Catalyst metal particles and catalysts

Abstract

The present invention provides a catalyst metal particle and a catalyst which can achieve both reduction of cost and improvement of N2 selectivity. The catalyst metal particle of the present invention purifies nitrogen oxides with hydrogen as a reducing agent, and is characterized by comprising an alloy represented by the formula Rh 1‑x M x (wherein M is a transition metal element, and 0 < x < 1).

Description

Technical Field

[0001] This invention relates to a catalyst metal particle and a catalyst, and more particularly to a catalyst metal particle for purifying nitrogen oxides using hydrogen as a reducing agent and a catalyst containing the same. Background Technology

[0002] Traditionally, in internal combustion engines (gasoline engines) that use gasoline as fuel, NOx purification catalysts that remove NOx (nitrogen oxides) from exhaust gases use catalyst metal particles containing Pd (palladium) or Rh (rhodium) and reduce NOx to N2 using CO as a reducing agent. On the other hand, in internal combustion engines (hydrogen fuel engines) that use H2 (hydrogen) as fuel, which are being developed as a new field, catalysts that use H2 as a reducing agent are used as NOx purification catalysts.

[0003] As a NOx purification catalyst using H2 as a reducing agent, one known catalyst is a nitrogen oxide purification catalyst in which platinum (Pt) as the active material and ruthenium (Ru) as the oxygen carrier are supported on a porous support such as Al2O3 or TiO2, and is used to purify NOx in exhaust gas at low temperatures (Patent Document 1). Furthermore, a nitrogen oxide removal catalyst is known to contain 0.05 to 10 wt% rhodium (Rh) supported on a silica support (Patent Document 2).

[0004] Patent Document 1: Japanese Patent Application Publication No. 2009-644

[0005] Patent Document 2: Japanese Patent Application Publication No. 2003-33654 Summary of the Invention

[0006] In known NOx purification catalysts using H2 as a reducing agent, catalyst metals such as Pt, Ru, or Pd (Patent Document 1) or Rh (Patent Document 2) are used. These catalyst metals are expensive, thus increasing the cost of NOx purification catalysts and potentially hindering practical application. On the other hand, in NOx purification catalysts using H2 as a reducing agent, NH3 or N2O are generated as byproducts during NOx purification, thus requiring high N2 selectivity. However, conventional catalyst metals such as Pt, Pd, and Rh have the following problem: the electronic or surface states exhibited during catalysis are not optimal for the formation of reaction pathways that selectively reduce NOx to N2, thereby promoting the formation of byproducts such as NH3 or N2O. Therefore, N2 selectivity cannot be sufficiently improved.

[0007] The present invention has been made in view of this, and its object is to provide a catalyst metal particle and catalyst that can achieve both cost reduction and improved N2 selectivity.

[0008] To address the aforementioned issues, the catalyst metal particles of the present invention use hydrogen as a reducing agent to purify nitrogen oxides. These catalyst metal particles comprise a composition expressed in atomic proportions as Rh. 1-x M x (where M is a transition metal element, and 0 < x < 1.) represents an alloy.

[0009] Furthermore, the catalyst of the present invention is characterized by comprising the above-mentioned catalyst metal particles and a support on which the above-mentioned catalyst metal particles are loaded.

[0010] Invention Effects

[0011] According to the present invention, both cost reduction and improved N2 selectivity can be achieved. Attached Figure Description

[0012] Figure 1 This is a schematic side view showing the catalyst metal particles and the catalyst according to the first embodiment.

[0013] Figure 2 This is a graph showing the test conditions, such as the temperature program, in the test of the catalytic reaction to evaluate the NOx purification performance of the catalysts involved in the examples and comparative examples using H2 as a reducing agent.

[0014] Figure 3 This is a graph showing the N2 selectivity at various temperatures during the heating process of the inlet gas temperature for the evaluation treatment gas of the catalysts involved in Examples 1 and 2 and Comparative Examples 1 to 3. Detailed Implementation

[0015] The following describes embodiments of the catalyst metal particles and catalyst of the present invention.

[0016] First, regarding the catalyst metal particles and catalyst involved in the embodiments, the catalyst metal particles and catalyst involved in the first embodiment will be described as examples. Figure 1 This is a schematic side view showing the catalyst metal particles and the catalyst according to the first embodiment.

