JP2015089958A - Mesoporous metal nanoparticle and method for producing the same as well as catalyst comprising mesoporous metal nanoparticle - Google Patents
Mesoporous metal nanoparticle and method for producing the same as well as catalyst comprising mesoporous metal nanoparticle Download PDFInfo
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Abstract
Description
本発明は大きな表面積を有するとともに、高い触媒活性を有するメソポーラス金属ナノ粒子及びその製造方法に関する。本発明はまた、このメソポーラス金属ナノ粒子を含む触媒にも関する。 The present invention relates to mesoporous metal nanoparticles having a large surface area and high catalytic activity, and a method for producing the same. The present invention also relates to a catalyst comprising the mesoporous metal nanoparticles.
近年、新規な物理的また化学的な特性を発揮する、形とサイズが制御された金属コロイドナノ粒子の合成に向けて多大な努力がなされてきた（非特許文献１、２）。従来の合成方法は、予備形成された金属シード、及び／または各種の化学添加剤を使用するものであった。これらほとんど場合において、何段階かのステップや高圧高温の処理条件などの複雑な処理が必要とされてきた（非特許文献３〜６）。特に、二金属ナノ粒子はそれを構成する第二金属の相乗効果により、単一金属の場合に比べてしばしば卓越した触媒活性を示している（非特許文献７〜１１）。しかしながら、コア−シェル構造などの異種二金属ナノ粒子は、単一金属ナノ粒子と比べると製造が困難である。その原因は、複数の金属塩の還元速度が互いに異なること、及び金属種と相互作用する構造規定剤（structural directing agent）の特性の違いにより引き起こされる複雑な合成条件によるものである。今日に至るまで、金属ナノ粒子上に制御された条件で浅い凹面を形成する方法がいくつか開発された（非特許文献１２〜１４）。しかし、均一なサイズのメソ細孔を有し，大きな表面積（対体積比）を持つ金属ナノ粒子を簡単に合成することは、依然として困難な問題である。 In recent years, great efforts have been made to synthesize metal colloidal nanoparticles with controlled shapes and sizes that exhibit new physical and chemical properties (Non-Patent Documents 1 and 2). Conventional synthesis methods have used preformed metal seeds and / or various chemical additives. In most of these cases, complicated processes such as several steps and high-pressure and high-temperature processing conditions have been required (Non-Patent Documents 3 to 6). In particular, bimetallic nanoparticles often exhibit superior catalytic activity compared to the case of a single metal due to the synergistic effect of the second metal constituting the bimetallic nanoparticles (Non-Patent Documents 7 to 11). However, dissimilar bimetallic nanoparticles such as a core-shell structure are difficult to manufacture compared to single metal nanoparticles. The cause is due to the complicated synthesis conditions caused by the different reduction rates of the plurality of metal salts and the difference in the characteristics of the structural directing agent that interacts with the metal species. To date, several methods for forming shallow concave surfaces under controlled conditions on metal nanoparticles have been developed (Non-Patent Documents 12 to 14). However, it is still a difficult problem to easily synthesize metal nanoparticles having uniformly sized mesopores and having a large surface area (vs. volume ratio).
現在、工業的な、また技術的な多様な用途があるため、メソポーラスシリカナノ粒子の合成の合成に多大な関心が向けられてきている。これらのメソポーラスナノ粒子の性質には粒子サイズが多くの影響を与え得る。粒子サイズをナノメートルレンジ（１００ｎｍ未満）まで小さくすると、特定の条件下において、興味深い性質を有する持続的なコロイド懸濁液を得ることができる。メソポーラスナノ粒子は、吸着、触媒、薬物送達及びエネルギー貯蔵分野の膨大な用途に大いに有用である（非特許文献１５〜１９）。しかしながら、これまでに報告されたメソポーラスナノ粒子の組成は金属酸化物（例えばシリカ，チタニア）（非特許文献２０〜２５）及びカーボン（非特許文献２６〜３０）に限定されている。このような制限は、メソポーラスナノ粒子の可能な用途範囲を著しく狭めている。多様な組成を有するメソポーラスナノ粒子が実現できれば、広範な機能を実現することができるようになる。とりわけ、全金属からなるメソポーラスナノ粒子は広い範囲の電気化学用途に大いに有用である。
従来，全金属メソポーラス材料を作成するには２つの主な方法、すなわちリオトロピック液晶法（lyotropic liquid crystalline approach）（非特許文献３１〜３４）及び転写法（又は，ハードテンプレート法）（非特許文献３５〜３９）がある。しかしながら、これらの方法は単分散メソポーラス金属ナノ粒子の合成のための一般的な方法に拡張することはできない。
At present, there is a great deal of interest in the synthesis of the synthesis of mesoporous silica nanoparticles due to a variety of industrial and technical applications. The particle size can have many effects on the properties of these mesoporous nanoparticles. Reducing the particle size to the nanometer range (less than 100 nm) can provide a sustained colloidal suspension with interesting properties under certain conditions. Mesoporous nanoparticles are very useful for enormous applications in the fields of adsorption, catalyst, drug delivery and energy storage (Non-Patent Documents 15 to 19). However, the composition of the mesoporous nanoparticles reported so far is limited to metal oxides (for example, silica and titania) (Non-Patent Documents 20 to 25) and carbon (Non-Patent Documents 26 to 30). Such limitations significantly narrow the possible application range of mesoporous nanoparticles. If mesoporous nanoparticles having various compositions can be realized, a wide range of functions can be realized. In particular, mesoporous nanoparticles consisting of all metals are very useful for a wide range of electrochemical applications.
Conventionally, there are two main methods for producing an all-metal mesoporous material: a lyotropic liquid crystalline approach (Non-patent Documents 31 to 34) and a transfer method (or a hard template method) (Non-patent Document 35). ~ 39). However, these methods cannot be extended to general methods for the synthesis of monodisperse mesoporous metal nanoparticles.
本発明の課題は、触媒反応とりわけ電気酸化反応（electro-oxidation reaction）のための高触媒活性サイトを提供できる高指数の面を有するメソポーラス金属ナノ粒子の合成を与えることである。 It is an object of the present invention to provide the synthesis of mesoporous metal nanoparticles having a high index surface that can provide highly catalytically active sites for catalytic reactions, especially electro-oxidation reactions.
