Patent Application
20210187482
The present disclosure relates to a method for producing a noble metal fine particle-supported catalyst, a method for producing noble metal fine particles, a noble metal fine particle-supported catalyst, and noble metal fine particles. The present application is based on Japanese Patent Application No. 2019-227955 filed on Dec. 18, 2019 and claims the benefit of the priority thereof, the entire content of which is incorporated herein by reference.
Active metal-supported catalysts are applied in sensors, petroleum refining, hydrogen production, and other fields such as environment-related fields and energy fields. Among them, a polymer electrolyte fuel cell, which has been researched and developed in recent years as a power source for automobiles and stationary cogeneration, is given as a typical example. As an electrode catalyst, a catalyst obtained by using a conductive material such as carbon or an oxide as a support and supporting an active metal such as platinum, which has been miniaturized to a nanometer size, on the conductive material is used. The performance of the catalyst depends on the particle diameter of the active metal, the uniformity of the particle diameter, and the degree of dispersion on the support. In particular, when catalysts have the same amount of the active metal supported, a catalyst in which a particle diameter of the active metal is smaller (a surface area of the active metal is larger) and the active metal is more highly dispersed has higher performance. Further, since platinum is expensive, it is required to make its particle diameter smaller in order to reduce the amount of platinum used.
As a method for producing an electrode catalyst composed of active metal nanoparticles, for example, a technique of irradiating with microwaves a mixture that contains a precursor containing a noble metal element and a support having microwave absorption for the purpose of reduction, as shown in JP 2010-253408 A (Patent Document 1), is known. Further, a technique of reducing a chloroplatinic acid solution to prepare a metal colloidal solution and supporting it on a support, as in JP 2001-224968 A (Patent Document 2), is also known. In addition, a technique of dispersing an organometallic complex of an active metal and a metal chloride in an organic solvent, adding a reducing agent, and pressurizing and heating the mixture according to need to prepare a product, as in JP 2015-17317 A (Patent Document 3), is known. A technique of supporting platinum group nanoparticles whose particle diameter is controlled by using a dispersant such as a microemulsion dispersion (JP 2018-44245 A, Patent Document 4) or an amphipathic polymer (JP 2009-164142 A, Patent Document 5) has also been proposed. However, in all the cases, the number of catalyst synthesis steps is large, and waste liquid treatment is also included within the steps. Additionally, it is very difficult to make the particle diameter infinitely small, which is the main purpose, and the distribution width of the particle diameter is wide in a range of from 2 nm to a dozen nm.
In JP 2010-253408 A (Patent Document 1), particles of a noble metal are controlled by microwave heating. Therefore, the support material that absorbs microwaves is limited, and the heat generated from the support is non-uniform. Thus, the distribution width of the particle size is so wide that it is difficult to control the particle size.
In JP 2001-224968 A (Patent Document 2), it is difficult to control the particle diameter because the colloid is unstable. In JP 2015-17317 A (Patent Document 3), an attempt is made to control the particle diameter by pressurization, heating, etc., but the preparation process is complicated, and, additionally, there is a problem in the distribution of particle diameter.
Further, in JP 2018-44245 A (Patent Document 4) and JP 2009-164142 A (Patent Document 5), the particle diameter, including its distribution state, is controlled by a polymer agent. However, the particle diameter depends on the size of the emulsion produced by the dispersant, making it difficult to control small particles. Further, it is difficult to remove the polymer agent after reduction, which contributes to an increase in the number of steps.
The present disclosure has been made for solving at least a part of the above problems, and can be realized in the following forms.
[1] A method for producing a noble metal fine particle-supported catalyst, including:
a step of mixing a noble metal salt, an alcohol having 1 to 5 carbon atoms, and a support to form a mixture; and
a heating step of the mixture at a temperature of 150° C. or higher and 800° C. or lower to produce a noble metal fine particle-supported catalyst.
According to the production method, a highly active noble metal fine particle-supported catalyst can be produced by a simplified method.
Here, other examples of the present disclosure are presented.
[2] The method for producing a noble metal fine particle-supported catalyst, wherein a concentration of the noble metal salt in an alcohol solution in which the noble metal salt is dissolved is 0.1 mol L−1 or more and 50 mol L−1 or less.
