Catalyst for exhaust gas purification

Abstract

To provide means that enhances catalytic activity of a catalyst metal in a catalyst for exhaust gas purification.SOLUTION: One aspect of the present invention relates to a catalyst for exhaust gas purification having a support and catalyst metal nanoparticles containing rhodium supported on the support, wherein the catalyst metal nanoparticles have iron in a range of 30 to 50 mol% and iridium in a range of 30 to 90 mol% dissolved in solid solution with respect to a total amount of rhodium.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a catalyst for purifying exhaust gases. [Background technology]

[0002] Catalysts for purifying exhaust gases from automobiles and other vehicles oxidize hydrocarbons (HC) and carbon monoxide (CO) contained in exhaust gases emitted from engines to water and carbon dioxide, and reduce nitrogen oxides (NOx) to nitrogen. Exhaust gas purification catalysts with this catalytic activity typically use precious metal-supported catalysts, which are formed by coating a heat-resistant substrate with a catalyst layer containing particles of catalytic precious metals such as palladium (Pd), rhodium (Rh), and platinum (Pt).

[0003] For example, Patent Document 1 describes an exhaust gas purification catalyst in which bimetallic particles, which are at least partially solid-dissolved rhodium and iridium, have a diffraction peak at 40.66°<2θ<41.07° in X-ray diffraction using CuKα rays, are supported on a catalyst carrier. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-180485 Summary of the Invention [Problem to be solved by the invention]

[0005] In exhaust gas purification catalysts, there is a demand for reducing the amount of catalytic metal used to cut raw material costs. In particular, the price of Rh has risen significantly in recent years, doubling. Therefore, there is a demand for improving the catalytic activity of catalytic metals, including Rh, in order to reduce the amount of catalytic metals used.

[0006] Therefore, an object of the present invention is to provide a means for improving the catalytic activity of catalytic metals in exhaust gas purification catalysts. [Means for solving the problem]

[0007] The present inventors have investigated various means for solving the above problems. They have found that by incorporating a predetermined amount of iron (Fe) and iridium (Ir) into catalytic metal nanoparticles containing Rh as a solid solution, it is possible to improve catalytic activity while reducing the amount of Rh used. Based on this finding, the present inventors have completed the present invention.

[0008] That is, the present invention includes the following aspects and embodiments. (Embodiment 1) A catalyst for purifying exhaust gases comprising a support and catalytic metal nanoparticles containing rhodium supported on the support, The catalytic metal nanoparticles are solid-dissolved with 30 to 50 mol% of iron and 30 to 90 mol% of iridium relative to the total substance amount of rhodium. The exhaust gas purification catalyst. [Effects of the Invention]

[0009] The present invention makes it possible to provide a means for improving the catalytic activity of catalytic metals in exhaust gas purification catalysts. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a scanning transmission electron microscope image of the exhaust gas purification catalyst of Test 5. [Figure 2] 1 is a graph showing the relationship between the reaction temperature and catalytic activity in the NO-CO reaction of the exhaust gas purification catalysts of Comparative Examples 1 and 2, and Test 2. In the figure, the horizontal axis represents the temperature (°C) at the time of measurement, and the vertical axis represents the conversion rate (%) of NO to N. [Figure 3] 1 is a graph showing the catalytic activity of the NO-CO reaction at 320° C. of the exhaust gas purification catalysts of Comparative Examples 1 and 2 and Tests 1 to 5. In the figure, the vertical axis represents the conversion rate (%) of NO to N2. DETAILED DESCRIPTION OF THE INVENTION

[0011] Preferred embodiments of the present invention will now be described in detail.

[0012] One aspect of the present invention relates to a catalyst for purifying exhaust gases. The catalyst for purifying exhaust gases of this aspect has a support and catalytic metal nanoparticles containing Rh supported on the support.

[0013] In the exhaust gas purifying catalyst of this embodiment, any material can be used as the support, for example, aluminum oxide (alumina), a composite oxide of ceria and zirconia (ACZ), a composite oxide of alumina and zirconia (AZ), a composite oxide of ceria and zirconia (CZ), and alumina.

[0014] The exhaust gas purification catalyst of this embodiment typically contains 25 to 50 mol% Fe and 25 to 150 mol% Ir, and particularly 30 to 50 mol% Fe and 30 to 90 mol% Ir, relative to the total substance amount of Rh, in the catalytic metal nanoparticles. The exhaust gas purification catalyst of this embodiment preferably contains 30 to 40 mol% Fe and 50 to 70 mol% Ir, and more preferably 30 to 35 mol% Fe and 60 to 70 mol% Ir, relative to the total substance amount of Rh, in the catalytic metal nanoparticles. By containing Fe and Ir in the above-mentioned ranges, the exhaust gas purification catalyst of this embodiment can improve catalytic activity while reducing the amount of Rh used.

