JP2011212666A - Platinum core-shell catalyst manufacturing method, and fuel cell using catalyst - Google Patents

Platinum core-shell catalyst manufacturing method, and fuel cell using catalyst Download PDF

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JP2011212666A
JP2011212666A JP2010195345A JP2010195345A JP2011212666A JP 2011212666 A JP2011212666 A JP 2011212666A JP 2010195345 A JP2010195345 A JP 2010195345A JP 2010195345 A JP2010195345 A JP 2010195345A JP 2011212666 A JP2011212666 A JP 2011212666A
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platinum
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shell catalyst
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JP5660603B2 (en
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Minoru Inaba
稔 稲葉
Hiroaki Tsuji
裕明 辻
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Doshisha Co Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8605—Porous electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H01M4/925—Metals of platinum group supported on carriers, e.g. powder carriers
    • H01M4/926—Metals of platinum group supported on carriers, e.g. powder carriers on carbon or graphite
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M2008/1095—Fuel cells with polymeric electrolytes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Abstract

PROBLEM TO BE SOLVED: To provide a method in which a platinum core-shell catalyst, which is suitably used as a catalyst for oxygen reduction reactions in a fuel cell, can be produced in large quantities through a simple manufacturing process.SOLUTION: In the method, a gold core particle is immersed in a solution containing a bivalent or tetravalent platinum ion in the absence of a reducing agent to deposit platinum directly onto the gold core particle. It is preferable that the gold core particle is deposited on the surface of a carrier and the solution is an aqueous solution containing the platinum ion of 0.1-100 mM concentration.

Description

本発明は、燃料電池において酸素還元反応の触媒として用いるのに適した白金コアシェル触媒を製造する方法、および当該触媒を用いた燃料電池に関する。   The present invention relates to a method for producing a platinum core-shell catalyst suitable for use as a catalyst for an oxygen reduction reaction in a fuel cell, and a fuel cell using the catalyst.

固体高分子形燃料電池(PEFC)は、アノード側で水素の酸化反応を、カソード側で酸素の還元反応を起こすことにより、水のみを生成するクリーンエネルギーデバイスとして知られており、PEFCのカソード側の触媒としては、カーボンブラック担体に白金微粒子を高分散担持させた白金担持カーボン触媒(Pt/C触媒)が一般的に用いられている。Pt/C触媒は、触媒活性が高く、電気伝導性が高いという利点を有し、また、貴金属であるため、周辺環境の状態や周辺環境に存在する物質により腐食や被毒を受けにくいという利点を有する。
しかし、白金は価格が高く、また資源量も少ないという問題があり、白金量の低減が求められている。 A polymer electrolyte fuel cell (PEFC) is known as 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 this catalyst, a platinum-supported carbon catalyst (Pt / C catalyst) in which platinum fine particles are supported in a highly dispersed manner on a carbon black carrier is generally used. Pt / C catalysts have the advantages of high catalytic activity and high electrical conductivity, and because they are noble metals, they are less susceptible to corrosion and poisoning due to the state of the surrounding environment and substances present in the surrounding environment. Have
However, there is a problem that platinum is expensive and has a small amount of resources, and a reduction in the amount of platinum is required.

この問題を解決するため、異種金属上に白金を原子レベルで被覆してなる白金コアシェル触媒が注目されている。白金コアシェル触媒は、白金原子層(シェル)で被覆された異種金属微粒子(コア金属)が担体(カーボンブラック等)に高分散担持された構成を有する。このような構成とすれば、白金量を少なくしつつ、表面積の増大を図ることができるため、白金の質量当たりの活性を向上させ、白金量の低減を図ることができる。   In order to solve this problem, a platinum core-shell catalyst in which platinum is coated on a dissimilar metal at an atomic level has attracted attention. The platinum core-shell catalyst has a configuration in which different metal fine particles (core metal) coated with a platinum atomic layer (shell) are supported in a highly dispersed manner on a carrier (carbon black or the like). With such a configuration, the surface area can be increased while reducing the amount of platinum, so that the activity per mass of platinum can be improved and the amount of platinum can be reduced.

金は貴金属であり、白金と同じく高価ではあるが、白金よりもイオン化傾向が小さく、酸化に対して安定であり、また白金よりもはるかに資源量が多いことから、コア金属の一つとして期待されている。水素や水素化ホウ素ナトリウムなどの還元剤を用いて溶液中の白金イオンを還元し、金(コア金属)上に白金を析出させて白金コアシェル触媒(Pt/Au/C触媒)を製造しようとすると、厚い白金(シェル)層が金(コア金属)に析出したり、また溶液中に析出してしまったりして、効率的に白金コアシェル触媒を製造することは難しい。そこで、金をコアに用いて効率的に白金コアシェル触媒を製造するために、アンダーポテンシャル析出法(UPD法)が利用されている(非特許文献１)。UPD法を利用した白金コアシェル触媒の製造方法を図１(ａ)に模式的に示す。UPD法によれば、特定の条件を用いることにより、金表面を銅の単原子層で被覆することができ、その後、これを塩化白金酸イオンが存在する塩酸溶液に浸漬すると、銅と白金が置換するため、白金の単原子層を形成することができる。
上記UPD法を利用した方法では、理論上モノレイヤーで金(コア金属)を被覆することができるが、銅等の置換される金属を必要とすることや、電気化学的な処理を経るため合成方法が複雑であり、大量合成が難しいという問題点があった。 Gold is a noble metal and is as expensive as platinum, but it is less prone to ionization than platinum, is stable to oxidation, and is much more resource-rich than platinum, so it is expected as one of the core metals Has been. When trying to produce a platinum core-shell catalyst (Pt / Au / C catalyst) by reducing platinum ions in solution using a reducing agent such as hydrogen or sodium borohydride, and depositing platinum on gold (core metal) Since a thick platinum (shell) layer is deposited on gold (core metal) or in a solution, it is difficult to efficiently produce a platinum core-shell catalyst. Therefore, in order to efficiently produce a platinum core-shell catalyst using gold as a core, an underpotential deposition method (UPD method) is used (Non-Patent Document 1). A method for producing a platinum core-shell catalyst using the UPD method is schematically shown in FIG. According to the UPD method, by using specific conditions, the gold surface can be coated with a copper monoatomic layer, and then immersed in a hydrochloric acid solution containing chloroplatinate ions, For substitution, a monoatomic layer of platinum can be formed.
In the method using the UPD method, gold (core metal) can be theoretically coated with a monolayer, but it is synthesized because it requires a metal to be replaced such as copper or undergoes an electrochemical treatment. There was a problem that the method was complicated and mass synthesis was difficult.

第16回燃料電池シンポジウム講演要旨集、東京 (2009) pp. 92-95.「固体高分子形燃料電池用高活性・高耐久白金コアシェル触媒の開発」稲葉稔等Abstracts of 16th Fuel Cell Symposium, Tokyo (2009) pp. 92-95. "Development of Highly Active and Durable Platinum Core-Shell Catalysts for Polymer Electrolyte Fuel Cells" Satoshi Inaba, et al.

したがって、本発明は、還元剤を用いずに、またアンダーポテンシャル析出法を利用した方法よりも工程がシンプルであり、低コストで大量に白金コアシェル触媒を製造できる方法を提供することを課題とする。   Accordingly, an object of the present invention is to provide a method that can produce a platinum core-shell catalyst in a large amount at a low cost without using a reducing agent and having a simpler process than a method using an underpotential deposition method. .

本発明者らは、前記課題を解決するために検討を繰り返す中で、アンダーポテンシャル析出法を用いなくとも、金コア粒子を二価白金イオンあるいは四価白金イオンを含む溶液に単に浸漬することにより、金コア粒子上に白金の単原子層(シェル)を精密にかつ容易に形成できることを見出すとともに、この方法で製造された白金コアシェル触媒が、アンダーポテンシャル析出法で製造した白金コアシェル触媒に匹敵する触媒活性を有することを確認し、本発明を完成した。   In the course of repeating the study to solve the above problems, the present inventors simply immersed the gold core particles in a solution containing divalent platinum ions or tetravalent platinum ions without using the underpotential precipitation method. The platinum core-shell catalyst produced by this method is comparable to the platinum core-shell catalyst produced by the underpotential deposition method, while it is found that a platinum monoatomic layer (shell) can be accurately and easily formed on the gold core particle. It was confirmed that the catalyst had catalytic activity, and the present invention was completed.

