Catalysts for waste gas purification, methods for purifying waste gas, and methods for manufacturing catalysts for waste gas purification.

Abstract

A catalyst for purifying exhaust gas, comprising: a three-dimensional structure (10), and the catalyst component (100) supported on the three-dimensional structure (10) and containing a noble metal composite (22) containing platinum and palladium and a porous body (21), wherein the surface enrichment rate C(Pt) of platinum is 0.00070 or more and 0.01000 or less, the surface enrichment rate C(Pd) of palladium is 0.00800 or more and 0.10000 or less, and the surface enrichment rate C(Pt) is represented by C(Pt) = P XPS (Pt) / (d 2 ×P TEM (Pt)×0.01), and the surface enrichment rate C(Pd) is represented by C(Pd) = P XPS (Pd) / (d 2 ×P TEM (Pd)×0.01).

Description

Technical Field

[0001] The present invention relates to an exhaust gas purification catalyst, an exhaust gas purification method, and a method for producing an exhaust gas purification catalyst. Background Art

[0002] Due to exhaust gas restrictions, a high level of exhaust gas treatment is required. Usually, in order to purify exhaust gas, precious metals such as platinum are carried on porous bodies such as alumina. However, precious metals tend to aggregate (sinter) and coarsen when exposed to high-temperature exhaust gas. The surface area of ​​the roughened precious metal that can contact the exhaust gas is reduced. Therefore, the problem of reduced performance of exhaust gas purification compared to before coarsening arises. In order to solve this problem, a core-shell catalyst comprising a metal and a porous body has been proposed (Non-Patent Document 1).

[0003] In addition, platinum nanoparticles are proposed, which are prepared by heating at 85° C. for 12 hours using a colloidal dispersion, particles in which 90% or more of platinum is completely reduced, a dispersant containing a polar solvent, a water-soluble polymer suspension stabilizer, and a reducing agent. A catalyst in which atomic platinum is more highly dispersed than a conventional platinum / alumina catalyst by applying the nanoparticles to an alumina carrier is proposed (Patent Document 1).

[0004] Exhaust gas regulations are becoming stricter every year around the world and are expected to become even stricter in the future. Therefore, it is necessary to develop catalysts that can achieve better exhaust gas purification performance.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application No. 2018-502982

[0008] Non-patent literature

[0009] Non-patent document 1: J. Hu, RSC Advances, August 17, 2016, Vol. 6, No. 85, pp. 81767-81773 Summary of the invention

[0010] Problems to be solved by the invention

[0011] However, when the core-shell catalyst is exposed to high-temperature exhaust gas, the core-shell particles collide with each other, which easily causes sintering and reduces the catalytic performance. In addition, the smaller the noble metal particle size, the easier it is for the noble metal particles to move when exposed to high-temperature exhaust gas. As a result, even if the noble metal nanoparticles are supported, the catalytic performance is not sufficient.

[0012] The present invention has been completed in view of the above-mentioned problems, and its purpose is to provide an exhaust gas purification catalyst that can increase the number of precious metals that can contact the exhaust gas by not forming a precious metal shell on the surface of a porous body and carrying a large amount of precious metal complexes having a predetermined size near the surface of the porous body (surface enrichment, surface concentration). In addition, another object of the present invention is to provide a method for preparing an exhaust gas purification catalyst that carries a large amount of precious metals near the surface of a porous body. In addition, the purpose is to provide a method for purifying exhaust gas using an exhaust gas purification catalyst prepared by the catalyst preparation method.

[0013] Means for solving problems

[0014] In order to solve the above problems, the inventors have repeatedly conducted serious research. As a result, it was found that when the catalyst metal exists in a specified state, the performance of the catalyst is improved. In view of this discovery, the present application adopts the following scheme to solve the above problems.

[0015] (1) A first aspect relates to an exhaust gas purification catalyst comprising: a three-dimensional structure, and a catalyst component supported on the three-dimensional structure, wherein the catalyst component comprises a noble metal composite containing platinum and palladium, and a porous body, wherein a surface enrichment ratio C(Pt) of the platinum is not less than 0.00070 and not more than 0.01000, a surface enrichment ratio C(Pd) of the palladium is not less than 0.00800 and not more than 0.10000, and the surface enrichment ratio C(Pt) is expressed by C(Pt)=P XPS (Pt) / (d 2 ×P TEM (Pt)×0.01), the surface enrichment rate C(Pd) is expressed as C(Pd)=P XPS (Pd) / (d 2 ×P TEM (Pd)×0.01), wherein d represents the crystallite diameter of the noble metal composite obtained by X-ray diffraction (XRD), and P XPS (Pt) represents the mass percentage concentration of the platinum relative to the catalyst component determined by X-ray photoelectron spectroscopy (XPS), and the P XPS (Pd) represents the mass percentage concentration of the palladium relative to the catalyst component determined by X-ray photoelectron spectroscopy (XPS), and the P TEM (Pt) represents the mass percentage concentration of the platinum relative to the noble metal composite determined by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS), and the P TEM (Pd) represents the mass percentage concentration of the palladium relative to the precious metal composite determined by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS).

[0016] (2) In the exhaust gas purification catalyst according to the above aspect, the d may be 1 nm or more and 50 nm or less.

[0017] (3) A second aspect relates to a method for purifying exhaust gas, comprising the step of allowing exhaust gas to flow through the exhaust gas purifying catalyst of the above aspect.

[0018] (4) The third scheme relates to a method for producing a catalyst for exhaust gas purification, which includes: a first step of obtaining a precious metal solution containing platinum, palladium and a protective agent; a second step of mixing the precious metal solution and a reducing agent to obtain a reducing solution; a third step of mixing the reducing solution and a porous body to obtain a slurry; a fourth step of applying the slurry to a three-dimensional structure; and a fifth step of heating the slurry, wherein in the second step, the temperature of the precious metal solution and the reducing agent is greater than 10°C and less than 40°C.

