JP2017168385A - Platinum catalyst and manufacturing method thereof, and fuel cell using platinum catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a platinum catalyst small in the decrease in electrochemical surface area (ECSA) owing to an accelerated durability test (ADT) and superior in durability; and a method for manufacturing the platinum catalyst.SOLUTION: A platinum catalyst for a fuel cell comprises: carbon carriers; and catalyst particles including platinum and a metal other than platinum and supported on the surface of each carbon carrier. On the surface of the catalyst particles, there is a connection structure part having a sulfur atom of a thiol group on one end side and an oxygen atom of a hydroxyl group on the other end side. In the connection structure part, a lone-electron pair of the sulfur atom of the thiol group is coordinated to the catalyst particle. The surface of the catalyst particle is covered by a coating of a silicon dioxide, titanium dioxide or aluminum oxide covalently bonded with the oxygen atom of the hydroxyl group. A part of the surface of the carbon carrier having no catalyst particle is exposed without the coating. A compound suitable for forming the connection structure part is 3-mercaptopropyl triethoxy silane.SELECTED DRAWING: Figure 7

Description

  The present invention relates to a platinum catalyst suitable for use as a catalyst for an oxygen reduction reaction in a fuel cell, a method for producing the same, and a fuel cell using the catalyst.

  A polymer electrolyte fuel cell (PEFC) is a clean energy device that produces only water by causing an oxidation reaction of hydrogen on the anode side and a reduction reaction of oxygen on the cathode side. As a catalyst on the cathode side, Those using platinum (Pt) are known. A catalyst using platinum, which is a noble metal, has high catalytic activity and high electrical conductivity, and has an advantage that it is less susceptible to corrosion and poisoning due to the state of the surrounding environment and substances present in the surrounding environment.

  On the other hand, platinum has a problem that the amount of resources is small and the price is high, and various studies are being conducted to improve the utilization efficiency and durability and reduce the amount of use. As one of the studies, a platinum core-shell catalyst obtained by coating platinum on a different metal has attracted attention. The platinum core-shell catalyst was devised by focusing on the fact that the platinum atoms exhibiting catalytic activity are only the platinum atoms exposed in the outermost layer of the catalyst particles. The dissimilar metal fine particles covered with the platinum atomic layer (shell) The (core) is configured to be highly dispersed and supported on a carrier such as carbon.

  Palladium (Pd) is known as one of the core metals of the platinum core-shell catalyst. Non-Patent Document 1 and Non-Patent Document 2 disclose that when Pd is used as a core metal, oxygen reduction reaction (ORR) activity in PEFC is enhanced. Since the lattice constant (0.38898 nm) of Pd is smaller than Pt (0.39231 nm), a slight compressive stress is generated in the Pt shell provided on the Pd core. It is thought that this compressive stress realizes a situation where the oxygen reduction reaction easily proceeds on the surface of the Pt shell and increases the ORR activity.

  In the core-shell catalyst with platinum as the shell and palladium as the core, the ORR activity is improved as described above, while the standard redox potential of Pd (0.92 V vs. NHE) is higher than that of Pt (1.19 V vs. NHE). Since it is low, there is a problem in its durability. In Non-Patent Document 3, in a PEFC using a carbon-supported Pd core / Pt shell catalyst (hereinafter also referred to as a Pt / Pd / C catalyst) as a cathode, a part of the Pd core is oxidized and dissolved by power generation. It has been reported that metal Pd is reprecipitated in the solid polymer electrolyte membrane and a Pd band appears.

  The inventors have already described that the oxidation elution of Pd core is a problem from the viewpoint of the durability of the catalyst, but the oxidation elution of Pd causes a change in the particle size and form of the Pt / Pd / C catalyst, resulting in an ORR activity. It has been found to increase. Patent Document 1 discloses that application of a voltage, which has been conventionally performed as an accelerated cycle durability test (ADT) of a catalyst, results in improving the activity of a Pt / Pd / C catalyst. ing. Further, Patent Document 1 discloses that a Pt / Pd / C catalyst is repeatedly given a potential higher than the Pt oxide reduction start potential and a potential lower than the Pt oxide formation start potential, whereby Pt / Pd / C. It is also disclosed that the activity of the C catalyst is improved.

  In the specific potential application method disclosed in Patent Document 1, a platinum core-shell catalyst is dispersed in an acidic solution containing protons, and a metal having a redox potential lower than the oxide generation start potential of the platinum core-shell catalyst is allowed to coexist. However, it was a method of stirring under oxygen supply. This method is a completely new method and has obtained a certain effect, but further improvement of ORR activity has been expected.

On the other hand, Patent Document 2 discloses a platinum alloy catalyst and a method for producing the same. In the production method disclosed in Patent Document 2, a mixed solution prepared by adding a reducing agent after dispersing an organometallic complex of platinum and a metal chloride in an organic solvent is pressurized and heated to have a diameter of 2 nm or less. The platinum alloy nanoparticles are synthesized, and the platinum alloy nanoparticles are heated (annealed) in a vacuum at a temperature of 300 ° C. or higher and 1000 ° C. or lower to have a diameter of 2 nm or more and 100 nm or less. In the invention of Patent Document 2, by synthesizing platinum alloy nanoparticles and then heating (annealing), the particle diameter of the alloy particles is adjusted to 2 nm or more and 100 nm or less to obtain platinum alloy particles having a specific crystal form. It is considered possible.
However, even in the case of such a specific crystal form of platinum alloy particles, when ADT is performed, the electrochemical surface area (ECSA) is greatly reduced, resulting in decreased ORR activity and insufficient durability. There was a problem that there was.