すなわち、本発明は、還元剤の不存在下で、金コア粒子を二価白金イオンあるいは四価白金イオンを含む溶液に浸漬することにより、前記金コア粒子上に白金を直接析出させることを特徴とする、白金コアシェル触媒の製造方法に関する。   That is, the present invention is characterized in that platinum is directly deposited on the gold core particles by immersing the gold core particles in a solution containing divalent platinum ions or tetravalent platinum ions in the absence of a reducing agent. And a method for producing a platinum core-shell catalyst.

また、前記金コア粒子は、適切な担体(好ましくはカーボンブラックなどの炭素粉末や、スズ酸化物やチタン酸化物など導電性酸化物粉末)の表面に担持されていることが好ましい。   The gold core particles are preferably supported on the surface of an appropriate carrier (preferably carbon powder such as carbon black, or conductive oxide powder such as tin oxide or titanium oxide).

本発明の方法によれば、高活性な白金コアシェル触媒を低コストで大量に製造することができる。この触媒は燃料電池の触媒として用いることができるため、燃料電池のコストの飛躍的低減が可能となる。   According to the method of the present invention, a highly active platinum core-shell catalyst can be produced in large quantities at a low cost. Since this catalyst can be used as a catalyst for a fuel cell, the cost of the fuel cell can be drastically reduced.

(ａ)はアンダーポテンシャル法を利用した従来の白金コアシェル触媒の製造方法を模式的に説明する図であり、(ｂ)は本発明の白金コアシェル触媒の製造方法を模式的に説明する図である。(a) is a figure explaining typically the manufacturing method of the conventional platinum core-shell catalyst using an underpotential method, (b) is a figure explaining the manufacturing method of the platinum core-shell catalyst of this invention typically. . (ａ)は本発明の方法に使用した装置を模式的に示す図であり、(ｂ)は酸素還元活性評価に使用した回転リングディスク電極を模式的に示す図である。(a) is a figure which shows typically the apparatus used for the method of this invention, (b) is a figure which shows typically the rotating ring disk electrode used for oxygen reduction activity evaluation. (ａ)は、実施例１および比較例２(UPD法)で作成したPt/Au/C触媒のサイクリックボルタモグラムを示す図であり、(ｂ)は(ａ)に示したボルタモグラムのピークを説明する図である。(a) is a figure which shows the cyclic voltammogram of the Pt / Au / C catalyst created in Example 1 and Comparative Example 2 (UPD method), (b) explains the peak of the voltammogram shown in (a). It is a figure to do. (ａ)は実施例１および比較例２(UPD法)で作成したPt/Au/C触媒、および、白金被覆処理を行わない金担持カーボン担体(Au/C)のサイクリックボルタモグラムを示す図であり、(ｂ)は(ａ)に示したボルタモグラムのピークを説明する一部拡大図である。(a) is a view showing a cyclic voltammogram of the Pt / Au / C catalyst prepared in Example 1 and Comparative Example 2 (UPD method) and a gold-supported carbon support (Au / C) not subjected to platinum coating treatment. (B) is a partially enlarged view for explaining the voltammogram peak shown in (a). 実施例１(1.0mM)・実施例２(0.1mM)および比較例２(UPD法)で作成したPt/Au/C触媒、および金担持カーボン担体(Au/C)の対流ボルタモグラムを示す図である。The figure which shows the convective voltammogram of the Pt / Au / C catalyst created in Example 1 (1.0 mM), Example 2 (0.1 mM), and Comparative Example 2 (UPD method), and a gold | metal | money carrying | support carbon support (Au / C). is there. 調製直後の白金イオン含有溶液のUVスペクトルと、白金コアシェル触媒作製後の白金イオン含有溶液の上澄み液のUVスペクトルを示す図であって、(ａ)はテトラクロロ白金(II)酸カリウム水溶液、(ｂ)はヘキサクロロ白金(IV)酸カリウム水溶液についての結果を示す。It is a figure which shows the UV spectrum of the platinum ion containing solution immediately after preparation, and the UV spectrum of the supernatant liquid of the platinum ion containing solution after the platinum core-shell catalyst preparation, (a) is a potassium tetrachloroplatinate (II) acid solution, b) shows the results for an aqueous solution of potassium hexachloroplatinate (IV). 透過型電子顕微鏡(TEM)画像であって、(ａ)は実施例７で用いた金担持カーボン担体(Au/C)、(ｂ)は実施例７で作成したPt/Au/C触媒の画像を示す。It is a transmission electron microscope (TEM) image, (a) is the gold | metal | money carrying | support carbon support (Au / C) used in Example 7, (b) is the image of the Pt / Au / C catalyst produced in Example 7. Indicates. 粒度分布を示すグラフであって、(ａ)は実施例７で用いた金担持カーボン担体(Au/C)、(ｂ)は実施例７で作成したPt/Au/C触媒のグラフを示す。It is a graph which shows a particle size distribution, Comprising: (a) is a gold | metal | money carrying | support carbon support | carrier (Au / C) used in Example 7, (b) shows the graph of the Pt / Au / C catalyst created in Example 7. FIG.

本発明に用いられる金コア粒子は、粒径1nm〜30nm程度が好ましい。また、金コア粒子は表面が金からなる粒子であればよく、内部に金や白金以外の金属が含まれていてもかまわない。
また、多数の金コア粒子が表面に担持された金担持担体を用いて本発明の方法を行うことが好ましく、この際、金コア粒子は担体の表面に分散された状態で、かつ金コア粒子と担体を合わせた重量の1％〜70％を占める量で担持されていることが好ましい。金コア粒子を高分散担時するために、担体の比表面積は10〜1000m2/gであることが好ましく、また担体の粒径は10nm〜1mmの範囲が好ましい。このような性質の担体を用いることで、金コア粒子を担体表面に担持させることにより、ナノメータースケールの金コア粒子を高密度にかつ安定に高分散された状態で保持できる。
なお、上記金コア粒径は、XRD法で測定された平均結晶子径を意味し、担体粒径は電子顕微鏡観察による一次粒子の平均粒径を意味する。
好ましい金担持担体の一例として、粒径1nm〜30nmの金微粒子が、カーボンブラック担体の表面に高分散状態で担持されている金担持担体が挙げられる。このような金担持担体として、従来の白金コアシェル触媒の製造法(UPD法を利用する方法)で使用されている金担持担体と同じものを用いることができる。 The gold core particles used in the present invention preferably have a particle size of about 1 nm to 30 nm. The gold core particle may be a particle whose surface is made of gold, and may contain a metal other than gold or platinum.
In addition, it is preferable to carry out the method of the present invention using a gold-supported carrier having a large number of gold core particles supported on the surface. At this time, the gold core particles are dispersed on the surface of the carrier and the gold core particles It is preferably supported in an amount that occupies 1% to 70% of the combined weight of the carrier and the carrier. In order to carry the gold core particles in a highly dispersed state, the specific surface area of the carrier is preferably 10 to 1000 m 2 / g, and the particle size of the carrier is preferably in the range of 10 nm to 1 mm. By using the carrier having such properties, the gold core particles are supported on the surface of the carrier, whereby the nanometer-scale gold core particles can be held in a highly dispersed state with high density.
The gold core particle size means an average crystallite size measured by the XRD method, and the carrier particle size means an average particle size of primary particles observed by an electron microscope.
As an example of a preferable gold-supporting carrier, a gold-supporting carrier in which gold fine particles having a particle size of 1 nm to 30 nm are supported in a highly dispersed state on the surface of a carbon black carrier. As such a gold-supported carrier, the same gold-supported carrier used in the conventional method for producing a platinum core-shell catalyst (method using the UPD method) can be used.