According to the production method, a noble metal fine particle-supported catalyst having a small particle diameter and high activity can be produced.
[3] The method for producing a noble metal fine particle-supported catalyst, wherein an average particle diameter of the noble metal fine particles is 0.7 nm or more and less than 2 nm.
According to the production method, a noble metal fine particle-supported catalyst having a small particle diameter and high activity can be produced.
[4] A method for producing noble metal fine particles, including:
a step of mixing a noble metal salt and an alcohol having 1 to 5 carbon atoms to form a mixture; and
a heating step of the mixture at a temperature of 150° C. or higher and 800° C. or lower to produce noble metal fine particles.
According to the production method, highly active noble metal fine particles can be produced by a simplified method.
[5] The method for producing noble metal fine particles, wherein a concentration of the noble metal salt in an alcohol solution in which the noble metal salt is dissolved is 0.1 mol L−1 or more and 50 mol L−1 or less.
According to the production method, noble metal fine particles having a small particle diameter and high activity can be produced.
[6] The method for producing noble metal fine particles, wherein an average particle diameter of the noble metal fine particles is 0.7 nm or more and less than 2 nm.
According to the production method, noble metal fine particles having a small particle diameter and high activity can be produced.
[7] A noble metal fine particle-supported catalyst in which noble metal fine particles are supported on a support,
wherein an average particle diameter of the noble metal fine particles is 0.7 nm or more and less than 2 nm.
The noble metal fine particle-supported catalyst has high activity.
[8] Noble metal fine particles having an average particle diameter of 0.7 nm or more and less than 2 nm.
The noble metal fine particles have high activity.
Hereinafter, embodiments of the present disclosure will be described in detail. In addition, in the present specification, a phrase about a numerical range using the word “to” includes a lower limit value and an upper limit value unless otherwise specified. For example, the phrase “10 to 20” includes both the lower limit “10” and the upper limit “20”. That is, the phrase “10 to 20” has the same meaning as “10 or more and 20 or less”.
A method for producing a noble metal fine particle-supported catalyst of the present disclosure includes: a step of mixing a noble metal salt, an alcohol having 1 to 5 carbon atoms, and a support to form a mixture; and a heating step of the mixture at a temperature of 150° C. or higher and 800° C. or lower to produce a noble metal fine particle-supported catalyst.
The noble metal contained in the noble metal salt is not particularly limited, but at least one selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir), and ruthenium (Ru) is preferably used. Among these noble metals, at least one selected from the group consisting of Pt, Pd, Rh, Ir, and Ru is more preferred, and at least one selected from the group consisting of Pt and Pd is further preferred, from the viewpoint of catalytic performance.
As the noble metal salt, at least one selected from the group consisting of hexachloroplatinic (IV) acid hexahydrate (H2PtCl6.6H2O), tetraamminedichloroplatinum (Pt(NH3)4Cl2.xH2O), platinum bromide (IV) (PtBr4), and bis(acetylacetonato)platinum (II) ([Pt(C5H7O2)2]) can preferably be used.
(2) Alcohol having 1 to 5 Carbon Atoms
As the alcohol having 1 to 5 carbon atoms, at least one selected from the group consisting of methanol, ethanol, propanol, isopropyl alcohol, 1-butanol, 2-butanol, t-butyl alcohol, 1-pentanol, and 3-pentanol can preferably be used. Among these alcohols, ethanol is preferred from the viewpoint of reducing the environmental load.
The amount ratio of the alcohol to the noble metal salt is not particularly limited. The concentration of the noble metal salt in an alcohol solution in which the noble metal salt is dissolved is not particularly limited. The concentration of the noble metal salt is preferably 0.1 mol L−1 or more and 50 mol L−1 or less, more preferably 5 mol L−1 or more and 40 mol L−1 or less, and further preferably 10 mol L−1 or more and 30 mol L−1 or less, from the viewpoint of producing highly active noble metal fine particles having a particle diameter of 0.7 nm to 2 nm and a uniform size.