Furthermore, in the following Patent Document 3, 3-aminopropylethoxysilane (APTES) is added to a solution in which a platinum-supported carbon catalyst is dispersed and stirred, and then tetraethoxysilane (TEOS) is added and stirred. Thereafter, there is disclosed a catalyst produced by hydrogen reduction, in which platinum supported on carbon is coated with silica (SiO ₂ component). In this catalyst, not only the surface of platinum but also carbon Since the surface is also coated with a thick uniform silica film, there is a problem that when the battery is incorporated in the battery, the electric resistance is increased by the silica covering the carbon, and the battery characteristics are deteriorated.

Japanese Patent Laid-Open No. 2015-398 JP 2015-17317 A JP 2012-22960 A

J. Zhang et al., J. Phys. Chem. B, 108, 10955 (2004) J. Zhang et al., Angew. Chem., Int. Ed., 44, 2132 (2005) K. Sasaki et al., Angew. Chem. Int. Ed., 49, 8602 (2010)

  The present invention relates to a platinum catalyst using a combination of platinum and a metal other than platinum, in which the catalyst particles do not easily aggregate due to ADT, have excellent durability, and a production method for obtaining such a catalyst. The purpose is to provide.

The inventors have determined that ORR mass activity (MA) indicating activity per Pt mass is ORR specific activity (SA) and electrochemical surface area (ECSA). Focusing on the fact that it is expressed by the product of the above, we thought that it is possible to further improve the ORR mass activity by increasing both SA and ECSA. And since aggregation has occurred in some of the particles of the platinum core-shell catalyst that has undergone the potential cycle durability test (ADT), by suppressing the aggregation, the decrease in ECSA is suppressed, thereby improving the ORR mass activity. Inspired by the plan, a catalyst particle (including platinum and a metal other than platinum) supported on carbon, a compound having a thiol group on one end and a trialkoxysilyl group on the other end (seeding agent) , The lone pair of sulfur atom of the thiol group is coordinated to the catalyst particle, and the catalyst particle is selectively coated, and the trialkoxysilyl group on the other end side is hydrolyzed to form a trihydroxysilyl group. Then, it is hydrolyzed by reacting with a tetraalkoxysilane compound, tetraalkoxytitanium compound or trialkoxyaluminum compound, and then heated in a non-oxidizing atmosphere, While the surface portion of the carbon support medium particles is not carried is exposed, only the supported surface of the catalyst particles to the carbon carrier is silica (SiO ₂₎ film, titania (TiO ₂₎ film, an alumina (Al ₂ O ₃ ) The carbon-supported platinum catalyst having such a structure is found to be covered with a coating, and the catalyst particles are hardly aggregated by ADT, have excellent durability, and are useful as a catalyst for oxygen reduction reaction. The present invention was confirmed.

  In the present specification, catalysts that use platinum as a substance having catalytic activity are collectively referred to as a platinum catalyst, and the platinum catalyst includes both a platinum core-shell catalyst and a platinum alloy catalyst.

That is, the present invention
[1] A platinum catalyst for a fuel cell, in which catalyst particles containing platinum and a metal other than platinum are supported on the surface of a carbon support, and a sulfur atom of a thiol group on one end side of the surface of the catalyst particles Having a hydroxyl group oxygen atom on the other end, the lone pair of sulfur atoms of the thiol group in the linkage structure is coordinated to the catalyst particle and bonded thereto, On the oxygen atom side, a coating of silicon dioxide, titanium dioxide, or aluminum oxide bonded to the oxygen atom exists, and the surface of the catalyst particles is covered with the coating, and the catalyst particles are not supported. The surface of the carbon support is not covered with the coating film, and the carbon support is exposed.

The present invention also provides
[2] The platinum catalyst according to [1], wherein the compound forming the connecting structure part is 3-mercaptopropyltriethoxysilane (MPTS).

The present invention also provides
[3] The catalyst particle according to [1] or [2], wherein the catalyst particle is a platinum core-shell catalyst having a core particle containing palladium and a platinum shell formed on a surface of the core particle. The platinum catalyst.

The present invention also provides
[4] The platinum catalyst according to [1] or [2], wherein the catalyst particle is a platinum alloy catalyst of platinum and palladium, cobalt, nickel, iron, or copper.

Furthermore, the present invention provides
[5] A method for producing a platinum catalyst for a fuel cell in which catalyst particles containing platinum and a metal other than platinum are supported on the surface of a carbon support,
Preparing a catalyst in which catalyst particles containing platinum and a metal other than platinum are supported on a carbon carrier, and dispersing the catalyst in an aqueous solution; in the presence of an inert gas, the dispersion solution obtained in the above step An alkaline agent is added to the dispersion solution so that the dispersion solution becomes alkaline (pH of the dispersion solution is 10.5 or more). Thereafter, a thiol group is provided on one end side, and a trialkoxysilyl group is provided on the other end side. Adding a connecting compound having a first hydrolysis,
A step of performing a second hydrolysis by adding a tetraalkoxysilane compound, a tetraalkoxytitanium compound or a trialkoxyaluminum compound to the dispersion solution after the hydrolysis;
The present invention relates to a method for producing a platinum catalyst, comprising the steps of filtering and drying the hydrolyzed catalyst, and performing a heat treatment in a non-oxidizing atmosphere.

