gas are becoming increasingly stringent. To address these demands, the need for improved performance in exhaust gas purification catalysts is also growing. The present inventors conducted various experiments and studies to meet these requirements, resulting in the discovery of rare earth-containing materials that exhibit distinct performance compared to conventional materials. The present invention is based on this discovery, and its purpose is to provide an exhaust gas purification catalyst that utilizes a rare earth-containing material and exhibits high exhaust gas purification performance.

[0011] Technical solutions to problems

[0012] As described above, the inventors conducted various experiments and studies, resulting in the discovery of rare earth-containing materials that exhibit distinct effects compared to conventional materials. Specifically, the inventors discovered that when particles containing rare earth elements (rare earth particles) are highly dispersed with extremely fine particle sizes, they not only exhibit higher NOx adsorption performance than conventional NOx adsorbents, but also exhibit superior CO adsorption performance. When these rare earth particles exhibit excellent CO adsorption performance, they can significantly reduce CO emissions in low-temperature environments (typically, immediately after starting an internal combustion engine), where chemical purification by precious metal catalysts is not fully effective. Furthermore, since CO adsorbed on the rare earth particles releases heat during oxidation, this also helps improve the catalyst's warm-up performance.

[0013] However, due to their low heat resistance, rare earth particles with a small particle size are prone to sintering, resulting in coarsening. Consequently, exhaust gas purification catalysts heated to temperatures exceeding 600°C during use struggle to maintain high levels of NOx and CO adsorption performance. Consequently, the inventors, after further research, came up with the idea of supporting rare earth particles in their fine primary particle form on an alumina carrier with excellent heat resistance, leading to the present invention.

[0014] The exhaust gas purification catalyst described in this specification is an invention made based on the above-mentioned cognition. The exhaust gas purification catalyst is arranged in the exhaust passage of an internal combustion engine and is used to purify the exhaust gas discharged from the internal combustion engine, wherein the catalyst layer includes a substrate and a catalyst layer formed on the surface of the substrate. Moreover, the catalyst layer of the exhaust gas purification catalyst described in this specification includes rare earth-supported alumina containing primary particles of rare earth particles containing at least one rare earth element on the surface of an alumina carrier containing alumina, and the average particle size D of the rare earth particles in the rare earth-supported alumina is 0.0447 W / cm2 as measured by TEM. 50Below 10nm.

[0015] In the exhaust gas purification catalyst described in this specification, D 50 Primary particles of tiny rare earth particles with an average particle size of 10 nm or less are supported on an alumina carrier. This allows the rare earth particles to function as active sites exhibiting excellent adsorption performance for NOx and CO. Therefore, the technology described herein can provide an exhaust gas purification catalyst that exhibits high NOx and CO purification performance in low-temperature environments and excellent catalyst warm-up performance. Thus, the technology described herein can provide an exhaust gas purification catalyst that utilizes a rare earth-containing material and exhibits high exhaust gas purification performance.

[0016] Furthermore, in one embodiment of the exhaust gas purification catalyst described herein, the catalyst layer further comprises a precious metal catalyst. Because the precious metal catalyst not only oxidizes CO adsorbed by the rare earth particles but also reduces NOx, achieving purification, it exhibits higher purification performance against harmful components in the exhaust gas.

[0017] Furthermore, in one embodiment of the exhaust gas purification catalyst described herein, the oxide-converted weight of the rare earth particles is between 5% and 20% by weight, based on 100% by weight of the alumina support. This allows the fine primary particles of the rare earth particles to be held in a highly dispersed state on the surface of the alumina support, thereby suppressing coarsening of the rare earth particles due to sintering and maintaining stable NOx and CO adsorption performance.

[0018] In one embodiment of the exhaust gas purification catalyst described in this specification, the specific surface area of the alumina carrier after a 10-hour durability test at 1000°C is 50 m 2 By using this highly heat-resistant alumina carrier that can maintain a large specific surface area even in high-temperature environments, the coarsening of rare earth particles caused by sintering can be better suppressed, further stabilizing the adsorption performance of NOx and CO.

[0019] In one embodiment of the exhaust gas purification catalyst described in this specification, the rare earth particles contain at least one rare earth element selected from Y, La, Ce, Pr, and Nd. Rare earth particles containing these rare earth elements are dispersed in the form of tiny primary particles and therefore exhibit excellent adsorption performance for NOx and CO. It should be noted that Ce is particularly preferred among the above-mentioned rare earth elements. Typically, when the total amount of rare earth element oxides is 100 wt%, the weight of the oxides of elements other than Ce in the rare earth particles is preferably less than 80 wt%. Such rare earth particles containing Ce as the main element can exhibit particularly good adsorption performance for NOx and CO.

[0020] Furthermore, in one embodiment of the exhaust gas purification catalyst described herein, the catalyst layer contains La as the primary element of the rare earth particles and Pd as the precious metal catalyst. In addition to the aforementioned NOx and CO adsorption performance, the La-based rare earth particles also enhance the reducing effect of the precious metal catalyst (Pd). Therefore, this embodiment can suppress NH3 emissions resulting from incomplete NOx reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of an internal combustion engine exhaust system equipped with an exhaust gas purification catalyst.

[0022] Figure 2 It is a perspective schematic diagram of the exhaust gas purification catalyst according to the first embodiment.

[0023] Figure 3 It is a schematic cross-sectional view of the exhaust gas purification catalyst according to the first embodiment, taken along the cylinder axis direction.

[0024] Figure 4 Schematic diagram of rare earth-supported alumina contained in a catalyst layer according to one embodiment.

[0025] Figure 5 It is a schematic cross-sectional view of the exhaust gas purification catalyst according to the second embodiment, taken along the cylinder axis direction.

[0026] Figure 6 This is a TEM photograph of sample 1 in the experimental example (magnification: 1,200,000 times).