The support is not particularly limited as long as it can support the noble metal fine particles. As the support, at least one selected from carbon black, amorphous carbon, carbon nanotubes, carbon nanohorns, and one or more metal oxides selected from rare earths, alkaline earths, transition metals, niobium, bismuth, tin, antimony, zirconium, molybdenum, indium, tantalum, and tungsten can preferably be used. Among these supports, carbon black is preferred from the viewpoint of surface area.
When carbon black is used as the support, the nitrogen adsorption specific surface area of carbon black is not particularly limited. The nitrogen adsorption specific surface area of carbon black is preferably 10 m2g−1 or more and 1800 m2g−1 or less, and more preferably 150 m2g−1 or more and 800 m2g−1 or less, from the viewpoint of supporting noble metal fine particles.
The mixing ratio of the support to the alcohol is not particularly limited. From the viewpoint of fully blending the support and the alcohol into highly active noble metal fine particles having a particle diameter of 0.7 nm to 2 nm and a uniform size, the support is preferably mixed at a ratio of 2 mg or more and 200 mg or less, more preferably mixed at a ratio of 10 mg or more and 100 mg or less, and further preferably mixed at a ratio of 30 mg or more and 80 mg or less, per mL of the alcohol.
The mixing method is not particularly limited. Pulverization mixing may be performed using a mortar and a pestle. For example, pulverization mixing may be performed using a dry crusher such as a ball mill, a vibration mill, a hammer mill, a roll mill, or a jet mill. For example, mixing may be performed using a mixer such as a ribbon blender, a Henschel mixer, or a V-type blender.
The mixing time is not particularly limited. Mixing is preferably performed until the alcohol volatilizes so that the mixture dries.
The heating temperature is 150° C. or higher and 800° C. or lower, preferably 150° C. or higher and 400° C. or lower, and more preferably 150° C. or higher and 250° C. or lower, from the viewpoint of producing highly active noble metal fine particles having a particle diameter of 0.7 nm to 2 nm and a uniform size.
Heating is preferably performed in an atmosphere of an inert gas. As the inert gas, a rare gas such as argon gas or nitrogen gas can preferably be used. Heating may be performed in air.
The average particle diameter of the noble metal fine particles is not particularly limited. The average particle diameter of the noble metal fine particles is preferably 0.7 nm or more and less than 2 nm, and more preferably 1.2 nm or more and 1.6 nm or less, from the viewpoint of increasing the activity.
The average particle diameter can be determined by the following method (way to determine the average particle diameter). A synthesized catalyst is observed by a transmission electron microscope (TEM). The TEM photograph is printed out on paper. The noble metal fine particles (black circular images) are regarded as spherical, and the length from end to end of each of the noble metal fine particles is regarded as diameter. A total of 300 particles are randomly measured from images of several fields of view (3 to 5 fields of view). The average of the diameters of the counted 300 particles is defined as average particle diameter.
Further, the noble metal fine particles preferably have a standard deviation value of 0% or more and 20% or less with respect to the average particle diameter value. The standard deviation value is calculated by creating a distribution map from the diameters of the 300 particles.
The production method of the present embodiment is a production method which enables production of an ultrafine and highly active metal-supported catalyst by a very simple technique of mixing a noble metal salt and a support material in a highly volatile alcohol (for example, ethanol) and heat-treating the mixture, and which is environment-friendly because it does not generate any waste liquid in the producing process.
Further, the production method of the present embodiment can be used to produce a noble metal fine particle-supported catalyst in which a highly active metal composed of nano-level structures, whose particle diameter can be controlled extremely accurately within the range of 0.7 nm and 2 nm only by the concentration of the noble metal salt and which have a uniform size, is highly dispersed and supported on a support such as carbon. This noble metal fine particle-supported catalyst is extremely useful as an electrode catalyst.
Further, since the active metal has a particle diameter of 2 nm or less and is highly dispersed and supported on the support in the noble metal fine particle-supported catalyst produced by the production method of the present embodiment, the metal utilization rate is high at the atomic level, and high performance is achieved. Therefore, the noble metal fine particle-supported catalyst is suitable, for example, as an electrode catalyst for a polymer electrolyte fuel cell used as a power source for households or automobiles for which reduction in the amount of noble metal used is required. The catalyst exhibits 10 times higher activity than that of a conventional product (Pt/C catalyst in which Pt nanoparticles of about 3 nm are supported on carbon).