本発明に用いられる二価白金イオンあるいは四価白金イオンを含む溶液としては、例えば、テトラクロロ白金(II)酸の水溶液、テトラクロロ白金(II)酸カリウムの水溶液、ジアンミンジクロロ白金(II)の水溶液、ジアンミンジニトロ白金(II)の水溶液、テトラアンミン白金(II)塩化物(一水和物)の水溶液、ヘキサクロロ白金(IV)酸の水溶液、ヘキサクロロ白金(IV)酸カリウムの水溶液等が挙げられる。また、これらの白金イオンは、陰イオン錯体、陽イオン錯体、非イオン錯体のいずれの状態で溶液中に存在していてもよい。
なお、本発明において、金コア粒子を、白金イオンを含む溶液に浸漬するとは、結果的に金コア粒子が白金イオン含有溶液に浸漬された状態になればよいことを意味する。すなわち、白金イオンをすでに含む溶液に金コア粒子を浸漬してもよく、金コア粒子を溶媒に浸漬させた後で白金錯体を加えてもよい。 Examples of the solution containing divalent platinum ions or tetravalent platinum ions used in the present invention include, for example, an aqueous solution of tetrachloroplatinum (II) acid, an aqueous solution of potassium tetrachloroplatinate (II), and diaminemineplatinum platinum (II). An aqueous solution, an aqueous solution of diamminedinitroplatinum (II), an aqueous solution of tetraammineplatinum (II) chloride (monohydrate), an aqueous solution of hexachloroplatinum (IV) acid, an aqueous solution of potassium hexachloroplatinum (IV), and the like. Further, these platinum ions may exist in the solution in any state of an anion complex, a cation complex, and a nonionic complex.
In the present invention, immersing the gold core particles in a solution containing platinum ions means that the gold core particles only need to be immersed in the platinum ion-containing solution as a result. That is, the gold core particles may be immersed in a solution that already contains platinum ions, or the platinum complex may be added after the gold core particles are immersed in a solvent.

前記溶液に含まれる白金イオンの量は、金コア粒子の量によって適宜調節すればよい。すなわち、金コア粒子の表面を白金単原子層のシェルで覆うことを目的とするため、これに足りる白金イオンが溶液中に含まれていればよい。具体的には、用いる金の表面積当たり、単原子層の白金原子が被覆する量の1倍〜1000倍程度の量の白金イオンが含まれるように溶液を調製することが好ましい。なお、溶液の濃度が薄すぎると、被覆に時間がかかるため、白金イオン濃度は0.05mM以上とすることが好ましい。一般的に、白金イオンを0.1mM〜100mM含む水溶液を用いればよい。   The amount of platinum ions contained in the solution may be appropriately adjusted according to the amount of gold core particles. That is, in order to cover the surface of the gold core particle with the shell of the platinum monoatomic layer, it is sufficient that sufficient platinum ions are included in the solution. Specifically, it is preferable to prepare the solution so that the amount of platinum ions is about 1 to 1000 times the amount covered by the platinum atoms of the monoatomic layer per surface area of the gold used. If the concentration of the solution is too thin, it takes time to coat, so the platinum ion concentration is preferably 0.05 mM or more. In general, an aqueous solution containing 0.1 mM to 100 mM of platinum ions may be used.

本発明において「還元剤の不存在下」とは、還元剤(水素や水素化ホウ素等)が意図的に溶液に加えられていないことを意味する。すなわち、還元作用を有する物質が溶液中に微量に存在していても、白金イオンが還元されない程度であれば「還元剤の不存在下」に含まれる。   In the present invention, “in the absence of a reducing agent” means that a reducing agent (such as hydrogen or borohydride) is not intentionally added to the solution. That is, even if a substance having a reducing action is present in a small amount in the solution, it is included in “in the absence of a reducing agent” as long as platinum ions are not reduced.

本発明にかかる方法では、金コア粒子上に白金原子を析出させることにより、金コア粒子の表面を白金単原子層で被覆することを目的とするが、金コア粒子の全表面を白金で被覆しなくても、十分な触媒活性が得られる。金コア粒子の表面積の60％以上、特に70%以上を白金原子で被覆することが好ましい。   The method according to the present invention is intended to coat the surface of the gold core particle with a platinum monoatomic layer by depositing platinum atoms on the gold core particle. Even if not, sufficient catalytic activity is obtained. It is preferable to cover 60% or more, particularly 70% or more, of the surface area of the gold core particles with platinum atoms.

本発明にかかる方法を行う際、溶液の温度は特に限定されず、室温でも反応を進行させることができる。   When performing the method concerning this invention, the temperature of a solution is not specifically limited, Reaction can be advanced also at room temperature.

本発明にかかる方法は、例えば以下のようにして実施することができる。まず、金微粒子(金コア粒子)を高分散担持させた担体を用意し、これを水中に加え、超音波処理を10〜60分程度行って担体を水中で分散させた後、金コア粒子を被覆するのに十分な量のPt錯体(二価または四価)を添加し、Ar雰囲気等の不活性雰囲気下で1〜24時間程度攪拌する。その後、遠心分離と超純水による洗浄を行って、得られた粉末(白金コアシェル触媒)を乾燥する。
このようにして得られた粉末は、担体上の金コア粒子の表面の60〜70％以上が、白金単原子層で被覆されており、高い触媒活性を有する。
なお、上述の金コア粒子の白金被覆率は、白金被覆前の金コア粒子(Au)と、白金被覆後の金コア粒子(Pt/Au)についてサイクリックボルタンメトリーを行い、得られたサイクリックボルタモグラムの金酸化物被膜の還元ピークから、以下の式を用いて求めることができる（測定の詳細は、実施例５参照）。
式(１) 被覆率(％)＝
{[(Auのピーク面積)−(Pt/Auのピーク面積)]/(Auのピーク面積)}×100 The method according to the present invention can be carried out, for example, as follows. First, a carrier on which gold fine particles (gold core particles) are highly dispersed and supported is prepared, added to water, and subjected to ultrasonic treatment for about 10 to 60 minutes to disperse the carrier in water. A sufficient amount of Pt complex (divalent or tetravalent) for coating is added and stirred for about 1 to 24 hours under an inert atmosphere such as an Ar atmosphere. Thereafter, centrifugation and washing with ultrapure water are performed, and the obtained powder (platinum core-shell catalyst) is dried.
In the powder thus obtained, 60 to 70% or more of the surface of the gold core particle on the support is coated with a platinum monoatomic layer, and has high catalytic activity.
The platinum coverage of the gold core particles described above is the cyclic voltammogram obtained by performing cyclic voltammetry on the gold core particles (Au) before platinum coating and the gold core particles (Pt / Au) after platinum coating. It can obtain | require from the reduction | restoration peak of a gold oxide film using the following formula | equation (for the details of a measurement, refer Example 5).
Formula (1) Coverage (%) =
{[(Au peak area) − (Pt / Au peak area)] / (Au peak area)} × 100

これまで行われてきたアンダーポテンシャル析出法では、金微粒子を高分散担持させたカーボンブラックを電極に担持させ、この電極を銅イオンを含む溶液中で電位をかけた状態で保持し、金微粒子上にまず銅の単原子層をアンダーポテンシャル析出させた後、塩化白金酸溶液に浸漬して銅原子を白金原子に置換する必要があった。このため、電極を作成し、電気化学的な処理を行う必要があり、また、粉末状態の触媒を直接製造することが難しかった。さらに、金コア粒子を直接白金の単原子層で覆うことができず、銅原子を白金原子と置換する必要があった(図１ａ参照)。
これに対し、本発明の方法では、上述したような簡単な設備と工程で、金コア粒子を直接単原子層の白金シェルで被覆することができ(図１ｂ参照)、且つ、粉末から製造できるので大量生産が可能である。白金と金では、金のほうがより貴な(イオン化傾向の小さな)金属であるため、当技術分野の常識からは、白金イオンの溶液に金コアを浸しても、金コアが白金原子で被覆されるとは予測できない。そのために、これまでアンダーポテンシャル析出法が使用されてきたのであり、本発明の方法における現象は、白金コアシェル触媒を改良するための研究過程において見い出された予想外の現象である。 In the conventional underpotential deposition method, carbon black in which gold fine particles are supported in a highly dispersed state is supported on an electrode, and this electrode is held in a state in which a potential is applied in a solution containing copper ions. First, it was necessary to deposit a copper monoatomic layer underpotentially and then immerse it in a chloroplatinic acid solution to replace the copper atoms with platinum atoms. For this reason, it is necessary to prepare an electrode and perform an electrochemical treatment, and it is difficult to directly produce a catalyst in a powder state. Furthermore, the gold core particles could not be directly covered with a platinum monoatomic layer, and copper atoms had to be replaced with platinum atoms (see FIG. 1a).
On the other hand, in the method of the present invention, the gold core particles can be directly coated with a monoatomic platinum shell (see FIG. 1b) and can be produced from powder with the simple equipment and steps described above. So mass production is possible. In platinum and gold, gold is a noble metal (less ionization tendency), so from the common knowledge in the art, even if a gold core is immersed in a solution of platinum ions, the gold core is covered with platinum atoms. It cannot be predicted. Therefore, the underpotential deposition method has been used so far, and the phenomenon in the method of the present invention is an unexpected phenomenon found in the research process for improving the platinum core-shell catalyst.

