Exhaust gas purifying catalyst

Abstract

This invention provides an exhaust gas purification catalyst formed by supporting an alkaline earth metal in a highly dispersed state on a porous support. The exhaust gas purification catalyst provided by this invention is characterized by: its catalyst layer having an alkaline earth metal supporting region, which comprises a porous support, a platinum group metal catalyst, and a sulfate of at least one alkaline earth metal supported on the porous support; the average particle size of the alkaline earth metal sulfate is less than 25 nm; the porous support is a secondary particle, on the outer surface and inside of which alkaline earth metal sulfate is dispersed and immobilized; for the cross-section of this region, surface analysis is performed using FE-EPMA under specific pixel size and number of measured pixels, and the intensity (α:cps) of the characteristic X-rays of the alkaline earth metal element (Ae) and the intensity (β:cps) of the characteristic X-rays of the main constituent element (M) of the inorganic compound constituting the porous support are measured for each pixel; the Pearson correlation coefficient R is calculated using the obtained α and β values ​​for each pixel. Ae／M The value is 0.7 or higher.

Description

[0001] This case is a divisional application filed on April 9, 2018, with application number 201880024001.6 (PCT / JP2018 / 014939) and invention titled Catalyst for Exhaust Gas Purification. Technical Field

[0002] This invention relates to a catalyst for exhaust purification installed in the exhaust system of an internal combustion engine. More specifically, it relates to an exhaust purification catalyst containing at least one metal belonging to the platinum group as a catalyst metal and alkaline earth metals such as barium (Ba) and strontium (Sr) as co-catalyst components. Furthermore, this invention claims priority based on Japanese Patent Application No. 2017-078366, filed on April 11, 2017, the entire contents of which are incorporated herein by reference. Background Technology

[0003] This process involves using oxidation or reduction reactions to remove hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) from exhaust gases emitted from internal combustion engines such as automobile engines. x Catalysts for purifying exhaust gases containing harmful components such as aluminum oxide (Al₂O₃) and zirconium oxide (ZrO₂) have been used, and so-called three-way catalysts have been employed. Three-way catalysts utilize metals (typically platinum group metals such as palladium (Pd) and rhodium (Rh; hereinafter sometimes referred to as "catalyst metals") that function as oxidation and / or reduction catalysts, supported on a porous support such as aluminum oxide (Al₂O₃) or zirconium oxide (ZrO₂). For example, three-way catalysts made by supporting Pd as an oxidation catalyst and Rh as a reduction catalyst on a porous support are widely used.

[0004] Furthermore, this exhaust purification catalyst uses co-catalyst components that enhance exhaust purification performance. Examples include alkaline earth metals such as barium (Ba) and strontium (Sr). For instance, by containing an appropriate amount (e.g., approximately 1% to 10% by mass of the entire area) of alkaline earth metals such as Ba in at least a portion of the catalyst layer, it is possible to temporarily absorb NO contained in the exhaust gas. x Furthermore, by coexisting with Pd, the sintering of Pd can be suppressed by donating electrons from Ba to Pd, thereby maintaining and improving the catalytic activity of Pd. For example, in the following patent documents 1 and 2, existing exhaust gas purification catalysts containing alkaline earth metals such as Ba and Sr as co-catalysts are described.

[0005] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. Hei 5-237390 Patent Document 2: Japanese Patent Application Publication No. Hei 11-285639 Summary of the Invention

[0006] The technical problem that the invention aims to solve However, for alkaline earth metals to exert their co-catalyst effect as described above, they need to be present near the catalyst metal in the catalyst layer of the exhaust gas purification catalyst. Furthermore, for them to function as co-catalysts throughout the entire exhaust gas purification catalyst, it is important that the alkaline earth catalyst and the catalyst metal exist in a highly dispersed state.

[0007] However, when using existing catalyst layer formation techniques and methods, alkaline earth metal components tend to be biased within the catalyst layer (alkaline earth metal supporting region), failing to be supported in a highly dispersed state on the outer surface and within the pores of the porous support. In other words, it is impossible to support (fix) the alkaline earth metal in the catalyst layer (alkaline earth metal supporting region) with the same high degree of dispersion as the catalyst metal supported in a highly dispersed state on the outer surface and within the pores of the porous support. For example, there is a situation where most of the alkaline earth metal is biased towards the outer surface of the porous support.

[0008] Therefore, the present invention was created to solve the above-mentioned problems of the prior art, and its object is to provide an exhaust gas purification catalyst in which an alkaline earth metal as a co-catalyst component is supported in a highly dispersed state on a porous carrier, and to provide a manufacturing method that can achieve such a highly dispersed support.

[0009] Technical solutions for solving technical problems The inventors conducted a detailed study on the existing forms of alkaline earth metals such as Ba as co-catalysts in the catalyst layer. It was then confirmed that if alkaline earth metals such as Ba are used as water-insoluble sulfates from the raw material stage, it leads to the bias of alkaline earth metals in the catalyst layer, making it impossible to achieve a highly dispersed state. Furthermore, when using a raw material slurry containing water-soluble compounds such as Ba nitrates, and supplying the slurry with sulfuric acid or ammonium sulfate solution to generate sulfates (insoluble substances) of alkaline earth metals such as Ba, the raw material slurry becomes excessively acidic during the subsequent drying and calcination stages. As a result, a highly dispersed state cannot be maintained, leading to the bias of alkaline earth metals and preventing the achievement of a highly dispersed state.

[0010] Therefore, the inventors started their research from the raw material stage of alkaline earth metals such as Ba, and discovered that by using a raw material formed by coexisting a water-soluble compound of alkaline earth metal with a certain water-soluble organic compound containing S, it is possible to configure (support) alkaline earth metal sulfates such as Ba and catalyst metal together in a highly dispersed state in a porous carrier in the catalyst layer (alkaline earth metal supporting region), until the present invention was completed.

[0011] According to the present invention, an exhaust purification catalyst can be provided that is disposed in the exhaust pipe of an internal combustion engine and purifies the exhaust gas discharged from the internal combustion engine.

[0012] That is, the exhaust gas purification catalyst disclosed herein has a substrate and a catalyst layer formed on the substrate. The catalyst layer has an alkaline earth metal supporting region, which has: Porous supports composed of inorganic compounds; At least one platinum group metal catalyst supported on the porous support, functioning as an oxidation and / or reduction catalyst; and At least one alkaline earth metal sulfate supported on the porous carrier.