[0027] Figure 7 This is a TEM photograph of sample 2 in the experimental example (magnification: 1,200,000 times).

[0028] Figure 8 This is a TEM photograph of sample 3 in the experimental example (magnification: 1,200,000 times).

[0029] Figure 9 This is the EPMA analysis result of Sample 1 in the test example (magnification: 300 times).

[0030] Figure 10 This is the EPMA analysis result of Sample 2 in the test example (magnification: 300 times).

[0031] Figure 11 This is the EPMA analysis result of Sample 3 in the test example (magnification: 300 times). DETAILED DESCRIPTION

[0032] Preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be noted that, except for matters specifically mentioned in this specification, matters necessary for implementing the present invention (e.g., the detailed structure of the substrate, etc.) are design matters that are readily understood by those skilled in the art based on prior art. The present invention can be implemented based on the contents of this specification and common technical knowledge in the art.

[0033] <Layout Position of Exhaust Gas Purification Catalyst>

[0034] Figure 1 This is a schematic diagram of an internal combustion engine exhaust system equipped with an exhaust gas purification catalyst. It should be noted that arrows A in the various figures in this specification indicate the flow of exhaust gas. For ease of explanation, the side where exhaust gas is supplied is referred to as the "upstream side (of exhaust gas flow)" and the side where exhaust gas is discharged is referred to as the "downstream side (of exhaust gas flow)."

[0035] like Figure 1 As shown, the exhaust purification catalyst 1 is arranged in the exhaust passage 3 of the internal combustion engine (engine) 2. The internal combustion engine 2 has a mechanism for burning a mixed gas containing oxygen and fuel gas and converting the combustion energy into mechanical energy. As an example, the internal combustion engine 2 is mainly composed of a gasoline engine. It should be noted that the internal combustion engine 2 can also be an engine other than a gasoline engine (such as a diesel engine). In addition, the exhaust passage 3 is a gas channel composed of an exhaust manifold and an exhaust pipe. The exhaust purification catalyst 1 is arranged in the exhaust passage 3 (a typical example is an exhaust pipe). The exhaust gas generated by the internal combustion engine 2 is purified in the exhaust purification catalyst 1 and then discharged to the outside.

[0036] The exhaust gas purification catalyst described in this specification is described below using the first through third embodiments as examples. It should be noted that the exhaust gas purification catalyst described in this specification encompasses modifications and variations made to the exhaust gas purification catalyst of each embodiment by those skilled in the art within the scope of design considerations. In other words, the exhaust gas purification catalyst described in this specification is not limited to the first through third embodiments.

[0037] <First embodiment>

[0038] Figure 2 It is a perspective schematic diagram of the exhaust gas purification catalyst according to the first embodiment. Figure 3 Schematic diagram of a cross section of the exhaust gas purification catalyst according to the first embodiment along the cylinder axis. Figure 2 and Figure 3 As shown, the exhaust gas purifying catalyst 1 of the present embodiment includes a substrate 10 and a catalyst layer 20 formed on the surface of the substrate 10 .

[0039] (1) Base material

[0040] like Figure 2 As shown, the substrate 10 of this embodiment is a cylindrical substrate having a honeycomb structure. That is, the substrate 10 has cells 12 as gas flow paths for exhaust gas to pass through, and partitions 14 that divide adjacent cells 12. The substrate 10 of this embodiment is a so-called direct current type substrate. Specifically, as Figure 3 As shown, the chamber 12 of the substrate 10 extends along the cylindrical axis direction of the substrate 10 (exhaust flow direction A), and both ends of the chamber 12 are open. It should be noted that in the exhaust gas purification catalyst described in this specification, the structure of the substrate is not particularly limited. For example, a so-called wall-through substrate can be used, in which an input side chamber is closed on the downstream side and an output side chamber is closed on the upstream side, and the input side chamber and the output side chamber are separated by a porous partition wall. In addition, as a substrate other than a honeycomb shape, a foam material-shaped substrate, a granular-shaped substrate, etc. can also be used. Moreover, the outer shape of the substrate is not limited to a cylindrical shape, and an elliptical cylindrical shape or a prismatic shape can also be used.

[0041] It should be noted that the substrate 10 can use various materials that have been used for this purpose in the past. For example, as the material of the substrate 10, ceramics with high heat resistance can be used. As a specific example, the substrate 10 can use aluminum oxide (Al2O3), cerium dioxide (CeO2), titanium dioxide (TiO2), zirconium dioxide (ZrO2), silicon dioxide (SiO2), aluminum titanate (Al2TiO5), etc. In addition, as other materials, composite oxide ceramics such as cordierite (2MgO·2Al2O3·5SiO2) and carbide ceramics such as silicon carbide (SiC) can be cited. Alternatively, as a material other than ceramics, alloys such as stainless steel can also be used.

[0042] (2) Catalyst layer

[0043] The catalyst layer 20 is formed on the surface of the substrate 10. Specifically, Figure 3As shown, the catalyst layer 20 is formed on the surface of the partition wall 14 for separating the small chamber 12 inside the substrate 10. The catalyst layer 20 is a porous layer containing a precious metal catalyst. By bringing the exhaust gas into contact with the precious metal catalyst in the catalyst layer 20, the harmful components (CO, HC, NOx) in the exhaust gas are chemically purified. As an example of the precious metal catalyst, a three-way catalyst that oxidizes CO and HC and reduces NOx can be cited. Using this three-way catalyst, the harmful components in the exhaust gas can be effectively purified. As an example of a three-way catalyst, particles containing platinum group elements such as platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os), and iridium (Ir) as the main body can be cited. It should be noted that among these platinum group elements, Pt, Pd, and Rh can show particularly excellent purification performance for harmful components.

