阅读说明：本技术 把贵金属薄层作为最上层予以包含的具多层结构的排气净化用催化剂及其制备方法 (Exhaust gas purifying catalyst having multilayer structure including thin noble metal layer as uppermost layer and method for producing the same ) 是由 李道均 韩尚润 罗承彻 于 2020-03-31 设计创作，主要内容包括：本发明揭示一种包括含Rh及/或Pd贵金属成分的20微米以内极薄层的具多层结构的排气净化用催化剂及制备它的方法,揭示一种对排气净化用多层结构催化剂适用具有可和Rh及/或Pd贵金属成分进行螯合的官能基团的高分子涂布液而形成作为催化剂最上层的20微米以内极薄层的方法。而且,相比于不具备薄层的催化剂,本发明的催化剂能在THC、CO及NOx的排放方面得到改善,因此,在使用同量贵金属的情形下把贵金属的一部分涂布成薄层的话能大幅提高三元催化剂的性能。(The invention discloses an exhaust gas purifying catalyst with a multilayer structure comprising a 20-micron inner layer containing Rh and/or Pd precious metal components and a method for preparing the same, and discloses a method for forming the 20-micron inner layer as the uppermost layer of the catalyst by applying a polymer coating solution with functional groups capable of chelating with the Rh and/or Pd precious metal components to the exhaust gas purifying multilayer structure catalyst. Further, the catalyst of the present invention can improve the emissions of THC, CO and NOx compared to a catalyst without a thin layer, and thus, the performance of the three-way catalyst can be greatly improved when a part of the noble metal is coated in a thin layer in the case of using the same amount of the noble metal.)

1. A method for producing a catalyst for purifying exhaust gas having a multilayer structure,

comprises the following steps:

forming a pre-catalyst body with a multilayer structure on a substrate by using the catalyst slurry; and a process for the preparation of a coating,

and coating a precious metal-containing coating liquid on the surface of the pre-catalyst body.

2. The method of producing a catalyst for purification of exhaust gas having a multilayer structure according to claim 1,

the noble metal-containing coating liquid is a polymer solution that forms a complex with a noble metal component.

3. The method of producing a catalyst for purification of exhaust gas having a multilayer structure according to claim 2,

the polymer solution that forms a complex with the noble metal component has a functional group capable of chelating with the noble metal component.

4. The method of producing a catalyst for purification of exhaust gas having a multilayer structure according to claim 3,

the functional group is a hydroxyl group or an ether group.

5. The method of producing a catalyst for purification of exhaust gas having a multilayer structure according to claim 2,

the polymer solution is hydroxyethyl cellulose (HEC) or polypropylene glycol.

6. An exhaust gas purifying catalyst having a multilayer structure, characterized in that,

the catalyst for purifying exhaust gas having a multilayer structure is produced by the method according to claim 1, wherein the thickness of the uppermost layer containing a noble metal is 20 μm or less.

Technical Field

The present invention relates to an exhaust gas purifying catalyst having a multilayer structure comprising a 20 μm or more inner layer containing a noble metal component of Rh and/or Pd and a method for producing the same, and more particularly, to a technique for forming a 20 μm or more inner layer as the uppermost layer of the catalyst by applying a polymer coating solution having a functional group capable of chelating a noble metal component of Rh and/or Pd to a multilayer structure catalyst for exhaust gas purification.

Background

The exhaust gas purifying catalyst (e.g., three-way catalyst) reduces CO and HC in the automobile exhaust gas by an oxidation reaction and reduces these harmful components by reducing NOx by a reduction reaction. The catalyst body comprises a support or substrate (substrate) made of ceramic, Al coated in a single layer or in multiple layers to the substrate₂O₃A coating comprising a precious metal-containing composition impregnated with alumina (alumina). In the three-way catalyst, a Pt/Pd/Rh three-way noble metal system containing Pt, Rh and palladium (Pd) is used as the noble metal component. Specifically, Pt mainly promotes the oxidation reaction of CO with HC to be reduced, and Rh promotes the NOx reaction. Pd favors the onset of CO and HC reaction (light-off) but is detrimental to NOx reactions and sulfur toxicity (reduced performance due to sulfur content in the fuel). In practice, the Pd and Rh components in the noble metal component can each be present as individual components in order to optimize the performance of the three-way catalyst. That is, Pd and Rh components can be arranged in the three-way catalyst as individual components without forming an alloy, and for example, Pd and Rh exist in a two-layer form to avoid mutual alloying, or exist in respective supports even when they exist in a single-layer form. The single layer or the plurality of layers constituting the catalyst are formed by a commonly used slurry coating method.