The method for producing noble metal fine particles of the present disclosure includes:
a step of mixing a noble metal salt and an alcohol having 1 to 5 carbon atoms to form a mixture; and
a heating step of the mixture at a temperature of 150° C. or higher and 800° C. or lower to produce noble metal fine particles.
The above descriptions of “(1) Noble metal salt”, “(2) Alcohol having 1 to 5 carbon atoms”, “(5) Mixing”, “(6) Heating”, and “(7) Average particle diameter of noble metal fine particles” explained in “1. Method for producing noble metal fine particle-supported catalyst” are applied to the method for producing noble metal fine particles of the present disclosure as they are, and these descriptions are omitted.
The production method of the present embodiment is a production method which enables production of ultrafine and highly active metal fine particles by a very simple technique of mixing a highly volatile alcohol (for example, ethanol) and a noble metal salt and heat-treating the mixture, and which is environment-friendly because it does not generate any waste liquid in the producing process.
Further, the production method of the present embodiment can be used to produce highly active noble metal fine particles composed of nano-level structures whose particle diameter can be controlled extremely accurately within the range of 0.7 nm and 2 nm only by the concentration of the noble metal salt and which have a uniform size. These noble metal fine particles are extremely useful in an electrode catalyst.
Further, since the active metal has a particle diameter of 2 nm or less in the noble metal fine particles produced by the production method of the present embodiment, the metal utilization rate is high at the atomic level, and high performance is achieved. Therefore, the noble metal fine particles are suitable, for example, for an electrode catalyst for a polymer electrolyte fuel cell used as a power source for households or automobiles for which reduction in the amount of noble metal used is required. The catalyst exhibits 10 times higher activity than that of a conventional product (Pt/C catalyst in which Pt nanoparticles of about 3 nm are supported on carbon).
In the noble metal fine particle-supported catalyst of the present disclosure, noble metal fine particles are supported on a support. The average particle diameter of the noble metal fine particles is 0.7 nm or more and less than 2 nm. The noble metal fine particle-supported catalyst can be produced by “1. Method for producing noble metal fine particle-supported catalyst”.
The above descriptions of “noble metal” in “(1) Noble metal salt”, “(3) Support”, and “way to determine the average particle diameter” in “(7) Average particle diameter of noble metal fine particles” explained in “1. Method for producing noble metal fine particle-supported catalyst” are applied to the noble metal fine particle-supported catalyst of the present disclosure as they are, and these descriptions are omitted.
The amount of the noble metal supported is not particularly limited, and a required amount of the noble metal may appropriately be supported in response to the target design and the like. From the viewpoint of catalyst performance and cost, the amount of the noble metal supported is preferably 5 parts by mass or more and 50 parts by mass or less, and more preferably 10 parts by mass or more and 30 parts by mass or less, in terms of metal, per 100 parts by mass of the support.
The noble metal fine particle-supported catalyst of the present embodiment can be produced by a very simple technique of mixing a noble metal salt and a support material in a highly volatile alcohol (for example, ethanol) and heat-treating the mixture, and can also be produced by an environment-friendly production method which does not generate any waste liquid in the producing process.
Since the noble metal fine particle-supported catalyst of the present embodiment has an average particle diameter of 0.7 nm or more and less than 2 nm, the metal utilization rate is high at the atomic level, and high performance is achieved. Therefore, the noble metal fine particle-supported catalyst is suitable, for example, as an electrode catalyst for a polymer electrolyte fuel cell used as a power source for households or automobiles for which reduction in the amount of noble metal used is required. The catalyst exhibits 10 times higher activity than that of a conventional product (Pt/C catalyst in which Pt nanoparticles of about 3 nm are supported on carbon).
The noble metal fine particles of the present disclosure have an average particle diameter of 0.7 nm or more and less than 2 nm. The noble metal fine particles can be produced by the above “2. Method for producing noble metal fine particles”.
The above descriptions of “noble metal” in “(1) Noble metal salt” and “the way to determine the average particle diameter” in “(7) Average particle diameter of noble metal fine particles” explained in “1. Method for producing noble metal fine particle-supported catalyst” are applied to the noble metal fine particles of the present disclosure as they are, and these descriptions are omitted.