[0044] It should be noted that the precious metal catalyst in the catalyst layer 20 is preferably supported on the surface of the rare earth-supported alumina described later (a typical example is on the surface of both the alumina carrier and the rare earth particles. In this way, NOx and CO adsorbed on the rare earth particles can be moderately reduced (or oxidized). It should be noted that, from the perspective of further demonstrating suitable purification performance, the content of precious metal catalyst per 1L capacity of the substrate 11 is preferably 0.1g / L or more, more preferably 0.5g / L or more, and particularly preferably 1g / L or more. On the other hand, from the perspective of material cost, the content of precious metal catalyst is preferably 10g / L or less, more preferably 8g / L or less, and particularly preferably 7g / L or less.

[0045] Furthermore, the catalyst layer 20 in the present embodiment contains rare earth-supported alumina. Figure 4 Schematic diagram of rare earth-supported alumina contained in the catalyst layer of this embodiment. Figure 4 As shown, the rare earth-carrying alumina 50 in this embodiment includes an alumina carrier 30 and rare earth particles 40. The materials constituting the rare earth-carrying alumina 50 are described below.

[0046] (a) Alumina support

[0047] The alumina carrier 30 is a particle containing aluminum oxide (Al2O3) as its primary component. In the rare earth-loaded alumina 50, the alumina carrier 30 supports the rare earth particles 40 and prevents the rare earth particles 40 from coarsening due to sintering. As long as the required heat resistance is achieved, the alumina carrier 30 may also contain components other than aluminum oxide (e.g., metal oxides such as lanthanum oxide (La2O3), barium oxide (BaO), silicon dioxide (SiO2), and titanium dioxide (TiO2)). It should be noted that, when the alumina content is 100 wt%, the content of components other than aluminum oxide is preferably 1 wt% to 10 wt%, more preferably 1 wt% to 8 wt%, further preferably 1 wt% to 5 wt%, and particularly preferably 1 wt% to 4 wt%. This ensures the heat resistance of the alumina carrier 30 and effectively prevents the rare earth particles 40 from coarsening due to sintering. It should be noted that the alumina carrier 30 may contain only aluminum oxide.

[0048] In addition, the average particle size D of the alumina support 30 is 50 It is preferably 1 μm or more, more preferably 5 μm or more, further preferably 10 μm or more, and particularly preferably 15 μm or more. This can fully ensure the heat resistance of the alumina carrier 30. On the other hand, the specific surface area of the alumina carrier 30 increases with the average particle size D 50 Therefore, there is a tendency to improve the dispersibility of the rare earth particles 40 described later. From this perspective, the average particle size of the alumina support 30 is preferably 100 μm or less, more preferably 70 μm or less, further preferably 50 μm or less, and particularly preferably 30 μm or less.

[0049] It should be noted that the “average particle size D 50 " is the arithmetic mean of the equivalent circular diameters of the primary particles of multiple (for example, 20) rare earth particles that can be confirmed in multiple TEM observation fields (for example, 4 fields of view). Specifically, the measurement object is dispersed in a solvent (for example, a mixed solvent of water and ethanol), dropped into the grid of a copper mesh, and then dried to prepare a measurement sample. Then, the measurement sample is observed with a transmission electron microscope (TEM: Transmission Electron Microscope), and the major axis and minor axis of the particles that can be confirmed within a specific field of view (for example, 200nm×200nm) are measured, and the middle value between the two is measured as the equivalent circular diameter. Then, the average value of the equivalent circular diameters of the 40 particles measured is calculated, thereby obtaining the "average particle size D 50 ”.

[0050] In addition, as described above, when the specific surface area of the alumina support 30 increases, the dispersibility of the rare earth particles 40 is improved, and the adsorption performance of NOx and CO is improved. Therefore, the specific surface area of the alumina support 30 is preferably 40 m 2 / g or more, more preferably 50m 2 / g or more, more preferably 55m 2 / g or more, particularly preferably 60m 2 / g or more. In addition, the upper limit of the specific surface area of the alumina support 30 is not particularly limited, and may be 200m 2 / g or less, can be 150m 2 / g or less, or 125m 2 / g or less, or 100m 2 / g or less. It should be noted that in the internal combustion engine 2 (see Figure 1 During operation, the exhaust gas purification catalyst 1 is exposed to a high temperature environment of 600°C or higher. Therefore, the alumina support 30 preferably has excellent heat resistance, which allows it to maintain a large specific surface area even in a high temperature environment. Specifically, the specific surface area of the alumina support 30 after a 10-hour endurance test at 1000°C is preferably 45 m 2 / g or more, more preferably 50m 2 / g or more, more preferably 55m 2 / g or more, particularly preferably 60m 2 This can effectively suppress the coarsening of the rare earth particles 40 due to sintering, thereby enabling the adsorption performance of NOx and CO to be more stably exhibited.

[0051] (b) Rare earth particles

[0052] The rare earth particles 40 are particles containing at least one rare earth element. The rare earth particles 40 are small primary particles (typically, the average particle size D 50 The rare earth particles 40 are supported on the surface of the alumina carrier 30 in the state of primary particles (less than 10 nm). Therefore, the rare earth particles 40 are highly dispersed in the interior of the catalyst layer 20 in the state of tiny primary particles. According to the experiments and research of the inventors, when the tiny rare earth particles 40 are highly dispersed in the catalyst layer 20 in this way without forming agglomerated particles (secondary particles), the rare earth particles 40 function as active sites for adsorbing NOx and CO, and can exhibit high NOx adsorption performance and CO adsorption performance. Therefore, the exhaust gas purification catalyst 1 of this embodiment can remove NOx and CO from the exhaust gas and reduce the emission of these harmful components even in a low-temperature environment where the chemical purification of the precious metal catalyst cannot fully function. Moreover, the CO adsorbed on the rare earth particles 40 releases heat when oxidized, which also helps to improve the warm-up performance of the catalyst.