Disclosure of Invention

Technical problem

On the other hand, in order to meet the continuously increasing exhaust gas regulation, it is still necessary to improve the HC reaction start temperature (light off), and particularly to increase the catalyst activity at low temperatures, so that it is necessary to collect a part of the Pd component, which is an active material suitable for the above purpose, on the catalyst surface in the form of a thin layer in the exhaust gas purifying catalyst. However, in the conventional coating method, when a part of the noble metal component is formed as a thin layer on the uppermost layer of the catalyst, it is difficult to adjust the coating amount and the coating shape.

The present inventors have surprisingly found that an exhaust gas purifying catalyst having a multilayer structure with an extremely thin uppermost layer having a thickness of 20 μm or less can be prepared by using a polymer solution capable of forming a composite with a noble metal component, and that the amount and shape of the coating can be easily adjusted in this manner to improve the catalyst performance.

Technical scheme

The preparation method of the catalyst with the multilayer structure for purifying the exhaust gas comprises the following steps: forming a multi-layered structure on a substrate using a catalyst paste; and applying a noble metal-containing coating liquid to the uppermost layer of the multilayer structure. In the present invention, the noble metal-containing coating liquid is a polymer solution that forms a complex with the noble metal component, and the polymer solution that forms a complex with the noble metal component has a functional group (for example, a hydroxyl group or an ether group) capable of chelating with the noble metal component.

Further, the present invention discloses an exhaust gas purifying catalyst having a multilayer structure, which is produced by the production method and contains a noble metal, wherein the thickness of the uppermost layer is 20 μm or less.

Although not limited thereto, the present invention discloses a three-way catalyst comprising an extremely thin layer containing a noble metal component as a main component, comprising a lower layer, an intermediate layer and an upper layer, each of which is composed of Pd and Rh components, and is a thin layer containing Pd components. Without limitation, the weight ratio of the Pd component of the lower layer and the upper layer in the three-way catalyst of the present invention is 50:50 to 90:10, and preferably, the thickness of the upper layer is an extremely thin layer of 20um or less.

The invention discloses a method for preparing a catalyst body with a thin layer containing precious metal components, which comprises the following steps: step i, generating a Pd impregnated support body and a Rh impregnated support body; step ii, mixing the impregnated support with an additive to prepare a slurry coating (wash coat); step iii, applying the slurry coating to a substrate (substrate) to form a lower layer and a middle layer, respectively; and, step iv, coating a polymer coating liquid containing a Pd component on the intermediate layer to form a thin layer with a thickness of 20 um. The thin layer (upper layer or uppermost layer) coated with the Pd component of the present invention is 10 to 50% by weight of the total Pd component suitable for the catalyst body. 10 wt% is the minimum amount for the three-way catalyst of the present invention to improve the HC reaction initiation temperature (light off) in a thin layer, and 50 wt% takes into account the palladium content and 20 μm thickness that are required to be present at least in the lower layer. It is within the contemplation of the person skilled in the art that not only palladium may be present in the layer, but also palladium-rhodium alloys may be present as the main constituent of the layer, in which case the rhodium content and/or palladium content may be adapted in the same way as described above. The noble metal component of the thin layer of the present invention will be described by taking palladium as an example, but the main component of the thin layer may be rhodium when the catalyst structure is made such that palladium and rhodium are arranged in reverse order according to the application.

Advantageous effects

The catalyst preparation method of the present invention can uniformly realize the thin layer of the upper layer or the uppermost layer, which is difficult to adjust the coating amount and/or the coating shape in the conventional coating method, and the exhaust gas purifying catalyst having the extremely thin layer of 20 μm or less formed by the coating method can improve the emission of THC, CO and NOx compared with the catalyst having no thin layer, and therefore, the performance of the three-way catalyst can be greatly improved by coating a part of the noble metal into the thin layer in the case of using the same amount of the noble metal.