[0017] like Figure 1 As shown, the catalyst 6 powder according to the first embodiment comprises the catalyst metal particle 2 powder according to the first embodiment and the support particle (support) 4 on which the catalyst metal particle 2 powder is supported. The catalyst metal particle 2 powder is a powder of nano-metal particles that use H2 (hydrogen) as a reducing agent to purify NOx (nitrogen oxides). The catalyst metal particle 2 powder comprises a composition expressed in atomic ratio (molar ratio) as formula Rh 1-x Cu x(Where 0.3≤x≤0.6.) represents an alloy formed by the solid solution of Rh (rhodium) and Cu (copper). The powder of support particle 4 contains γ-Al2O3 (alumina). The powder of catalyst 6 is a NOx purification catalyst that removes NOx by reducing NOx to N2 using H2 as a reducing agent.

[0018] The catalyst metal particle 2 powder involved in the first embodiment is a powder of nano-metal particles, comprising a composition expressed in atomic ratio as Rh 1-x Cu x (Where 0.3 ≤ x ≤ 0.6.) represents an alloy formed by solid solution of Rh and Cu. Compared with precious metals such as Rh, transition metals such as Cu are very inexpensive. Therefore, compared with conventional powders containing catalyst metal particles containing Rh, the powder of catalyst metal particles 2 according to the first embodiment can significantly reduce material costs, and depending on the situation, can reduce material costs by more than 10%.

[0019] Furthermore, it is known that alloys combining transition metals such as Rh and Cu are difficult to form solid solutions in bulk, but can form solid solutions in nano-sized metal particles. Moreover, it is believed that alloys formed through solid solution of transition metals such as Rh and Cu are atomically close, resulting in electron donation from transition metals such as Cu to Rh (Palina, N., Sakata, O., Kumara, L. et al. Electronic Structure Evolution with CompositionAlteration of RhxCuy Alloy Nanoparticles. Sci Rep 7, 41264 (2017).). Therefore, in the powder of catalyst metal particles 2 according to the first embodiment, the electronic and surface states arising from the reaction mechanism based on these premises enable a higher N2 selectivity than conventional powders containing Rh-containing metals.

[0020] Therefore, the powder of catalyst metal particles 2 and the powder of catalyst 6 according to the first embodiment can achieve both cost reduction and improved N2 selectivity. Even the powder of catalyst metal particles and the powder of catalyst according to the embodiments can achieve both cost reduction and improved N2 selectivity, similar to the first embodiment. Next, the detailed structure of the catalyst metal particles and catalyst according to the embodiments will be described.

[0021] 1. Catalyst metal particles

[0022] The catalyst metal particles involved in the embodiments purify NOx (nitrogen oxides) using H2 (hydrogen) as a reducing agent, characterized in that they comprise a composition expressed in atomic ratio (molar ratio) as formula Rh 1-x M x (Where M is a transition metal element, and 0 < x < 1.) represents an alloy formed by the solid solution of Rh and M (an alloy containing Rh and M). Catalyst metal particles remove NOx by reducing NOx to N2 using H2 as a reducing agent, thereby purifying NOx.

[0023] Catalyst metal particles only need to contain a composition expressed in atomic proportions as Rh 1-x M x There are no particular limitations on the alloy being described and capable of achieving higher N2 selectivity than catalyst metal particles containing Rh monomer metals. For example, it is preferable to have Cu as the M (transition metal element) and a composition expressed in atomic ratio as Rh. 1-x Cu x The alloy is indicated by (where 0.3 ≤ x ≤ 0.6). This is because Cu is particularly inexpensive among transition metals. Furthermore, Cu readily donates electrons to Rh among transition metals, and the atomic ratio (x) of Cu in the alloy satisfies 0.3 ≤ x ≤ 0.6, thus the electronic or surface state of the alloy during catalysis is particularly favorable for N2 selectivity. Additionally, there are no particular limitations on M (the transition metal element), as long as it can achieve a higher N2 selectivity than catalyst metal particles containing Rh monomer metals; for example, Cu, Mn, Fe, Co, Ni, etc., are preferred, with Cu being the most preferred.

[0024] Catalyst metal particles are typically nano-sized metal particles, existing in the form of catalyst metal particle powder. When the catalyst metal particles exist in the form of catalyst metal particle powder, there is no particular limitation on the average particle size of the catalyst metal particles. For example, it can be 0.5 nm or more, 1 nm or more, 2 nm or more, 3 nm or more, 4 nm or more, and can be less than 50 nm, less than 10 nm, less than 8 nm, less than 7 nm, less than 6 nm.