[0019] (5) In the method for producing an exhaust gas-purifying catalyst according to the above aspect, the noble metal solution may contain 2 g / L or more and 50 g / L or less of the protective agent.

[0020] (6) In the method for producing an exhaust gas-purifying catalyst according to the above aspect, in the reducing solution, a molar ratio of the reducing agent to the precious metal may be 0.2 or more and 1.6 or less.

[0021] Effects of the Invention

[0022] The exhaust gas purification catalyst according to the above-mentioned embodiment can provide excellent exhaust gas purification performance.

[0023] Furthermore, the exhaust gas purification method according to the above aspect can purify the exhaust gas efficiently even after being exposed to high-temperature exhaust gas.

[0024] Furthermore, the method for producing an exhaust gas-purifying catalyst according to the above aspect can provide an exhaust gas-purifying catalyst having excellent exhaust gas purification performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1A It is a schematic cross-sectional view of the exhaust gas purifying catalyst according to the present embodiment.

[0026] Figure 1B is a schematic enlarged view of the catalyst component 100 .

[0027] Figure 2 C(Pt) and C(Pd) are plotted relative to the molar ratio of ascorbic acid relative to the total moles of Pt and Pd (AA / (Pt+Pd)).

[0028] Figure 3The graph plots T50 against the molar ratio of ascorbic acid to the total molar number of Pt and Pd (AA / (Pt+Pd)). DETAILED DESCRIPTION

[0029] Hereinafter, the present embodiment will be described in detail with reference to the accompanying drawings as appropriate. For the accompanying drawings used in the following description, in order to facilitate understanding of the features of the present invention, for the sake of convenience, sometimes the features are enlarged to show, and sometimes the dimensional ratios of the various components are different from the actual ones. The materials, dimensions, etc. illustrated in the following description are examples, but the present invention is not limited to these examples, and can be appropriately changed and implemented within the scope of not changing the gist of the present invention.

[0030] Reference Figure 1A The cross-sectional diagram and Figure 1B The enlarged view shown explains the exhaust gas purifying catalyst of the present embodiment. Figure 1B yes Figure 1A Schematic enlarged view of the portion indicated by "A". The exhaust gas purification catalyst of the present embodiment includes a three-dimensional structure 10 and a catalyst component 100 supported on the three-dimensional structure 10. The catalyst component 100 of the present embodiment includes: a precious metal composite 22 containing platinum (Pt) and palladium (Pd), and a porous body 21. The catalyst component 100 has a layer structure and is preferably coated (supported) on the three-dimensional structure 10. Hereinafter, the product in which the catalyst component 100 is coated (supported) on the three-dimensional structure 10 is set as an exhaust gas purification catalyst, and the component coated (supported) on the three-dimensional structure 10 and including the precious metal composite 22 and the porous body 21 is set as the catalyst component 100. The catalyst component 100 may also include a co-catalyst component.

[0031] Figure 1B Schematic enlarged views of three catalyst components 100 are drawn in the figure. The central enlarged view in which the precious metal composite 22 covers the porous body 21 to form a core-shell structure and the right enlarged view in which the precious metal composite 22 is mainly supported inside the porous body 21 are schematic cross-sectional views of catalysts prepared by the conventional impregnation method and pore filling method. On the other hand, the left enlarged view in which the fine particles of the precious metal composite 22 are mainly supported near the surface of the porous body 21 is a schematic cross-sectional view of the catalyst of this embodiment.

[0032] Carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas of the internal combustion engine come into contact with the precious metal composite 22 during diffusion in the pores in the catalyst layer, and a purification reaction is carried out. Therefore, when a large amount of the precious metal composite 22 exists near the surface compared with the inside of the porous body 21, the contact probability between the exhaust gas and the precious metal composite 22 increases. However, in the conventional catalyst, the precious metal composite 22 easily penetrates into the inside of the porous body 21 during the preparation process, and the proportion of the precious metal composite 22 existing near the surface of the porous body 21 is low.

[0033] On the other hand, in the catalyst of the present embodiment, the precious metal complex 22 may be present in a relatively small amount inside the porous body 21 and in a relatively large amount near the surface of the porous body 21. From the viewpoint of the dispersibility of the precious metal, it is preferred not to have a structure (core-shell structure) in which the precious metal complex 22 is present only on the surface and the porous body 21 is present only inside. Therefore, the catalyst involved in the present embodiment preferably has the precious metal complex 22 present in a large amount near the surface relative to the porous body 21, but is preferably not in a core-shell state. In addition, it is preferred that the average concentration of the precious metal complex 22 relative to the porous body 21 is higher than the concentration of the precious metal complex 22 at the center of the porous body 21. Here, the index of the ratio of the precious metal complex 22 present near the surface of the porous body 21 to the precious metal complex 22 present in the entire porous body is referred to as the surface enrichment rate, which is defined by the following formula.

[0034] (Surface enrichment rate)

[0035] In the catalyst component according to the present embodiment, palladium and platinum have a predetermined surface enrichment ratio. The surface enrichment ratio C(M) of the noble metal M to be analyzed is given by C(M)=P XPS (M) / (d 2 ×P TEM (M) × 0.01). Here, d represents the crystallite diameter of the noble metal composite obtained by X-ray diffraction (XRD). XPS (M) represents the mass percentage concentration of the noble metal M relative to the catalyst component determined by X-ray photoelectron spectroscopy (XPS). TEM (M) represents the mass percentage concentration of the noble metal M relative to the noble metal complex determined by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS). In order to measure the surface enrichment rate in the state of being exposed to high-temperature exhaust gas, the above-mentioned XRD, XPS and TEM-EDS were measured on the catalyst in which air containing 10% by volume of water at 700°C was circulated for 40 hours.