The noble metal fine particles of the present embodiment can be produced by a very simple technique of mixing a noble metal salt and a highly volatile alcohol (for example, ethanol) and heat-treating the mixture, and can also be produced by an environment-friendly production method which does not generate any waste liquid in the producing process.
Since the noble metal fine particles of the present embodiment have an average particle diameter of 0.7 nm or more and less than 2 nm, the metal utilization rate is high at the atomic level, and high performance is achieved. Therefore, the noble metal fine particles are suitable, for example, for an electrode catalyst for a polymer electrolyte fuel cell used as a power source for households or automobiles for which reduction in the amount of noble metal used is required.
The present invention will be described more specifically by way of Examples.
Hexachloroplatinic (IV) acid hexahydrate (H2PtCl6.6H2O: Kanto Chemical Co., Inc., 98.5%) was collected in a beaker in an amount as shown in Table 1 below, and ethanol (C2H5OH) was added thereto in an amount as shown in Table 1 below to dissolve hexachloroplatinic (IV) acid hexahydrate. After collection of 85.6 g of graphitized carbon black (GCB, specific surface area of 150 m2g−1: LION) in a mortar, an ethanol solution in which the above Pt salt was dissolved was added thereto, and the mixture was stirred and mixed until ethanol volatilized to dryness. The obtained powder was transferred to a ceramic boat and heat-treated in an argon (Ar) atmosphere at 200° C. for 2 hours in a tubular furnace. After the temperature was lowered to room temperature, the heat-treated powder was taken out from the tubular furnace and evaluated as a catalyst.
2. Way to determine particle diameter distribution (average particle diameter and standard deviation of particle diameter distribution)
Synthesized Pt particles (noble metal fine particles) were observed by a transmission electron microscope (TEM). The TEM photograph was printed out on paper. The Pt particles (black circular images) were regarded as spherical, and the length from end to end of each of the Pt particles was regarded as diameter. A total of 300 particles were randomly measured from images of several fields of view (3 to 5 fields of view). A value obtained by averaging the diameters of the counted 300 particles was defined as average particle diameter. In addition, a distribution map was created from the particle diameters of the 300 particles to calculate the standard deviation value. The distribution width of the particle diameter of the synthesized Pt particles was very narrow, and the standard deviation value was between 0 and 20% of the average particle diameter value.
As an example, a Pt1.4 nm/C catalyst having an average particle diameter d=1.4 nm in Example 3 will be described (see
First, Pt particles are defined as spherical, and the specific surface area (SA) of the spheres is calculated from Equation (3.1) with the value of the average particle diameter as the radius.
SA=((4/3)πr3)/(ρ4πr2)=6/(ρ×d) (3.1)
From this, when the average particle diameter is d=1.4 nm, SA=200 m2g−1.
On the other hand, there is a technique of determining the specific surface area from an electrochemical reaction (area that is actually active as a fuel cell reaction).
Then, an appropriate amount of Pt1.4 nm/C catalyst is dispersed and fixed on an electrode substrate, and CV is measured in the same manner using it as a working electrode. The hydrogen adsorption wave region at this time is integrated to determine the electrical quantity QH. Since the electrical quantity of 210 μC flows per cm2 of Pt, the surface area SPt (m2) of the Pt1.4 nm/C catalyst that actually contributes to the reaction can be calculated from Equation (3.2). The electrochemical specific surface area ECA can be calculated by dividing the obtained SPt value by the mass (mPt) of Pt placed on the electrode substrate, as shown in Equation (3.3).
S
Pt
=Q
H(C)/210(C/m2) (3.2)
ECA=S
Pt(m2)/mPt(g) (3.3)
Here, since the specific surface area value (SA=200 m2g−1) of the spheres and the electrochemical specific surface area value (ECA=200 m2g−1) match, the synthesized Pt particles can be considered as spherical.
4.1. Formation of Pt/C Catalyst into Electrode
A predetermined amount of Pt/C (e.g., 2 mg) is ultrasonically dispersed in ethanol (e.g., 2 mL). This solution is used as a catalyst ink.