Drawings

Fig. 1 shows a conceptual diagram of a catalyst of the present invention in which a thin layer contains palladium as a noble metal component.

Fig. 2(a) is a coating shape when an upper layer is coated in a conventional method, and it can be seen that the upper layer is not uniformly formed due to a capillary phenomenon, and fig. 2(b) is a photograph measured with an electron probe X-ray microanalyzer (EPMA) showing a uniform noble metal thin film upper layer formed in a coating method of the present invention.

FIG. 3 is a graph comparing the performance test results of a Pd/Rh two-layer three-way catalyst and the performance test results of a Pd (90%)/Rh (100%) multi-layer three-way catalyst of the present invention forming a Pd thin layer (10%).

Fig. 4 is a summary of the EMPA photograph and thickness of the catalyst completed in the example.

Detailed Description

Definition of

The term "catalyst" as used herein refers to a powder form impregnated with an active component such as Pd, Rh or the like on a support such as alumina powder, and "catalyst body" refers to a structure in which the "catalyst" is coated on a substrate or carrier such as cordierite. "coating" refers to a slurry of the catalyst mixed with other ingredients. The coating is applied to the substrate to form the catalyst body. However, as will be appreciated by those skilled in the art, the term "catalyst" or the term "catalyst bulk" may be used interchangeably. In the present application, the upper layer and the uppermost layer are used in the same sense, and the lower layer means a layer closest to the substrate, and the intermediate layer is a layer disposed on the upper portion of the lower layer, and any structural layer other than the upper layer may be collectively referred to as the lower layer.

The "heat treatment step" refers to a heating step for inducing a raw material component in a precursor state into a stable structure, and particularly refers to a heating step in an exhaust atmosphere. "exhaust atmosphere" means an atmosphere containing emissions such as O emitted by a gasoline internal combustion engine₂、CO₂、CO、H₂HC (AHC (aromatic hydrocarbons), propane/propylene, etc.), NOx, H₂O and the like, and is referred to in the art as containing 5 to 10 wt.% of H₂O and O₂、CO₂、CO、H₂HC (AHC (aromatic hydrocarbons), propane/propylene, etc.), NOx, N₂An atmosphere having a composition varying from 0 to 15% by weight. By "precatalyst" in this application is meant a compound which is not structurally subjected to a heat treatment step before it is appliedThe catalyst in an alloyed state means, specifically, a catalyst in a state in which the support is simply impregnated with the precursor.

In the present application, the thin layer of a noble metal such as palladium means that the thin layer constituting the upper layer is not formed of palladium alone but includes an inorganic oxide such as alumina, an oxygen storage material, and the like which are understood in the art, but is referred to as a thin layer of palladium for convenience.

With respect to the arrangement of palladium and rhodium in the noble metal component of the three-way catalyst, it is practically accepted in the art to arrange palladium and rhodium in the respective components. That is, the palladium and rhodium constitute a catalyst so as not to be close to or adjacent to each other. For example, a three-way catalyst body is prepared by impregnating a support with palladium and then thermally fixing the support, and impregnating another support with rhodium and then thermally fixing the support, and then mixing the coated support with cordierite or coating the coated support with a plurality of layers, and the body is installed in an automobile exhaust system by canning (canning).

The present inventors have found that the noble metal thin layer concept introduced in the present application can be applied to the existing three-way catalyst arrangement, that is, that a part of the palladium component applied to the existing lower layer, preferably 10 to 50 wt% of the total Pd component applied to the catalyst is laminated as a thin layer (e.g., 20um or less) to the upper layer regardless of the arrangement of the lower layer disposed below the noble metal component thin layer, to improve the three-way catalyst performance.