以下、実施例を用いて本発明をより具体的に説明するが、本発明は実施例に限定されるものではない。   EXAMPLES Hereinafter, although this invention is demonstrated more concretely using an Example, this invention is not limited to an Example.

［実施例１］白金コアシェル触媒の製造
0.1gの金担持カーボン担体(Au/C、金平均粒径：5nm、カーボン担体：粒径50nm、比表面積800m2/gのケッチェンブラック、金担持密度28.6重量％)を、30分間超純水１リットル中で超音波分散させた後、アルゴンガスで十分に脱気し、1.0mMとなるようにテトラクロロ白金(II)酸カリウム(K2PtCl4)を添加した。これにアルゴンガスを流通させながら30℃で24時間攪拌し、Pt/Au/C懸濁液を得た(図２ａ参照)。この懸濁液を遠心分離と超純水による洗浄(×３回)を行った後、常温で乾燥させてPt/Au/C粉末を製造した。 [Example 1] Production of platinum core-shell catalyst
0.1 g of gold-supported carbon support (Au / C, gold average particle diameter: 5 nm, carbon support: 50 nm particle diameter, ketjen black with a specific surface area of 800 m 2 / g, gold support density of 28.6% by weight), ultrapure for 30 minutes After ultrasonically dispersing in 1 liter of water, it was sufficiently deaerated with argon gas, and potassium tetrachloroplatinate (II) (K 2 PtCl 4 ) was added so as to be 1.0 mM. The mixture was stirred at 30 ° C. for 24 hours while flowing argon gas, to obtain a Pt / Au / C suspension (see FIG. 2a). This suspension was centrifuged and washed with ultrapure water (× 3 times), and then dried at room temperature to produce Pt / Au / C powder.

得られたPt/Au/C粉末を、直径6mmのグラッシーカーボンディスクより構成される回転ディスク電極(幾何面積：0.283cm2)上に14.1μg(Au)cm-2となるように均一に分散担持した。さらにバインダーとして5％ Nafion(登録商標)溶液(Aldrich社製)を膜厚0.1mmとなるようキャストして、乾燥して性能評価用電極を作製した。 The Pt / Au / C powder obtained, rotating disk electrode composed of glassy carbon disk with a diameter of 6 mm (geometrical area: 0.283cm 2) uniformly dispersed and supported so as to 14.1μg (Au) cm -2 on did. Further, a 5% Nafion (registered trademark) solution (manufactured by Aldrich) as a binder was cast to a film thickness of 0.1 mm and dried to prepare a performance evaluation electrode.

［実施例２］白金コアシェル触媒の製造
テトラクロロ白金(II)酸カリウム(K2PtCl4)の濃度を0.1mMとした以外は、実施例１と同様の方法でPt/Au/C粉末を製造した。
得られたPt/Au/C粉末を、実施例１と同様の方法で回転ディスク電極に均一に分散担持して、性能評価用電極を作製した。 [Example 2] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 1 except that the concentration of potassium tetrachloroplatinate (II) (K 2 PtCl 4 ) was 0.1 mM. did.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

［実施例３］白金コアシェル触媒の製造
反応温度を60℃とした以外は、実施例１と同様の方法でPt/Au/C粉末を製造した。
得られたPt/Au/C粉末を、実施例１と同様の方法で回転ディスク電極に均一に分散担持して、性能評価用電極を作製した。 [Example 3] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 1 except that the reaction temperature was 60 ° C.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

［実施例４］白金コアシェル触媒の製造
用いる白金源をヘキサクロロ白金(IV)酸カリウム(K2PtCl6)とした以外は、実施例１と同様の方法でPt/Au/C粉末を製造した。
得られたPt/Au/C粉末を、実施例１と同様の方法で回転ディスク電極に均一に分散担持して、性能評価用電極を作製した。 [Example 4] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 1 except that the platinum source used was potassium hexachloroplatinate (IV) (K 2 PtCl 6 ).
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

［比較例１］
市販の白金担持カーボン触媒(Pt/C、白金平均粒径：2.8nm、カーボン担体：粒径50nmのケッチェンブラック、白金担持密度：50重量％)を、直径6mmのグラッシーカーボンディスクより構成される回転ディスク電極(幾何面積：0.283cm2)上に14.1μg(Pt)cm-2となるように均一に分散担持した。さらにバインダーとして5％Nafion(登録商標)溶液(Aldrich社製)を膜厚0.1μmとなるようキャストして、乾燥して性能評価用電極を作製した。 [Comparative Example 1]
A commercially available platinum-supported carbon catalyst (Pt / C, platinum average particle size: 2.8 nm, carbon support: Ketjen black with a particle size of 50 nm, platinum support density: 50% by weight) is composed of a glassy carbon disk with a diameter of 6 mm. rotating disk electrode (geometric area: 0.283cm 2) was uniformly dispersed and supported so as to 14.1μg (Pt) cm -2 on. Further, a 5% Nafion (registered trademark) solution (manufactured by Aldrich) as a binder was cast so as to have a film thickness of 0.1 μm, and dried to produce a performance evaluation electrode.

［比較例２］
金担持カーボン担体(Au/C、金平均粒径：5nm、カーボン担体：粒径50nm、比表面積800m2/gのケッチェンブラック、金担持密度28.6重量％)を、直径6mmのグラッシーカーボンディスクより構成される回転ディスク電極(幾何面積：0.283cm2)上に14.1μg(Au)cm-2となるように均一に分散担持した。この電極をアルゴンガスで脱気した2mM硫酸銅を含む0.5M硫酸水溶液中で、可逆水素電極(RHE)に対して0.3Vで10分間保持することにより、金コア粒子上に銅の単原子層をアンダーポテンシャル析出させた。この電極を素早く水洗し、あらかじめアルゴンガスで脱気した5mMテトラクロロ白金(II)酸カリウム水溶液に10分間浸漬することで、銅原子を白金原子に置換して、白金モノレイヤーコアシェル触媒(Pt/Au/C)を得た。さらにバインダーとして5％ Nafion(登録商標)溶液(Aldrich社製)を膜厚0.1mmとなるようキャストして、乾燥して性能評価用電極を作製した。 [Comparative Example 2]
Gold-supported carbon support (Au / C, gold average particle diameter: 5 nm, carbon support: particle diameter 50 nm, specific surface area 800 m 2 / g ketjen black, gold support density 28.6% by weight) from a glassy carbon disk with a diameter of 6 mm composed rotating disk electrode (geometric area: 0.283cm 2) was uniformly dispersed and supported so as to 14.1μg (Au) cm -2 on. This electrode was degassed with argon gas in 0.5M sulfuric acid aqueous solution containing 2mM copper sulfate, held at 0.3V for 10 minutes against a reversible hydrogen electrode (RHE), thereby forming a copper monoatomic layer on the gold core particle. Underpotential deposition. This electrode was quickly washed with water and immersed in a 5 mM aqueous potassium tetrachloroplatinate (II) solution that had been degassed with argon gas for 10 minutes to replace the copper atoms with platinum atoms, resulting in a platinum monolayer core-shell catalyst (Pt / Au / C) was obtained. Further, a 5% Nafion (registered trademark) solution (manufactured by Aldrich) as a binder was cast to a film thickness of 0.1 mm and dried to prepare a performance evaluation electrode.