[0013] Furthermore, the exhaust gas purification catalyst disclosed herein is characterized in that: for the cross-section of the alkaline earth metal supporting region of the aforementioned catalyst layer, in Pixel (area) size: 0.34μm × 0.34μm The number of pixels (regions) measured is 256×256. Under the given conditions, surface analysis was performed using FE-EPMA. For each pixel, the intensity (α: cps) of the characteristic X-rays of the alkaline earth metal element (Ae) and the intensity (β: cps) of the characteristic X-rays of the main constituent element (M: i.e., the main constituent metal or half-metal element of the inorganic compound constituting the porous carrier) were measured. The Pearson correlation coefficient calculated using the obtained α and β values ​​for each pixel was set as R0. Ae／M At that time, the R Ae／M The value is 0.5 or higher.

[0014] The exhaust gas purification catalyst disclosed herein is a catalyst product that can be manufactured using the manufacturing method for the exhaust gas purification catalyst provided by the present invention (details will be described later). As described above, it is characterized by: R being the Pearson correlation coefficient (product-moment correlation coefficient) calculated based on the results of surface analysis using FE-EPMA. Ae／M It is above 0.5.

[0015] The above correlation coefficient R Ae／M When the first variable is set as the characteristic X-ray intensity (α) of the alkaline earth metal element (Ae) in the surface analysis using FE-EPMA, and the second variable is set as the characteristic X-ray intensity (β) of the main constituent element (M) of the inorganic compound constituting the porous support in the same surface analysis, then... R Ae／M = (codispersion) / (standard deviation of α × standard deviation of β) The correlation coefficient was calculated.

[0016] In the exhaust gas purification catalyst of this configuration, the characteristic feature is that, in the alkaline earth metal supporting region of the catalyst layer, there is a high correlation between the location (distribution) of the main constituent element constituting the porous support and the location (distribution) of the alkaline earth metal element. In other words, alkaline earth metal (sulfate) exists in a highly dispersed state throughout all porous support particles (i.e., both the outer surface and the interior (pores) of the support particles). Therefore, in the alkaline earth metal supporting region of the catalyst layer, the alkaline earth metal can exist at a high frequency near the catalyst metal particles, thus enabling the alkaline earth metal to exert its effect as a co-catalyst component at a high level.

[0017] Therefore, for example, alkaline earth metal components such as Ba can be used to temporarily absorb NO contained in exhaust gas. x The catalyst can be effectively reduced and purified using catalyst metals such as Rh. Alternatively, since alkaline earth metals can be frequently placed near Pd, the sintering of catalyst metals such as Pd can be suppressed, thus maintaining and improving catalytic activity.

[0018] The average particle size of the alkaline earth metal sulfate supported on the porous support, as determined by X-ray diffraction, is particularly preferably below 25 nm. Such a fine average particle size allows the alkaline earth metal component to exhibit exceptionally high performance as a co-catalyst.

[0019] In another preferred embodiment of the exhaust gas purification catalyst disclosed herein, the characteristic is that the Pearson correlation coefficient R calculated above is... Ae／M The value is above 0.7.

[0020] For example, R Ae／M As indicated by a value of 0.7 or higher, the alkaline earth metal component of the exhaust gas purification catalyst of this configuration exhibits high dispersibility. Therefore, it is able to perform its function as a co-catalyst component at a high level within the catalyst layer (alkaline earth metal support region).

[0021] Especially when Ae is Ba and M is Al or Zr, high R can be appropriately achieved. Ae／M (That is, it can be recorded as R) Ba／Al Or R Ba／Zr 。 ).

[0022] In addition, in a preferred embodiment of the exhaust gas purification catalyst disclosed herein, it is characterized by having at least palladium (Pd) and / or rhodium (Rh) as catalyst metals, and at least barium sulfate (BaSO4) as the alkaline earth metal sulfate.

[0023] When using an exhaust gas purification catalyst with this configuration, NO can be stably and temporarily adsorbed using the highly dispersed barium component (barium sulfate). xFurthermore, it is possible to utilize Rh to temporarily absorb NO from this component. x The components undergo effective reduction and purification treatment. Furthermore, by being highly dispersed and supported on a carrier, the NO content can be increased. x Reduction reaction. Therefore, the exhaust gas purification catalyst of this composition can serve as a high-performance NO reduction agent. x Purify the catalyst and use it appropriately.

[0024] Furthermore, by allowing Pd and Ba to coexist, the sintering of Pd can be suppressed through the electronic interaction of Ba, thus maintaining the activity of Pd. Therefore, the exhaust gas purification catalyst of this configuration can be appropriately used as a high-performance exhaust gas purification catalyst with excellent durability.

[0025] Furthermore, to achieve the above objectives, the present invention provides a method for suitably manufacturing the exhaust gas purification catalyst disclosed herein. That is, the manufacturing method disclosed herein is a method for manufacturing an exhaust gas purification catalyst disposed in the exhaust pipe of an internal combustion engine and for purifying the exhaust gas discharged from the internal combustion engine, the method comprising: The process of forming a catalyst layer on a substrate; and The process of firing the substrate on which the above-mentioned catalyst layer is formed. At least a portion of the catalyst layer described above has an alkaline earth metal supporting region, wherein the alkaline earth metal supporting region has: Porous supports composed of inorganic compounds; At least one platinum group metal catalyst supported on the porous support, functioning as an oxidation and / or reduction catalyst; and At least one alkaline earth metal sulfate supported on the porous carrier.

[0026] Furthermore, the manufacturing method disclosed herein includes the following steps in the catalyst layer formation process: (1) The following ingredients: The inorganic compound particles constituting the above porous carrier Precursor material used to precipitate the above-mentioned catalyst metal particles or the catalyst metal; The above-mentioned water-soluble compounds of alkaline earth metals, and Water-soluble organic compounds containing sulfur as a constituent element and capable of forming sulfates of the aforementioned alkaline earth metals. The step of mixing with an aqueous solvent to prepare a raw material suspension; (2) The step of preparing a powder material containing the above-mentioned porous support, catalyst metal and alkaline earth metal sulfate by drying and calcining the above-mentioned raw material suspension; (3) The step of preparing a slurry for forming an alkaline earth metal support region, containing at least the above-mentioned powder material and an aqueous solvent; and (4) The step of forming the alkaline earth metal support region on the substrate using the above slurry.