[0015] The exhaust gas purifying catalyst of this embodiment may contain one or more additional components in addition to the support and catalytic metal nanoparticles containing Rh, Fe, and Ir, such as La2O3, YO3, Nd2O3, and BaSO4.

[0016] In the exhaust gas purifying catalyst of this embodiment, Fe and Ir are usually contained in a composite form with Rh, particularly in a solid solution form (hereinafter also referred to as a "solid solution"). By containing Fe and Ir in the form exemplified above, the exhaust gas purifying catalyst of this embodiment can improve catalytic activity while reducing the amount of Rh used.

[0017] In the exhaust gas purifying catalyst of this embodiment, the content and form of components such as Rh, Fe, and Ir can be determined, for example, by analyzing catalytic metal nanoparticles contained in the exhaust gas purifying catalyst by energy dispersive X-ray spectroscopy (EDX), or by dissolving the exhaust gas purifying catalyst using an acid or the like and then subjecting the components in the resulting solution to inductively coupled plasma (ICP) emission spectroscopy.

[0018] The particle size of the catalytic metal nanoparticles contained in the exhaust gas purification catalyst of this embodiment is usually in the range of 0.1 to 10 nm, particularly in the range of 0.1 to 5 nm. By including catalytic metal nanoparticles having a particle size in this range, the exhaust gas purification catalyst of this embodiment can improve catalytic activity while reducing the amount of Rh used.

[0019] In the exhaust gas purification catalyst of this embodiment, the particle size of the catalytic metal nanoparticles can be determined, for example, by imaging the exhaust gas purification catalyst using a scanning transmission electron microscope (STEM), measuring the particle sizes of multiple catalytic metal nanoparticles in the obtained STEM image, and calculating the average value.

[0020] The catalytic activity of the exhaust gas purification catalyst of this embodiment can be evaluated, for example, by the following procedure. The exhaust gas purification catalyst is loaded into a fixed-bed flow-type testing apparatus. While controlling the temperature of the testing apparatus, an NO-CO mixed gas (e.g., NO: 0.3 vol %, CO: 0.3 vol %) is passed through the testing apparatus at a predetermined space velocity, and the exhaust gas concentration is measured. In the NO-CO reaction, the percentage of the mass of N2 produced relative to the total mass of NO introduced is calculated as the conversion rate (%) of NO to N2. The catalytic activity of the NO-CO reaction can be evaluated based on the conversion rate of NO to N2.

[0021] As described above, the exhaust gas purification catalyst of this embodiment can improve the catalytic activity while reducing the amount of Rh used. Therefore, by applying the exhaust gas purification catalyst of this embodiment to an automobile or the like, it is possible to provide an automobile or the like that can meet the exhaust gas emission regulations while reducing the raw material cost.

Example

[0022] Hereinafter, the present invention will be described more specifically using examples. However, the technical scope of the present invention is not limited to these examples.

[0023] <I: Preparation of catalyst> Fe-Rh-Ir composite metal nanoparticles were synthesized by the so-called "impregnation method". The ratio of the catalytic metals was adjusted by changing the amount of metal salts charged. For example, a catalyst with the ratio of Fe, Rh, and Ir adjusted to a molar ratio of 2:2:2 is represented as "Fe2-Rh2-Ir2". The specific preparation procedure is shown below. In a flask, a predetermined ratio of catalytic metal salts (iron(III) chloride hexahydrate, rhodium chloride, and iridium(IV) chloride) and distilled water were mixed and stirred and dissolved. To the resulting solution, a catalyst support (a composite oxide of aluminum oxide or cerium oxide and zirconium oxide) was added and stirred for 3 hours. Using an evaporator, the moisture of the resulting mixture was dried and then dried at 120 °C overnight. The dried powder was calcined at 500 °C for 3 hours. Under a 3% hydrogen atmosphere, reduction treatment of the calcined product was performed under the condition of 600 °C for 1 hour to obtain an Fe-Rh-Ir composite metal nanoparticle-supported catalyst. The obtained catalyst powder was put into a bag for cold isostatic pressing (CIP) and vacuum-packed. This was pressed at a pressure of 1 ton / cm 2 of pressure. After sieving the obtained powder, it was pelletized by hitting with a pestle. The obtained pellets were used as samples of the exhaust gas purification catalyst. The metal loading amount of the exhaust gas purification catalyst was unified to 50 μmol / g as the total mass of the metal relative to the total mass of the catalyst.