[0053] It should be noted that the rare earth particles 40 supported on the surface of the alumina carrier 30 do not need all particles to be primary particles, and may also contain a portion of secondary particles. Specifically, when more than 40% (preferably more than 50%, more preferably more than 60%, and further preferably more than 70%) of the rare earth particles 40 observed by TEM are primary particles, suitable NOx adsorption performance and CO adsorption performance can be exhibited. In addition, there is no special limit on the upper limit of the proportion of primary particles, and it can be 100% (all primary particles) or less than 80%. It should be noted that the above-mentioned "proportion of primary particles of rare earth particles" can be measured by TEM observation. Specifically, in order to determine the above-mentioned average particle size D by TEM observation, 50 When preparing a measurement sample, the number of primary and secondary rare earth particles identified within a specific field of view (e.g., 200 nm x 200 nm) is counted. The "primary particle ratio of rare earth particles" can then be determined by calculating the ratio of the number of primary particles to the total number of particles (the sum of the number of primary and secondary particles) within multiple fields of view (e.g., four fields of view).

[0054] Furthermore, as described above, the average particle size D of the primary particles of the rare earth particles 40 in this embodiment is 50 By dispersing such tiny rare earth particles 40, it is possible to exhibit good adsorption performance for NOx and CO. It should be noted that, from the perspective of further improving the adsorption performance of NOx and CO, the average particle size D of the primary particles of the rare earth particles 40 is 50 It is preferably 9 nm or less, more preferably 8 nm or less, further preferably 7 nm or less, and particularly preferably 6 nm or less. 50 The lower limit of is not particularly limited and may be 0.01 nm or more, or 0.05 nm or more. However, since the cohesive force between the primary particles increases with the average particle size D of the primary particles of the rare earth particles 40, 50 Considering this, the average particle size D of the primary particles of the rare earth particles 40 is 50 It is preferably 0.1 nm or more, more preferably 0.5 nm or more, further preferably 1 nm or more, and particularly preferably 3 nm or more.

[0055] Moreover, the maximum particle size of the primary particles of the rare earth particles 40 is less than 30 nm (more preferably less than 25 nm, further preferably less than 23 nm, and particularly preferably less than 20 nm). Thus, most of the rare earth particles 40 carried on the surface of the alumina carrier 30 can function as active sites for adsorbing NOx and CO. In addition, the lower limit of the maximum particle size of the primary particles is not particularly limited, and can be greater than 1 nm, greater than 5 nm, greater than 10 nm, or greater than 15 nm. It should be noted that the "maximum particle size of the rare earth particles" in this specification refers to the average particle size D 50 The largest equivalent circle diameter among the equivalent circle diameters of the primary particle sizes of multiple rare earth particles measured at the time.

[0056] It should be noted that rare earth elements contained in the rare earth particles 40 of this embodiment include yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd). Rare earth particles 40 containing at least one of these rare earth elements can exhibit suitable adsorption performance for NOx and CO by being highly dispersed in the form of tiny primary particles. Furthermore, the rare earth particles 40 may also contain elements other than rare earth elements. Examples of such elements other than rare earth elements include transition metal elements such as zirconium (Zr), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), niobium (Nb), and molybdenum (Mo).

[0057] Among the above-mentioned rare earth elements, Ce can show particularly high adsorption performance for NOx and CO. Therefore, the rare earth particles 40 are preferably particles with Ce as the main element. Generally, when the main element of the rare earth particles 40 is Ce, the oxide conversion weight of elements other than Ce is preferably 80wt% or less, more preferably 60wt% or less, further preferably 40wt% or less, and particularly preferably 20wt% or less, relative to 100wt% of the total amount of rare earth element oxide conversion. In this way, the adsorption performance of Ce can be better demonstrated. On the other hand, the lower limit value of the oxide conversion weight of elements other than Ce is not particularly limited. That is, the rare earth particles 40 can be composed only of Ce. It should be noted that "elements other than Ce" in this specification also include other rare earth elements other than Ce (Y, La, Pr, Nd, etc.).

[0058] Among the rare earth elements, La has the function of improving the chemical purification performance of the noble metal catalyst (Pd), which will be described in detail later in the third embodiment. Therefore, among the rare earth elements, Ce and La are suitable as the main elements constituting the rare earth particles 40 .

[0059] (c) Other additives

[0060] In addition, materials other than precious metal catalysts and precious metal-supported alumina may be added to the catalyst layer 20. For example, when a three-way catalyst is used as the precious metal catalyst, it is preferred to add an OSC material to the catalyst layer 20. The OSC material is a metal oxide (for example, a cerium zirconium oxide composite oxide, etc.) that has the function of absorbing and releasing oxygen. By adding such an OSC material to the catalyst layer 20, the air-fuel ratio of the exhaust gas can be maintained near the ideal value (theoretical air-fuel ratio), and the chemical purification effect of the precious metal catalyst can be stably exhibited. In addition, as other examples of additives contained in the catalyst layer 20, stabilizers, adhesives, etc. can be cited.

[0061] The first embodiment of the exhaust gas purification catalyst described in this specification has been described above. However, the exhaust gas purification catalyst described in this specification is not limited to this embodiment, and includes various modifications and variations.

[0062] <Second embodiment>

[0063] Next, a second embodiment of the exhaust gas purification catalyst described in this specification will be described. Figure 5 It is an enlarged cross-sectional view of the exhaust gas purification catalyst according to the second embodiment, taken along the cylinder axis direction.