[0027] In the method for manufacturing an exhaust gas purification catalyst with such a configuration, in the application of forming an alkaline earth metal support region, a raw material (suspension) is used, which is a mixture of a water-soluble compound of alkaline earth metal and the aforementioned S-containing water-soluble organic compound with a carrier component (inorganic compound particles) and a catalyst metal component (catalyst metal particles or a compound that is a precursor of the catalyst metal).

[0028] In the prepared raw material suspension, both the alkaline earth metal water-soluble compound and the sulfur-containing water-soluble organic compound are dissolved in an aqueous solvent. At this time, the sulfur-containing water-soluble organic compound does not cause a sharp drop in the pH of the raw material suspension (i.e., acidification of the suspension), and both the alkaline earth metal water-soluble compound and the sulfur-containing water-soluble organic compound can maintain their water solubility and reach the interior (pores) of the inorganic compound particles (secondary particles) that serve as carrier components.

[0029] The prepared raw material suspension is then dried and calcined. During this process, the water-soluble compound of the alkaline earth metal, which is a component, reacts with the aforementioned water-soluble organic compound containing sulfur. Insoluble alkaline earth metal sulfates are formed inside and outside the inorganic compound particles and are fixed at their sites.

[0030] Therefore, by using the manufacturing method of this configuration, it is possible to suitably manufacture an exhaust gas purification catalyst in which all or part of the catalyst layer has an alkaline earth metal carrying region, characterized in that the alkaline earth metal is present in a highly dispersed state throughout all porous carrier particles (i.e., both the outer surface and the interior (pores) of the carrier particles).

[0031] In a preferred embodiment of the method for manufacturing a catalyst for exhaust gas purification disclosed herein, the method is characterized in that: as a water-soluble organic compound containing sulfur, a water-soluble organic compound having at least one functional group selected from sulfonyl (-SO3H), sulfonyl (-S(=O)2-), and thionyl (-S(=O)-) is used.

[0032] Such organic compounds having an S-containing functional group are preferred as S-containing water-soluble organic compounds for preparing the above-mentioned raw material suspension.

[0033] In another preferred embodiment of the method for manufacturing the catalyst for exhaust gas purification disclosed herein, the method is characterized in that: as the water-soluble compound of the alkaline earth metal, a hydroxide, acetate, or nitrate of any alkaline earth metal selected from Ba, Sr, and Ca is used.

[0034] Such hydroxides, acetates, and nitrates have good water solubility and are preferably water-soluble compounds of alkaline earth metals used to prepare suspensions of the above-mentioned raw materials. Attached Figure Description

[0035] Figure 1 This is a perspective view schematically representing an exhaust gas purification catalyst according to one embodiment.

[0036] Figure 2 This is a schematic cross-sectional view of the catalyst layer of an exhaust gas purification catalyst according to one embodiment.

[0037] Figure 3 This is an image showing the Ba elemental mapping results from a FE-EPMA surface analysis (256×256 pixels) performed on the catalyst layer (alkaline earth metal support region) of the exhaust gas purification catalyst involved in Example 14.

[0038] Figure 4 This is an image showing the S element mapping results from a FE-EPMA surface analysis (256×256 pixels) performed on the catalyst layer (alkaline earth metal support region) of the exhaust gas purification catalyst involved in Example 14.

[0039] Figure 5 This is an image showing the Ba elemental mapping results from a FE-EPMA surface analysis (256×256 pixels) performed on the catalyst layer (alkaline earth metal supported region) of the exhaust gas purification catalyst involved in Comparative Example 1.

[0040] Figure 6 This is an image showing the S element mapping results from a FE-EPMA surface analysis (256×256 pixels) performed on the catalyst layer (alkaline earth metal supported region) of the exhaust gas purification catalyst involved in Comparative Example 1.

[0041] Figure 7 This is an image showing the Ba elemental mapping results from a FE-EPMA surface analysis (256×256 pixels) performed on the catalyst layer (alkaline earth metal supported region) of the exhaust gas purification catalyst involved in Comparative Example 2.

[0042] Figure 8 This is an image showing the S element mapping results from a FE-EPMA surface analysis (256×256 pixels) performed on the catalyst layer (alkaline earth metal supported region) of the exhaust gas purification catalyst involved in Comparative Example 2.

[0043] Figure 9 This involves removing HC, CO, and NO from the exhaust gas purification catalyst described in one embodiment. x The graph is obtained by comparing the purification performance (T50: °C) of the sample with that of the comparative example. Detailed Implementation

[0044] Hereinafter, preferred embodiments of the present invention will be described with appropriate reference to the accompanying drawings. Matters not specifically mentioned in this specification but necessary for the implementation of the present invention can be understood by those skilled in the art based on existing technology in the field. The present invention can be implemented based on the disclosures in this specification and technical knowledge in the field. The following descriptions... Figures 1-2 These are schematic diagrams for the purpose of understanding the content of this invention. The dimensional relationships (length, width, thickness, etc.) in the diagrams do not reflect the actual dimensional relationships.

[0045] The exhaust gas purification catalyst disclosed herein is characterized by the following: at least a portion of the catalyst layer (i.e., a pre-designed alkaline earth metal support region within the catalyst layer) contains highly dispersed alkaline earth metal sulfates exhibiting the aforementioned properties; other components are not particularly limited. By appropriately selecting the types of catalyst metal, support, and substrate described later, and forming them into desired shapes according to the application, the exhaust gas purification catalyst disclosed herein can be configured in the exhaust systems (exhaust pipes) of various internal combustion engines, particularly automobile engines.

[0046] In the following description, the exhaust purification catalyst of the present invention is primarily described under the premise of being applicable to a three-way catalyst installed in the exhaust pipe of a gasoline engine of an automobile, but there is no intention to limit the exhaust purification catalyst disclosed herein to the above-described uses.

[0047] <Substrate> The substrate constituting the framework of the exhaust gas purification catalyst disclosed herein can be made from various raw materials and in various forms currently used for this purpose. For example, ceramics such as cordierite and silicon carbide (SiC), which have high heat resistance, are preferred. Alternatively, an alloy substrate (such as stainless steel) can be used. Regarding the shape, it can also be the same as existing exhaust gas purification catalysts. As an example, examples such as... Figure 1 The exhaust gas purification catalyst 10 shown has a cylindrical honeycomb substrate 1 with through holes (divided chambers) 2 along its cylindrical axis, serving as an exhaust flow path. The exhaust gas can contact the partition walls (ribs) 4 that separate the partition chambers 2. Besides a honeycomb shape, the substrate 1 can also be foam-shaped, granular, etc. Alternatively, it can be a so-called through-wall type (also called wall-flow type) substrate, where exhaust gas flows from the inlet partition chamber on one side through the partition wall to the outlet partition chamber on the other side. Furthermore, regarding the overall shape of the substrate, an elliptical or polygonal cylindrical shape can be used instead of a cylindrical shape.