The electrode substrate is a glassy carbon disc (0, geometric area: 0.196 cm2). An appropriate amount of catalyst ink is dropped onto the surface of the substrate to disperse and support the catalyst.
After drying, a 0.2 wt% Nafion solution (dry film thickness: 0.1 μm) is added dropwise. The catalyst layer is covered on the substrate. This product is used as a working electrode.
Also in the CV measurement in the above “3. Validity of definition of spherical form”, the catalyst is formed into an electrode by the same method.
The measurement principle is described at the following URL.
https://www.bas.co.jp/xdata/200706_mail_news/hydrodynamic_voltammetry_with_rde_and_rrde.pdf
The above electrode was used as a working electrode. An ORR reaction was carried out in an O2-saturated 0.1 M perchloric acid (HClO4) solution (30° C.) to evaluate the activity.
First, the convection voltammogram is measured by sweeping the potential from 0.3V to 1.0 V (vs. reversible hydrogen electrode (RHE)) while changing the rotation speed of the electrode.
Since ORR is a reaction system (irreversible system) in which the electron transfer rate that occurs on the electrode surface is slow, the active control current (Ik) from which the effect of mass transfer is removed can be determined by the following equation (Koutecky-Levich equation).
1/I=1/Ik+1/(0.62nFSD2/3Coυ−1/6ω1/2)
wherein n is the number of reaction electrons, F is the Faraday constant, S is the electrochemically active surface area, D is the diffusion coefficient of O2, Co is the oxygen solubility, v is the viscosity of the electrolytic solution, and co is the angular velocity.
5.1 Observation with Transmission Electron Microscope (Comparison with Patent Document 1 (JP 2010-253408 A))
The present invention is characterized in that it is possible to synthesize an electrode catalyst in which a highly active metal composed of nano-level structures having a uniform particle size is highly dispersed and supported on a support such as carbon. The left figure of
In the TEM image of Patent Document 1, as compared with the catalyst synthesized in Example 3, the size of the Pt particles (black dots) on carbon (gray part) varies, the dispersed state is poor, and there are many places where the particles are aggregated. In this way, when the particles of the active metal are controlled to be small, the distribution width is usually widened so that the particles are likely to be aggregated. Further, in such a state, the performance as a catalyst is significantly deteriorated.
In the Examples, it was confirmed that when only the noble metal reagent concentration (noble metal salt concentration) is controlled while all other conditions during synthesis (heat treatment temperature: 200° C., atmosphere: in inert gas) are the same, the average particle diameter can be controlled extremely accurately within the range of 0.7 nm and 2 nm. In all of the techniques described in the patent documents, the particle size ranges from 2 nm to a dozen nm, which is larger than that of the catalyst particles of the Examples. It is a feature of the present invention that particles of about 1 nm can be easily controlled and synthesized.
In the catalyst produced in the present invention, since the active metal can be controlled to have a particle diameter of 2 nm or less and is highly dispersed and supported on the support, the metal utilization rate is high at the atomic level, and high performance is achieved. Thus, the catalyst can be used, for example, as an electrode catalyst for a polymer electrolyte fuel cell (PEFC) used as a power source for households or automobiles for which reduction in the amount of noble metal used is required. Then, the activity of the oxygen reduction reaction (ORR) when the catalyst was used as a catalyst for an air electrode of PEFC was investigated.
The effect of the heating temperature in the heating process was investigated.
The influence of the atmosphere in the heating process was investigated. As an example,
A catalyst was synthesized under the same conditions as in Example 3 using the noble metal salts having the concentrations shown in Table 2.
It was confirmed that a catalyst in which platinum particles of about 1 nm are highly dispersed and supported on carbon can be prepared, whichever of the two platinum salts, tetraammineplatinum (II) chloride hydrate; Pt(NH3)4Cl2.xH2O and hexachloroplatinic (IV) acid hexahydrate; H2PtCl6.6H2O, is used.
It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the scope of the appended claims, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular structures, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, which are within the scope of the appended claims.
The present invention is not limited to the embodiments described in detail above, and can be modified or changed in various manners within the scope as set forth in the claims of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-227955 | Dec 2019 | JP | national |