本発明の一側面によれば、外径が３０ｎｍ〜２００ｎｍであり、少なくとも表面に凹面が形成された、メソポーラス金属ナノ粒子が与えられる。
ここで、前記凹面の径が２ｎｍ〜５０ｎｍであってよい。
また、前記金属はめっき可能な金属または半金属であってよい。
また、前記めっき可能な金属はＰｔ、Ｐｄ、Ａｕ、Ｎｉ、Ｃｕ、Ａｇ、Ｓｎからなる群から選択される一または複数の金属であってよい。
また、前記メソポーラス金属ナノ粒子は、コアと前記コアの周囲のシェルとを有し、前記コアは第１の金属を前記第１の金属と異なる第２の金属よりも多く含み、前記シェルは前記第２の金属を前記第１の金属よりも多く含んでよい。
また、前記第１の金属はＰｄであり、前記第２の金属はＰｔであってよい。
本発明の他の側面によれば、臨界ミセル濃度以上の界面活性剤と、金属イオンを含む金属塩と、前記金属イオンを還元する還元剤とを含む溶液中で前記金属イオンを還元する、メソポーラス金属ナノ粒子の製造方法が与えられる。
ここで、前記溶液中における前記還元の速度を低下させてよい。
また、前記還元の速度の低下は前記溶液に前記還元の速度を低下させる成分を添加することによってよい。
また、前記還元の速度を低下させる成分は酸であってよい。
あるいは、前記還元速度の低下は、前記溶液を冷却することによってよい。
また、前記還元剤はアスコルビン酸、水素化ホウ素ナトリウム、ジメチルアミンボラン、トリメチルアミンボラン及び蟻酸からなる群から選択される少なくとも一であってよい。
また、前記金属イオンはめっき可能な金属または半金属のイオンであってよい。
また、前記めっき可能な金属のイオンはＰｔ、Ｐｄ，Ａｕ、Ｎｉ、Ｃｕ、Ａｇ、Ｓｎからなる群から選択される一または複数の金属のイオンであってよい。
また、前記金属イオンは前記還元の速度が互いに異なる複数の金属イオンであってよい。
また、前記界面活性剤はPluronic F127、水素化ホウ素ナトリウム、ジメチルアミンボラン、トリメチルアミンボラン及び蟻酸からなる群から選択された少なくとも一であってよい。
また、前記還元は超音波を照射しながら行ってよい。
本発明の更に他の側面によれば、上記何れかのメソポーラス金属ナノ粒子を含む触媒が与えられる。
ここで、前記触媒は電気化学触媒またはメタノール酸化触媒として用いられてよい。
According to one aspect of the present invention, mesoporous metal nanoparticles having an outer diameter of 30 nm to 200 nm and having at least a concave surface formed are provided.
Here, the diameter of the concave surface may be 2 nm to 50 nm.
The metal may be a metal or metalloid that can be plated.
The metal that can be plated may be one or more metals selected from the group consisting of Pt, Pd, Au, Ni, Cu, Ag, and Sn.
The mesoporous metal nanoparticles have a core and a shell around the core, and the core contains more first metal than a second metal different from the first metal, The second metal may be included more than the first metal.
The first metal may be Pd, and the second metal may be Pt.
According to another aspect of the present invention, a mesoporous material that reduces a metal ion in a solution containing a surfactant having a critical micelle concentration or more, a metal salt containing a metal ion, and a reducing agent that reduces the metal ion. A method for producing metal nanoparticles is provided.
Here, the reduction rate in the solution may be reduced.
The reduction rate may be reduced by adding a component that reduces the reduction rate to the solution.
The component that reduces the reduction rate may be an acid.
Alternatively, the reduction rate may be decreased by cooling the solution.
The reducing agent may be at least one selected from the group consisting of ascorbic acid, sodium borohydride, dimethylamine borane, trimethylamine borane, and formic acid.
The metal ions may be metal ions or metalloid ions that can be plated.
The metal ions that can be plated may be one or more metal ions selected from the group consisting of Pt, Pd, Au, Ni, Cu, Ag, and Sn.
The metal ions may be a plurality of metal ions having different reduction rates.
The surfactant may be at least one selected from the group consisting of Pluronic F127, sodium borohydride, dimethylamine borane, trimethylamine borane, and formic acid.
The reduction may be performed while irradiating ultrasonic waves.
According to still another aspect of the present invention, there is provided a catalyst containing any one of the above mesoporous metal nanoparticles.
Here, the catalyst may be used as an electrochemical catalyst or a methanol oxidation catalyst.
本発明によれば、非常に簡単なプロセスで、新規な構造のメソポーラス金属ナノ粒子を高い収率で作製することができる。また、本発明で与えられるメソポーラス金属ナノ粒子は、大きな比表面積を有するとともに、比表面積あたりで評価しても非常に高い触媒活性を有する。 According to the present invention, mesoporous metal nanoparticles having a novel structure can be produced in a high yield by a very simple process. In addition, the mesoporous metal nanoparticles provided in the present invention have a large specific surface area and have a very high catalytic activity even when evaluated per specific surface area.
熱力学的に一般に不利な凹面形態を自発的に形成させることが困難なことである。従来の溶液相合成では、金属ナノ粒子は｛１１１｝面や｛１００｝面などの低指数表面で観察されるファセット結晶形態を示す傾向がある（非特許文献４０）。これに対して，本発明の方法では、界面活性剤が自己形成により球形ミセルとなり、それがメソポーラス金属ナノ粒子のテンプレート（鋳型）として機能することを利用する。本発明の新規な方法では、液相中で行われる非常に簡単な手順により、新規でかつこれまでにない強い触媒活性を有するメソポーラス金属ナノ粒子を得ることができる。また、その収率も極めて高い（以下の実施例ではほぼ１００％）。 It is difficult to spontaneously form a concave surface form that is generally disadvantageous in thermodynamics. In conventional solution phase synthesis, metal nanoparticles tend to exhibit facet crystal morphology observed on low index surfaces such as {111} faces and {100} faces (Non-Patent Document 40). In contrast, the method of the present invention utilizes the fact that the surfactant becomes a spherical micelle by self-formation and functions as a template of the mesoporous metal nanoparticles. In the novel method of the present invention, mesoporous metal nanoparticles having novel and unprecedented strong catalytic activity can be obtained by a very simple procedure performed in a liquid phase. Also, the yield is extremely high (in the following examples, almost 100%).