[0048] <Catalyst Metals> The catalyst layer of the exhaust gas purification catalyst disclosed herein may contain at least one platinum group metal (hereinafter also referred to as "PGM") capable of functioning as an oxidation or reduction catalyst. Typical examples include rhodium (Rh), palladium (Pd), and platinum (Pt). Ruthenium (Ru), osmium (Os), iridium (Ir), or other metals other than PGMs with catalytic function may also be used. Substances formed by alloying two or more PGMs may also be used. Particularly preferred is the combination of Rh, which has high reduction activity, and Pd or Pt, which has high oxidation activity, to construct a three-way catalyst.

[0049] From the viewpoint of increasing the contact area with the exhaust gas, the catalyst metal is preferably used in the form of particles with sufficiently small particle sizes. Typically, the average particle size (preferably the average particle size obtained by TEM observation or the average particle size based on X-ray diffraction) is about 1 nm to 15 nm, and particularly preferably 10 nm or less, 7 nm or less, and even 5 nm or less.

[0050] The loading rate of the catalyst metal (the PGM content when the support is set to 100% by mass) is not particularly limited, but it is appropriate to be 2% by mass or less, for example, 0.05% by mass or more but less than 2% by mass, and preferably about 0.2% by mass or more but less than 1% by mass. When the loading rate is too low compared to the above range, it is difficult to obtain the catalytic effect of the catalyst metal. When the loading rate is too high compared to the above range, it is disadvantageous in terms of cost.

[0051] <Carrier> The porous support that constitutes the catalyst layer and carries the aforementioned catalyst metal and other components (such as alkaline earth metals) uses the same inorganic compound as existing exhaust gas purification catalysts.

[0052] Porous supports with a sufficiently large specific surface area (specific surface area measured using the BET method, hereinafter the same) are preferred. Preferred examples include alumina (Al₂O₃), zirconium oxide (ZrO₂), cerium oxide (CeO₂), silicon dioxide (SiO₂), titanium oxide (TiO₂), their solid solutions (e.g., cerium oxide-zirconia composite oxides (CZ composite oxides)), or combinations thereof. As mentioned above, in this specification, "the main constituent element (M) of the inorganic compound constituting the porous support" refers to the main metallic or half-metallic element that determines the inorganic compound. Therefore, it is readily understood by those skilled in the art that the main constituent element (M) of the aforementioned alumina, zirconium oxide, cerium oxide, silicon dioxide, titanium oxide, and CZ composite oxides is Al, Zr, Ce, Si, Ti, Ce & Zr, respectively.

[0053] From the perspective of improving the thermal stability of catalysts used for exhaust gas purification, it is preferable that the catalyst layer contains inorganic compounds such as alumina and zirconium oxide, which have good heat resistance, as a support or non-support (i.e., the composition of the catalyst layer without supporting catalyst metals or alkaline earth metals. The same applies below.).

[0054] For particles used as carriers or non-carriers (such as alumina powder or CZ composite oxide powder), considering heat resistance and structural stability, a specific surface area of ​​50–500 m² is preferred. 2 / g (e.g., 200-400mg) 2 / g). In addition, the average particle size of the carrier particles based on TEM observation is preferably about 1 nm to 500 nm (more preferably 5 nm to 300 nm).

[0055] Furthermore, when using such an inorganic compound (ceramic) as a carrier, it is preferable that the catalyst metal content per 1L catalyst volume is approximately 0.1 to 5 g / L, preferably approximately 0.2 to 2 g / L. Excessive catalyst metal content is not cost-effective; insufficient content reduces exhaust gas purification capacity, and is therefore also undesirable. In this specification, a catalyst volume of 1L refers to a loosely packed volume of 1L, including the volume of the internal voids (dividing chambers) (i.e., the catalyst layer formed within these voids (dividing chambers)).

[0056] <Catalyst layer and alkaline earth metal support region> The catalyst layer formed on the substrate serves as the part for purifying exhaust gas and forms the main body of the exhaust gas purification catalyst. However, in the exhaust gas purification catalyst disclosed herein, as described above, at least a portion (or all) of the catalyst layer constitutes an alkaline earth metal support region.

[0057] In this specification, "alkaline earth metal supported region" refers to a portion or all of a catalyst layer having a porous support, a catalyst metal, and alkaline earth metal sulfates (barium sulfate, strontium sulfate, etc.). "A portion of the catalyst layer" refers to a region capable of functioning as a catalyst for exhaust gas purification; this term does not refer to a microscopic part, such as several to dozens of support particles, that cannot functionally be considered a region. For example, as an example... Figure 2 As shown in the catalyst layer 6, when a catalyst layer 6 is a stacked structure with upper and lower layers having different contents formed on a substrate 1, either one or both of the lower layer 6B close to the substrate 1 and the upper layer 6A constituting the surface portion of the catalyst layer 6 can be formed as an alkaline earth metal supporting region. Alternatively, in a stacked or single-layer catalyst layer as shown in the figure, a portion (e.g., all or more of 10 vol%) on the upstream (or downstream) side can be made into an alkaline earth metal supporting region along the direction of its exhaust flow.

[0058] Furthermore, barium (Ba), strontium (Sr), and calcium (Ca) are examples of preferred alkaline earth metal elements that constitute the alkaline earth metal supporting region and are suitable for supporting sulfates. From the viewpoint of being able to perform highly functionally as a co-catalyst component, Ba and Sr are preferred, with Ba being particularly preferred. Barium sulfate (BaSO4) has an extremely high melting point and is stable, and has extremely low solubility in water, making it suitable as an alkaline earth metal supported on a carrier.