Furthermore, the present inventors have found that the conventional coating method cannot achieve a desired coating amount and a desired coating shape when a thin palladium layer is formed as the uppermost layer of the catalyst. For example, the palladium content distribution exceeds a reference value or the coating shape cannot be uniformly maintained. For this reason, an exhaust gas-purifying catalyst structure having a Thin layer was prepared by applying a new coating method (hereinafter referred to as Post PGM Thin-layer Coat (PPTC)). Briefly, after preparing a double-layer precatalyst in a conventional manner, if a polymer coating solution is allowed to contain a part of the noble metal component constituting the catalyst (for example, a part of the palladium component constituting the lower layer) and coated on the upper part of the prepared double-layer precatalyst in a conventional manner, the coating solution can be impregnated into the surface of the three-way catalyst within 20 μm to form a thin layer of the noble metal.

The present invention will be described in detail below by way of examples, but it is apparent that the spirit of the present invention is not limited thereto. For example, in the three-way catalyst of the present invention, the noble metal component of the lower layer may contain Pt component, but in the present application, only Pd and Rh components are described for the sake of simplicity and clarity of the description of the thin film characteristics of the upper layer, and the Pd and Rh components in the lower layer may be formed in a non-alloy or alloy form. In the present application, the coating liquid containing palladium is a polymer solution forming a complex with palladium, and although not limited by theory, hydroxyethyl cellulose (HEC) or polypropylene glycol having a functional group (for example, a hydroxyl group or an ether group) capable of chelating with palladium is exemplified, but the invention is not limited thereto.

Pre-catalyst

Allowing an aqueous solution of Palladium Nitrate (Palladium Nitrate) and an aqueous solution of Rhodium Nitrate (Rhodium Nitrate) to impregnate the support Al₂O₃And (3) powder. The alumina powder is dried in a furnace at 150 ℃ for 5 hours and sintered at 400 ℃ to 650 ℃ for 5 hours to prepare a pre-catalyst powder. After preparing a slurry using each of the obtained pre-catalyst powders and applying it to a substrate in two passes according to a general coating method, a pre-catalyst body is prepared by heat treatment at 500 to 1100 ℃ for 1 to 50 hours, preferably 12 hours, in a reducing gas atmosphere (e.g., an exhaust gas atmosphere).

Comparative example

Separately prepared palladium slurry, in which alumina is impregnated into an aqueous palladium solution and dispersed in a solvent, is pulverized (Ball Milling), is applied as a thin layer to the pre-catalyst body according to the conventional coating method, and the catalyst body is dried and sintered. At this time, the amount of Pd used for the thin layer corresponded to 10 wt% of the total amount of Pd used for the catalyst body. Specifically, 90% of the palladium component of the lower layer used as the precatalyst was applied and the remaining 10% was applied as the upper layer of the thin layer (the upper layer was applied at a palladium coating thickness of 28. mu.m, and the coating amount was 10 g/L). If the upper layer coating thickness deviation exceeds the reference value, the coating shape becomes uneven (see fig. 2 (a)).

First embodiment

The palladium aqueous solution was dispersed in a commercially available polypropylene glycol solution and applied to the pre-catalyst body in the form of a thin layer, and dried and sintered to complete the catalyst body (upper layer palladium applied thickness 8.61um, 10 wt% of the total amount of Pd used for the catalyst body, see fig. 2 (b)). The Pd content for the thin layer at this time is applied identically to the comparative example.

Second embodiment

Prepared identically to the first example, but the Pd content for the thin layer corresponds to 20 wt% of the total amount of Pd used for the catalyst body.

Third embodiment

Prepared identically to the first example, but the Pd content for the thin layer corresponds to 30 wt% of the total amount of Pd used for the catalyst body.

Fourth embodiment

Prepared identically to the first example, but the Pd content for the thin layer corresponds to 50 wt% of the total amount of Pd used for the catalyst body.

The thin layer (upper layer) configurations of the comparative examples and examples are summarized below.

[ Table 1]

Fig. 4 shows a photograph and thickness observed at 400 magnifications using an electron probe X-ray microanalyzer (EPMA). Fig. 3 shows the vehicle evaluation results obtained with the catalyst body prepared in the first embodiment as the subject. The present invention gave excellent results in all of HC, CO and NOx compared to the comparative examples. Therefore, it was confirmed that, when the same amount of noble metal was used, the performance of the three-way catalyst could be greatly improved by disposing only a part of the noble metal as a thin layer.