このようにして製造されたメソポーラス金属ナノ粒子は、外径が３０ｎｍから２００ｎｍの範囲であり、またその表面や内部に２ｎｍ〜５０ｎｍ程度のサイズの凹面や空孔が形成されているという、これまでにない新規な構造を有する。このようなメソポーラス構造により、単に比表面積が大きくなるだけではなく、このような凹面や空所により高指数面や原子ステップが多数形成され、これらが触媒作用を発現する活性サイトとして機能することで、更に高い触媒活性を得ることができる。 The mesoporous metal nanoparticles thus produced have an outer diameter in the range of 30 nm to 200 nm, and a concave surface or a hole having a size of about 2 nm to 50 nm is formed on the surface or inside thereof. It has a new structure that is not present. Such a mesoporous structure not only increases the specific surface area, but also forms a large number of high index surfaces and atomic steps due to such concave surfaces and voids, which function as active sites that exhibit catalytic action. Higher catalytic activity can be obtained.
この形状をより具体的に説明すれば、例えば図２の（ａ）、（ｃ）及び（ｄ）に示すＴＥＭ像からわかるように、本発明のメソポーラス金属ナノ粒子は単純な外形ではなく、例えば球面などの粒子の表面に抉られたような凹面が形成されている。このような凹面は好ましくは複数形成され、互いにほぼ一様なサイズになっている。従って、粒子の表面上に形成されている互いに隣接した凹面には，多くの高指数面や原子ステップの形成がされている。更に、粒子内部にも、特にある程度大きく成長した粒子では、凹面の別形態として、粒子表面には一部しか開口していない空孔が形成される場合がある。このような場合でも、空孔の表面（凹面）などには多くの高指数面や原子ステップが形成される。 If this shape is described more specifically, for example, as can be seen from the TEM images shown in FIGS. 2 (a), (c) and (d), the mesoporous metal nanoparticles of the present invention do not have a simple outer shape, A concave surface is formed such that it is struck by the surface of a particle such as a spherical surface. A plurality of such concave surfaces are preferably formed and are substantially uniform in size. Therefore, many high index surfaces and atomic steps are formed on the concave surfaces adjacent to each other formed on the surface of the particle. Furthermore, in the case of particles that have grown to a certain extent inside the particles, there are cases where, as another form of concave surface, pores that are only partially opened are formed on the particle surface. Even in such a case, many high-index surfaces and atomic steps are formed on the surface (concave surface) of the holes.
なお、以下の実施例ではＰｄ及びＰｔを使用し、コア−シェル構造を有する二金属メソポーラス金属ナノ粒子を取り上げたが、本発明はこのような特定の実施例に限定されるものではなく、特許請求の範囲によってのみ規定されるものであることに注意する必要がある。例えば、金属種はＰｄ及びＰｔ以外でもよく、また使用する金属種の数は単一でも、あるいは３種類以上でもよい。また、使用する界面活性剤及、還元剤、酸等も実施例に記載したものに限定されず、その他、実施例に記載した反応条件も必要に応じて修正することができるなど、本発明の範囲内で多様な変更が可能である。 In the following examples, Pd and Pt were used and bimetallic mesoporous metal nanoparticles having a core-shell structure were taken up, but the present invention is not limited to such specific examples, It should be noted that it is defined only by the claims. For example, the metal species may be other than Pd and Pt, and the number of metal species to be used may be single or three or more. Further, the surfactant, reducing agent, acid and the like to be used are not limited to those described in the examples, and other reaction conditions described in the examples can be modified as necessary. Various changes are possible within the scope.
［実施例の概要］
以下で説明する本発明の実施例では、コア部分がＰｄに富んでおり、シェル部分がＰｔに富んでいるコア−シェル構造を有するメソポーラス金属ナノ粒子（以下、二金属メソポーラスＰｄ＠Ｐｔナノ粒子、あるいは単にメソポーラスＰｄ＠Ｐｔナノ粒子、Ｐｄ＠Ｐｔナノ粒子と称する）の合成を例に挙げて説明する。Ｐｄ＠Ｐｔナノ粒子中のメソポーラス構造により、大きな表面積及び豊富な高触媒活性サイトが与えられる。これを不規則形態のもの、つまり集塊状であって全体としては特定の形状を取らないもの、と比較すれば、メソ細孔により反応物質及び生成物の移送の促進に貢献することができ、これによってメソポーラスＰｄ＠Ｐｔナノ粒子の電気化学活性が大幅に増大する。
[Summary of Example]
In the embodiments of the present invention described below, mesoporous metal nanoparticles having a core-shell structure in which a core portion is rich in Pd and a shell portion is rich in Pt (hereinafter, bimetallic mesoporous Pd @ Pt nanoparticles, Alternatively, the synthesis of simply mesoporous Pd @ Pt nanoparticles and Pd @ Pt nanoparticles) will be described as an example. The mesoporous structure in Pd @ Pt nanoparticles provides a large surface area and abundant high catalytic activity sites. Compared to the irregular form, that is, agglomerated and not taking a specific shape as a whole, mesopores can contribute to the promotion of the transfer of reactants and products, This greatly increases the electrochemical activity of mesoporous Pd @ Pt nanoparticles.
メソポーラス金属ナノ粒子を作製するため、Ｐｔ源及びＰｄ源並びに塩酸を含む前駆体溶液中にPluronic F127を、超音波を使用して溶解した。還元剤としてアスコルビン酸（ＡＡ）を添加した後、この混合液を連続して超音波処理した。反応が進行するにつれて、溶液の色は図１に示すように、黒く変色した。得られたナノ粒子は沈殿することなく水中に充分に分散しており、高品質のコロイド溶液であることが示された。最終生成物を集めて、洗浄−遠心分離サイクルによりアセトン及び水で洗浄して、界面活性剤を除去した。 To make mesoporous metal nanoparticles, Pluronic F127 was dissolved using ultrasonic waves in a precursor solution containing a Pt source and a Pd source and hydrochloric acid. After ascorbic acid (AA) was added as a reducing agent, the mixture was sonicated continuously. As the reaction proceeded, the color of the solution turned black as shown in FIG. The resulting nanoparticles were well dispersed in water without precipitation, indicating a high quality colloidal solution. The final product was collected and washed with acetone and water in a wash-centrifugation cycle to remove the surfactant.