The present invention also provides
[6] The method for producing a platinum catalyst according to [5], wherein the linking compound is 3-mercaptopropyltriethoxysilane (MPTES).

The present invention also provides
[7] The method for producing a platinum catalyst according to [5] or [6], wherein the compound added in the second hydrolysis step is tetraethoxysilane (TEOS). .

The present invention also provides
[8] Any one of [5] to [7], wherein the catalyst particle is a platinum core-shell catalyst having palladium-containing core particles and a platinum shell formed on the surface of the core particles. This invention relates to a method for producing a platinum catalyst according to claim 1.

The present invention also provides
[9] The platinum catalyst according to any one of [5] to [7], wherein the catalyst particle is a platinum alloy catalyst of platinum and palladium, cobalt, nickel, iron, or copper. It relates to the manufacturing method.

Furthermore, the present invention provides
[10] The present invention relates to a fuel cell using the platinum catalyst according to any one of [1] to [4] as an oxygen reduction reaction catalyst.

  According to the present invention, only the surface of the catalyst particles supported on the carbon support is coated with a silica coating or the like, and the carbon support is not present on the surface portion of the carbon support on which the catalyst particles are not supported. Thus, a platinum catalyst having a structure in which the catalyst is exposed is obtained, and the platinum catalyst does not have an insulating coating on the surface of the carbon support, so that the surface of the catalyst particles is not degraded without deterioration of battery characteristics. The coated film is less likely to agglomerate catalyst particles in the potential cycle durability test (ADT), suppresses the decrease in electrochemical surface area (ECSA), and exhibits excellent durability. Useful.

It is a synthetic scheme in a preferable example of the manufacturing method of the platinum catalyst (SH-SiO ₂ Pt / Pd / C catalyst) of the present invention. 4 is a TEM image after ADT for four types of Pt / Pd / C catalysts listed in Table 1. FIG. It is a chart showing a TEM-EDX analysis result of the conventional product _{(NH 2 -SiO 2 Pt / Pd} / C catalyst). The TEM-EDX analysis result of the present invention product _{(SH-SiO 2 Pt / Pd} / C catalyst) is a chart showing. 4 is an XRD pattern for four types of Pt / Pd / C catalysts listed in Table 1. 4 is a graph showing changes in ECSA after ADT for four types of Pt / Pd / C catalysts listed in Table 1. FIG. The upper side is a schematic view showing the SiO ₂ coating state of a Pt catalyst by hydrolysis of conventional 3-aminopropyltriethoxysilane (APTES) → tetraethoxysilane (TEOS), and the lower side is 3-mercaptopropyl according to the present invention. FIG. 3 is a schematic diagram showing a SiO ₂ coating state of a Pt catalyst by hydrolysis of triethoxysilane (MPTES) → tetraethoxysilane (TEOS).

As shown in the schematic diagram on the lower side of FIG. 7, the platinum catalyst of the present invention has a catalyst particle containing platinum and a metal other than platinum, and the surface of the catalyst particle is an oxide film (SiO ₂ , TiO ₂ or Al ₂ O ₃ coating) is supported on the surface of the carbon support, and the surface of the carbon support on which the catalyst particles are not supported is not covered with the oxide coating and the carbon support is exposed. It has a structure. And between this catalyst particle and the oxide film, there is a connecting structure part having a specific chemical structure (a chemical structure having a sulfur atom of a thiol group on one end side and an oxygen atom of a hydroxyl group on the other end side) The lone electron pair of the sulfur atom of the thiol group in the connecting structure part is coordinated to the surface of the catalyst particle, and the oxygen atom of the hydroxyl group constitutes the oxide film. Bonded with Si atom, Ti atom or Al atom.
In the present invention, the connecting structure portion that can be selectively bonded to the surface of the catalyst particles is preferably a connecting structure portion derived from 3-mercaptopropyltriethoxysilane (MPTES), but is not limited thereto. Any substance having a thiol group (—SH) and a hydroxyl group (—OH) at both ends can be used. The oxide film is preferably a SiO ₂ film, but is not limited to this, and may be TiO ₂ or Al ₂ O ₃ , or may be SiO ₂ and a mixed oxide thereof. Good.

The catalyst particles in the platinum catalyst of the present invention may be those containing platinum and a metal other than platinum, and may be a conventionally known platinum catalyst, for example, core particles containing palladium, and the surface of the core particles. A platinum core-shell catalyst having a platinum shell formed on a platinum alloy or a platinum alloy catalyst of platinum and palladium, cobalt, nickel, iron or copper is preferable.
In the case of a platinum core-shell catalyst, the method of generating the core and / or shell is not particularly limited, and the platinum shell is formed with an improved type that does not require precise potential control using an external power source and a counter electrode or reference electrode. It is preferable to use a Cu-UPD method. In the improved Cu-UPD method, a Pd / C core is put into an acidic aqueous solution containing Cu ²⁺ ions in which a solid made of Cu is immersed, and stirred in an inert gas atmosphere such as argon or nitrogen. In this way, a monoatomic film made of Cu is formed on the surface of the Pd core. Subsequently, Cu on the surface of the obtained Pd core particles is substituted with Pt, and this step can be performed by a known substitution plating method.