[0025] Here, "average particle size of catalyst metal particles" refers to the average particle size obtained by methods such as STEM (scanning transmission electron microscopy) or TEM (transmission electron microscopy) to observe the powder containing catalyst metal particles in the catalyst powder described below. For example, it is the arithmetic mean of the equivalent circle diameters of more than 30 catalyst metal particles randomly selected in the TEM image.

[0026] The method for manufacturing the catalyst metal particle powder is not particularly limited. For example, a so-called co-impregnation method is preferred, in which the catalyst metal particle powder is loaded onto the carrier particle powder using a mixed solution containing a metal salt of Rh and a metal salt of M (a transition metal element) (a solution containing both an Rh source and an M source) and a dispersion of carrier particles. The Rh metal salt used in the co-impregnation method is not particularly limited, and examples include rhodium chloride (RhCl3), rhodium acetate, and rhodium nitrate. The M (transition metal element) metal salt used in the co-impregnation method is particularly not limited to a Cu metal salt, and examples include copper chloride (CuCl3), copper acetate, and copper nitrate.

[0027] 2. Catalyst

[0028] The catalyst involved in the embodiments uses H2 (hydrogen) as a reducing agent to purify NOx (nitrogen oxides), characterized in that it includes catalyst metal particles involved in the embodiments and a support on which the catalyst metal particles are loaded.

[0029] The support is not particularly limited as long as it can support catalyst metal particles. For example, it may include at least one material selected from the group consisting of Al₂O₃ (alumina), SiO₂ (silicon dioxide), MgO (magnesium oxide), ZrO₂ (zirconia), CeO₂ (cerium oxide), and TiO₂ (titanium dioxide) and their composite oxides, with Al₂O₃ being preferred, and γ-Al₂O₃ being particularly preferred. The support usually exists in the form of powdered support particles. The particle size of the support particles is not particularly limited and can be appropriately set.

[0030] The catalyst is not particularly limited as long as it possesses the aforementioned characteristics, and it typically exists in the form of a powder containing catalyst metal particles and a powder of carrier particles supporting the catalyst metal particles. As for the catalyst, there is no particular limitation as long as it is a NOx purification catalyst that uses H2 as a reducing agent to purify NOx; for example, a NOx purification catalyst used in an internal combustion engine (hydrogen fuel cell engine) that uses H2 as fuel is preferred. This is because a significantly improved N2 selectivity can be obtained.

[0031] The loading of catalyst metal particles relative to the support (the mass ratio of catalyst metal particles supported on the support when the mass ratio of the support is set to 100% by mass) is not particularly limited. For example, it can be 0.01% or more by mass, 0.1% or more by mass, 0.5% or more by mass, or 1% or more by mass, and it can be less than 10% by mass, less than 5% by mass, less than 3% by mass, or less than 2% by mass.

[0032] There are no particular limitations on the method for manufacturing the catalyst; for example, the so-called co-impregnation method is preferred. Regarding the co-impregnation method, it is the same as the co-impregnation method described in the section "1. Catalyst Metal Particles" above.

[0033] Example

[0034] The following examples and comparative examples provide a more detailed description of the catalyst metal particles and catalysts involved in the implementation.

[0035] [Example 1]

[0036] Particles of the catalyst (catalyst powder) according to the first embodiment were prepared, wherein the catalyst metal particle powder comprises a composition expressed in atomic ratio as Rh 0.7 Cu 0.3 The alloy represented is formed by solid solution of Rh and Cu, the powder of the support particles contains γ-Al2O3 (alumina), and the loading of the catalyst metal particles relative to the powder of the support particles (the mass ratio of the catalyst metal particles loaded on the powder of the support particles when the mass ratio of the powder of the support particles is set to 100% by mass) is 1 mass.

[0037] First, a solution containing both Rh and Cu sources was prepared by adding the required amounts of RhCl3·3H2O (rhodium(III) chloride trihydrate) and CuCl2·2H2O (copper(II) chloride dihydrate) to distilled water in an atomic ratio (molar ratio) of Rh:Cu = 0.7:0.3, and stirring at room temperature for 1 hour. Then, a required amount of γ-Al2O3, acting as a carrier particle, was added to distilled water and stirred at room temperature for 10 minutes to prepare a dispersion of γ-Al2O3.