［メソポーラスＰｄ＠Ｐｔナノ粒子の作成］
１．８ｍｌのＫ２ＰｔＣｌ４（２０ｍＭ）、０．２ｍｌのＮａ２ＰｄＣｌ４（２０ｍＭ）溶液、及び４４μｌの塩酸（６Ｍ）を含む水溶液中にPluronic F127（２０ｍｇ）を超音波で溶解した。還元剤として１００ｍＭアスコルビン酸（ＡＡ）を２．０ｍｌ添加した後、この混合物を水浴中で４時間連続的に超音波処理した。この超音波処理の間、温度を３５℃に保持した。最終生成物を収集してアセトン及び水で洗浄する連続的洗浄／遠心分離サイクルを５回繰り返した後、室温で乾燥した。
[Creation of mesoporous Pd @ Pt nanoparticles]
Pluronic F127 (20 mg) was ultrasonically dissolved in an aqueous solution containing 1.8 ml of K 2 PtCl 4 (20 mM), 0.2 ml of Na 2 PdCl 4 (20 mM), and 44 μl of hydrochloric acid (6M). After adding 2.0 ml of 100 mM ascorbic acid (AA) as a reducing agent, the mixture was sonicated continuously for 4 hours in a water bath. The temperature was kept at 35 ° C. during this sonication. The final product was collected and washed 5 times with acetone and water and then repeated 5 times before drying at room temperature.
［測定用機器及び測定方法］
ＴＥＭによる観察は、エネルギー分散分光（ＥＤＳ）分析機能を備えたJEOL Model JEM-2100F TEMシステムによって行った。高角度ＸＲＤ回折パターンは、Rigaku Model Ring 2500X回折計をステップ走査プログラム（ステップ幅０．０５）を使用して動作させることで測定した。窒素吸脱着データは、７７ＫでBelsorp 28装置（日本ベル株式会社）を使用して得た。紫外線−可視光（ＵＶ−ｖｉｓ）吸収スペクトルは、紫外可視近赤外分光光度計（日本分光株式会社のＶ−５７０）によって測定した。
[Measurement equipment and measurement method]
Observation by TEM was performed with a JEOL Model JEM-2100F TEM system equipped with an energy dispersive spectroscopy (EDS) analysis function. The high angle XRD diffraction pattern was measured by operating a Rigaku Model Ring 2500X diffractometer using a step scanning program (step width 0.05). Nitrogen adsorption / desorption data was obtained using a Belsorp 28 apparatus (Bell Japan) at 77K. The ultraviolet-visible light (UV-vis) absorption spectrum was measured with an ultraviolet-visible near-infrared spectrophotometer (V-570, JASCO Corporation).
電気化学特性は、ＣＨＩ８４２Ｂ電気化学ワークステーション（米国のCH Instrument Co.）を使って、従来型のＡｇ／ＡｇＣｌ電極（参照電極）、白金ワイヤー（対電極）及び修飾グラッシーカーボン電極（ＧＣＥ）（作用電極）を設けた３電極セルにより調べた。ＧＣＥを３．０μｇという同じ量の金属ロードを有する試料でコーティングした。次に、このようにして準備された電極の表面に０．５ｗ％Ｎａｆｉｏｎ溶液（Ｎａｆｉｏｎはイー アイ デュポン ドゥ ヌムール アンド カンパニーの登録商標）を３．０μｌだけ滴下し、室温で乾燥した。メタノール電気酸化測定を、０．５Ｍ Ｈ２ＳＯ４を含む０．５Ｍメタノール溶液中で５０ｍＶ・ｓ−１の走引レートで行った。 Electrochemical properties were measured using a conventional Ag / AgCl electrode (reference electrode), platinum wire (counter electrode) and modified glassy carbon electrode (GCE) (action) using the CHI842B electrochemical workstation (CH Instrument Co., USA). This was investigated by a three-electrode cell provided with an electrode). GCE was coated with a sample having the same amount of metal load of 3.0 μg. Next, 0.5 μl of a 0.5 w% Nafion solution (Nafion is a registered trademark of EI DuPont de Nemours and Company) was dropped on the surface of the electrode thus prepared, and dried at room temperature. Methanol electrooxidation measurement was performed at a running rate of 50 mV · s −1 in a 0.5 M methanol solution containing 0.5 MH 2 SO 4 .
［評価及び検討］
図２はこのようにして得られた、典型的な条件下で合成したナノ粒子の走査電子顕微鏡（ＳＥＭ）像及び透過電子顕微鏡（ＴＥＭ）像である。このナノコロイドのサイズ分布は４５±５．０ｎｍであり、平均サイズは４５ｎｍであった。図２（ｃ）、（ｄ）に示すように、ナノ粒子表面に一様なサイズのメソ細孔が確認された。ここで、その外表面上のメソポアは半球状であり、これにより明確に規定された凹面が示されていることに注意されたい。メソポーラスＰｄ＠Ｐｔナノ粒子の低角Ｘ線回折（low-angle X-ray diffraction）は一つのブロードなピーク（図３（ａ））を示したが、これは一様なサイズの穴が互いに近接して充填されていることを意味している。また、図３（ｂ）に示すそのＮ２吸脱着等温線から、４０ｍ２・ｇ−１もの大きな表面積を持つことがわかる。
[Evaluation and examination]
FIG. 2 is a scanning electron microscope (SEM) image and a transmission electron microscope (TEM) image of the nanoparticles synthesized under typical conditions thus obtained. The size distribution of the nanocolloid was 45 ± 5.0 nm, and the average size was 45 nm. As shown in FIGS. 2 (c) and 2 (d), mesopores of uniform size were confirmed on the nanoparticle surface. Note that the mesopores on the outer surface are hemispherical, thereby indicating a well-defined concave surface. The low-angle X-ray diffraction of the mesoporous Pd @ Pt nanoparticles showed one broad peak (Fig. 3 (a)), which indicates that the holes of uniform size are close to each other. It means that it is filled. Further, from the N 2 adsorption / desorption isotherm shown in FIG. 3B, it can be seen that the surface area is as large as 40 m 2 · g −1 .