In the case of a platinum core-shell catalyst having a palladium-containing core particle and a platinum shell formed on the surface of the core particle, the particle size of the Pd core particle of the platinum core-shell catalyst is suitably 3.0 nm to 7.0 nm. When Pd core particles having a particle size of less than 3.0 nm are used, there is a problem that the particle size of the platinum core-shell catalyst becomes small and aggregation due to potential fluctuations is likely to occur. On the other hand, when the particle size exceeds 7.0 nm, the particle size of the platinum core-shell catalyst becomes large, the catalyst particles become porous due to potential fluctuations, and the ORR activity does not increase. The particle diameter of the Pd core particle means an average particle diameter obtained from a TEM image or a value calculated by applying the Scherrer equation to the X-ray diffraction peak of the (220) plane of Pd.
The average thickness of the Pt shell of such a platinum core-shell catalyst is preferably a monoatomic layer (corresponding to 1 ML) to a triatomic layer (corresponding to 3 ML), that is, about 0.3 nm to 0.9 nm. Pt atoms that exhibit activity as an oxygen reduction catalyst are only Pt atoms located in the outermost layer (outermost surface) of the shell, but a Pt monoatomic layer is considered insufficient from the viewpoint of corrosion resistance. According to the present invention, it is important from the viewpoint of corrosion resistance that the Pt shell atoms are rearranged to form a thick film, and a Pt shell thickness of a diatomic layer to a triatomic layer is suitable. In a Pt shell exceeding the triatomic layer, Pt atoms not participating in the oxygen reduction reaction increase, leading to a decrease in ORR mass activity. The Pt shell in the platinum core-shell catalyst may consist of Pt alone, Pd and Pt may be mixed, or a Pt—Pd alloy shell. Further, it may be an alloy shell with a different metal other than Pd. As the dissimilar metal, a metal having a lower redox potential than platinum is preferable, and examples thereof include silver (Ag), copper (Cu), nickel (Ni), and cobalt (Co).

In the platinum catalyst of the present invention, the above catalyst particles are supported in a dispersed state on the surface of a carbon support (support made of a carbonaceous material) as shown in the schematic diagram on the lower side of FIG. Preferably, the carbonaceous material as a carrier includes carbon black, ketjen black, acetylene black, carbon nanotubes and the like. The carrier preferably has a specific surface area of about 10 to 1000 m ² / g. The platinum catalyst is thought to be supported on the surface of the support mainly by electrostatic interaction. However, in order to reduce the amount of catalyst falling off from the support surface by supporting it more firmly, the platinum catalyst and the support A chemical bond can be formed between and supported.
The Pd core particles supported on the carbon support can be synthesized by a known synthesis method. Examples include palladium chloride (PdCl ₂ ), palladium nitrate (Pd (NO ₃ ) ₂ ), palladium acetate (Pd (CH ₃ COO) ₂ ), palladium (II) chloride sodium trihydrate (Na ₂ [PdCl ₄ ] · 3H ₂ O), dinitrodiamminepalladium (II) ([Pd (NH ₃ ) ₂ (NO ₂ ) ₂ ]), etc. There is a method for reducing carbon to obtain a carbon-supported Pd nanoparticle core.

Next, a method for producing the platinum catalyst of the present invention in which the catalyst particles whose surface is coated with the oxide film is supported on the surface of the carbon support will be described.
In FIG. 1, a platinum core-shell catalyst having palladium-containing core particles and a platinum shell formed on the surface of the core particles is used as catalyst particles, and 3-mercaptopropyltriethoxysilane (MPTES) is used as a linking compound. In a preferred example of the method for producing a platinum catalyst (SH-SiO ₂ Pt / Pd / C catalyst) of the present invention when tetraethoxysilane (TEOS) is used as a compound for forming a silica film. A synthetic scheme is shown.

  The first step in the production method of the present invention is a step of preparing a catalyst in which catalyst particles containing platinum and a metal other than platinum are supported on a carbon carrier, and dispersing the catalyst in an aqueous solution. In this case, it is preferable to put the above catalyst in a water-alcohol mixed solution and carry out ultrasonic stirring dispersion using an ultrasonic stirrer, but this dispersion method is not limited to the above method.

In the second step of the production method of the present invention, in the presence of an inert gas, an alkaline agent is added to the dispersion obtained in the above step to make the aqueous solution alkaline (for example, the pH is set to 10.5 or more). And then performing a first hydrolysis (alkali hydrolysis) by adding a connecting compound having a thiol group on one end side and a trialkoxysilyl group on the other end side as a seeding agent. In general, while supplying an inert gas (for example, nitrogen gas) to the dispersion solution obtained in the first step, heating (for example, 50 to 70 ° C.) and organic amine compounds such as triethylamine and trimethylamine are performed. Is added to make the dispersion alkaline (for example, after adjusting the pH to 10.5 or higher), and then a connecting compound such as 3-mercaptopropyltriethoxysilane (MPTES) is added, for example, 5 Stirred at ~70 ℃ of temperature.
In this step, the lone electron pair of the sulfur atom of the thiol group located on one end side of the linking compound having the above chemical structure is coordinated to the surface of the catalyst particles supported on the carbon support, while the other end side The group located at (triethoxysilyl group in the case of MPTES) undergoes hydrolysis to produce a hydroxyl group (in the case of MPTES, a trihydroxysilyl group is produced). In the present invention, due to the chemical structure of the linking compound, the linking compound is selectively coordinated to the catalyst particles and hardly adheres to the portion of the carbon support where no catalyst particles are present.