[0059] The type and configuration (distribution) of the catalyst metal supported on the catalyst layer, similar to the determination of the alkaline earth metal support area, can be appropriately set according to various purposes. For example, in Figure 2 In the catalyst layer 6 of the stacked structure shown, the type of support, the type of catalyst metal supported on the support, or the content ratio can be different in the upper layer 6A and the lower layer 6B, similar to existing products. For example, when the lower layer 6B is used as an alkaline earth metal support region, the alkaline earth metal component (e.g., barium sulfate) can coexist with at least one catalyst metal (e.g., Pd or Pt) in the PGM. This improves the durability of Pd and the like during sintering.

[0060] Additionally, Rh, acting as a catalyst metal, can be contained in the upper 6A layer, which is not an alkaline earth metal-supported region. This prevents Rh from approaching alkaline earth metals such as Ba (especially Ba), thus preventing the formation of NO from Rh. x The purification function is reduced or Rh is excessively oxidized.

[0061] In addition to catalyst metals and alkaline earth metals, various auxiliary components can be incorporated into the catalyst layer. A typical example is the oxygen storage component (OSC). Zirconia, cerium oxide, and zeolite are preferred OSC materials. Furthermore, from the viewpoint of high heat resistance and storage / release rate, the aforementioned cerium oxide-zirconia composite oxide (CZ composite oxide) is preferred as the OSC material.

[0062] By calculating the Pearson correlation coefficient above: R Ae／M This allows for easy assessment of the dispersion of alkaline earth metals in the alkaline earth metal support region of the exhaust gas purification catalyst disclosed herein. Surface analysis using FE-EPMA is employed, measuring the intensity (α: cps) of characteristic X-rays of the alkaline earth metal element (Ae) and the intensity (β: cps) of characteristic X-rays of the main constituent element (M) of the inorganic compound constituting the porous support for each pixel. Using the obtained α and β values ​​for each pixel, the aforementioned correlation coefficient can be calculated.

[0063] FE (Field Emission)-EPMA (Electron Probe Micro Analysis), also known as field emission electron beam microscopy, is an analytical method capable of high-precision elemental analysis and imaging of specified regions of a sample. Using FE-EPMA, the α and β values ​​in the catalyst layer (alkaline earth metal supported region) of an exhaust gas purification catalyst are measured at a specified number of pixels. By calculating the obtained data, R can be determined. Ae／M .

[0064] That is, R is the Pearson correlation coefficient (product rate correlation coefficient). Ae／M for: R Ae／M = (co-dispersion) / (standard deviation of α × standard deviation of β), specifically, it can be obtained using the following formula (1). Regarding the correlation coefficient R based on the above equation (1) Ae／M The calculations can be exported without requiring particularly difficult manual calculations by using commercially available spreadsheet software. For example, the CORREL function in Excel (a Microsoft product) can be used for simple export.

[0065] In addition, regarding the data collection for calculating the correlation coefficient, surface analysis using FE-EPMA can be performed by following the manual of the commercially available device.

[0066] In general, firstly, the catalyst layer (alkaline earth metal supported region) of an exhaust gas purification catalyst for surface analysis is cut out and embedded using a curing resin such as epoxy resin, or the aforementioned powder material is used to prepare an embedded sample for surface analysis. After the resin cures, the surface to be analyzed is ground and a conductive material (typically carbon) is vapor-deposited to form an EPMA analysis sample. Then, surface analysis is performed using a commercially available device (such as an electron beam microanalyzer manufactured by Nippon Electronics Corporation, model JXA-8530F).

[0067] The pixel (region) size is 0.34μm × 0.34μm, and the number of pixels (regions) measured can be 200 × 200 or higher, for example, 256 × 256. Measurement conditions depend on the analytical device and are therefore not particularly limited. Several typical measurement conditions can be listed as follows: Accelerating voltage: 10kV~30kV (e.g. 20kV) Irradiation current: 50nA~500nA (e.g. 100nA) Minimum probe diameter: below 500nm (e.g., 100nm) Unit measurement time: 40ms~100ms (e.g. 50ms).

[0068] Alternatively, the application program (computer software) included with the commercially available device can be used to represent the results of surface analysis using FE-EPMA as an elemental image (see the accompanying figure below).

[0069] The method for manufacturing an exhaust gas purification catalyst according to this embodiment includes: a step of forming a catalyst layer on a substrate, wherein at least a portion of the catalyst layer has an alkaline earth metal support region, the alkaline earth metal support region having a porous support, a catalyst metal containing at least one PGM and a sulfate of at least one alkaline earth metal supported on the porous support; and a step of firing the substrate on which the catalyst layer is formed.

[0070] In the catalyst layer formation process, inorganic compound particles that serve as the porous support material, catalyst metal (PGM) particles that serve as the catalyst metal material or precursors for the precipitation of the metal (such as water-soluble salts of the catalyst metal), water-soluble compounds of alkaline earth metals, and water-soluble organic compounds containing sulfur are first mixed with an aqueous solvent to prepare a raw material suspension.

[0071] Although the material used is water-soluble, various salts of alkaline earth metals can be listed, such as hydroxides, acetates, nitrates, and nitrites of Ba, Sr, or Ca. Compounds with high water solubility (e.g., acetates or nitrites for Ba) are particularly preferred.

[0072] In addition, water-soluble coordination compounds or salts of Pd, Rh or Pt can be listed as precursors for the precipitation of catalyst metals.

[0073] As a water-soluble organic compound containing sulfur, there are no particular restrictions as long as it can form an alkaline earth metal sulfate during the preparation, drying, and calcination of the above-mentioned raw material suspension. Preferred examples include taurine (2-aminoethanesulfonic acid), aminobenzenesulfonic acid, aminomethanesulfonic acid, 1-amino-2-naphthol-4-sulfonic acid, cysteine, methionine, cystine, dimethyl sulfate, methyl sulfide, dimethyl trisulfide, 2-mercaptoethanol, diphenyl sulfide, dithiothreitol, allyl disulfide, sulfolane, furfuryl mercaptan, dipropyl disulfide, dimethyl sulfone, and dimethyl sulfoxide.

[0074] Among these, the reactivity of water-soluble organic compounds having at least one functional group among sulfonyl (-SO3H), sulfonyl (-S(=O)2-), and thionyl (-S(=O)-) in the molecule to generate sulfates is good and preferred.

[0075] In addition, compounds with basic groups such as amino (-NH2) are highly effective in preventing the pH of the raw material suspension from decreasing (i.e., becoming strongly acidic), and are therefore preferred.