［実施例５］触媒の性能評価
実施例１〜４および比較例１，２で得た電極に関して、以下のように電気化学的評価を行い、比較例１，２で得た電極と比較して酸素還元活性評価を行った。本実施例で使用した回転リングディスク電極の構造を図２(ｂ)に示す。ディスク部分に触媒を担持させ、作用電極を回転させることにより電解液中に一定の対流を発生させて物質移動を制御した。
各々の電極に対して、アルゴンガスで飽和した25℃の0.1M過塩素酸中で、可逆水素電極(RHE)に対して0.05-1.2Vの電位範囲で50mVs-1の走査速度でサイクリックボルタンメトリーを行った。
図３(ａ)に実施例１および比較例２(UPD法)で作成したPt/Au/C触媒のサイクリックボルタモグラムを示す。横軸は電位、縦軸は電流値を表す。図３(ｂ)は(ａ)に示したボルタモグラムのピークを説明する図である。実施例１および比較例２で作成したPt/Au/CはどちらもPt特有の水素吸脱着および白金酸化物被膜の生成・還元ピークを示した。実施例２〜４で作成したPt/Au/Cも同様のピークを示した。
得られたボルタモグラムの0.05-0.4Vに現れる水素脱着ピークの面積より、Pt/Au/C触媒(実施例１〜４・比較例２)およびPt/C触媒(比較例１)の電気化学的表面積(cm2)を算出した。 [Example 5] Performance evaluation of catalyst The electrodes obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were subjected to electrochemical evaluation as follows, and compared with the electrodes obtained in Comparative Examples 1 and 2. Oxygen reduction activity was evaluated. The structure of the rotating ring disk electrode used in this example is shown in FIG. The catalyst was supported on the disk portion and the working electrode was rotated to generate a constant convection in the electrolyte solution to control mass transfer.
For each electrode, cyclic voltammetry at a scan rate of 50 mVs -1 in a potential range of 0.05-1.2 V against a reversible hydrogen electrode (RHE) in 0.1 M perchloric acid at 25 ° C saturated with argon gas Went.
FIG. 3A shows a cyclic voltammogram of the Pt / Au / C catalyst prepared in Example 1 and Comparative Example 2 (UPD method). The horizontal axis represents the potential, and the vertical axis represents the current value. FIG. 3B is a diagram for explaining the peak of the voltammogram shown in FIG. Both Pt / Au / C prepared in Example 1 and Comparative Example 2 exhibited Pt-specific hydrogen adsorption / desorption and platinum oxide coating generation / reduction peaks. Pt / Au / C prepared in Examples 2 to 4 also showed the same peak.
From the area of the hydrogen desorption peak appearing at 0.05-0.4 V in the obtained voltammogram, the electrochemical surface areas of the Pt / Au / C catalyst (Examples 1 to 4 and Comparative Example 2) and the Pt / C catalyst (Comparative Example 1) (cm 2 ) was calculated.

また、比較例１以外については、白金で修飾していないAu/Cおよび修飾後のPt/Au/Cについて、RHEに対して0.05-1.7Vの電位で50mVs-1の走査速度でサイクリックボルタンメトリーを行った。
図４(ａ)に実施例１、比較例２(UPD法)で作成したPt/Au/C触媒、および、白金被覆処理を行わない金担持カーボン担体(Au/C)のサイクリックボルタモグラムを示す。横軸は電位、縦軸は電流値を表す。図４(ｂ)は(ａ)に示したボルタモグラムの還元ピーク部分の一部拡大図である。図４から分かるように、Ptによる被覆により、Au/Cでみられる1.2 V付近のAu酸化物皮膜の還元ピークが減少した。実施例２〜４で作成したPt/Au/Cでも同じように、Au酸化物皮膜の還元ピークの減少が見られた。
1.2V付近に現れる金酸化物の還元ピークを用いて白金の被覆率(％)を下記の式(１)を用いて算出した。
式(１) 被覆率(％)＝
{[(Au/Cのピーク面積)−(Pt/Au/Cのピーク面積)]/(Au/Cのピーク面積)}×100 In addition, except for Comparative Example 1, with respect to Au / C not modified with platinum and Pt / Au / C after modification, cyclic voltammetry at a scanning speed of 50 mVs −1 at a potential of 0.05 to 1.7 V with respect to RHE. Went.
FIG. 4 (a) shows cyclic voltammograms of the Pt / Au / C catalyst prepared in Example 1 and Comparative Example 2 (UPD method), and the gold-supported carbon support (Au / C) not subjected to the platinum coating treatment. . The horizontal axis represents the potential, and the vertical axis represents the current value. FIG. 4B is a partially enlarged view of the reduction peak portion of the voltammogram shown in FIG. As can be seen from FIG. 4, the reduction peak of the Au oxide film near 1.2 V seen with Au / C was reduced by the coating with Pt. Similarly, in the Pt / Au / C prepared in Examples 2 to 4, a reduction in the reduction peak of the Au oxide film was observed.
The platinum coverage (%) was calculated using the following formula (1) using the reduction peak of gold oxide appearing near 1.2 V.
Formula (1) Coverage (%) =
{[(Au / C peak area) − (Pt / Au / C peak area)] / (Au / C peak area)} × 100

次いで、酸素で飽和した25℃の0.1M過塩素酸中で、回転リングディスク電極を1600rpm(回転／分)の速度で回転させ、10mVs-1の走査速度でRHEに対して0.1Vから1.0Vまで流れる酸素還元電流を測定した。図５に、実施例１(1.0mM)・実施例２(0.1mM)および比較例２(UPD法)で作成したPt/Au/C触媒、および金担持カーボン担体(Au/C)の対流ボルタモグラムを示す。横軸は電位、縦軸は電流値を表す。図５に示すように、本発明の方法で製造したコアシェル触媒は、1.0V付近から酸素還元電流が立ち上がり、UPD法で作製したコアシェル触媒と同様の酸素還元反応の挙動を示した。また、実施例３〜４で作成したPt/Au/Cでも同様の挙動が見られた。
0.9Vにおける電流Iより、0.4Vにおける限界電流値ILを用いて、下記の式(２)に従い0.9Vにおける活性支配電流IKを算出した。
式(２) IK＝(I×IL)/(IL−I) The rotating ring disk electrode is then rotated at a rate of 1600 rpm (rev / min) in 0.1 M perchloric acid saturated with oxygen at 25 ° C. and 0.1 V to 1.0 V relative to RHE at a scan rate of 10 mVs −1. The oxygen reduction current flowing up to was measured. FIG. 5 shows convective voltammograms of the Pt / Au / C catalyst prepared in Example 1 (1.0 mM), Example 2 (0.1 mM) and Comparative Example 2 (UPD method), and a gold-supported carbon support (Au / C). Indicates. The horizontal axis represents the potential, and the vertical axis represents the current value. As shown in FIG. 5, the core-shell catalyst produced by the method of the present invention exhibited an oxygen reduction reaction behavior similar to that of the core-shell catalyst produced by the UPD method, with the oxygen reduction current rising from around 1.0 V. Moreover, the same behavior was seen also in Pt / Au / C created in Examples 3-4.
Than the current I in 0.9V, using a limiting current I L in 0.4V, it was calculated activity dominant current I K in 0.9V according the following equation (2).
Formula (2) I K = (I × I L ) / (I L −I)

得られたIK値を白金の電気化学表面積で除した表面積比活性(μA cm-2)、および触媒中の白金の質量で除した質量活性(A mg(Pt)-1)を酸素還元活性の指標として求めた。各触媒中の白金の質量は、触媒粉末を王水(濃塩酸3mLと濃硝酸1mLの混合物)に溶解させ、誘導結合プラズマ発光分析法(ICP)により分析することで見積もった。実施例１〜４および比較例１、２の電極に対して得られた白金の電気化学的表面積、被覆率、表面積比活性、質量活性を表１に示す。 Surface area specific activity (μA cm -2 ) obtained by dividing the obtained IK value by the electrochemical surface area of platinum, and mass activity (A mg (Pt) -1 ) divided by the mass of platinum in the catalyst for oxygen reduction activity It was calculated as an index. The mass of platinum in each catalyst was estimated by dissolving the catalyst powder in aqua regia (a mixture of 3 mL of concentrated hydrochloric acid and 1 mL of concentrated nitric acid) and analyzing by inductively coupled plasma emission spectrometry (ICP). Table 1 shows the electrochemical surface area, coverage, surface area specific activity, and mass activity of platinum obtained for the electrodes of Examples 1 to 4 and Comparative Examples 1 and 2.