[0024] The prepared samples of the exhaust gas purification catalyst are as follows. Comparative Example 1: Rh6 Comparative Example 2: Ir6 Test 1: Fe1-Rh4-Ir1 Test 2: Fe1-Rh3-Ir2 Test 3: Fe1-Rh2-Ir3 Test 4: Fe1-Rh1-Ir4 Test 5: Fe2-Rh2-Ir2

[0025] <II: Analysis of Catalysts> The scanning transmission electron microscope (STEM) image of the exhaust gas purification catalyst of Test 5 is shown in FIG. 1. As shown in FIG. 1, it was revealed that the Fe2-Rh2-Ir2 composite metal nanoparticles supported on aluminum oxide in Test 5 had a particle size of about 1.5 nm. As a result of the energy dispersive X-ray spectroscopy (EDX) analysis of the Fe2-Rh2-Ir2 composite metal nanoparticles supported on aluminum oxide in Test 5, the composite metal nanoparticles on the carrier had an atomic ratio of 42:34:24. This atomic ratio was in good agreement with the charging ratio of the materials during catalyst preparation. From this result, it was revealed that the Fe2-Rh2-Ir2 composite metal nanoparticles in Test 5 were a solid solution in which Fe, Rh, and Ir were compounded at the atomic level. Similar analysis results were obtained for the exhaust gas purification catalysts of Tests 1 to 4.

[0026] <III: Evaluation of Catalytic Activity> The pellets of the exhaust gas purification catalysts of Comparative Examples 1 and 2, and Tests 1 to 5 were loaded into a fixed-bed flow-type test apparatus. The test apparatus was pretreated at 400 °C and then cooled to 200 °C. While controlling the temperature of the test apparatus, a NO-CO mixed gas (NO: 0.3 vol%, CO: 0.3 vol%) was passed through the test apparatus at a space velocity of 200,000 h -1 to measure the exhaust gas concentration. The temperature control of the test apparatus was carried out based on a program in which the temperature was increased stepwise from 200 °C to 240 °C at a rate of 10 °C every 10 minutes, and from 240 °C to 360 °C at a rate of 40 °C every 10 minutes. In the NO-CO reaction, the percentage of the mass of the generated N2 with respect to the total mass of the input NO was calculated as the conversion rate (%) from NO to N2. The catalytic activity of the NO-CO reaction was evaluated based on the conversion rate from NO to N2.

[0027] Fig. 2 is a graph showing the relationship between the reaction temperature and catalytic activity in the NO-CO reaction for the exhaust gas purification catalysts of Comparative Examples 1 and 2 and Test 2. In the graph, the horizontal axis represents the temperature (°C) at the time of measurement, and the vertical axis represents the conversion rate (%) of NO to N2. As shown in Fig. 2, the exhaust gas purification catalyst of Test 2 (Fe1-Rh3-Ir2) exhibited catalytic activity equal to or greater than that of the exhaust gas purification catalyst of Comparative Example 1 (Rh6) under all temperature conditions.

[0028] FIG. 3 shows a graph illustrating the catalytic activity of the NO-CO reaction at 320°C for the exhaust gas purification catalysts of Comparative Examples 1 and 2 and Tests 1 to 5. In the graph, the vertical axis represents the conversion rate (%) of NO to N2. As shown in FIG. 3, the exhaust gas purification catalyst of Test 2 (Fe1-Rh3-Ir2) exhibited higher catalytic activity than the exhaust gas purification catalyst of Comparative Example 1, which consists of Rh alone, despite the amount of Rh being half that of the exhaust gas purification catalyst of Comparative Example 1. It is presumed that the catalytic activity of the exhaust gas purification catalyst of Test 2 was improved by the ensemble effect that was produced by forming a solid solution by combining Rh with Fe and Ir.

[0029] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to add, delete, and / or replace part of the configuration of each embodiment with other configurations.

Claims

[Claim 1] A catalyst for purifying exhaust gases comprising a support and catalytic metal nanoparticles containing rhodium supported on the support, The catalytic metal nanoparticles are solid-dissolved with 30 to 50 mol% of iron and 30 to 90 mol% of iridium relative to the total substance amount of rhodium. The exhaust gas purification catalyst.

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