[0036] When the crystallite diameter d of the precious metal composite is obtained by XRD, the crystallite diameter d is calculated by writing the Scherrer formula based on the half-width (half-maximum width) of the (111) peak of Pt or Pd. More specifically, the half-width β at the peak of 2θ=39.60° is obtained, and the X-ray wavelength λ, Bragg angle θ, and half-width β are substituted into the following formula to obtain the crystallite diameter d.

[0037] d=kλ / βcosθ

[0038] In the XPS measurement, the catalyst component is peeled off from the three-dimensional structure, the granular material coarsely crushed in a mortar is fixed on a carbon tape, the device is evacuated, and then the measurement is performed. Since the beam diameter of the XPS measurement is significantly larger than the size of the porous body and the precious metal complex, the average information of the entire surface of the catalyst component can be obtained according to the XPS measurement. Specifically, when the precious metal complex is supported inside the porous body, the peak of the precious metal complex in the XPS measurement becomes weak or cannot be measured. Therefore, P XPS (M) is related to the average composition of the entire surface of the catalyst component, which reflects the distribution of the positions of the noble metal complex in the catalyst component. The mass percentage concentration of the noble metal M near the surface of the catalyst component can be obtained by XPS analysis.

[0039] Composition analysis based on TEM-EDS can analyze a micro area, so it is possible to analyze the composition of only the noble metal complex. TEM (M) is not affected by the position distribution of the precious metal complex in the catalyst component. As a result, the surface enrichment rate C(M) becomes a parameter indicating the degree of bias (degree of uneven distribution) of the precious metal M set as the analysis object on the surface. That is, a large surface enrichment rate C(M) means that the precious metal M set as the analysis object is more biased near the surface of the catalyst component. TEM-EDS measurement can be performed on any 30 precious metal complexes, and their average value can be used to calculate P TEM (M).

[0040] The surface enrichment rate C(Pt) of platinum involved in the present embodiment is 0.00070 or more and 0.01000 or less, preferably 0.00090 or more and 0.00600 or less, more preferably 0.00110 or more and 0.00400 or less, and most preferably 0.00190 or more and 0.00350 or less. In addition, the surface enrichment rate C(Pd) of palladium is 0.00800 or more and 0.10000 or less, preferably 0.01100 or more and 0.08000 or less, more preferably 0.01500 or more and 0.07000 or less, and most preferably 0.03300 or more and 0.06500 or less.

[0041] When the surface enrichment rate C(Pt) of platinum and the surface enrichment rate C(Pd) of palladium are within the above ranges, the catalyst metal can easily contact the exhaust gas, and high catalytic performance can be obtained.

[0042] The ratio of the surface enrichment rate C (Pd) of palladium to the surface enrichment rate C (Pt) of platinum involved in this embodiment is preferably greater than 4.7 and less than 49, preferably greater than 12.5 and less than 26, more preferably greater than 13.5 and less than 23, and most preferably greater than 17 and less than 20.

[0043] (Precious Metal Complex)

[0044] The precious metal complex contains at least platinum and palladium. The precious metal complex may also contain precious metals other than platinum and palladium, such as rhodium. The composition of the precious metals may be appropriately changed according to the gas components in the exhaust gas. There is no particular limitation on the ratio of platinum to palladium contained in the precious metal complex. For example, the mass of platinum relative to palladium may be greater than 0.1 and less than 50, preferably greater than 1.0 and less than 10. The precious metal complex may also be a mixture of multiple precious metals.

[0045] The crystallite diameter d of the precious metal complex determined by XRD is preferably 1 nm or more and 50 nm or less, more preferably 1 nm or more and 30 nm or less, and further preferably 1 nm or more and 10 nm or less. By adopting the crystallite diameter d in this range, the active sites per unit mass of the precious metal complex can be increased, and as a result, high catalytic performance can be effectively obtained.

[0046] The amount of precious metal can be determined based on the engine exhaust volume, the exhaust gas flow rate per unit volume of the catalyst (SV (h -1 )), or the composition of the exhaust gas, etc. can be appropriately changed.

[0047] The amount of palladium relative to the catalyst component is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 5% by mass or less, and still more preferably 0.2% by mass or more and 2% by mass or less.

[0048] The amount of platinum relative to the catalyst component is preferably 0.01 mass % to 10 mass %, more preferably 0.1 mass % to 5 mass %, and even more preferably 0.3 mass % to 3 mass %.

[0049] The amount of the noble metal complex is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.5% by mass or more and 10% by mass or less, and further preferably 1% by mass or more and 5% by mass or less relative to the amount of the porous body. When the amount of the noble metal complex is within the above range relative to the amount of the porous body, the noble metal complex can be sufficiently dispersed and high catalytic performance can be obtained.

[0050] In the case where the catalyst component is supported on the three-dimensional structure, the supported amount of the noble metal complex is preferably 0.01 g / L or more and 30 g / L or less, more preferably 0.01 g / L or more and 10 g / L or less, and further preferably 0.1 g / L or more and 5 g / L or less, in terms of metal relative to the volume of the three-dimensional structure. By adopting the supported amount within this range, aggregation of the noble metal complex can be avoided and sufficient purification performance can be obtained.

[0051] The palladium loading relative to the volume of the three-dimensional structure can be 0.01 g / L or more and 10 g / L or less, preferably 0.1 g / L or more and 5 g / L or less, and more preferably 0.2 g / L or more and 2 g / L or less. By adopting the loading within this range, the aggregation of palladium can be avoided and sufficient purification performance can be obtained.