表１に示した結果より、本発明の方法に従って作製した白金コアシェル触媒(Pt/Au/C：実施例１〜４)は、市販の白金カーボン担持触媒(Pt/C：比較例１)と比較して、いずれも高い表面積比活性および質量活性を有していることが分かる。また、従来のUPD法を用いて作製した白金コアシェル触媒(Pt/Au/C-UPD法：比較例２)と比較した場合にも、白金被覆率は低くなるものの、表面積比活性および質量活性は高くなることが確認された。このため、本発明の方法で製造した白金コアシェル触媒は、酸素還元反応において、UPD法で製造した白金コアシェル触媒と比べて遜色のない触媒活性を発揮することが分かった。   From the results shown in Table 1, the platinum core-shell catalyst (Pt / Au / C: Examples 1 to 4) prepared according to the method of the present invention was compared with a commercially available platinum carbon supported catalyst (Pt / C: Comparative Example 1). Thus, both have high surface area specific activity and mass activity. In addition, when compared with the platinum core-shell catalyst prepared using the conventional UPD method (Pt / Au / C-UPD method: Comparative Example 2), the platinum coverage is low, but the surface area specific activity and mass activity are It was confirmed that it would be higher. For this reason, it was found that the platinum core-shell catalyst produced by the method of the present invention exhibits a catalytic activity comparable to that of the platinum core-shell catalyst produced by the UPD method in the oxygen reduction reaction.

また、実施例１と実施例２の結果から、白金イオンの濃度を変えても、被覆率や触媒活性に大きな変化は見られないことが分かった。したがって、溶液中の白金イオン量については、金コア粒子の表面を白金単原子層で覆うために十分な白金イオンが溶液中に存在していれば、濃度は特に制限されないと考えられる。   Further, from the results of Example 1 and Example 2, it was found that even if the concentration of platinum ions was changed, no significant change was observed in the coverage ratio or the catalytic activity. Therefore, regarding the amount of platinum ions in the solution, the concentration is not particularly limited as long as sufficient platinum ions are present in the solution to cover the surfaces of the gold core particles with the platinum monoatomic layer.

また、実施例１と実施例３の結果から、溶液の温度を高めても、被覆率や触媒活性に大きな変化は見られないことが分かった。したがって、本発明の方法では、温度を操作する必要はなく、室温で実施しても優れた触媒活性を有する白金コアシェル触媒を製造できることが分かった。なお、実施例１、実施例２および実施例４では、製造条件を完全に同じにするために温度を30℃に保って実験を行ったが、室温で実施しても高活性の白金コアシェル触媒が製造できることが確認されている。   In addition, from the results of Example 1 and Example 3, it was found that even when the temperature of the solution was increased, no significant change was observed in the coverage and the catalytic activity. Therefore, in the method of the present invention, it was found that a platinum core-shell catalyst having excellent catalytic activity could be produced even if it was carried out at room temperature without the need to manipulate the temperature. In Example 1, Example 2 and Example 4, the experiment was carried out while keeping the temperature at 30 ° C. in order to make the production conditions completely the same. It has been confirmed that can be manufactured.

また、実施例１と実施例４の結果から、浸漬時間を同一とした場合、白金錯体の価数を上げると被覆率が減少することが分かった。したがって、２価の白金イオンより４価の白金イオンのほうが白金の析出速度が遅い可能性が考えられる。なお、上述したように、２価でも４価でも、サイクリックボルタモグラムでは白金特有のピークが見られ、同様の酸素還元電流の挙動を示した。   Moreover, from the results of Example 1 and Example 4, it was found that when the immersion time was the same, the coverage decreased when the valence of the platinum complex was increased. Therefore, there is a possibility that the deposition rate of platinum is slower for tetravalent platinum ions than for divalent platinum ions. In addition, as described above, the cyclic voltammogram showed a peak peculiar to platinum regardless of whether it was divalent or tetravalent, and showed the same behavior of oxygen reduction current.

［実施例６］上澄み液のUVスペクトルの測定
実施例１と同じ手順に基づいて、金担持カーボン担体を、テトラクロロ白金(II)酸カリウム(K2PtCl4)の水溶液中で24時間攪拌した後放置し、上澄み液を採取し、ＵＶスペクトルを測定した。このＵＶスペクトルを、調製直後のテトラクロロ白金(II)酸カリウムの水溶液(金担持カーボン担体なし)のＵＶスペクトルと比較した。
また、ヘキサクロロ白金(IV)酸カリウム(K2PtCl6)水溶液についても同じ実験を行った。測定結果を図６に示す。(ａ)はテトラクロロ白金(II)酸カリウム水溶液、(ｂ)はヘキサクロロ白金(IV)酸カリウムの水溶液のＵＶスペクトルである。
図に示すように、215 nm付近にみられるPt-Clの電荷移動遷移に起因するピークおよび、300 nm付近の０価Ptに起因するピークは、触媒作製の有無にかかわらずほぼ同一であった。したがって、溶液中の白金錯体の一部が選択的に金コア粒子上で還元されて白金コアシェル触媒をなし、残りの白金錯体は還元されずにそのまま上澄み液に残っていると考えられる。 [Example 6] Measurement of UV spectrum of supernatant liquid Based on the same procedure as in Example 1, the gold-supported carbon support was stirred in an aqueous solution of potassium tetrachloroplatinate (II) (K 2 PtCl 4 ) for 24 hours. After standing, the supernatant was collected and the UV spectrum was measured. This UV spectrum was compared with the UV spectrum of an aqueous solution of potassium tetrachloroplatinate (II) immediately after preparation (no gold-supported carbon support).
The same experiment was also conducted on an aqueous solution of potassium hexachloroplatinate (IV) (K 2 PtCl 6 ). The measurement results are shown in FIG. (a) is a UV spectrum of an aqueous solution of potassium tetrachloroplatinate (II), and (b) is an aqueous solution of potassium hexachloroplatinate (IV).
As shown in the figure, the peak due to the Pt-Cl charge transfer transition near 215 nm and the peak due to zero-valent Pt near 300 nm were almost the same regardless of whether or not the catalyst was prepared. . Therefore, it is considered that a part of the platinum complex in the solution is selectively reduced on the gold core particle to form a platinum core-shell catalyst, and the remaining platinum complex is not reduced but remains in the supernatant.

これに対し、還元剤の存在下で金コア粒子を白金含有溶液に浸漬した場合、白金は、金コア粒子上だけでなく溶液中でも析出した。また、還元剤の存在下で作製したPt/Au/C触媒のTEM画像を観察したところ、白金イオンが高濃度になるにつれ、粒子が巨大化することが分かった。これは、金コア粒子が単層ではなく、何層にも重なった白金原子層で覆われたためと考えられる。   In contrast, when gold core particles were immersed in a platinum-containing solution in the presence of a reducing agent, platinum precipitated not only on the gold core particles but also in the solution. Observation of a TEM image of the Pt / Au / C catalyst prepared in the presence of a reducing agent revealed that the particles became larger as the concentration of platinum ions increased. This is presumably because the gold core particles were not covered by a single layer but were covered with a platinum atomic layer that overlapped many layers.