[0064] like Figure 5 As shown, in the exhaust gas purification catalyst 1 of the first embodiment, a catalyst layer 20 consisting of a single layer is formed on the surface of the partition wall 14 of the substrate 10. However, the catalyst layer is not limited to this form and may have a multilayer structure of two or more layers. For example, Figure 5 As shown, in the exhaust gas purification catalyst 1 of the second embodiment, a catalyst layer 20 having a double-layer structure having a lower layer 22 and an upper layer 24 is formed. In this case, rare earth-loaded alumina may be added to either the lower layer 22 or the upper layer 24, or to both layers. It should be noted that when rare earth-loaded alumina is added to either the lower layer 22 or the upper layer 24, the rare earth-loaded alumina is preferably added to the lower layer 22. Since the exhaust gas flow rate is low immediately after the internal combustion engine is started, it is easy to supply exhaust gas to the lower layer side of the catalyst layer 20. Therefore, by adding rare earth-loaded alumina that can remove NOx and CO by adsorption to the lower layer 22, the emission of NOx and CO in a low-temperature environment immediately after the internal combustion engine is started can be effectively suppressed.

[0065] In addition, when the formation Figure 5When the catalyst layer 20 has a two-layer structure as shown, the types and contents of the precious metal catalysts in the lower layer 22 and the upper layer 24 can be made different from each other. In this case, it is preferred to adjust the types and contents of the precious metal catalysts in each layer in consideration of the exhaust flow rate and the content of harmful components. It should be noted that, according to the research of the inventors, rare earth-loaded alumina (Ce-loaded alumina) containing rare earth particles whose main element is Ce exhibits excellent adsorption performance for NOx and CO on the one hand, but on the other hand, it may hinder the catalytic activity of rhodium (Rh). Therefore, when using Ce-loaded alumina, it is preferred to add Ce-loaded alumina to the catalyst layer to which Rh is not added. As an example, when the precious metal catalyst of the upper layer 22 is Rh and the precious metal catalyst of the lower layer 24 is Pd, it is preferred to add Ce-loaded alumina to the lower layer 24. In addition, either the lower layer 22 or the upper layer 24 constituting the catalyst layer 20 may be a catalyst-free layer that does not contain a precious metal catalyst. In this case, by adding rare earth-supported alumina to the catalyst-free layer, the catalyst-free layer can function as a NOx and CO adsorption layer.

[0066] <Third embodiment>

[0067] As described above, the present inventors' research has confirmed that rare earth-supported alumina (La-supported alumina) containing rare earth particles whose main element is La has the function of improving the chemical purification performance of precious metal catalysts. In the third embodiment, an exhaust gas purification catalyst having this La-supported alumina added to the catalyst layer will be described.

[0068] As described above, in the exhaust gas purification catalyst described in this specification, rare earth particles containing rare earth elements (Y, La, Ce, Pr, Nd, etc.) are supported on an alumina carrier in the form of tiny primary particles. However, among these rare earth particles, rare earth particles containing La as the main element have the function of improving the chemical purification performance of the Pd when they coexist with palladium (Pd) in the catalyst layer. Specifically, in a conventional exhaust gas purification catalyst, the electronic state of Pd present in the catalyst layer is 0-valent to 2-valent. However, in a catalyst layer where tiny La particles are highly dispersed, the electronic state of Pd can be increased to 4-valent. Since this 4-valent Pd has a strong reducing effect on NOx, it can better purify NOx. For example, when the reduction of NOx in the exhaust gas is incomplete, ammonia (NH3), a harmful component, may be discharged. However, according to this embodiment, the incomplete reduction of NOx can be suppressed and the emission of NH3 can be greatly reduced.

[0069] It should be noted that, when the La oxide-converted weight in the rare earth particles is set to 100 wt%, the oxide-converted weight of other elements is 80 wt% or less (i.e., when La is contained as the main element in the rare earth particles), a good NH3 reduction effect can be achieved by the coexistence of La-supported alumina and Pd. It should be noted that, from the perspective of achieving a better NH3 reduction effect, the oxide-converted weight of other elements is suitably 60 wt% or less, preferably 40 wt% or less, more preferably 20 wt% or less, further preferably 15 wt% or less, and particularly preferably 10 wt% or less, for example 0 wt% (rare earth particles composed only of La).

[0070] Furthermore, in this embodiment, the catalyst layer may further contain a noble metal catalyst (e.g., Rh, Pt) other than Pd. However, from the perspective of achieving a better NH3 reduction effect, a higher Pd content is preferred. For example, when the Pd content in the catalyst layer is 100 wt%, the content of the other noble metal catalyst is preferably 80% or less, more preferably 60 wt% or less, further preferably 40 wt% or less, and particularly preferably 20 wt% or less. For example, 0 wt% (adding only Pd as the noble metal catalyst) is preferred.

[0071] Furthermore, the NH3 reduction effect varies depending on factors such as the weight ratio of La in the La-supported alumina to the alumina support, the Pd content relative to the specific surface area of the alumina support, and the Pd content relative to the pore volume of the catalyst layer. For example, when the content of the rare earth particles (La) is 100 wt% of the alumina support, the content is preferably 1 wt% to 50 wt%, more preferably 2 wt% to 40 wt%, even more preferably 3 wt% to 30 wt%, and particularly preferably 4 wt% to 15 wt%. This further enhances the NH3 reduction effect.

[0072] <Method for Manufacturing Exhaust Gas Purifying Catalyst>

[0073] Next, a method for producing the exhaust gas purification catalyst of this embodiment will be described. This production method includes the steps of preparing rare earth-supported alumina and forming a catalyst layer on the surface of a substrate.

[0074] (1) Preparation of rare earth-supported alumina

[0075] In this process, rare earth-loaded alumina is prepared by supporting rare earth particles on the surface of an alumina support. First, a solution containing a fully dissolved rare earth element (rare earth solution) is prepared. The rare earth solution is then allowed to adhere to the surface of the alumina support, followed by drying and firing. This produces rare earth-loaded alumina, in which primary rare earth particles are supported on the surface of the alumina support.