このナノ粒子を更に検討するため、図４（ａ）に示す高分解のＴＥＭにより、個々の粒子の特徴を調べた。コントラストの違いから、その穴と壁とを明確に識別できた。壁は細かなナノ粒子（２〜３ｎｍ）を組み上げたものでできていて、十分発達した枝分かれ構造が図４（ａ）、（ｂ）で観察される。従って、これは当該ナノ粒子における２種類の穴、すなわち小さな穴（壁内の高度に枝分かれした構造に由来）及び大きなメソ細孔、の存在を示している。図４（ｃ）の差し込み図に示す個々のナノ粒子の電子線回折（electron diffraction；ＥＤ）パターンは、Ｐｔ面心立方（ｆｃｃ）結晶に帰属できる，ほぼ単結晶の状態を確認した。観測された格子縞はナノ粒子全体にわたってコヒーレントに広がっていた。図４（ｃ）に示す、間隔が０．２３ｎｍの観測された縞模様は、Ｐｔのｆｃｃ結晶の（１１１）面間隔と一致した。ＰｔとＰｄの格子が非常によく一致している（９９．２３％）ため（非特許文献４１、４２）、ＰｔとＰｄとの間の明確な粒界は観測されなかった。図３（ｃ）に示されるように、広角ＸＲＤプロファイルも単相のｆｃｃ結晶構造の（１１１）、（２００）、（２２０）、（３１１）及び（２２２）回折に割り当てられた。図３（ｄ）に示す大倍率ＴＥＭ像に見られるように、これらのナノ粒子がコヒーレントな結晶格子状に接続されていることで、多数の段差を有する凹面形状が与えられる。 In order to further examine the nanoparticles, the characteristics of the individual particles were examined by a high-resolution TEM shown in FIG. The hole and the wall could be clearly identified from the difference in contrast. The walls are made of fine nanoparticles (2 to 3 nm), and a fully developed branch structure is observed in FIGS. 4 (a) and 4 (b). This therefore indicates the presence of two types of holes in the nanoparticles: small holes (derived from highly branched structures in the walls) and large mesopores. The electron diffraction (ED) pattern of each nanoparticle shown in the inset of FIG. 4 (c) confirmed a substantially single crystal state that can be attributed to a Pt face-centered cubic (fcc) crystal. Observed lattice fringes were coherently spread throughout the nanoparticle. The observed striped pattern with an interval of 0.23 nm shown in FIG. 4C coincided with the (111) plane interval of the Pt fcc crystal. Since the lattices of Pt and Pd agree very well (99.23%) (Non-Patent Documents 41 and 42), no clear grain boundary between Pt and Pd was observed. As shown in FIG. 3 (c), the wide angle XRD profile was also assigned to the (111), (200), (220), (311) and (222) diffractions of the single phase fcc crystal structure. As can be seen from the large-magnification TEM image shown in FIG. 3 (d), these nanoparticles are connected in a coherent crystal lattice shape to give a concave shape having a large number of steps.
図４（ｄ−１）及び（ｄ−２）に示すナノ粒子の元素マッピングデータ並びに図４（ｅ）に示す線走査（line-scanning）データは、Ｐｄが粒子中央に集まっている一方で、Ｐｔはナノ粒子の外縁部に集まっていることをはっきりと示している。ここに示されたＰｄ及びＰｔの元素分布により、Ｐｄ＠Ｐｔコア−シェル構造の構成を確認できた。ナノ粒子中のＰｄ成分は１５原子％と測定されたが、これは誘導結合プラズマ（inductively coupled plasma；ＩＣＰ）分析の測定結果と一致した。図５に示す低倍率のＴＥＭ像及びＳＥＭ像から、ナノコロイドはサイズと形状が顕著に一様であり副生成物が何も形成されなかったことを明確に見て取ることができた。このことは、Ｐｄ＠Ｐｔナノ粒子を高収率（１００％）で合成できたことを示している。 The elemental mapping data of the nanoparticles shown in FIGS. 4 (d-1) and (d-2) and the line-scanning data shown in FIG. 4 (e) show that while Pd is gathered at the center of the particle, Pt clearly shows gathering at the outer edge of the nanoparticles. From the element distribution of Pd and Pt shown here, the configuration of the Pd @ Pt core-shell structure could be confirmed. The Pd component in the nanoparticles was measured to be 15 atomic%, which was consistent with the measurement result of inductively coupled plasma (ICP) analysis. From the low-magnification TEM and SEM images shown in FIG. 5, it was possible to clearly see that the nanocolloid was remarkably uniform in size and shape and no by-products were formed. This indicates that Pd @ Pt nanoparticles could be synthesized with a high yield (100%).
Ｐｄ＠Ｐｔナノ粒子の形成機構を解明するため、図６に示すＴＥＭ像を使用して、その反応を、段階を追って精査した。これらの得られた像はナノ粒子の形成機構に対する洞察を与えるものである。ここにおける反応系では、Ｐｄ種の還元は短時間のうちに優先的に起こり、図６（ａ）に示すように、Ｐｄ成分を多く含む金属の種粒子を形成する。これと同様な特異な挙動については、以前Ｐｄ種とＰｔ種との共析出（co-deposition）について報告されたことがある（非特許文献４３、４４）。疎水性のＰＰＯ基と金属表面との相互作用により（非特許文献４５）最初にできた種粒子上に吸着したＦ１２７分子は、単分散ナノ粒子の安定化剤として働く。この界面活性剤の濃度は臨界ミセル濃度（ＣＭＣ）を超えているため、溶液中に溶解している界面活性剤は球形のミセルを形成する。溶液中に溶解している金属錯体は，このミセルのＰＥＯシェル領域に組み込まれる（非特許文献４６、４７）。Ｐｄ種粒子上へＰｔが更に析出してそれに続いて過剰成長が起こる間において、金属錯体が入ったＦ１２７ミセルは種粒子に近づいて構造規定剤として直接働き、メソ細孔を有する、形状が明確に規定された凹面形態を形成する。粒子サイズが大きくなった場合には、粒子内部に球状の空孔を形成する。 To elucidate the formation mechanism of Pd @ Pt nanoparticles, the reaction was examined step by step using the TEM image shown in FIG. These obtained images provide insight into the mechanism of nanoparticle formation. In the reaction system here, reduction of the Pd species occurs preferentially within a short time, and as shown in FIG. 6A, metal seed particles containing a large amount of the Pd component are formed. A unique behavior similar to this has been reported previously for co-deposition of Pd species and Pt species (Non-patent Documents 43 and 44). The F127 molecule adsorbed on the seed particles first formed by the interaction between the hydrophobic PPO group and the metal surface (Non-patent Document 45) serves as a stabilizer for monodisperse nanoparticles. Since the concentration of this surfactant exceeds the critical micelle concentration (CMC), the surfactant dissolved in the solution forms spherical micelles. The metal complex dissolved in the solution is incorporated into the PEO shell region of the micelle (Non-patent Documents 46 and 47). While Pt is further deposited on the Pd seed particles and subsequent overgrowth occurs, the F127 micelle containing the metal complex approaches the seed particles and acts directly as a structure-directing agent, has mesopores, and has a clear shape The concave shape defined in (1) is formed. When the particle size increases, spherical pores are formed inside the particle.