[0052] The amount of platinum carried relative to the volume of the three-dimensional structure can be 0.01 g / L or more and 10 g / L or less, preferably 0.1 g / L or more and 5 g / L or less, and more preferably 0.3 g / L or more and 3 g / L or less. By adopting a carrying amount within this range, aggregation of platinum can be avoided and sufficient purification performance can be obtained.

[0053] (porous body)

[0054] The porous body carries a precious metal composite. The porous body may be a porous body commonly used for exhaust gas purification catalysts. For example, aluminum oxides such as γ-alumina, δ-alumina, η-alumina, θ-alumina, single oxides such as zirconium oxide and silica, composite oxides or mixtures such as zeolite and zirconium oxide-alumina, and mixtures thereof may also be used. Preferably, γ-alumina, θ-alumina, zirconium oxide, zeolite, and zirconium oxide are used.

[0055] The porous body may contain a rare earth element such as lanthanum, yttrium, neodymium, or praseodymium. The porous body containing the rare earth element improves the heat resistance of the porous body. In particular, it is preferred that the porous body contain lanthanum.

[0056] The shape of the porous body is not particularly limited. The BET (Brunauer-Emmett-Teller) specific surface area of ​​the porous body is preferably 30 m 2 / g and above 1000m 2 / g or less, more preferably 40m 2 / g and above and 500m 2 / g or less, more preferably 50m 2 / g and above and 300m 2 / g or less. When the porous body has a BET specific surface area within the above range, the noble metal complex can be supported in a dispersed manner. As a result, the catalytic performance of the catalyst component is improved.

[0057] There is no particular restriction on the particle size of the porous body. If the uniformity of the slurry is taken into consideration, the particle size of the porous body is preferably 0.5 μm or more and 100 μm or less, more preferably 1 μm or more and 50 μm or less, and further preferably 2 μm or more and 30 μm or less. Here, in this specification, the particle size of the porous body is the median particle size (d50) of the particle size measured by the laser diffraction method.

[0058] When the catalyst component is supported on a three-dimensional structure, the supported amount of the porous body can be any amount commonly used in exhaust gas purification catalysts. Specifically, the supported amount of the porous body is preferably 20 g / L or more and 300 g / L or less relative to the volume of the three-dimensional structure, more preferably 50 g / L or more and 200 g / L or less, and further preferably 80 g / L or more and 130 g / L or less. By adopting a supported amount within this range, exhaust gas easily enters the catalyst component, and as a result, high catalytic performance can be obtained.

[0059] "Three-dimensional structure"

[0060] The catalyst component involved in this embodiment is supported on a three-dimensional structure. The three-dimensional structure is a structure having openings inside that allow exhaust gas to flow. The three-dimensional structure may be the same structure as that used in a general exhaust gas purification catalyst. The three-dimensional structure is preferably a refractory three-dimensional structure. A refractory three-dimensional structure refers to a three-dimensional structure whose volume change after heating is less than 5% relative to the volume before heating, even when heated to above 1000°C in an atmospheric atmosphere.

[0061] The total length of the three-dimensional structure is not particularly limited, but is preferably 10 mm to 1000 mm, more preferably 15 mm to 500 mm, and further preferably 20 mm to 300 mm. The three-dimensional structure may also have a honeycomb structure. The “total length of the three-dimensional structure” refers to the length of the three-dimensional structure from the inflow side to the outflow side of the exhaust gas.

[0062] The number of holes in the opening of the three-dimensional structure can be set in an appropriate range considering the type of waste gas to be treated, gas flow rate, pressure loss, removal efficiency, etc. For example, the pore density (cell density) (number of holes / unit cross-sectional area) is 100 holes / square inch or more and 1200 holes / square inch or less, and can be fully used, preferably 200 holes / square inch or more and 900 holes / square inch or less, and more preferably 300 holes / square inch or more and 700 holes / square inch or less. The shape of the gas passage port (hole shape, cell shape) of the three-dimensional structure can be set to hexagonal, quadrilateral, triangular or corrugated. Each opening is divided by a partition wall, and the thickness of the partition wall is preferably 1 mil (mil: one thousandth of an inch) or more and 15 mils or less, more preferably 2 mils or more and 13 mils or less, and further preferably 2.5 mils or more and 8 mils or less.

[0063] As a three-dimensional structure, any type of a flow type (open flow type) and a wall flow type can be used. In the flow type three-dimensional structure, the gas flow path is connected from the gas inlet side to the gas outlet side, and the gas can directly pass through the flow path. On the other hand, in the wall flow type three-dimensional structure, the gas inlet side is sealed with a square hole pattern, and when one side of the gas flow path is open, the other side of the same flow path is closed. In the wall flow type three-dimensional structure, the gas can flow to other gas flow paths through the fine holes existing on the wall surface of the gas flow path, and the exhaust gas introduced from the opening flows out of the three-dimensional structure through other flow paths. The air resistance of the flow type three-dimensional structure is small, and the pressure loss of the exhaust gas is small. In addition, the wall flow type three-dimensional structure can filter out (extract) particulate components contained in the exhaust gas.

[0064] The material of the three-dimensional structure can be the same as the material used in the general exhaust gas purification catalyst. The three-dimensional structure can be made of metal, ceramic, etc., preferably cordierite, stainless steel, silicon carbide (SiC), mullite, alumina (α-alumina) or silicon dioxide, and more preferably cordierite, stainless steel or SiC. The material of the three-dimensional structure is cordierite, stainless steel or SiC, so that the durability is improved.

[0065] (Other ingredients)

[0066] The catalyst component may contain other components depending on the object to be purified. For example, when purifying NOx, it may contain a Group 2 element that can store NOx.