  In the third step of the production method of the present invention, a tetraalkoxysilane compound, a tetraalkoxytitanium compound or a trialkoxyaluminum compound is added to the dispersion solution after the first hydrolysis and heated, It is a step of performing hydrolysis, and by this step, the hydroxyl group generated in the above step reacts with one alkoxy group in the above compound, and Si, Ti or Al atom is bonded to the oxygen atom in the linking compound, The remaining alkoxy group that has not reacted with the hydroxyl group undergoes hydrolysis to form a hydroxyl group.

The fourth step in the production method of the present invention is a step in which the catalyst after the second hydrolysis is filtered and dried and heated in a non-oxidizing atmosphere. In this step, the catalyst taken out from the dispersion solution is removed. By heating at a temperature of 300 to 400 ° C. (preferably 350 ° C.) in a non-oxidizing atmosphere, only the surface of the catalyst particles supported on the carbon support as shown in the schematic diagram on the lower side of FIG. Is coated with a SiO ₂ component (a TiO ₂ component when a tetraalkoxytitanium compound is added in the above step, or an Al ₂ O ₃ component when a trialkoxyaluminum compound is added in the above step), and a carbon carrier In the portion where the catalyst particles do not exist, a platinum catalyst is produced in which the surface of the carbon support is exposed as it is.

  In the present invention, after the fourth step, after preparing an acidic solution in which the platinum catalyst obtained by the above step is dispersed, a plurality of kinds of chemical species are alternately added to the acidic solution with a certain duration. A step of repeatedly applying a predetermined potential to the platinum catalyst by repeatedly feeding may be performed. The said process is a post-processing process in manufacture of a platinum catalyst, and is a catalyst activity improvement processing process for improving the activity of a catalyst. By this process, a part of different metals (palladium, cobalt, nickel, iron, copper, etc.) having a redox potential lower than that of platinum are oxidized and eluted, and the rearrangement of platinum atoms occurs on the surface of the catalyst particles. Aggregation of the catalyst particles is suppressed. As a result, it is considered that a catalyst having a high ORR mass activity can be obtained by increasing the ORR area specific activity of the catalyst and maintaining the electrochemical surface area.

In the above catalytic activity improving treatment step, a platinum catalyst is dispersed in an acidic solution containing protons (for example, nitric acid, sulfuric acid, hydrochloric acid, perchloric acid, etc.), and (I) the platinum catalyst in the platinum catalyst is dispersed in the dispersed solution. A step of causing a chemical species to give a potential higher than an oxide generation start potential; and a step of (II) causing a chemical species to give a potential lower than the oxide reduction start potential of platinum of the platinum catalyst to be present.
The above step (I) is typically a step of (A) feeding a gas (for example, oxygen) giving a higher potential than the platinum oxide formation start potential of the platinum catalyst. The above step (II) is typically (B-1) a step of feeding a gas (for example, hydrogen) that gives a potential lower than the platinum oxide reduction start potential of the platinum catalyst, or (B -2) An inert gas (for example, nitrogen gas or argon gas) is sent while a solid (for example, metallic copper) that gives a potential lower than the platinum oxide reduction starting potential of the platinum catalyst is present in the dispersion. Process.

The step (A) and the step (B-1) or (B-2) are preferably performed while stirring, and the step (A) and the step (B-1) or (B-2) ) And step (B-1) or (B-2) are preferably repeated alternately several times (preferably 20 times to 500 times). Further, it is preferable that the method further includes (C) a step of feeding an inert gas such as argon gas or nitrogen gas between the step (A) and the step (B-1). Also in the manufacturing method including (B-2), it is preferable to further include a step (C) of feeding an inert gas between the step (A) and the step (B-2).
In addition, when employ | adopting a process (B-2), it is preferable to contain Cu2 ⁺ ion in the acidic solution which disperse | distributed the platinum catalyst, However, Solid copper is made into a platinum catalyst except at the time of implementation of a process (B-2). Remove from dispersion. In particular, when the step (A) is performed, solid copper should not be present in the platinum catalyst dispersion solution.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely using an Example, this invention is not limited to an Example.

[Example 1] Production example of SiO ₂ coated Pt / Pd / C catalyst according to the present invention (I) Production of Pd / C core
1.4 × 10 ⁻³ mol of Pd (NO ₃ ) ₂ was dissolved in 300 ml of pure water, and 0.35 g of carbon support (Ketjen Black EC 300J, specific surface area 800 m ² / g) was ultrasonically dispersed in this aqueous solution. Water was evaporated by a hot stirrer and Pd (NO ₃ ) ₂ was supported on the carbon support. Thereafter, the solid was reduced in an H ₂ atmosphere at 400 ° C. for 1 hour, and treated with a carbon-supported Pd core (Pd / C) in an N ₂ atmosphere at 300 ° C. for 1 hour in order to remove hydrogen stored in the Pd particles. )

(II) Analysis of Pd / C core As a result of observing the produced Pd / C core with TEM (JEM2100F, manufactured by JEOL Ltd.), Pd fine particles supported on a carbon support were confirmed. As a result of measuring the diameter of 200 Pd core particles in the TEM image, the average particle size was 4.8 nm. Further, the loading ratio of metal Pd was examined by thermogravimetric analysis (Rigaku, Thermo Plus TG-8120). As a result, it was 34.2 wt.%.