[0038] Next, a solution containing Rh and Cu sources was added to the γ-Al2O3 dispersion in such a way that the total mass ratio of Rh and Cu to γ-Al2O3 was 1% by mass and the mixture was stirred at room temperature for 1 hour to prepare a catalyst slurry.

[0039] Next, the solvent was removed from the catalyst slurry by distillation at 50°C using an evaporator. Then, the concentrate after solvent removal was vacuum dried in a vacuum drying furnace at 80°C to obtain a catalyst precursor. The catalyst precursor was then pulverized and calcined at 350°C for 2 hours in a reducing gas mixture consisting of 1% H2 and the remainder (by volume) to obtain catalyst powder. Finally, the catalyst powder was granulated to produce catalyst particles.

[0040] [Example 2]

[0041] Catalyst particles different from those in Example 1 were prepared, the difference being that the catalyst metal particle powder contained a composition expressed in atomic ratio as Rh 0.4 Cu 0.6 The alloy is represented as shown. In this case, when preparing a solution containing Rh source and Cu source, the required amounts of RhCl3·3H2O and CuCl2·2H2O were added in such a way that the atomic ratio (molar ratio) of Rh and Cu was Rh:Cu=0.4:0.6. Otherwise, catalyst particles were prepared by the same method as in Example 1.

[0042] [Comparative Example 1]

[0043] Catalyst particles different from those in Example 1 were prepared, the difference being that the catalyst metal particle powder contained a composition expressed in atomic ratio as Rh 0.9 Cu 0.1 The alloy is represented as shown. In this case, when preparing a solution containing Rh source and Cu source, the required amounts of RhCl3·3H2O and CuCl2·2H2O were added in such a way that the atomic ratio (molar ratio) of Rh and Cu was Rh:Cu=0.9:0.1. Otherwise, catalyst particles were prepared by the same method as in Example 1.

[0044] [Comparative Example 2]

[0045] Catalyst particles different from those in Example 1 were prepared, the difference being that the catalyst metal particle powder contained Rh monomer metal. Instead of preparing a solution containing both Rh and Cu sources, the required amount of RhCl3·3H2O was added to distilled water and stirred at room temperature for 1 hour, thereby preparing a solution containing the Rh source. Then, the required amount of the Rh source-containing solution was added to the γ-Al2O3 dispersion at a mass ratio of Rh to γ-Al2O3 of 1% by mass and mixed, stirred at room temperature for 1 hour, thereby preparing a catalyst slurry. Except for these points, the catalyst particles were prepared using the same method as in Example 1.

[0046] [Comparative Example 3]

[0047] Catalyst particles different from those in Example 1 were prepared. The difference lies in that the catalyst metal particle powder contained Cu monomer metal and the loading of the catalyst metal particle powder relative to the carrier particle powder was 10% by mass. Instead of preparing a solution containing both Rh and Cu sources, the required amount of CuCl2·2H2O was added to distilled water and stirred at room temperature for 1 hour to prepare a Cu source solution. Then, the required amount of the Cu source solution was added to the γ-Al2O3 dispersion at a Cu-to-γ-Al2O3 mass ratio of 10% by mass and mixed. The mixture was stirred at room temperature for 1 hour to prepare a catalyst slurry. Except for these points, catalyst particles were prepared using the same method as in Example 1.

[0048] [Catalyst Evaluation]

[0049] Regarding the catalyst particles prepared in Examples 1 and 2 and Comparative Examples 1 to 3, catalytic reaction tests were conducted using a gas flow continuous catalytic reaction test apparatus on 2g test samples of each catalyst, thereby evaluating the NOx purification performance (N2 selectivity) using H2 as a reducing agent. Figure 2 This is a graph showing the test conditions, such as the temperature program, in the test of the catalytic reaction to evaluate the NOx purification performance of the catalysts involved in the examples and comparative examples using H2 as a reducing agent.