[0076] It should be noted that in order to make the rare earth particles with a sufficiently small particle size be supported on the surface of the alumina support in a highly dispersed state, a spray drying method is preferably used in this process. Specifically, the alumina support is floated inside a flow channel where an inert gas flows, and a rare earth solution is sprayed into the flow channel. In this way, since very small droplets of rare earth solution adhere to the surface of the alumina support, the average particle size D can be easily reduced by firing such a support. 50 Primary rare earth particles of 10 nm or less are supported on the surface of the alumina support. It should be noted that when using this spray drying method, the rare earth solution is preferably sprayed so that the droplet diameter is 800 μm or less (more preferably 600 μm or less, even more preferably 500 μm or less, and particularly preferably 400 μm or less). This allows the rare earth particles of microscopic size to be stably supported on the alumina support.

[0077] (2) Formation of catalyst layer

[0078] Next, in this process, a catalyst layer containing rare earth-supported alumina is formed on the surface of the substrate. There is no particular limitation on the method of forming the catalyst layer in this process, and previously known techniques can be used without restriction. For example, first, a slurry is prepared in which various materials (precious metal catalysts, OSC materials, etc.) including rare earth-supported alumina are dispersed in a dispersion medium (water, etc.). Then, after the slurry is introduced into the small chamber of the substrate, the slurry is attached to the surface of the partition wall of the substrate by means of air supply, etc. Then, drying and firing are performed under predetermined conditions. Thus, a catalyst layer containing rare earth-supported alumina is formed on the surface of the partition wall of the substrate to prepare a catalyst for exhaust purification. It should be noted that from the perspective of reliably preventing the coarsening caused by sintering of rare earth particles contained in the rare earth-supported alumina, the slurry drying and firing in this process are preferably carried out at a temperature below 600°C.

[0079] The above describes the method for producing the exhaust gas purification catalyst of the present embodiment. However, the exhaust gas purification catalyst described in this specification is not limited to the product produced by the production method. 50 Rare earth-supported alumina having rare earth particles of less than 10 nm may be added to the catalyst layer.

[0080] [Test Example]

[0081] Hereinafter, test examples of the present invention will be described, but the present invention is not intended to be limited to the following test examples.

[0082] Experiment 1

[0083] In this experiment, the effects of the particle size and dispersion of rare earth particles in rare earth-loaded alumina on the NOx and CO adsorption performance were studied.

[0084] 1. Sample Preparation

[0085] (1) Sample 1

[0086] In this sample, rare earth-supported alumina (Ce-supported alumina) prepared by a spray drying method is added to the lower layer of an exhaust gas purifying catalyst having a two-layer catalyst structure of upper and lower layers.

[0087] Specifically, a compound containing a rare earth element (Ce) (cerium nitrate hexahydrate) is first dissolved in a solvent (water) to prepare a rare earth solution (Ce solution). Next, a La2O3 composite alumina containing 5wt% lanthanum oxide (La2O3) is prepared as an alumina carrier. Then, after the Ce solution is attached to the alumina carrier by a spray drying method, it is calcined (500°C, 2 hours) to prepare Ce-loaded alumina. It should be noted that in this sample, the supply amount of Ce solution in the spray drying method was adjusted so that 15wt% of Ce was loaded relative to 100wt% of the alumina carrier.

[0088] Next, palladium nitrate, Ce-loaded alumina, OSC material (CeO2-ZrO2 composite oxide), barium sulfate (BaSO4), binder (Al2O3-based binder) and solvent (water) are mixed to prepare a slurry for the lower layer. The slurry for the lower layer is introduced into the small chamber of the substrate and air is blown, and then dried (120°C, 2 hours) and fired (500°C, 2 hours) to form the lower layer of the catalyst layer on the surface of the substrate partition. It should be noted that the substrate used in this experiment is a cordierite direct current type substrate with a honeycomb structure with a capacity of 850cc, a partition thickness of 2mm, and a number of small chambers of 900. In addition, in this sample, the composition of the slurry for the lower layer and the amount of slurry supplied to the interior of the substrate are adjusted so that the lower layer after forming contains 2.0g / L of Pd, 50g / L of Ce-loaded alumina, 20g / L of OSC material and 5.0g / L of barium sulfate.

[0089] Next, rhodium nitrate, an alumina carrier (La2O3 composite alumina), an OSC material (CeO2-ZrO2 composite oxide), a binder (Al2O3-based binder) and a solvent (water) are mixed to prepare a slurry for the upper layer. The slurry for the upper layer is then introduced into a small chamber of the substrate and air is blown, followed by a drying treatment (120°C, 2 hours) and a firing treatment (500°C, 2 hours) to form an upper layer on the surface of the lower layer. In this way, an exhaust gas purification catalyst having a two-layer catalyst layer and Ce-supported alumina added to the lower layer is prepared. It should be noted that in the formation of the upper layer, the composition of the slurry for the upper layer and the amount of slurry supplied to the interior of the substrate are adjusted so that the formed upper layer contains 0.13 g / L of Rh, 55 g / L of alumina carrier and 50 g / L of OSC material.

[0090] (2) Sample 2

[0091] In Sample 2, an exhaust gas purification catalyst was prepared by the same procedures as Sample 1, except that a Ce-loaded alumina was prepared with a lower Ce loading than in Sample 1. It should be noted that in this sample, the Ce solution feed rate during the spray drying process was adjusted so that 5 wt% of Ce was loaded per 100 wt% of the alumina support.

[0092] (3) Sample 3

[0093] Sample 3 was prepared using the same exhaust gas purification catalyst as Sample 1, except that Ce-supported alumina was prepared using the impregnation method. To prepare the Ce-supported alumina in this sample, the alumina support (La2O3 composite alumina) was first impregnated with a rare earth solution (Ce solution) prepared in the same manner as Sample 1. This solution was then dried (250°C for 2 hours) and calcined (500°C for 2 hours) to produce the Ce-supported alumina. The resulting powder was then pulverized and its particle size adjusted.