[0067] [Method for producing exhaust gas purifying catalyst]

[0068] The method for manufacturing a catalyst for exhaust gas purification involved in the present embodiment includes a first step, a second step, a third step, a fourth step and a fifth step. The first step is a step of obtaining a precious metal solution containing platinum, palladium and a protective agent. The second step is a step of mixing the obtained precious metal solution and a reducing agent to obtain a reducing solution. The third step is a step of mixing the obtained reducing solution and a porous body to obtain a slurry. The fourth step is a step of applying the obtained slurry to a three-dimensional structure. The fifth step is a step of heating the applied slurry. In the second step, the temperature of the precious metal solution and the reducing agent is above 10°C and below 40°C.

[0069] (1st step)

[0070] The first step is a step of protecting the precursor of the precious metal composite by a protective agent. The first step may include a step of mixing a solution containing platinum, a solution containing palladium, and a protective agent, or a step of mixing a solution containing platinum and palladium, and a protective agent, or a step of mixing a solution containing platinum and a protective agent, and a solution containing palladium and a protective agent. It is preferred to sequentially add a solution containing platinum and a solution containing palladium to a solution containing a protective agent. For example, a solution containing platinum may be dripped into a solution containing a protective agent, and then a solution containing palladium may be dripped into the solution, thereby preparing a precious metal solution.

[0071] There is no particular limitation on the solvent of the noble metal solution, and water or any organic solvent may be used. The solvent of the noble metal solution is preferably water or alcohol. Platinum and palladium are dissolved in the noble metal solution. That is, platinum and palladium exist as ions in the noble metal solution. In the noble metal solution, preferably all the noble metals exist as ions.

[0072] The protective agent prevents the aggregation of the nanoparticles of the noble metal and promotes the full dispersion of the noble metal complex. There is no particular limitation on the protective agent of this embodiment, and any protective agent that can prevent the aggregation of the nanoparticles of the noble metal can be used. For example, the protective agent can be a polymer, a surfactant, a compound with a ligand, etc. The protective agent preferably does not contain metal elements, and the protective agent more preferably contains hydrogen, carbon, oxygen and nitrogen. Examples of preferred protective agents are polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyethyleneimine (PEI), and polyacrylic acid (PA).

[0073] The concentration of the protective agent in the noble metal solution is preferably 0.2 mass % to 5.0 mass %, more preferably 0.5 mass % to 3.0 mass %. When the concentration of the protective agent is within the above range, the particle size of the noble metal composite can be sufficiently small, and the active sites of the catalyst component can be increased.

[0074] (Second step)

[0075] In the second step, the precursor of the noble metal complex protected in the first step is reduced by a reducing agent to generate noble metal nanoparticles. The generated noble metal nanoparticles are protected by the protecting agent, thereby preventing the noble metal nanoparticles from aggregating. There is no particular limitation on the reducing agent used in the second step, and it is preferred that the reducing agent does not contain a metal element. Examples of suitable reducing agents are hydrazine, sodium borohydride (NaBH4) and organic acids. For example, ascorbic acid is preferably used as an organic acid. At this time, it is preferred that the palladium source is reduced to palladium by using a reducing agent, while platinum is not reduced. Since the reducing agent does not contain a metal element, it is possible to avoid mixing of metal impurities into the catalyst component. In the second step, the reducing agent can be added to the noble metal solution, or the noble metal solution can be added to the reducing agent.

[0076] In the reducing solution, the molar ratio of the reducing agent to the noble metal is preferably 0.1 or more and 1.6 or less, more preferably 0.2 or more and 1.5 or less, and further preferably 0.5 or more and 1.3 or less. By having the molar ratio of the reducing agent to the noble metal within the above range, the noble metal ions can be fully reduced without hindering the protective agent from protecting the noble metal nanoparticles. In addition, when the reducing agent is ascorbic acid and the noble metals are Pt and Pd, the molar ratio of ascorbic acid to the total amount of Pt and Pd (AA / (Pt+Pd)) is preferably 0.1 or more and 1.6 or less, more preferably 0.2 or more and 1.5 or less, and further preferably 0.5 or more and 1.3 or less.

[0077] In the second step, the temperature of the noble metal solution and the reducing agent is 10°C or higher and 40°C or lower. The second step is preferably performed at a temperature of 15°C or higher and 35°C or lower, and more preferably performed at 20°C or higher and 30°C or lower. By performing the second step at a temperature of 40°C or lower, the palladium source can be reduced while suppressing the reduction of the platinum source. By suppressing the reduction of the platinum source and performing the reduction of the palladium source, a preferred surface enrichment state can be obtained.

[0078] (Step 3)

[0079] In the third step, the noble metal nanoparticles are attached to the surface of the porous body. The noble metal nanoparticles are protected by the protective agent, so it is difficult to enter the center of the porous body compared to the case where they are not protected by the protective agent. As a result, it is believed that the proportion of the noble metal nanoparticles attached to the surface of the porous body increases.

[0080] The pH of the slurry is preferably 5.0 or more and 7.0 or less. The third step may include a step of adding a pH adjusting agent for adjusting the pH of the slurry. There is no particular limitation on the pH adjusting agent, and an alkali metal hydroxide, an organic ammonium salt, an alkaline compound, etc. can be used as the pH adjusting agent. For example, tetraethylammonium hydroxide (TEAH) can be used as the pH adjusting agent.

[0081] (Step 4)

[0082] In the fourth step, the slurry is applied to the three-dimensional structure. The slurry can be applied by any known method. For example, the slurry can be applied to the three-dimensional structure by a wash coating method or a doctor blade method.