[0076] Then, the aforementioned materials (carrier constituent materials, catalyst metal materials, water-soluble compounds of alkaline earth metals, and S-containing water-soluble organic compounds) are added to an aqueous solvent (typically water, such as pure water or deionized water), and thoroughly stirred using a mixer to prepare a slurry-like raw material suspension. For example, initially, inorganic compound particles (powder) constituting the porous carrier are mixed with water and stirred. Then, precursors of the catalyst metal are added, followed by water-soluble compounds of alkaline earth metals, and the mixture is stirred thoroughly for a specified time (e.g., 10 to 60 minutes). After that, water-soluble organic compounds containing S are added, and the mixture is dried thoroughly (e.g., for more than 6 hours, preferably more than 8 hours) in a temperature range of about 90 to 130°C, and then calcined at a temperature range of about 400 to 600°C for several hours (e.g., about 1 to 3 hours).

[0077] Using this process, it is possible to modulate a powder material in a highly dispersed state, supporting (fixing) the catalyst metal and alkaline earth metal sulfate on the outer surface and interior (pores) of porous carrier particles (secondary particles) before forming the catalyst layer (including the alkaline earth metal supported region). The resulting powder material can then be subjected to pulverization to adjust to the desired particle size (e.g., a particle size of less than 10 μm).

[0078] Using this process, the particle size of alkaline earth metal sulfates supported on the outer surface and in the pores of porous carrier particles can be made extremely small compared to existing methods.

[0079] Typically, using the techniques disclosed herein, fine alkaline earth metal sulfate (e.g., barium sulfate) particles with an average particle size of 25 nm or less (e.g., 10 nm or more but less than 25 nm), preferably 20 nm or less, based on X-ray diffraction, can be carried in a highly dispersed state on the outer surface and within the pores of porous carrier particles.

[0080] Furthermore, using the above-described process, the aforementioned correlation coefficient, R, can be achieved. Ae／M The dispersion is of a degree of 0.5 or higher, more preferably 0.6 or higher, and even more preferably 0.7 or higher.

[0081] Next, using the obtained powder material (a powder material that has been appropriately pulverized), a slurry for forming the catalyst layer (alkaline earth metal support region) is prepared. The preparation of such a slurry can be the same as that for forming the catalyst layer of existing exhaust gas purification catalysts, without any particular limitations.

[0082] For example in Figure 2 When a lower layer 6B, which serves as an alkaline earth metal support region, is formed on a substrate 1 of an exhaust gas purification catalyst 10 with two different structures, as shown, a slurry containing the powder material prepared above and a carrier powder (such as an OSC material containing alumina, zirconium oxide, or CZ composite oxide) that does not support alkaline earth metals is applied to the honeycomb substrate 1 using a known washing and coating method or the like.

[0083] Next, using a washing and coating method or similar method, an upper layer forming slurry containing the desired catalyst metal component (typically a solution containing other PGM ions different from those used in the formation of the alkaline earth metal support region (lower layer) 6B) and the desired support powder (including alumina, zirconium oxide, CZ composite oxide OSC material, etc.) is laminated onto the surface of the lower layer 6B.

[0084] Then, by drying and firing at a specified temperature and time, a catalyst layer 6 with a stacked structure having an alkaline earth metal supporting region (lower layer) 6B and an upper layer 6A is formed on the substrate 1.

[0085] Alternatively, a two-stage firing process can be used instead of such a single firing process. The two-stage firing process is as follows: after applying a slurry for forming the alkaline earth metal support region (lower layer) to the surface of the substrate, it is dried and fired to first form the alkaline earth metal support region (lower layer). Then, a slurry for forming the upper layer is applied to the surface of the lower layer, dried and fired to form the upper layer of the catalyst layer.

[0086] The firing conditions for the washed slurry vary depending on the shape and size of the substrate or carrier, and are therefore not particularly limited. Typically, firing at around 400–1000°C for about 1–5 hours can form a catalyst layer in the target alkaline earth metal-supported region and other regions. Furthermore, there are no particular limitations on the drying conditions before firing, but drying at 80–300°C for about 1–12 hours is preferred.

[0087] Furthermore, when forming the catalyst layer 6 using the washing and coating method, it is preferable that the slurry contains an adhesive in order to ensure proper adhesion between the slurry and the surface of the substrate 1, and further, the surface of the lower layer 6B in the case of a stacked catalyst layer. For this purpose, aluminum sol, silica sol, etc., are preferred adhesives. Additionally, the viscosity of the slurry can be appropriately adjusted so that it can easily flow into the compartments 2 of the substrate (e.g., a honeycomb substrate) 1.

[0088] The following describes several embodiments of the present invention, but it is not intended to limit the invention to the specific examples shown.

[0089] <Experimental Example 1: Preparation of Catalyst for Exhaust Gas Purification> In this experimental example, using Figure 1 A cylindrical honeycomb substrate (i.e., a cordierite honeycomb substrate with a catalyst volume of 0.875L) with a diameter of 103mm and a total length of 105mm, as shown, is fabricated as described below. Figure 2 The catalyst shown is an exhaust gas purification catalyst with a two-layer structure.

[0090] As shown in the corresponding column of Table 1, any one of (1) barium hydroxide, (2) barium acetate, (3) barium nitrate, and (4) strontium hydroxide is used as the water-soluble compound of alkaline earth metal, and any one of (1) taurine, (2) dimethyl sulfone, (3) sulfolane, (4) cysteine, (5) dimethyl sulfoxide, (6) aminobenzenesulfonic acid, and (7) aminomethanesulfonic acid is used as the water-soluble organic compound containing S. First, the slurry for forming the lower layer (alkaline earth metal support region) of the catalyst layer is prepared as described below.

[0091] That is, 600g of alumina or 600g of zirconium oxide is added to 2L of pure water and stirred for 30 minutes to prepare a uniform suspension. Then, 535g of a 2wt% Pd-concentrated palladium nitrate aqueous solution is added to the suspension, followed by 107g of any water-soluble Ba compound from (1) to (3) above or water-soluble Sr compound from (4) above. Then, the mixture is stirred for 30 minutes. After that, 63g of any S-containing water-soluble organic compound from (1) to (7) above is added and stirred to prepare a total of 23 slurry-like raw material suspensions corresponding to the combinations of each raw material in Examples 1 to 23 of Table 1. The pH of all raw material suspensions is above 5.