[0094] 2. Evaluation Test

[0095] (1) Microscope observation

[0096] In the above manufacturing process, a portion of the Ce-supported alumina before being added to the catalyst layer (lower layer) was collected and added to a mixed solvent of ethanol and water. Then, after dispersion using an ultrasonic dispersing device, the dispersion was dropped onto the grid of the Cu mesh and dried to prepare a measurement sample. Then, a transmission electron microscope (JEOL Ltd., model: JEM-F200) was used to observe and take a TEM photograph. An example of a TEM photograph of sample 1 (magnification: 1,200,000 times) is shown in FIG. Figure 6 An example of a TEM image of sample 2 is shown in Figure 7An example of a TEM image of sample 3 is shown in Figure 8 In these TEM images, the white portions show Ce particles.

[0097] Then, in TEM observation, the equivalent circle diameters of 10 Ce particles were measured in each of the four fields of view, and the average particle size D of Ce particles was calculated based on the measurement results. 50 The measurement results are shown in Table 1. In addition, the maximum particle size D confirmed by measuring the equivalent circle diameter of the Ce particles was Max This is also shown in Table 1. Furthermore, in this test, the number of primary and secondary Ce particles in four fields of view was counted, and the ratio of the number of primary particles to the total number of particles (primary particles + secondary particles) was calculated to determine the "primary particle ratio of Ce particles." This measurement result is also shown in Table 1.

[0098] In addition, in this experiment, EPMA analysis was performed on the measurement sample containing Ce-supported alumina to study the distribution of Ce particles supported on the surface of the alumina support. The EPMA analysis results of sample 1 (magnification: 300 times) are shown in Figure 9 The EPMA analysis results of sample 2 are shown in Figure 10 The EPMA analysis results of sample 3 are shown in Figure 11 It should be noted that in these photos, the white portions represent Ce particles.

[0099] (2) Adsorption performance evaluation

[0100] The exhaust gas purification catalyst of each sample was assembled on an engine test stand, and the NOx adsorption performance and CO adsorption performance were evaluated by supplying exhaust gas containing NOx and CO. Specifically, hydrogen-containing N2 gas (flow rate: 8 L / min) with a heating rate of 35°C / min was supplied for 6 minutes to heat the exhaust gas purification catalyst to 200°C. Then, while maintaining the temperature, a reduction treatment was performed by supplying hydrogen-containing N2 gas for 2 minutes. Then, after a gas replacement treatment by supplying N2 gas for 1 minute, 2000 ppm of nitrogen monoxide (NO) and 3000 ppm of carbon monoxide (CO) were mixed into the N2 gas and supplied for 4 minutes. Then, the NO concentration on the upstream side and the NO concentration on the downstream side of the sample were measured, and the NO adsorption rate was calculated according to the following formula (1) to evaluate the NOx adsorption performance. Similarly, the CO concentration on the upstream side and the CO concentration on the downstream side of the sample were measured, and the CO adsorption rate was calculated according to the following formula (2) to evaluate the CO adsorption performance. The measurement results are shown in Table 1.

[0101] NO adsorption rate = (upstream NO concentration - downstream NO concentration) / upstream NO concentration × 100 (1)

[0102] CO adsorption rate = (upstream CO concentration - downstream CO concentration) / upstream CO concentration × 100 (2)

[0103] [Table 1]

[0104]

[0105] 3. Evaluation results

[0106] like Figures 6 to 11 As shown in Table 1, in Ce-supported alumina (samples 1 and 2) prepared by spray drying, the average particle size D 50 The tiny Ce particles with a size of 10 nm or less were supported on the surface of the alumina support in a highly dispersed state. On the other hand, in sample 3, a large number of secondary particles formed by the aggregation of Ce primary particles were confirmed to exist. The average particle size D 50 With the maximum particle size D Max Both results were greater than those of Samples 1 and 2. Furthermore, after confirming the NOx and CO adsorption performance, it was confirmed that Samples 1 and 2 exhibited higher performance than Sample 3. This demonstrates that adding Ce-supported alumina, such as Samples 1 and 2, to the catalyst layer of an exhaust gas purification catalyst exhibits higher NOx and CO purification performance. This Ce-supported alumina comprises fine Ce particles with an average particle size of 10 nm or less supported in a highly dispersed state on the surface of the alumina support.

[0107] Experiment 2

[0108] In this experiment, the effects of other elements (elements other than Ce) contained in rare earth particles (Ce particles) on the NOx adsorption performance and CO adsorption performance were studied.

[0109] 1. Sample Preparation

[0110] (1) Sample 4

[0111] In Sample 4, an exhaust gas purification catalyst was prepared by the same procedure as Sample 1, except that a Ce-loaded alumina was prepared with a greater Ce loading than that of Sample 1 in Experiment 1. Note that in this sample, the Ce solution feed rate during the spray drying process was adjusted so that 20 wt% of Ce was loaded per 100 wt% of the alumina support.

[0112] (2) Sample 5

[0113] In Sample 5, an exhaust gas purification catalyst was prepared using the same procedures as Sample 4, except that Pr was added to the rare earth solution (Ce solution) sprayed during the spray drying process. It should be noted that in this sample, the concentrations of Ce and Pr in the rare earth solution were adjusted, and the amount of rare earth solution supplied during the spray drying process was also adjusted, resulting in a loading rate of 4 wt% Ce and 16 wt% Pr per 100 wt% of the alumina support. In Sample 5, the total loading of rare earth particles was 20 wt%, and the ratio of elements other than Ce (Pr) to this total loading was 80%.

[0114] (3) Sample 6

[0115] In Sample 6, an exhaust gas purification catalyst was prepared according to the same procedures as Sample 5, except that the amount of rare earth particles supported was less than that of Sample 5. Note that in this sample, the amount of rare earth solution supplied during the spray drying process was adjusted so that 10 wt% of the rare earth particles were supported per 100 wt% of the alumina support.