(III) Formation of a Pt shell on the Pd / C core 800 mg of the Pd / C core obtained in (I) was added to an 800 ml aqueous solution containing 50 mM H ₂ SO ₄ and 10 mM CuSO _4. Dispersed in. Ar was bubbled at a flow rate of 500 ml / min., And the Cu sheet was allowed to coexist in the aqueous solution, and then stirred at 5 ° C. for 5 hours to form a Cu shell on the surface of the Pd core particles. After that, the Cu sheet is removed from the aqueous solution, and a K ₂ PtCl ₄ aqueous solution equivalent to 2 monolayers, in which the dissolved oxygen is removed by bubbling Ar in advance, is immediately added, and the Cu shell layer is replaced with the Pt shell layer to replace the Pt / Pd / C core shell. A catalyst was obtained. The obtained Pt / Pd / C core-shell catalyst was filtered off, redispersed in 300 ml of pure water, stirred for 30 minutes, and then filtered off. This operation was repeated 3 times to wash the Pt / Pd / C core-shell catalyst. Thereafter, it was dried in an oven at 60 ° C. in the atmosphere for 6 hours.

(IV) Analysis of Pt / Pd / C catalyst The composition of the Pt / Pd / C catalyst obtained in (III) was analyzed by XRF (SII, SEA1200VX). As a result, Pt: Pd = 30.9: 69.1 (at. As a result of examining the metal loading by thermogravimetric analysis (Rigaku, Thermo Plus TG-8120), it was 42.4 wt.%. The Pt shell layer thickness calculated from the Pd core particle size and XRF composition analysis value was equivalent to 1.2 atomic layers. The catalyst particle size was calculated to be 5.2 nm by applying the Scherrer equation to the (220) plane of X-ray diffraction.

(V) Preparation of SiO ₂ coated Pt / Pd / C catalyst (this product, pretreatment agent: MPTES)
In a separable flask containing 200 ml of water / ethanol mixed solution (50/50 vol.%), 100 mg of the Pt / Pd / C catalyst obtained in (III) was ultrasonically dispersed for 10 minutes. While bubbling N ₂ gas through the separable flask at a flow rate of 500 ml / min., The temperature was raised to 60 ° C. using a water bath, and triethylamine was added to adjust the pH of the mixture to 10.5. Then, 0.0113 mmol of 3-mercaptopropyltriethoxysilane (MPTES) was added as a pretreatment agent and stirred for 1 hour, and then 1.440 mmol of tetraethoxysilane (TEOS) as a silica source was added and stirred for 3 hours. Hydrolyzed. The obtained sample was washed by filtration and dried in the atmosphere at 60 ° C. for 6 hours. Thereafter, the sample was subjected to heat treatment at 350 ° C. for 2 hours in a 10% H ₂ / Ar atmosphere to perform dehydration condensation to obtain a SiO ₂ coated Pt / Pd / C catalyst.

(VI) Analysis of SiO ₂ coated Pt / Pd / C catalyst (present product, pretreatment agent: MPTES) Thermogravimetric analysis of silica loading in the SiO ₂ coated Pt / Pd / C catalyst obtained in (V) (Rigaku, Thermo Plus TG-8120) was examined and found to be 17.0 wt.%. The catalyst particle size was calculated by applying the Scherrer equation to the (220) plane of X-ray diffraction. As a result, it was 5.6 nm.

[Comparative Example 1] Example of production of SiO ₂ coated Pt / Pd / C catalyst by conventional method (VII) Production of SiO ₂ coated Pt / Pd / C catalyst (conventional product, pretreatment agent: APTES)
In a separable flask containing 200 ml of water / ethanol mixed solution (50/50 vol.%), 100 mg of the Pt / Pd / C catalyst obtained in (III) was ultrasonically dispersed for 10 minutes. While bubbling N ₂ gas through the separable flask at a flow rate of 500 ml / min., The temperature was raised to 60 ° C. using a water bath, and triethylamine was added to adjust the pH of the mixture to 10.5. Add 0.0113 mmol of 3-aminopropyltriethoxysilane (APTES) as a pretreatment agent and stir for 1 hour, then add 1.440 mmol of tetraethoxysilane (TEOS) as a silica source and stir for 3 hours to hydrolyze I let you. The obtained sample was washed by filtration and dried in the atmosphere at 60 ° C. for 6 hours. Thereafter, the sample was subjected to heat treatment at 350 ° C. for 2 hours in a 10% H ₂ / Ar atmosphere to perform dehydration condensation to obtain a SiO ₂ coated Pt / Pd / C catalyst.

(VIII) Analysis of SiO ₂ coated Pt / Pd / C catalyst (conventional product, pretreatment agent: APTES) Thermogravimetric analysis of silica loading in the SiO ₂ coated Pt / Pd / C catalyst obtained in (VII) ( It was 33.0 wt.% As a result of investigation using Thermo Plus TG-8120) manufactured by Rigaku. The catalyst particle size was calculated by applying the Scherrer equation to the (220) plane of X-ray diffraction. As a result, it was 6.1 nm.