[0092] Next, the raw material suspensions were dried at 110°C for at least 8 hours, and then calcined at 500°C for 2 hours. Afterward, appropriate pulverization was carried out until the particle size was below 10 μm, and powder materials corresponding to each raw material suspension (Examples 1 to 23 in Table 1) were prepared.

[0093] In addition, as a comparative example, the following is also prepared: From the outset, barium sulfate particles were used instead of water-soluble compounds of alkaline earth metals, and no water-soluble organic compounds containing sulfur were used. The powder material was prepared using the same process (Comparative Example 1). A powder material prepared using barium acetate as a water-soluble Ba compound and ammonium sulfate instead of a water-soluble S-containing organic compound was prepared using the same process (Comparative Example 2). A powder material prepared using barium acetate as a water-soluble Ba compound and sulfuric acid instead of a water-soluble S-containing organic compound was prepared using the same process (Comparative Example 3). A powder material prepared using barium hydroxide as a water-soluble Ba compound and sulfuric acid instead of a water-soluble S-containing organic compound was prepared using the same process (Comparative Example 4). Barium acetate was used as a water-soluble Ba compound, and no raw materials related to water-soluble organic compounds containing S were used. The powder material was prepared using the same process (Comparative Example 5).

[0094] Then, for each powder material (Examples 1-23 and Comparative Examples 1-5 in Table 1), 860g of alumina powder 970g of CZ composite oxide powder with a Ce:Zr molar ratio (Ce:Zr, etc.) of 3:7 (containing trace amounts of La, Y, and other rare earth elements). 30g of alumina adhesive The mixture was added to 2.4L of pure water and wet-milled using a magnetic ball mill until the particle size was below 5μm, thereby preparing a total of 28 types (i.e., 28 types corresponding to Examples 1 to 23 and Comparative Examples 1 to 5 in Table 1) of slurry for lower layer formation.

[0095] On the other hand, the slurry for forming the upper layer of the catalyst layer is modulated as described below. That is, the slurry for forming the upper layer of the catalyst layer is... 104g of a 2wt% rhodium nitrate aqueous solution 400g of alumina powder, and 1240g of the above CZ composite oxide powder Add to 5L of pure water and wet grind using a magnetic ball mill until the particle size is below 5μm, thereby preparing the slurry for upper layer formation involved in this test example.

[0096] First, the substrate is washed with a slurry for forming the lower layer and dried at 150°C for about 1 hour, thereby forming a lower layer (non-fired coating) on ​​the surface of the substrate (the rib wall surface of the partition chamber). Next, the substrate is washed with a slurry for forming the upper layer and dried at 150°C for about 1 hour, thereby forming an upper layer (non-fired coating) on ​​the surface of the lower layer. Then, it is fired at 500°C for 1 hour to obtain an exhaust gas purification catalyst with a catalyst layer consisting of two layers (coating amount of 210 g / L for both layers).

[0097] That is, by using different slurries for forming the lower layer, 28 types (Examples 1 to 23 and Comparative Examples 1 to 5) of exhaust gas purification catalysts with different compositions of the lower layer were produced.

[0098] <Experimental Example 2: Confirmation of Barium Sulfate Dispersibility using FE-EPMA> Using the apparatus (JXA-8530F) manufactured by Nippon Electronics Co., Ltd., surface analysis was performed on each powder material prepared in Test Example 1 in accordance with the manual.

[0099] Specifically, a specified amount of each powder material is embedded in epoxy resin. After the resin cures, the surface to be analyzed is ground, and then carbon as a conductive material is vapor-deposited onto the ground surface using a commercially available carbon coating machine (Vacuum Device Co., Ltd.: VC-100W). Then, the region corresponding to the lower layer of the catalyst layer in the carbon-deposited surface is appropriately determined, and this region is subjected to surface analysis using FE-EPMA. The measurement conditions are as follows: Pixel size: 0.34μm × 0.34μm Pixel count measured: 256×256 Accelerating voltage: 20kV Irradiation current: 100nA Probe diameter: Set to the minimum value for this measurement condition. Unit measurement time: 50ms / pixel Magnification: ×1000.

[0100] Then, for each pixel, the intensity (α:cps) of the characteristic X-rays of Ba (Sr in Example 24) and the main constituent elements of the inorganic compounds constituting the porous carrier were measured. Figure 3 The table shows the intensity (β: cps) of characteristic X-rays of Al or Zr. Additionally, in this experimental example, the intensity (γ: cps) of characteristic X-rays of S element was also measured for each pixel.

[0101] In this type of surface analysis, the threshold for the X-ray intensity of each pixel (region) is set to 15 cps, and pixels with intensities below the threshold are removed from the data used to calculate the correlation coefficient.

[0102] In this facet analysis, the correlation coefficient R is calculated using the CORREL function in Excel, a spreadsheet software, based on the obtained data. Ba／Al Or R Ba／Zr (Only in Example 24 is R) Sr／Zr ).

[0103] Meanwhile, in this experimental example, the intensity (γ:cps) of the characteristic X-rays of the S element calculated for each pixel is used as a third variable to calculate R. Ba／S (Only in Example 24 is R) Sr／S The results are shown in the corresponding columns of Table 1.

[0104] In addition, regarding the sample of Example 14 and the samples of Comparative Examples 1 and 2, the elemental mapping data (images) for Ba and S are shown below. Figures 3-8 .

[0105] [Table 1] As clearly shown in the elemental mapping, it can be seen that in the exhaust gas purification catalyst of Example 14, Ba and S elements are present in a highly dispersed state throughout the porous carrier (secondary particles) shown in the image. Figures 3-4 On the other hand, it can be seen that in the exhaust gas purification catalysts of Comparative Examples 1 and 2, Ba and S elements are present by agglomeration on the outer surface along the contour of the porous support (secondary particles) or at a location close to the support particles. Figures 5-8 ).

[0106] Based on the correlation coefficient R of the exhaust gas purification catalysts for each embodiment and comparative example listed in Table 1 Ba／Al Or R Ba／Zr (Only in Example 24 is R) Sr／Zr The value of ) can also clarify this point.

[0107] That is, the correlation coefficient R involved in each embodiment Ba／Al Or R Ba／Zr (Only in Example 24 is R) Sr／Zr All of them showed high correlations of 0.5 or higher, preferably 0.6 or higher, and particularly preferably 0.7 or higher. On the other hand, except for Comparative Example 5 which did not contain Ba sulfate, the correlation coefficients R involved in the comparative examples where Ba sulfate was not highly dispersed were not high. Ba／Al The correlation coefficients were all extremely low, ranging from 0.1 to 0.33.