［白金の析出状態の考察］
本発明の方法において、もし白金が、金担持カーボン担体のカーボン上に析出したとすると、反応に寄与しない白金が増加するため、質量活性が小さくなり、また、実施例１〜４の対流ボルタモグラムの挙動が、比較例２(UPD法でモノレイヤー被覆した白金コアシェル触媒)の対流ボルタモグラムと一致しないはずである。しかし、本実施例により製造した触媒は、UPD法により製造した触媒と、質量活性及び対流ボルタモグラムの挙動が一致するため、UPD法で製造した触媒と同様に、金コア粒子が白金単原子層で被覆されている触媒が製造されたと考えられる。
また、サイクリックボルタモグラムで観察されたように、本発明の方法によって製造された触媒は、無処理の金担持担体(Au/C)と比較して、金酸化物被膜の還元ピークが減少しており、さらに水素吸脱着ピークおよび白金酸化物皮膜の生成および還元ピークが表れている。金酸化物被膜還元ピーク減少は、白金により金表面が被覆されたことに基づき、水素吸脱着ピークおよび白金酸化物皮膜の生成および還元ピークは、金表面上に析出した白金に基づくと考えられるため、サイクリックボルタモグラムからも金が白金で被覆されたことが分かる。さらに、白金イオンの濃度を減らして実験を行ったところ、白金イオンの濃度の減少、すなわち、白金量の減少に伴って、水素吸脱着ピークおよび白金酸化物皮膜の生成および還元ピークが減少し、それに伴って金酸化物被膜の還元ピークが増加することが観察された。このことからも、金表面上に白金が析出したことが分かる。
これらのことから、本発明にかかる方法で製造される触媒は、UPD法で製造した触媒と同様、金粒子が白金で被覆されたコアシェル構造を有すると考えられる。 [Consideration of platinum deposition]
In the method of the present invention, if platinum is deposited on the carbon of the gold-supported carbon support, platinum that does not contribute to the reaction increases, so that the mass activity decreases, and the convective voltammograms of Examples 1 to 4 The behavior should not be consistent with the convective voltammogram of Comparative Example 2 (a platinum core-shell catalyst monolayer coated by the UPD method). However, since the catalyst produced by this example has the same mass activity and convective voltammogram behavior as the catalyst produced by the UPD method, the gold core particle is a platinum monolayer as in the catalyst produced by the UPD method. It is believed that a coated catalyst was produced.
Further, as observed in the cyclic voltammogram, the catalyst produced by the method of the present invention has a reduced reduction peak of the gold oxide film as compared with the untreated gold-supported support (Au / C). Furthermore, a hydrogen adsorption / desorption peak and a production and reduction peak of a platinum oxide film appear. The reduction in the reduction peak of the gold oxide film is based on the fact that the gold surface was coated with platinum, and the hydrogen adsorption / desorption peak and the formation and reduction peak of the platinum oxide film are considered to be based on platinum deposited on the gold surface. From the cyclic voltammogram, it can be seen that gold was coated with platinum. Furthermore, when the experiment was carried out by reducing the concentration of platinum ions, the decrease in platinum ion concentration, that is, the decrease in the amount of platinum, the hydrogen adsorption / desorption peak and the formation and reduction peak of the platinum oxide film decreased, Along with this, it was observed that the reduction peak of the gold oxide film increased. This also shows that platinum was deposited on the gold surface.
From these, it is considered that the catalyst produced by the method according to the present invention has a core-shell structure in which gold particles are coated with platinum, like the catalyst produced by the UPD method.

なお、触媒作製の過程で遠心分離と超純水による洗浄を行わず、Pt/Au/C触媒が沈むまで放置した後、上澄み液を除去し、乾燥してPt/Au/C触媒を作製した場合、サイクリックボルタモグラムにおいて、水素吸脱着ピークおよび白金酸化皮膜形成還元ピークに変化が見られた。これは、白金イオン含有溶液を洗浄除去しなかったため、生成した触媒上に白金錯体が残り、電位サイクルによって白金錯体が還元されて白金が析出したため、白金がモノレイヤーではなく、何層かに重なって析出したと考えられる。白金系触媒の質量活性向上のためには白金を単原子で析出することが望ましい。したがって、白金イオン含有溶液中で白金コアシェル触媒を製造した後は、遠心分離と洗浄などの方法により、未反応の白金錯体を触媒から除去した後、触媒を乾燥することが好ましいことが分かった。なお、白金錯体の除去には必ずしも遠心分離を用いる必要はなく、通常のろ過と洗浄を十分に繰り返すことによっても可能である。   In the process of catalyst preparation, centrifugation and washing with ultrapure water were not performed, and the Pt / Au / C catalyst was allowed to settle, then the supernatant was removed and dried to produce a Pt / Au / C catalyst. In the cyclic voltammogram, changes were observed in the hydrogen adsorption / desorption peak and the platinum oxide film-forming reduction peak. This was because the platinum ion-containing solution was not washed and removed, so the platinum complex remained on the generated catalyst, and the platinum complex was reduced by the potential cycle, and platinum was deposited. It is thought that it precipitated. In order to improve the mass activity of the platinum-based catalyst, it is desirable to deposit platinum as a single atom. Therefore, after manufacturing a platinum core-shell catalyst in a platinum ion containing solution, it turned out that it is preferable to dry a catalyst, after removing an unreacted platinum complex from a catalyst by methods, such as centrifugation and washing | cleaning. In addition, it is not always necessary to use centrifugation for the removal of the platinum complex, and it is possible to sufficiently repeat normal filtration and washing.

［実施例７］白金コアシェル触媒の製造
実施例１で用いたものとは製造ロットの異なる金担持カーボン担体(Au/C、金平均粒径：5nm、カーボン担体：粒径50nm、比表面積800m2/gのケッチェンブラック、金担持密度：29.5重量％)を用いて、実施例１と同様の方法でPt/Au/C粉末を製造した。
得られたPt/Au/C粉末を、実施例１と同様の方法で回転ディスク電極に均一に分散担持して、性能評価用電極を作製した。 [Example 7] Production of platinum core-shell catalyst A gold-supported carbon support (Au / C, gold average particle size: 5 nm, carbon support: particle size 50 nm, specific surface area 800 m 2) in a different production lot from that used in Example 1 Pt / Au / C powder was produced in the same manner as in Example 1, using Ketjen black / g, gold loading density: 29.5 wt%.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

［実施例８］透過型電子顕微鏡観察および粒度分布、平均粒径の算出
実施例７で用いたAu/C担体および実施例７で得られたPt/Au/C粉末を透過型電子顕微鏡(TEM)を用いて観察し、得られたTEM画像中の500個の貴金属微粒子の直径を測定し、粒度分布を得た。また、粒度分布より貴金属微粒子の平均粒径を算出した。
得られたTEM画像を図７に示す。(ａ)はAu/C担体の画像を、(ｂ)はPt/Au/C粉末の画像を示す。Au/C担体およびPt/Au/C粉末上の貴金属微粒子はいずれもカーボン担体上に分散良く担持されていることがわかる。TEM画像から得た貴金属微粒子の粒度分布を図８に示す。(ａ)はAu/C担体、(ｂ)はPt/Au/C粉末の粒度分布を示す。白金原子被覆後は、粒径分布は全体的に大粒径側にシフトしているが分布状態には大きな変化がなく、特に小粒径の粒子が増加している現象も見られないため、白金がカーボン上に析出することなく、金微粒子上に析出することにより全体的に粒径が大きくなったことがわかる。粒径分布より算出した平均粒径はAu/C担体上の金粒子が4.4nmであり、またPt/Au/C粉末上の貴金属粒子は5.1nmであった。この平均粒径の差0.7nmは白金の原子直径0.27nmの2倍に相当する0.54nmに近く、金コア粒子上に白金が単原子層で析出していることを示す。 [Example 8] Observation with a transmission electron microscope and calculation of particle size distribution and average particle diameter The Au / C carrier used in Example 7 and the Pt / Au / C powder obtained in Example 7 were measured with a transmission electron microscope (TEM). ) And the diameters of 500 noble metal fine particles in the obtained TEM image were measured to obtain a particle size distribution. Further, the average particle size of the noble metal fine particles was calculated from the particle size distribution.
The obtained TEM image is shown in FIG. (a) shows an image of Au / C support, and (b) shows an image of Pt / Au / C powder. It can be seen that the noble metal fine particles on the Au / C support and the Pt / Au / C powder are both supported on the carbon support with good dispersion. FIG. 8 shows the particle size distribution of the noble metal fine particles obtained from the TEM image. (a) shows the particle size distribution of the Au / C support, and (b) shows the particle size distribution of the Pt / Au / C powder. After the platinum atom coating, the particle size distribution is shifted to the large particle size as a whole, but there is no significant change in the distribution state, especially the phenomenon that the small particle size is increasing, It can be seen that the overall particle size was increased by depositing on the gold fine particles without depositing platinum on the carbon. The average particle size calculated from the particle size distribution was 4.4 nm for gold particles on the Au / C support and 5.1 nm for noble metal particles on the Pt / Au / C powder. This average particle size difference of 0.7 nm is close to 0.54 nm, which is twice the atomic diameter of platinum of 0.27 nm, indicating that platinum is deposited in a monoatomic layer on the gold core particles.