[Comparative Example 2] Preparation Example of Heat-treated Pt / Pd / C Catalyst (Comparative Product 1) (IX) Preparation of Heat-treated Pt / Pd / C Catalyst The Pt / Pd / C catalyst obtained in (III) was converted to H ₂ / Ar. Heat treatment was performed at 350 ° C. for 2 hours in an atmosphere.
(X) Analysis of heat-treated Pt / Pd / C catalyst As a result of calculating the particle size of the heat-treated Pt / Pd / C catalyst obtained in (IX) by applying the Scherrer equation to the (220) plane of X-ray diffraction, It was 6.3 nm.

  Further, as a comparative product 2, an untreated Pt / Pd / C catalyst (obtained in (III) above) was prepared.

The above four Pt / Pd / C catalysts:
(1) Untreated Pt / Pd / C catalyst (Comparative product 2),
(2) Untreated Pt / Pd / C catalyst heat-treated in H ₂ / Ar atmosphere (Comparative product 1),
(3) APTES added to Pt / Pd / C catalyst for hydrolysis, TEOS added for hydrolysis, and hydrogen reduction under heating (conventional product, NH ₂ -SiO ₂ Pt / Pd / C catalyst),
(4) MPTES added to Pt / Pd / C catalyst for hydrolysis, then TEOS added for hydrolysis, and heat treated in a non-oxidizing atmosphere (this product, SH-SiO ₂ Pt / Pd / C catalyst)]
Was subjected to a potential cycle durability test (ADT), and the durability of each catalyst was evaluated.
In the ADT, each Pt / Pd / C catalyst was immersed in an aqueous solution of HClO ₄ saturated with argon gas at 80 ° C. and at a concentration of 0.1 M, and 0.6 V (3 s) /1.0 against the reversible hydrogen electrode (RHE). V (3 s) square wave was applied with a potential at 3,000 cycles and 10,000 cycles.
Table 1 below summarizes the production conditions for the above four types of Pt / Pd / C catalysts: NH ₂ —SiO ₂ Pt / Pd / C catalyst (conventional product), and SH—SiO ₂ Pt / Pd. For the / C catalyst (product of the present invention), the amount of SiO ₂ is expressed in weight%.
From Table 1, the amount of SiO ₂ of the SH—SiO ₂ Pt / Pd / C catalyst (product of the present invention) is about ½ of the amount of SiO ₂ of the NH ₂ —SiO ₂ Pt / Pd / C catalyst (conventional product). It can be seen that it is.

Fig. 2 shows TEM images of the four types of Pt / Pd / C catalysts observed after ADT (10,000 cycles). The electrochemical properties of each Pt / Pd / C catalyst are shown in Fig. 2. Surface area (ECSA) is also listed. The ECSA of each catalyst was prepared by applying each catalyst on a rotating ring disk electrode GC electrode (diameter 6 mm) so that Pt was 14.1 μg / cm ² , and this working electrode was saturated with argon gas. Soaked in 25 ° C, 0.1 M HClO ₄ aqueous solution, using reversible hydrogen electrode (RHE) as standard electrode and Pt wire as counter electrode, potential range 0.05 V to 1.2 V, potential sweep rate 50 mV / s A cyclic voltammogram (CV) was measured and calculated from the hydrogen desorption wave of the obtained CV.
The top two TEM images in Fig. 2 show the ADT for Comparative Product 2 (untreated Pt / Pd / C catalyst) and Comparative Product 1 (Pt / Pd / C catalyst heat-treated in H ₂ / Ar atmosphere). It is a later TEM image, and in the case of these catalysts, in addition to the catalyst particles having a round shape alone, it is confirmed that there are also agglomerated catalyst particles, and the catalyst particles may be aggregated by ADT. all right. When the catalyst particles agglomerate in this way, their electrochemical surface area (ECSA) decreases, leading to a decrease in ORR mass activity.
On the other hand, the two TEM images on the lower side of FIG. 2 are for the conventional product (NH ₂ —SiO ₂ Pt / Pd / C catalyst) and the product of the present invention (SH—SiO ₂ Pt / Pd / C catalyst). It is a TEM image after ADT, and in the case of these catalysts, it was confirmed that agglomeration of catalyst particles by ADT was suppressed and ECSA decrease was also suppressed.

Next, for the conventional product (NH ₂ —SiO ₂ Pt / Pd / C catalyst) and the product of the present invention (SH—SiO ₂ Pt / Pd / C catalyst), the location of the catalyst particles and the catalyst particles TEM-EDX analysis was performed at a location that did not exist. FIG. 3 is a chart showing the TEM-EDX analysis result of the conventional product, and FIG. 4 is a chart showing the TEM-EDX analysis result of the product of the present invention. The position indicated by the horizontal line in the TEM image on the upper side The intensities of Si, S, Pd, and Pt are shown in the lower chart.
The TEM-EDX analysis results of the NH ₂ —SiO ₂ Pt / Pd / C catalyst in FIG. 3 indicate that Si is also present at the place where the catalyst particles are not present (carbon support). It was confirmed that the surface of the support was also covered with the SiO ₂ coating.
On the other hand, the TEM-EDX analysis result of the SH-SiO ₂ Pt / Pd / C catalyst of the present invention in FIG. 4 shows that S and Si are not present where the catalyst particles are not present (carbon support). This indicates that the catalyst particles are present where the catalyst particles are present, and it is confirmed that the surface of the carbon support where the catalyst particles are not present is exposed without being covered with the SiO ₂ coating. It was.