[0108] As these results clearly demonstrate, the process disclosed herein shows that by forming a catalyst layer (alkaline earth metal supported region), alkaline earth metal sulfates such as BaSO4 and SrSO4 can be supported in a highly dispersed state throughout the outer surface and the entire interior (within the pores) of the porous support. Therefore, regardless of where the catalyst metal particles are supported on the porous support, alkaline earth metals are present in their vicinity, enabling the alkaline earth metal to function more effectively as a co-catalyst component.

[0109] <Experimental Example 3: Determination of the average particle size of alkaline earth metal sulfates supported on porous carriers> The average particle size of barium sulfate (strontium sulfate in Example 24) contained in the exhaust gas purification catalysts of each example and comparative example was determined by X-ray diffraction using a commercially available X-ray diffraction apparatus (Rigaku Corporation: RINT-2500). Specifically, the particle size was calculated using analytical software (Rigaku Corporation: PDXL) based on the characteristic peaks of each sulfate (e.g., around 22–25° for barium sulfate). The results are shown in the corresponding columns of Table 1.

[0110] As shown in the table, in all embodiments, the average particle size of barium sulfate is less than 25 nm, which is a desirable fine size. In a particularly preferred embodiment, the average particle size is less than 20 nm (minimum 15 nm). On the other hand, in the comparative examples, such a fine size was not achieved, and the particle size was greater than 30 nm.

[0111] <Experimental Example 4: NO> x Evaluation of purification performance > For the exhaust gas purification catalysts of all examples and comparative examples other than Example 24, the NO levels after the following durability tests were evaluated. x Purification performance.

[0112] That is, firstly, the exhaust purification catalysts of each embodiment and comparative example are placed in the exhaust system of a 2.5L engine, the engine is run, and the catalyst bed temperature is maintained at 1000°C for 46 hours.

[0113] Following this durability test, the exhaust purification catalysts were transferred to the exhaust systems of other evaluation 2.5L engines. Then, after switching the air-fuel ratio (A / F) of the mixture supplied to the evaluation 2.5L engines from a lean mixture of 15.1 to a rich mixture of 14.1, the average NO for 3 minutes was measured. x Discharge volume. Such a concentrated mixed atmosphere NO. x Purification rate (%) Utilization (NO from engine) x Emissions – NO from catalyst x Displacement) / (NO from engine) x Calculate the discharge volume. Show the results in the corresponding column of Table 1.

[0114] The NO shown in Table 1 x The purification rate (%) indicates that the NO content of the exhaust gas purification catalyst in all embodiments after the experiment was [data missing]. x The purification rate exceeded that of all comparative examples of exhaust gas purification catalysts for NO reduction. x Purification rate. This demonstrates the performance of finely dispersed barium sulfate as a co-catalyst component (NO) within the catalyst layer (alkaline earth metal supported region). xThe purification performance has been improved.

[0115] <Example 5: Evaluation of Exhaust Gas Purification Performance (Temperature Characteristics)> Using the exhaust gas purification catalyst of Example 14 and the exhaust gas purification catalyst of Comparative Example 1, the temperature characteristics (T50: °C) were investigated as an indicator of exhaust gas purification performance. Specifically, in engine bench testing, after each exhaust gas purification catalyst was subjected to a durability test at 1000 °C for 46 hours, the temperature of the inlet gas of the catalyst was increased from 200 °C to 450 °C at a heating rate of 50 °C / min using a heat exchanger, while simulated exhaust gas (Ga = 23 g / s, 2600 rpm) was flowing into it, and the concentrations of HC (propylene in this case), CO, and NO at the outlet of the catalyst were measured. x Concentration. Then, the temperature at which the side gas concentration reaches 50 mol% relative to the inflow gas concentration is evaluated (50% purification rate is achieved at temperature °C; T50). The results are presented in... Figure 9 The lower the temperature of T50, the better its purification performance.

[0116] like Figure 9 As shown, it is possible to confirm the relationship with HC, CO, and NO. x Regarding T50, the exhaust gas purification catalyst of Example 14 is lower than that of Comparative Example 1.

[0117] This demonstrates that, compared to exhaust gas purification catalysts using barium sulfate produced by existing methods, the exhaust gas purification catalysts provided by the manufacturing method disclosed herein, characterized by finely dispersed barium sulfate supported on a fully porous carrier, can exhibit high purification performance.

[0118] The specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of protection claimed. The technology described in the scope of protection includes various modifications and alterations to the specific examples illustrated above.

Claims

1. A catalyst for exhaust gas purification, characterized in that: It is installed in the exhaust pipe of an internal combustion engine to purify the exhaust gas discharged from the engine. The catalyst used for exhaust gas purification has the following characteristics: Substrate; and The catalyst layer formed on the substrate, The catalyst layer has an alkaline earth metal supporting region, and the alkaline earth metal supporting region has: Porous supports composed of inorganic compounds; At least one platinum group metal catalyst supported on the porous support functions as an oxidation and / or reduction catalyst. and At least one alkaline earth metal sulfate supported on the porous carrier The average particle size of the alkaline earth metal sulfates is less than 25 nm. The porous carrier composed of inorganic compounds is a secondary particle, on the outer surface and inside of which the sulfate of the alkaline earth metal is dispersed and immobilized. Wherein, for the cross-section of the alkaline earth metal supporting region of the catalyst layer, in Pixel size 0.34μm × 0.34μm Pixel count measured: 256×256 Under the given conditions, surface analysis was performed using FE-EPMA. For each pixel, the intensity (α: cps) of the characteristic X-rays of the alkaline earth metal element (Ae) and the intensity (β: cps) of the characteristic X-rays of the main constituent element (M) of the inorganic compound constituting the porous carrier were measured. The Pearson correlation coefficient calculated using the obtained α and β values ​​for each pixel was set as R0. Ae／M hour, The R Ae／M The value is above 0.

7.

2. The catalyst for exhaust gas purification as described in claim 1, characterized in that: The average particle size of the alkaline earth metal sulfates supported on the porous carrier is less than 20 nm.

3. The catalyst for exhaust gas purification as described in claim 1, characterized in that: The catalyst metal has at least palladium (Pd) and / or rhodium (Rh) as its metal. It contains at least barium sulfate (BaSO4) as the alkaline earth metal sulfate.

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