[0083] (5th step)

[0084] The fifth step may include a drying step and a calcining step. The drying step is a step for mainly removing unnecessary solvents. The calcining step is a step for mainly generating a noble metal composite suitable for a catalytic reaction from the noble metal nanoparticles attached to the porous body. The drying step and the calcining step may be performed as separate steps or may be performed continuously as the same step. Drying and calcining may be performed independently in any atmosphere. For example, drying and calcining may be performed in the air, in a reducing atmosphere containing a reducing gas such as hydrogen, in an inert gas atmosphere, or in a vacuum. Drying may be performed at a temperature of 0°C to 200°C, preferably at a temperature of 50°C to 150°C, for 10 minutes to 10 hours. Calcination may be performed at a temperature of 200°C to 1000°C, preferably at a temperature of 300°C to 600°C, for 10 minutes to 3 hours.

[0085] [Exhaust gas purification method]

[0086] In one embodiment, a method for purifying exhaust gas includes the step of allowing exhaust gas to flow through the exhaust gas purifying catalyst.

[0087] The exhaust gas purification method of the present embodiment is particularly useful for a specified exhaust gas. The specified exhaust gas refers to an exhaust gas containing CO of 10 ppm or more and 50,000 ppm or less, hydrocarbons of 10 ppm or more and 50,000 ppm or less in terms of carbon (C1), and nitrogen oxides of 10 ppm or more and 50,000 ppm or less. The exhaust gas having such a composition can be rendered harmless by oxidation, hydrocarbons can be rendered harmless by oxidation, and nitrogen oxides can be rendered harmless by reduction. In this specification, the amount of hydrocarbons refers to the amount converted based on carbon (C1).

[0088] The CO contained in the exhaust gas is preferably 100 ppm or more and 10,000 ppm or less, and more preferably 500 ppm or more and 5,000 ppm or less. The hydrocarbon contained in the exhaust gas is preferably 100 ppm or more and 30,000 ppm or less, and more preferably 300 ppm or more and 20,000 ppm or less in terms of carbon. The nitrogen oxides contained in the exhaust gas are preferably 100 ppm or more and 10,000 ppm or less, and more preferably 300 ppm or more and 3,000 ppm or less.

[0089] The exhaust gas purification method of this embodiment can be used to purify the exhaust gas from an internal combustion engine, and in particular, can be used to purify the exhaust gas from a diesel engine. -1 Above and 500000h -1 The exhaust gas purifying catalyst is supplied at a space velocity of 5000 h / min. -1 Above and 150000h -1 In addition, the exhaust gas can be supplied at a linear velocity of 0.1 m / s to 8.5 m / s, preferably 0.2 m / s to 4.2 m / s. By supplying the exhaust gas at such a flow rate, the exhaust gas can be effectively purified.

[0090] In addition, in the exhaust gas purification method involved in the present embodiment, in order to promote the purification of the exhaust gas, high-temperature exhaust gas can be supplied. For example, exhaust gas at a temperature of 100°C or higher and 1000°C or lower can be supplied to the catalyst, preferably exhaust gas at a temperature of 200°C or higher and 600°C or lower. By supplying exhaust gas at such a temperature, thermal degradation of the catalyst can be suppressed and the exhaust gas can be purified with high efficiency.

[0091] Example

[0092] Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated concretely, this invention is not limited to an Example, as long as the effect of this invention is exhibited.

[0093] <Manufacturing of catalysts for exhaust gas purification>

[0094] [Example 1]

[0095] Platinum nitrate aqueous solution, palladium nitrate aqueous solution, La-containing Al2O3 (containing 4 parts by weight of La2O3, with a median particle size d50 of 5 μm and a BET surface area of ​​172.4 m2) were weighed in such a manner that the mass ratio of Pt:Pd:La-containing Al2O3 (lanthanum-containing alumina):polyvinyl pyrrolidone (PVP):ascorbic acid (AA) became the ratio shown in Table 1. 2 / g), PVP, AA. 5.1g of PVP was weighed and dissolved in 250ml of distilled water. Platinum nitrate aqueous solution was dripped into the PVP solution with a pipette, and then palladium nitrate aqueous solution was dripped into it with a pipette to obtain a precious metal solution containing platinum ions and palladium ions. The temperature of the precious metal solution is 25°C. The concentration of PVP in the precious metal solution is 20g / L. After ascorbic acid is dissolved in 70°C warm water, it is cooled to 25°C. A 25°C ascorbic acid aqueous solution is added to the 25°C precious metal solution, stirred for 15 minutes, and a reduced solution is obtained. Lanthanum-containing alumina is added to the obtained reduced solution, stirred for two hours, and slurry a1 is obtained. The pH of slurry a1 is 5.0. At this time, palladium nitrate is reduced, but platinum nitrate is not reduced. Next, the slurry a1 is wash-coated on a three-dimensional structure made of cordierite (diameter 24mm, length 67mm, 400 pores / square inch, wall thickness 4 mils). After that, the catalyst A for exhaust gas purification supported on the three-dimensional structure made of cordierite was obtained by drying at 150°C for 8 hours and calcining at 550°C for 30 minutes. The drying and calcining were performed in air. The supported amount of each component of the exhaust gas purification catalyst relative to the volume of the three-dimensional structure is shown in Table 2. The unit of Table 2 is [g / L].

[0096] [Example 2]

[0097] Platinum nitrate aqueous solution, palladium nitrate aqueous solution, La-containing Al2O3, PVP, and AA are weighed in such a manner that the ratio of Pt:Pd:La-containing Al2O3:PVP:AA becomes as shown in Table 1. 2.62 g of PVP weighed is dissolved in 250 ml of distilled water. According to the same steps as in Example 1, a slurry with a pH of 4.7 containing each raw material is obtained. At this time, palladium nitrate is reduced, but platinum nitrate is not reduced. Ammonia is added to the slurry to obtain slurry b with a pH of 5.1. Next, using slurry b, according to the same steps as in Example 1, a catalyst B for exhaust gas purification supported on a three-dimensional structure made of cordierite is obtained.