[0050] In the catalytic reaction experiment, after the catalyst particles were placed in the reactor of the experimental device, such as Figure 2 As shown, after pretreatment, an evaluation of N2 selectivity performance was performed. Specifically, in the pretreatment, a pretreatment gas consisting of H2:1% and N2:the remainder (by volume) was passed through the system at a flow rate of 15 L / min for approximately 127,000 hours. -1 The space velocity (calculated with catalyst particles having a diameter of 3 cm and a height of 1 cm) circulates within the reactor of the experimental apparatus while the temperature of the pretreatment gas entering the catalyst particles (the inlet gas temperature of the pretreatment gas) is raised from room temperature to 600°C and maintained for 10 minutes before being lowered back to room temperature. Next, in the N2 selectivity performance evaluation treatment, instead of the pretreatment gas, an evaluation treatment gas consisting of NO: 3200 ppm, H2: 3200 ppm (by volume) and N2: the remainder is used at a flow rate of 15 L / min for approximately 127,000 hours. -1The space velocity (calculated with catalyst particles having a diameter of 3 cm and a height of 1 cm) circulates within the reactor of the experimental apparatus. Simultaneously, the temperature of the evaluation gas entering the catalyst particles (the inlet gas temperature) is measured at a rate of 20 °C / min, increasing from room temperature to 600 °C. At each temperature during the temperature increase process, the concentrations (volume ratios) of each component in the gas after passing through the catalyst particles are measured using an FT-IR (Fourier Transform Infrared Spectrometer), an MPD (Magnetic Pressure Detector), and a gas chromatograph. Then, based on the NOx concentrations in the evaluation gas and the NOx concentrations in the gas after passing through the catalyst particles, the reduction in NOx concentration (volume ratio) due to the passage of the catalyst particles is calculated as the amount of NOx reacted. Furthermore, the amount of NOx converted to N2 was calculated based on the amount of NOx reacted and the concentrations of NH3 and N2O in the gas after passing through the catalyst particles (the concentrations of NH3 and N2O generated), and the N2 selectivity was calculated based on the amount of NOx reacted and the amount of NOx converted to N2. Figure 3 This is a graph showing the N2 selectivity at various temperatures during the heating process of the inlet gas temperature for the evaluation treatment gas of the catalysts involved in Examples 1 and 2 and Comparative Examples 1 to 3. Table 1 below shows the N2 selectivity when the inlet gas temperature for the evaluation treatment gas of the catalysts involved in Examples 1 and 2 and Comparative Examples 1 to 3 is 400°C.

[0051] [Table 1]

[0052]

[0053] like Figure 3 As shown, in the catalyst of Example 1, the N2 selectivity was higher than that of the catalysts of Comparative Examples 1-3 at a temperature of 375°C or higher for the inlet gas of the evaluation process gas. Furthermore, in the catalyst of Example 2, the N2 selectivity was higher than that of the catalysts of Comparative Examples 1-3 at a temperature of 300°C or higher for the inlet gas of the evaluation process gas. As shown in Table 1, in the catalysts of Examples 1 and 2, the N2 selectivity was higher than that of the catalysts of Comparative Examples 1-3 at an inlet gas temperature of 400°C. Based on these results, it can be seen that the catalyst metal particle powder contains a composition expressed in atomic ratio as Rh... 1-x Cu x In the catalysts (Examples 1 and 2) formed by solid solution of Rh and Cu in the form of (0.3≤x≤0.6), a higher N2 selectivity can be achieved than that of the catalyst containing Rh monomer metal in the powder of catalyst metal particles (Comparative Example 1).

[0054] The embodiments of the catalyst metal particles and catalysts involved in the present invention have been described in detail above. However, the present invention is not limited to the embodiments described above, and various design changes can be made without departing from the spirit of the present invention as set forth in the claims.

[0055] Symbol Explanation

[0056] 2-Catalyst metal particles, 4-Support particles (support), 6-Catalyst (NOx purification catalyst).

Claims

1. A catalyst metal particle that uses hydrogen as a reducing agent to purify nitrogen oxides, said catalyst metal particle being characterized in that... The composition, expressed in atomic ratios, is represented by the formula Rh 1-x M x The alloy is represented, wherein, M is a transition metal element, and 0 < x < 1.

2. The catalyst metal particles according to claim 1, characterized in that, The composition, expressed in atomic ratios, is represented by the formula Rh 1-x Cu x The alloy is represented by x, where 0.3 ≤ x ≤ 0.

6.

3. A catalyst, characterized in that, It comprises the catalyst metal particles as described in claim 1 or 2 and a support on which the catalyst metal particles are loaded.

4. The catalyst according to claim 3, characterized in that, The carrier comprises at least one selected from the group consisting of Al2O3, SiO2, MgO, ZrO2, CeO2 and TiO2 and their composite oxides.

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