［実施例９］白金コアシェル触媒の製造
反応時間を１時間とした以外は、実施例７と同様の方法でPt/Au/C粉末を製造した。
得られたPt/Au/C粉末を、実施例１と同様の方法で回転ディスク電極に均一に分散担持して、性能評価用電極を作製した。 [Example 9] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 7 except that the reaction time was 1 hour.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

［実施例１０］白金コアシェル触媒の製造
反応時間を３時間とした以外は、実施例７と同様の方法でPt/Au/C粉末を製造した。
得られたPt/Au/C粉末を、実施例１と同様の方法で回転ディスク電極に均一に分散担持して、性能評価用電極を作製した。 [Example 10] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 7 except that the reaction time was 3 hours.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

［実施例１１］白金コアシェル触媒の製造
反応時間を６時間とした以外は、実施例７と同様の方法でPt/Au/C粉末を製造した。
得られたPt/Au/C粉末を、実施例１と同様の方法で回転ディスク電極に均一に分散担持して、性能評価用電極を作製した。 [Example 11] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 7 except that the reaction time was 6 hours.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

［実施例１２］白金コアシェル触媒の製造
反応時間を４８時間とした以外は、実施例７と同様の方法でPt/Au/C粉末を製造した。
得られたPt/Au/C粉末を、実施例１と同様の方法で回転ディスク電極に均一に分散担持して、性能評価用電極を作製した。 [Example 12] Production of platinum core-shell catalyst Pt / Au / C powder was produced in the same manner as in Example 7 except that the reaction time was 48 hours.
The obtained Pt / Au / C powder was uniformly dispersed and supported on the rotating disk electrode in the same manner as in Example 1 to produce a performance evaluation electrode.

［実施例１３］白金の被覆率の測定
実施例７および９〜１２で得た電極に関して、実施例５に記載したものと同様の方法で白金の被覆率(％)を求めた。 [Example 13] Measurement of platinum coverage The platinum coverage (%) of the electrodes obtained in Examples 7 and 9-12 was determined in the same manner as described in Example 5.

［実施例１４］白金、金含有量および白金／金原子比の測定
実施例７および９〜１２で得たPt/Au/C粉末を少量とり、王水(塩酸と硝酸を体積比３：１で混合した液体)に溶解させた後、誘導結合高周波プラズマ発光(ICP)分析法を用いて溶液中の白金と金の濃度を求めた。得られた結果よりPt/Au/C粉末の貴金属中の白金および金の含有量(重量％)および白金／金原子比を算出した。 [Example 14] Measurement of platinum, gold content and platinum / gold atomic ratio A small amount of the Pt / Au / C powder obtained in Examples 7 and 9 to 12 was taken, and aqua regia (hydrochloric acid and nitric acid in a volume ratio of 3: 1). The concentration of platinum and gold in the solution was determined using inductively coupled high-frequency plasma emission (ICP) analysis. From the obtained results, the platinum and gold contents (% by weight) and the platinum / gold atomic ratio in the noble metal of the Pt / Au / C powder were calculated.

［比較例３］単原子層モデル粒子
粒径4.4nmの球状の金コア粒子上に単原子層のPt層(厚さ：0.27nm、白金原子直径に相当)が析出したと仮定し、金の密度(19.3g/cm3)および白金の密度(21.5g/cm3)を用いて、単原子層の白金で覆われた金微粒子の白金および金の含有量(重量％)および白金／金原子比の理論値を求めた。 [Comparative Example 3] Monoatomic layer model particle It is assumed that a monoatomic Pt layer (thickness: 0.27 nm, equivalent to platinum atom diameter) is deposited on a spherical gold core particle having a particle size of 4.4 nm. Using the density (19.3 g / cm 3 ) and the density of platinum (21.5 g / cm 3 ), the platinum and gold content (wt%) of the fine gold particles covered with monoatomic platinum and the platinum / gold atom The theoretical value of the ratio was obtained.

実施例７、９〜１２で得られた電極の白金の被覆率、ならびに実施例７，９〜１２で得られたPt/Au/C触媒および比較例３で得られた白金単原子層モデル粒子の白金および金の含有量、白金／金原子比を表２に示す。   The platinum coverage of the electrodes obtained in Examples 7 and 9-12, and the Pt / Au / C catalyst obtained in Examples 7 and 9-12 and the platinum monoatomic layer model particles obtained in Comparative Example 3 Table 2 shows the platinum and gold content and platinum / gold atomic ratio.

表２に示した結果より、白金被覆率および白金含有量は６時間までは反応時間と共に増加するが、それ以降反応をつづけても白金被覆率および白金含有量は増加せず、一定となる。また、白金／金原子比で比較しても同様に反応６時間までは増加傾向にあるが、それ以降はほぼ40%程度で一定となる。この40%という値は、比較例３で求めた白金単原子層で被覆された粒径4.4nmの金コア粒子に対して得られた理論的な白金／金原子比40.6%に極めて近い値であることがわかる。すなわち本発明の方法に従って作製した白金コアシェル触媒では白金被覆量を反応時間で制御することが可能であり、かつ白金原子層が単原子層を越えて被覆しないことを示す。   From the results shown in Table 2, although the platinum coverage and the platinum content increase with the reaction time up to 6 hours, the platinum coverage and the platinum content do not increase even if the reaction is continued thereafter, and become constant. Further, even when compared in terms of platinum / gold atomic ratio, it tends to increase up to 6 hours of reaction, but after that, it becomes constant at about 40%. This value of 40% is very close to the theoretical platinum / gold atomic ratio of 40.6% obtained for the gold core particle having a particle diameter of 4.4 nm coated with the platinum monoatomic layer obtained in Comparative Example 3. I know that there is. That is, in the platinum core-shell catalyst produced according to the method of the present invention, the platinum coating amount can be controlled by the reaction time, and the platinum atomic layer does not cover the monoatomic layer.

以上の実施例から、本発明の方法によれば、UPD法を用いる方法に比べて、非常に簡単な工程で高活性の白金コアシェル触媒を製造することができること、また、還元剤を使用する方法と比べて、効率的に白金コアシェル触媒を製造することができ、白金量の低減化に資することが実証された。   From the above examples, according to the method of the present invention, it is possible to produce a highly active platinum core-shell catalyst by a very simple process compared to the method using the UPD method, and the method using a reducing agent. As compared with the above, it was proved that a platinum core-shell catalyst can be produced efficiently and contributes to a reduction in the amount of platinum.

Claims (7)

還元剤の不存在下で、金コア粒子を二価白金イオンあるいは四価白金イオンを含む溶液に浸漬することにより、前記金コア粒子上に白金を直接析出させることを特徴とする、白金コアシェル触媒の製造方法。 A platinum core-shell catalyst characterized in that platinum is directly deposited on the gold core particles by immersing the gold core particles in a solution containing divalent platinum ions or tetravalent platinum ions in the absence of a reducing agent. Manufacturing method. 前記析出した白金により、前記金コア粒子上に白金単原子層が形成され、金コア粒子表面の６０％以上が当該層で被覆されることを特徴とする、白金コアシェル触媒の製造方法。 A method for producing a platinum core-shell catalyst, wherein a platinum monoatomic layer is formed on the gold core particle by the deposited platinum, and 60% or more of the gold core particle surface is covered with the layer. 前記金コア粒子が担体の表面に担持されていることを特徴とする、請求項１または２に記載の白金コアシェル触媒の製造方法。 The method for producing a platinum core-shell catalyst according to claim 1 or 2, wherein the gold core particles are supported on the surface of a support. 前記金コア粒子が粒径1nm〜30nmの金微粒子であり、前記担体がカーボンブラックであることを特徴とする、請求項３に記載の白金コアシェル触媒の製造方法。 The method for producing a platinum core-shell catalyst according to claim 3, wherein the gold core particles are gold fine particles having a particle diameter of 1 nm to 30 nm, and the carrier is carbon black. 前記溶液が、前記白金イオンを0.1mM〜100mM含む水溶液であることを特徴とする、請求項１〜４のいずれか１項に記載の白金コアシェル触媒の製造方法。 The method for producing a platinum core-shell catalyst according to any one of claims 1 to 4, wherein the solution is an aqueous solution containing 0.1 to 100 mM of the platinum ions. ａ）金コア粒子を担持させた担体を水中に添加し、超音波分散させる工程、
ｂ）前記水中に、二価あるいは四価の白金錯体を添加し、不活性雰囲気下で攪拌する工程
を含むことを特徴とする、請求項３〜５のいずれか１項に記載の白金コアシェル触媒の製造方法。 a) a step of adding a carrier carrying gold core particles to water and ultrasonically dispersing the carrier;
The platinum core-shell catalyst according to any one of claims 3 to 5, comprising a step of b) adding a divalent or tetravalent platinum complex to the water and stirring in an inert atmosphere. Manufacturing method. 請求項１〜６のいずれか１項に記載の方法によって製造された白金コアシェル触媒を酸素還元反応の触媒として利用する燃料電池。 A fuel cell using the platinum core-shell catalyst produced by the method according to claim 1 as a catalyst for an oxygen reduction reaction.
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