なお、ここで「凹面」とはナノ粒子の表面に完全に露出した構造だけではなく、あるいはナノ粒子の内部に包含されている球面であるが、ナノ粒子外部に連通する経路を有する、「ほぼ閉じているが狭い経路で外部に接続されている、球面に近い凹面」まで含む概念であることに注意されたい。実際、界面活性剤のミセルが種粒子上につみあがることでメソポーラスナノ粒子が成長する過程において、メソポーラスナノ粒子のサイズが小さい間はメソポア、つまり凹面は粒子表面に大きく開口しているが、サイズが大きくなるにつれて初期段階に形成された凹面は次第に粒子内部に入りこむことで、外部へは狭い経路で接続されるだけの、ほぼ閉じられた球面状の内表面となる傾向がある。上にも述べたように、この凹面は界面活性剤により形成され、集積した球面状のミセルを鋳型とするものであるので、隣接した凹面には高指数面や原子ステップが形成されやすい。また、凹面はナノ粒子の外部へ向かって開口しているだけではなく、隣接した凹面やナノ粒子の反対側の表面に存在する凹面、あるいは大きく成長したナノ粒子ではナノ粒子内部の空孔へも連通している。 Here, the “concave surface” is not only a structure that is completely exposed on the surface of the nanoparticle, or a spherical surface that is included inside the nanoparticle, but has a path that communicates with the outside of the nanoparticle, It should be noted that the concept includes a concave surface close to a spherical surface that is closed but connected to the outside through a narrow path. In fact, in the process of growth of mesoporous nanoparticles by surfactant micelles growing on seed particles, while the size of mesoporous nanoparticles is small, the mesopore, that is, the concave surface is greatly open to the particle surface. The concave surface formed in the initial stage gradually enters the inside of the particle as the value of the diameter increases, and tends to become a substantially closed spherical inner surface that is connected to the outside through a narrow path. As described above, since this concave surface is formed of a surfactant and uses an accumulated spherical micelle as a template, a high index surface or an atomic step is likely to be formed on the adjacent concave surface. In addition, the concave surface is not only open to the outside of the nanoparticle, but also the concave surface that exists on the adjacent concave surface or the opposite surface of the nanoparticle, or in the case of a large grown nanoparticle, the void inside the nanoparticle Communicate.
図６に示したところの時間軸に沿ったＴＥＭ観察結果から明確にわかるように、個々の粒子は徐々に成長した。３時間後、Ｐｔシェルの成長は終結し、ナノ粒子のサイズと形態はそれ以上変化しなかった。上述したように、図４（ｃ）に示す１個の粒子から得た電子線回折（ＥＤ）は、単一のｆｃｃ結晶に完全に割り当てられる。種粒子からの連続的なエピタキシャル成長が起こったことの重要な証拠である。 As can be clearly seen from the TEM observation results along the time axis as shown in FIG. 6, the individual grains gradually grew. After 3 hours, the growth of the Pt shell was terminated and the size and morphology of the nanoparticles remained unchanged. As described above, the electron diffraction (ED) obtained from one particle shown in FIG. 4C is completely assigned to a single fcc crystal. This is important evidence that continuous epitaxial growth from seed grains has occurred.
最終生成物に対する界面活性剤の濃度の影響も調べた（図７）。前駆体溶液中の界面活性剤濃度が減少する（０．０３ｗ％まで）と、図７（ａ）、（ｂ）に示すように、多孔質粒子と非多孔質粒子とが混在しているのが観察された。大部分の粒子が集塊状になっていたが、これは界面活性剤（これは安定化剤として働く）の量が粒子の凝集を防止するのに十分でなかったためである。極端に希釈された界面活性剤濃度（ＣＭＣ未満）、あるいは界面活性剤を全く添加せずに前駆体溶液を作製した時には、図７（ｃ）、（ｄ）に示すように、表面上のメソポーラス構造は完全に消失した。従って、本願発明者の観察したところでは、十分に分散されたメソポーラスナノ粒子を充分に分散させるに当たってＦ１２７の濃度は重要なパラメーターである。
この手法では、Ｐｄに富んでいるコアのサイズは前駆体溶液中のＰｔとＰｄとのモル比を変化させることによって容易に調節できた。図８（ａ）、（ｂ）に、ＰｔとＰｄとのモル比を９．０（典型的な条件）から４．０へ変更したときのＰｄ＠Ｐｔナノ粒子のＴＥＭ像を示す。図８（ｃ−１）、（ｃ−２）、（ｄ）の元素分析によって示されるように、当該モル比が４．０の場合に得られた、Ｐｄに富んでいるコアのサイズはほぼ４７ｎｍであったが、このサイズは典型的な試料の約１．８倍大きかった。
The effect of surfactant concentration on the final product was also examined (FIG. 7). When the surfactant concentration in the precursor solution decreases (up to 0.03 w%), as shown in FIGS. 7A and 7B, porous particles and non-porous particles are mixed. Was observed. Most of the particles were agglomerated because the amount of surfactant (which acts as a stabilizer) was not sufficient to prevent particle agglomeration. When the precursor solution was prepared without adding an extremely diluted surfactant concentration (below CMC) or without adding any surfactant, as shown in FIGS. 7 (c) and 7 (d), the mesoporous material on the surface was obtained. The structure has completely disappeared. Therefore, as observed by the inventors of the present application, the concentration of F127 is an important parameter in fully dispersing mesoporous nanoparticles that are sufficiently dispersed.
In this approach, the size of the Pd-rich core could be easily adjusted by changing the molar ratio of Pt to Pd in the precursor solution. 8A and 8B show TEM images of Pd @ Pt nanoparticles when the molar ratio of Pt to Pd is changed from 9.0 (typical conditions) to 4.0. As shown by the elemental analysis of FIGS. 8 (c-1), (c-2), and (d), the size of the core rich in Pd obtained when the molar ratio is 4.0 is almost equal to Although it was 47 nm, this size was about 1.8 times larger than a typical sample.