[0098] [Example 3]

[0099] An exhaust gas-purifying catalyst C supported on a three-dimensional structure made of cordierite was obtained in the same manner as in Example 1 except that the raw material components were changed as shown in Table 1. The pH of the slurry c formed by mixing the reducing solution and the lanthanum-containing alumina was 5.1.

[0100] [Example 4]

[0101] An exhaust gas-purifying catalyst D supported on a three-dimensional structure made of cordierite was obtained in the same manner as in Example 1 except that the raw material components were changed as shown in Table 1. The pH of the slurry d obtained by mixing the reducing solution and the lanthanum-containing alumina was 5.1.

[0102] [Comparative Example 1]

[0103] The exhaust gas purifying catalyst E supported on a three-dimensional structure made of cordierite was obtained in the same manner as in Example 1 except that AA was not used. At this time, neither palladium nitrate nor platinum nitrate was reduced. The pH of the slurry e formed by mixing the noble metal solution and lanthanum-containing alumina was 5.1.

[0104] [Comparative Example 2]

[0105] Catalyst F for exhaust gas purification supported on a three-dimensional structure made of cordierite was obtained in the same manner as in Example 1 except that PVP and AA were not used. At this time, neither palladium nitrate nor platinum nitrate was reduced. The pH of the slurry f formed by mixing the noble metal solution and lanthanum-containing alumina was 4.9.

[0106] [Comparative Example 3]

[0107] The catalyst G for exhaust gas purification supported on a three-dimensional structure made of cordierite was obtained in the same manner as in Example 1, except that the temperature of the precious metal solution was heated to 80°C, ascorbic acid was added to the precious metal solution at 80°C, and stirred for 15 minutes to obtain a reduced solution. At this time, both palladium nitrate and platinum nitrate were reduced. The pH of the slurry g formed by mixing the precious metal solution and the lanthanum-containing alumina was 4.9.

[0108] Table 1

[0109]

[0110] Table 2

[0111] catalyst Pt Pd <![CDATA[Al2O3]]> <![CDATA[La2O3]]> total A 1.3 0.7 96.0 4.0 102.0 B 1.3 0.7 96.0 4.0 102.0 C 1.3 0.7 96.0 4.0 102.0 D 1.3 0.7 96.0 4.0 102.0 E 1.3 0.7 96.0 4.0 102.0 F 1.3 0.7 96.0 4.0 102.0 G 1.3 0.7 96.0 4.0 102.0

[0112] [Durability test]

[0113] The exhaust gas purifying catalysts obtained in Examples and Comparative Examples were subjected to a durability test The durability test was conducted by flowing air at 700° C. containing 10% by volume of water through the exhaust gas purifying catalyst for 40 hours.

[0114] <Evaluation of Exhaust Gas Purifying Catalyst>

[0115] [X-ray diffraction (XRD) measurement]

[0116] X-ray diffraction (XRD) measurements were performed on each catalyst after the durability test, and the crystallite diameter d of the precious metal complex was calculated. In the measurement, Expert Pro manufactured by Spectris plc was used, and a copper tube was used for the X-ray tube. The X-ray wavelength λ is XRD measurement was performed on the exhaust gas purifying catalyst both before and after the durability test. The crystallite diameter d was measured in accordance with JISH 7805. Table 3 shows the calculated crystallite diameters of the precious metal composite.

[0117] [X-ray photoelectron spectroscopy (XPS) measurement]

[0118] XPS measurement was performed on each catalyst after the durability test to calculate the mass percentage concentration (P XPS (Pt) and P XPS (Pd)).

[0119] XPS measurement was performed using Quantera SXM (X-ray source: Al kα) manufactured by ULVAC-PHI. The beam diameter was set to 100 μm, the beam output was set to 25 W-15 kV, and the beam irradiation time was set to 200 ms per point. The peak intensity of 2p of Al and 4d of Pt were measured. 5 / 2 The peak intensity of Pd 3d 5 / 2 and 3D 3 / 2 The molar ratio of Al, Pt and Pd was determined by summing the peak intensities and dividing each peak intensity by the sensitivity coefficient of each peak. It should be noted that the peak intensity of Pd is the sum of the peaks of Pd and PdO. The mass ratio of Al2O3, Pt and Pd was calculated based on the obtained molar ratio of Al, Pt and Pd. The mass ratio of Pt, Pd, Al2O3 and La2O3 was calculated based on the lanthanum content (4 mass% La2O3) of the lanthanum-containing alumina used as a raw material. The calculated P XPS (Pt) and P XPS (Pd) is shown in Table 3.

[0120] [Transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS) measurement]

[0121] The mass percentage concentrations of platinum and palladium relative to the precious metal complex (P TEM (Pt) and P TEM For each sample, TEM-EDS measurement was performed on 30 arbitrary noble metal composites, and the average value of the 30 measured values ​​was defined as P. TEM (Pt) and P TEM (Pd). The obtained P TEM (Pt) and P TEM (Pd) is shown in Table 3.

[0122] [Surface enrichment rate]

[0123] According to the crystallite diameter d, P of the noble metal composite obtained as described above XPS (Pt), P XPS (Pd), P TEM (Pt) and P TEMThe surface enrichment rates of platinum and palladium (C(Pt) and C(Pd)) of each catalyst after the durability test were calculated. Table 3 shows C(Pt) and C(Pd) in each example and comparative example.

[0124] Table 3

[0125]

[0126] Referring to Table 3, it can be seen that the surface enrichment rate of the Example is greater than that of the Comparative Example. That is, the ratio of platinum and palladium present on the surface of the porous body of the exhaust gas purifying catalyst according to the Example is greater than that of the exhaust gas purifying catalyst according to the Comparative Example.