FIG. 5 is an XRD pattern for the four types of Pt / Pd / C catalysts listed in Table 1. From the bottom, the untreated Pt / Pd / C catalyst (Comparative product 2), H ₂ / Ar atmosphere. Pt / Pd / C catalyst (Comparative product 1), NH ₂ —SiO ₂ Pt / Pd / C catalyst (conventional product), and SH—SiO ₂ Pt / Pd / C catalyst (present product) heat-treated below.
From the result of FIG. 5, in the case of the present invention product in which only the Pt / Pd / C catalyst particles are coated with the SiO ₂ coating, H ₂ in the final step for proceeding the dehydration condensation reaction of silanol groups (Si—OH). Although the increase in catalyst particle size after heat treatment at 350 ° C for 2 hours in an Ar atmosphere is suppressed (particle size 5.2 nm → 5.6 nm), Were aggregated and the particle size increased (particle size 5.2 nm → 6.1 nm).

FIG. 6 is a graph showing changes in ECSA by ADT for the four types of Pt / Pd / C catalysts listed in Table 1. The right side of the graph shows the ECSA of each catalyst after ADT 10,000 cycles and ADT. The rate of decrease from the previous ECSA value is also shown.
The result of FIG. 6 shows that by changing the component used in the first hydrolysis step from APTES to MPTES, excellent durability equivalent to or better than APTES can be realized with about 1/2 SiO ₂ amount. Is shown.

Figure 7 is a schematic diagram showing a SiO ₂ coating state of the art APTES → Pt-based catalyst by TEOS hydrolysis and (upper side in the drawing), the SiO ₂ coating state of the Pt-based catalyst according to MPTES → TEOS hydrolysis according to the invention A schematic diagram (lower diagram) is shown.
In the case of Pt-based catalyst obtained by the conventional method, although the durability is improved since the outer peripheral surface of the catalyst particles is covered with the SiO ₂ film, a carbon support SiO ₂ coating of insulative on the surface of Therefore, when the battery is assembled, there is a disadvantage that the resistance loss is large and the battery characteristics are deteriorated. On the other hand, in the case of the Pt-based catalyst obtained by using the production method of the present invention, the SiO ₂ coating selectively coats the outer peripheral surface of the catalyst particles to improve durability, and at the same time, the surface of the carbon support is made of SiO ₂ Since it is not covered with a film, the resistance loss when the battery is assembled is suppressed, and the battery characteristics are less deteriorated.

  By using the production method of the present invention, a Pt / Pd / C catalyst having excellent durability can be produced, and such a Pt / Pd / C catalyst is used as a catalyst for an oxygen reduction reaction particularly in a fuel cell. Can be used.

Claims (10)

  A platinum catalyst for a fuel cell in which catalyst particles containing platinum and a metal other than platinum are supported on the surface of a carbon carrier, and the surface of the catalyst particles has a sulfur atom of a thiol group on one end side. The other end side of the linking structure portion having a hydroxyl group oxygen atom is bonded to the catalyst particle by a lone pair of sulfur atoms of the thiol group in the linking structure portion. On the side, there is a coating of silicon dioxide, titanium dioxide or aluminum oxide bonded to the oxygen atom, the surface of the catalyst particles is covered with the coating, and the carbon carrier on which the catalyst particles are not supported The platinum catalyst is characterized in that the carbon support is exposed without being covered with the coating.   2. The platinum catalyst according to claim 1, wherein the compound forming the connecting structure portion is 3-mercaptopropyltriethoxysilane.   The platinum catalyst according to claim 1 or 2, wherein the catalyst particle is a platinum core-shell catalyst having core particles containing palladium and a platinum shell formed on the surface of the core particles.   The platinum catalyst according to claim 1 or 2, wherein the catalyst particle is a platinum alloy catalyst of platinum and palladium, cobalt, nickel, iron, or copper. A method for producing a platinum catalyst for a fuel cell in which catalyst particles containing platinum and a metal other than platinum are supported on the surface of a carbon carrier,
Preparing a catalyst in which catalyst particles containing platinum and a metal other than platinum are supported on a carbon carrier, and dispersing the catalyst in an aqueous solution; in the presence of an inert gas, the dispersion solution obtained in the above step An alkali agent is added to make the dispersion solution alkaline, and then a first compound is added by adding a linking compound having a thiol group at one end and a trialkoxysilyl group at the other end. A process of performing;
A step of performing a second hydrolysis by adding a tetraalkoxysilane compound, a tetraalkoxytitanium compound or a trialkoxyaluminum compound to the dispersion solution after the hydrolysis;
A method for producing a platinum catalyst, comprising a step of filtering and drying the hydrolyzed catalyst and heat-treating it in a non-oxidizing atmosphere.   The method for producing a platinum catalyst according to claim 5, wherein the linking compound is 3-mercaptopropyltriethoxysilane.   The method for producing a platinum catalyst according to claim 5 or 6, wherein the compound added in the second hydrolysis step is tetraethoxysilane.   The said catalyst particle is a platinum core-shell catalyst which has the core particle containing palladium, and the platinum shell formed in the surface of the said core particle, The any one of Claims 5-7 characterized by the above-mentioned. A method for producing a platinum catalyst.   The said catalyst particle is a platinum alloy catalyst of platinum and palladium, cobalt, nickel, iron, or copper, The manufacturing method of the platinum catalyst of any one of Claims 5-7 characterized by the above-mentioned.   The fuel cell which utilizes the platinum catalyst of any one of Claims 1-4 as a catalyst of oxygen reduction reaction.