この反応系では、還元剤としてアスコルビン酸を使用する。アスコルビン酸は以下の平衡式に従って酸化される：
Ｃ６Ｈ８Ｏ６ ←→ Ｃ６Ｈ６Ｏ６＋２Ｈ＋＋２ｅ−
従って、アスコルビン酸の還元力は、前駆体溶液のｐＨ（酸性度）を精密に制御することによって調節できる。合成プロセスを、可視光−紫外線吸収分光により図９（ａ）に示すように反応中のいくつかの時点でモニターした。前駆体溶液の相対濃度を、３９０ｎｍを中心としたＰｔ錯体の特性吸収ピークから評価した。ｐＨが高いときには相対濃度は５分間ですぐに減少したが、これは金属イオンが急速に還元されたことを表している。反応時間を更に長くすると、Ｐｔ濃度は徐々に減少した。１時間反応させたところ、Ｐｔイオンが完全に消費された。これとは対照的に、ｐＨを低くした場合には還元速度は非常に遅かった。その相対濃度は４時間の間徐々に低下した。つまり、低ｐＨでの反応時間（４時間）は高ｐＨでの反応時間（１時間）よりも非常に長かった。これら２つのｐＨにおける反応速度を、図９（ｂ−１）、（ｂ−２）に示すように、反応溶液の変化からも視覚的にモニターした。ｐＨが高い場合（図９（ｂ−１））には反応溶液の色は５分間で初期状態の黄色からナノ粒子が形成されたことを示す黒へと、５分以内と言う極短時間のうちに変化した。一方、低ｐＨの場合（図９（ｂ−２））は、溶液の色は６０分かけて徐々に黒に変わった。上記結果から、金属イオンの還元速度は前駆体溶液に酸を添加することで劇的に低下することが明らかになった。
In this reaction system, ascorbic acid is used as a reducing agent. Ascorbic acid is oxidized according to the following equilibrium equation:
C 6 H 8 O 6 ← → C 6 H 6 O 6 + 2H + + 2e −
Therefore, the reducing power of ascorbic acid can be adjusted by precisely controlling the pH (acidity) of the precursor solution. The synthesis process was monitored at several points during the reaction by visible light-ultraviolet absorption spectroscopy as shown in FIG. 9 (a). The relative concentration of the precursor solution was evaluated from the characteristic absorption peak of the Pt complex centered on 390 nm. When the pH was high, the relative concentration immediately decreased in 5 minutes, indicating that the metal ions were rapidly reduced. As the reaction time was further increased, the Pt concentration gradually decreased. When reacted for 1 hour, Pt ions were completely consumed. In contrast, the reduction rate was very slow when the pH was lowered. The relative concentration gradually decreased during 4 hours. That is, the reaction time at low pH (4 hours) was much longer than the reaction time at high pH (1 hour). The reaction rates at these two pHs were also visually monitored from changes in the reaction solution, as shown in FIGS. 9 (b-1) and (b-2). When the pH is high (FIG. 9 (b-1)), the color of the reaction solution is changed from yellow in the initial state to black indicating that nanoparticles are formed in 5 minutes, and within 5 minutes. It changed. On the other hand, in the case of low pH (FIG. 9 (b-2)), the color of the solution gradually changed to black over 60 minutes. From the above results, it has been clarified that the reduction rate of metal ions is drastically decreased by adding acid to the precursor solution.
図１０（ａ）は、高ｐＨ状態での前駆体溶液により作製された生成物のＳＥＭ像を示す。この生成物は、一様なサイズのメソポアが全くない、高度に分岐した（つまり樹枝状の）構造を有していた。これは、図１０（ｂ）に示すところのｐＨが低い状態の系で作製して得られたナノ粒子とは対照的である。低ｐＨでは金属の成長速度が低いため、界面活性剤ミセルが金属の種粒子上に付着することによって最初の種粒子が成長する際のテンプレートとして効果的に働くための十分な時間が与えられる。この低速成長機構は、高ｐＨにおける高速成長条件下での樹枝状のＰｔベースナノ構造の形成とは完全に別物である。ｐＨが高い場合には金属イオンの還元速度が相対的に大きいことにより、即座の核形成が短時間で生起し高度に分岐した構造で構成される樹枝状粒子が形成される。この機構は樹枝状Ｐｔナノ構造の合成のために本願発明者が最近開発したものである（非特許文献４８〜５０）。従って、前駆体溶液のこのようなｐＨ制御は、一様なサイズのメソポアの形成にとって重要である。
なお、上で説明した実施例では前駆体溶液のｐＨを低下させることで還元速度を低下させることにより、金属の析出速度を低下させたが、これ以外の手段でももちろんこの目的を達成することができる。たとえば、適切な析出速度となるまで溶液の温度を低下させても良い。また、還元速度の低下を実現する複数の手段を併用しても良い。更には、金属イオン、還元剤、その他の反応条件の組み合わせによっては、特に還元速度を低下させる処置をとらなくても本発明のメソポーラス金属ナノ粒子を作成できる場合がありえる。
また、還元剤としては、アスコルビン酸以外に水素化ホウ素ナトリウム、ジメチルアミンボラン、トリメチルアミンボラン、蟻酸等も使用することができる。
FIG. 10 (a) shows an SEM image of the product produced by the precursor solution in a high pH state. The product had a highly branched (ie, dendritic) structure with no uniform sized mesopores. This is in contrast to the nanoparticles obtained from the low pH system shown in FIG. 10 (b). Since the growth rate of the metal is low at low pH, the surfactant micelles are deposited on the metal seed particles to provide sufficient time to effectively serve as a template for the initial seed particle growth. This slow growth mechanism is completely separate from the formation of dendritic Pt-based nanostructures under fast growth conditions at high pH. When the pH is high, the reduction rate of the metal ions is relatively high, so that immediate nucleation occurs in a short time and dendritic particles composed of highly branched structures are formed. This mechanism was recently developed by the present inventor for the synthesis of dendritic Pt nanostructures (Non-Patent Documents 48 to 50). Thus, such pH control of the precursor solution is important for the formation of uniformly sized mesopores.
In the examples described above, the metal deposition rate was reduced by lowering the reduction rate by lowering the pH of the precursor solution. Of course, other means can achieve this purpose. it can. For example, the temperature of the solution may be lowered until an appropriate deposition rate is reached. A plurality of means for realizing a reduction in the reduction rate may be used in combination. Furthermore, depending on the combination of the metal ion, the reducing agent, and other reaction conditions, the mesoporous metal nanoparticles of the present invention may be produced without particularly taking a measure for reducing the reduction rate.