[0116] (4) Sample 7

[0117] In Sample 7, an exhaust gas purification catalyst was prepared using the same procedures as Sample 5, except that the amount of Pr added to the rare earth solution was reduced. Note that in this sample, the concentrations of Ce and Pr in the rare earth solution were adjusted so that the amount of Pr was 10 wt% relative to the total amount of rare earth particles supported.

[0118] (5) Sample 8

[0119] In Sample 8, an exhaust gas purification catalyst was prepared according to the same procedures as Sample 4, except that Ce and Nd were added to the rare earth solution sprayed by the spray drying method. Note that in this sample, the concentrations of Ce and Nd in the rare earth solution were adjusted so that the amount of Nd was 20 wt% relative to the total amount of rare earth particles supported.

[0120] (6) Sample 9

[0121] In Sample 9, an exhaust gas purification catalyst was prepared by the same procedure as Sample 8, except that the elements added to the rare earth solution were changed to Ce and Y.

[0122] (7) Sample 10

[0123] In Sample 10, an exhaust gas purification catalyst was prepared by the same procedure as Sample 8, except that the elements added to the rare earth solution were changed to Ce and Zr.

[0124] (7) Sample 11

[0125] In Sample 11, an exhaust gas purifying catalyst was prepared by the same procedure as Sample 4, except that an alumina carrier (La2O3 composite alumina) was added instead of Ce-supported alumina when forming the lower layer of the catalyst layer.

[0126] (8) Samples 12 to 14

[0127] Samples 12 to 14 were prepared using the same procedure as Sample 4, except that rare earth-supported alumina containing rare earth particles containing La as the main element, i.e., La-supported alumina, was used instead of Ce-supported alumina. Note that, as shown in Table 2, the amount of La solution supplied during the spray drying process was adjusted in Samples 12 to 14 to achieve different La loadings relative to 100 wt% of the alumina support.

[0128] 2. Evaluation Test

[0129] The NO adsorption rate and CO adsorption rate of each sample were determined using the same procedure as in Experiment 1. The results are shown in Table 2. Table 2 also lists the NO adsorption rates and CO adsorption rates of Samples 1 to 3 measured in Experiment 1.

[0130] [Table 2]

[0131]

[0132] 3. Evaluation results

[0133] Although not shown in the figure, TEM observation of each sample revealed that, in samples 4 to 10, the average particle size D 50 Fine Ce particles of 10 nm or less are supported on the surface of the alumina support in a highly dispersed state.

[0134] Secondly, as shown in Table 2, the results of Samples 1, 2, and 4 indicate that when the Ce particle loading relative to the alumina support is in the range of 5 to 20 wt%, excellent NOx and CO adsorption performance is achieved. Furthermore, the results of Samples 5 to 9 indicate that when the primary element of the rare earth particles is Ce, even with the addition of other rare earth elements, excellent NOx and CO adsorption performance is achieved. Furthermore, it is understood that the addition of other elements relative to the total loading of the rare earth particles is preferably 80 wt% or less. Furthermore, the results of Sample 10 indicate that even with the addition of Zr, a metal element other than the rare earth element, excellent NOx and CO adsorption performance is achieved. Furthermore, as shown in Samples 12 to 14, when the primary element of the rare earth particles is La, an exhaust gas purification catalyst with particularly high NOx adsorption performance can be constructed, and at a loading of 15 wt%, CO adsorption performance comparable to that of Ce is achieved.

[0135] While the specific examples of the present invention have been described in detail above, these descriptions are merely illustrative and do not limit the scope of the present invention. The technology described in the present invention includes various modifications and variations of the specific examples described above.

[0136] Industrial Applicability

[0137] According to the present invention, an exhaust gas purification catalyst exhibiting high exhaust gas purification performance can be provided. Because this exhaust gas purification catalyst exhibits high NOx and CO purification performance in low-temperature environments and excellent catalyst warm-up performance, it can effectively purify exhaust gas emitted in low-temperature environments, where chemical purification by precious metal catalysts is not fully effective.

Claims

1. A catalyst for exhaust gas purification, characterized in that: It is arranged in the exhaust passage of an internal combustion engine to purify the exhaust gas discharged from the internal combustion engine. The exhaust gas purification catalyst comprises a substrate and a catalyst layer formed on the surface of the substrate. The catalyst layer comprises rare earth-supported alumina on the surface of an alumina carrier containing alumina, and primary particles of rare earth particles containing at least one rare earth element are supported. The rare earth particles are supported on an alumina carrier by spray drying. The average particle size D of the rare earth particles in the rare earth-supporting alumina obtained by TEM observation is 50 Below 10 nm, more than 60% of the rare earth particles are primary particles.

2. The exhaust gas purification catalyst according to claim 1, wherein The catalyst layer further contains a noble metal catalyst.

3. The exhaust gas purification catalyst according to claim 1 or 2, wherein: When the oxide-converted weight of the alumina support is 100 wt %, the oxide-converted weight of the rare earth particles is greater than or equal to 5 wt % and less than or equal to 20 wt %.

4. The exhaust gas purification catalyst according to claim 1 or 2, wherein: The specific surface area of the alumina carrier after a 10-hour 1000°C durability test is 50 m 2 / g or above.

5. The exhaust gas purification catalyst according to claim 1 or 2, wherein The rare earth particles contain at least one rare earth element selected from Y, La, Ce, Pr, and Nd.

6. The exhaust gas purification catalyst according to claim 5, wherein The rare earth particles contain Ce as a main element.

7. The exhaust gas purification catalyst according to claim 6, wherein: When the total amount of the rare earth element oxides is 100 wt %, the weight of the elements other than Ce in the rare earth particles in terms of oxide conversion is less than 80 wt %.

8. The exhaust gas purification catalyst according to claim 5, wherein The catalyst layer contains La as a main element of the rare earth particles and Pd as a noble metal catalyst.

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