[0127] For catalysts A to D of the examples using PVP and ascorbic acid, an approximate curve was obtained by the least square method using the values ​​of the molar ratios of C(Pt) and C(Pd) to ascorbic acid to the total of Pt and Pd (AA / (Pt+Pd)). Figure 2 In addition, regarding the catalysts E, F and G of the comparative examples, Figure 2 Shown in.

[0128] [Exhaust gas purification performance evaluation]

[0129] The exhaust gas purification performance of the exhaust gas purification catalysts obtained in the examples and comparative examples after the durability test was evaluated. In the evaluation, each catalyst formed into a cylindrical shape with a diameter of 24 mm and a length of 66 mm was used. As the evaluation gas, a gas containing 1000 ppm of carbon monoxide (CO), 350 ppm of hydrocarbons (HC) calculated as carbon (C1), 6% of H2O, 80 ppm of NO, 12% of oxygen, 6% of CO2 and the balance of nitrogen was used. The exhaust gas purification performance of the exhaust gas purification catalysts obtained in the examples and comparative examples after the durability test was evaluated. The exhaust gas purification performance of the exhaust gas purification catalysts obtained in the examples and comparative examples after the durability test was evaluated. The exhaust gas purification performance of the exhaust gas purification catalysts obtained in the examples and comparative examples after the durability test was evaluated was the ... -1 ) is used to make the evaluation gas flow through each catalyst at a space velocity (SV). The catalyst is heated to 100°C to 400°C, and the composition of the gas after passing through the catalyst is measured to calculate the purification rates of CO and HC. The temperatures when the purification rates of CO and HC reach 50% are denoted as T50(CO) and T50(HC), respectively. The lower the T50, the higher the purification performance of the catalyst. For catalysts A to D in the examples, the values ​​of T50(CO) and T50(HC) relative to the molar ratio of ascorbic acid to the total of Pt and Pd (AA / (Pt+Pd)) are used to obtain an approximate curve by the least squares method. The results are respectively plotted in Figure 3 In addition, regarding the catalysts E, F and G of the comparative examples, Figure 3 Shown in.

[0130] As can be roughly understood from the drawings, the exhaust gas-purifying catalyst of the example has a lower T50 than the exhaust gas-purifying catalyst of the comparative example.

[0131] Industrial Applicability

[0132] The exhaust gas purification catalyst according to the present disclosure can provide excellent exhaust gas purification performance.

[0133] The exhaust gas purification method according to the present disclosure can effectively purify the exhaust gas even after being exposed to high-temperature exhaust gas.

[0134] The method for producing an exhaust gas-purifying catalyst according to the present disclosure can provide an exhaust gas-purifying catalyst having excellent exhaust gas purification performance.

[0135] Description of Reference Numerals

[0136] 10 Three-dimensional structures

[0137] 21 porous body

[0138] 22 Precious Metal Complex

[0139] 100 Catalyst ingredients

Claims

1. A catalyst for exhaust gas purification, comprising: three-dimensional structures, and a catalyst component supported on the three-dimensional structure, the catalyst component comprising a noble metal complex containing platinum and palladium, and a porous body, in, The average concentration of the noble metal complex relative to the porous body is higher than the concentration of the noble metal complex at the center of the porous body, and is not in a core-shell state, The surface enrichment rate of platinum C(Pt) is greater than or equal to 0.00110 and less than or equal to 0.00400, The surface enrichment rate C(Pd) of palladium is greater than or equal to 0.01500 and less than or equal to 0.07000. The ratio C(Pd) / C(Pt) of the surface enrichment rate of palladium C(Pd) to the surface enrichment rate of platinum C(Pt) is 13.5 or more and 23 or less, The surface enrichment rate C(Pt) is expressed as C(Pt) = P XPS (Pt) / (d 2 ×P TEM (Pt) × 0.01) means, The surface enrichment rate C(Pd) is expressed as C(Pd) = P XPS (Pd) / (d 2 ×P TEM (Pd) × 0.01) means, The d represents the crystallite diameter of the noble metal composite obtained by X-ray diffraction (XRD), The P XPS (Pt) represents the mass percentage concentration of the platinum relative to the catalyst component determined by X-ray photoelectron spectroscopy (XPS), The P XPS (Pd) represents the mass percentage concentration of the palladium relative to the catalyst component determined by X-ray photoelectron spectroscopy (XPS), The P TEM (Pt) represents the mass percentage concentration of the platinum relative to the noble metal composite determined by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS), The P TEM (Pd) represents the mass percentage concentration of the palladium relative to the noble metal composite determined by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS), The d is greater than or equal to 1 nm and less than or equal to 50 nm.

2. A method for purifying exhaust gas, comprising the step of allowing exhaust gas to flow through the exhaust gas purifying catalyst according to claim 1.

3. The method for producing the exhaust gas purification catalyst according to claim 1, comprising: A first step of obtaining a precious metal solution containing platinum, palladium and a protective agent; A second step of mixing the noble metal solution and a reducing agent including ascorbic acid to obtain a reduced solution; A third step of mixing the reducing solution and the porous body to obtain a slurry; A fourth step of applying the slurry to a three-dimensional structure; and The fifth step of heating the slurry, In the second step, a molar ratio of ascorbic acid to the total of platinum and palladium (AA / (Pt+Pd)) is 0.5 to 1.5, and the temperature of the noble metal solution and the reducing agent is 10° C. to 40° C.

4. The method for producing an exhaust gas-purifying catalyst according to claim 3, wherein: The noble metal solution contains 2 g / L or more and 50 g / L or less of the protective agent.

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