In addition to ascorbic acid, sodium borohydride, dimethylamine borane, trimethylamine borane, formic acid and the like can be used as the reducing agent.
メタノール酸化のためのメソポーラスＰｄ＠Ｐｔナノ粒子の電気触媒活性を予備的に実証するため、図１１に示す０．５Ｍ Ｈ２ＳＯ４＋０．５Ｍ ＣＨ３ＯＨ電解液中でのサイクリックボルタモグラムを使用してその電気触媒活性を調べた。比較を行うため、商業的に入手可能な白金黒（ＰｔＢ）及び樹枝状Ｐｔナノ粒子（図１２に示す）（非特許文献５１）についても調べた。測定された全ての電流値を、ロードされたＰｔの質量で正規化した。図１１（ａ）に示すように、メソポーラスＰｔ＠Ｐｄナノ粒子は最も大きな電流密度を示した（３９５ｍＡ・ｍｇ−１）。この値は樹枝状Ｐｔナノ粒子の場合（２３４ｍＡ・ｍｇ−１）の１．７倍大きく、またＰｔＢの場合（９５ｍＡ・ｍｇ−１）の４．２倍大きいものであった。図１１（ｂ）に示すように、メソポーラスＰｔ＠Ｐｄナノ粒子の電流密度は、Ｐｔ−Ｒｕナノ粒子（非特許文献５２）やＡｕ＠Ｐｔナノ粒子（非特許文献５３）などのような、これまでに報告された他の樹枝状Ｐｔナノ粒子のものよりも格段に大きかった。 To preliminary demonstrate the electrocatalytic activity of mesoporous Pd @ Pt nanoparticles for methanol oxidation, a cyclic voltammogram in 0.5 MH 2 SO 4 +0.5 M CH 3 OH electrolyte shown in FIG. 11 was used. Then, the electrocatalytic activity was examined. For comparison purposes, commercially available platinum black (PtB) and dendritic Pt nanoparticles (shown in FIG. 12) (Non-Patent Document 51) were also examined. All measured current values were normalized by the mass of loaded Pt. As shown in FIG. 11 (a), the mesoporous Pt @ Pd nanoparticles showed the highest current density (395 mA · mg −1 ). This value is 1.7 times that of the dendritic Pt nanoparticles (234mA · mg -1) increases, also were those 4.2 times greater in the case of PtB (95mA · mg -1). As shown in FIG. 11B, the current density of mesoporous Pt @ Pd nanoparticles is such as Pt-Ru nanoparticles (Non-patent Document 52) and Au @ Pt nanoparticles (Non-patent Document 53). It was much larger than that of other dendritic Pt nanoparticles reported so far.
Ｉｆ／Ｉｂ値（順方向電流密度と逆方向電流密度との比の値として定義）を、メタノール酸化反応（ＭＯＲ）におけるＰｔ触媒のＣＯ耐性の指標として用いることができる（非特許文献５４、５５）。メソポーラスＰｄ＠Ｐｔナノ粒子は優れた被毒耐性を示し、そのＩｆ／Ｉｂ値は樹枝状Ｐｔナノ粒子の場合（０．９６）及びＰｔＢの場合（０．８５）に比べて高かった。更には、樹枝状Ｐｔナノ粒子及びＰｔＢと比較して、オンセット電位が顕著に負側にシフトすること（約１００ｍＶ）も観測された。上述したように、Ｐｄ原子とＰｔ原子とは高度に混和可能（miscible）であり、かつＰｄに富んでいるコア中のＰｄ原子は外側にあるＰｔに富んでいるシェルの格子構造とコヒーレントにマッチしている結果、疑似Ｐｔ−Ｐｄ合金（pseudo Pt-Pd alloy）が形成される。そのヘテロ界面でＰｄが親酸素性（oxophilic）元素として酸素を含む種を吸着することによって、近傍のＰｔ元素上に吸着されたＣＯの除去を増進することができる（非特許文献５６）。また，Ｐｔ−Ｐｄ合金が形成されることは，メタノールの分解におけるＣ−Ｈ開裂を促進するのに有用であるかもしれない（非特許文献５７）。以上の２つの作用がメタノール酸化を加速するにあたって重要な役割を演ずる。 The I f / I b value (defined as the ratio between the forward current density and the reverse current density) can be used as an indicator of the CO tolerance of the Pt catalyst in the methanol oxidation reaction (MOR) (Non-Patent Document 54). 55). The mesoporous Pd @ Pt nanoparticles showed excellent poisoning resistance and their If / Ib values were higher compared to dendritic Pt nanoparticles (0.96) and PtB (0.85). Furthermore, it was also observed that the onset potential was significantly shifted to the negative side (about 100 mV) compared to dendritic Pt nanoparticles and PtB. As mentioned above, Pd atoms and Pt atoms are highly miscible, and the Pd atoms in the Pd rich core are coherently matched with the lattice structure of the outer Pt rich shell. As a result, a pseudo Pt-Pd alloy is formed. Removal of CO adsorbed on nearby Pt elements can be promoted by adsorbing species containing oxygen as an oxyphilic element at the heterointerface (Non-patent Document 56). In addition, the formation of a Pt—Pd alloy may be useful for promoting C—H cleavage in the decomposition of methanol (Non-patent Document 57). These two actions play an important role in accelerating methanol oxidation.
これに加えて、メタノール酸化反応についての活性に基づいて本発明のメソポーラスナノ粒子のトポロジー的な利点、つまりその形状の位相幾何学的な特徴によってもたらされる利点についても検討した。Ｎ２吸着等温線から、ＰｔＢの表面積は約２０ｍ２・ｇ−１であると測定された。電気触媒活性を表面積で正規化しても、メソポーラスＰｄ＠Ｐｔナノ粒子の活性は依然として白金黒のほぼ２倍であった。この結果から、ここで観測された大きな活性は活性サイトの増加に帰着すると解釈できる。一般的に、欠陥サイト及び原子ステップは，水及びメタノール分子の解離を増進する（非特許文献５８〜６０）。図３（ｄ）からわかるように、そのメソポーラス構造は大きな表面積を与えるだけではなく、凹面状表面のトポロジーにより豊富な原子ステップも提供される。これがメタノール分解のＣ−Ｈ結合及びＯ−Ｈ結合の解離を増進したものと考えられる。 In addition to this, the topological advantages of the mesoporous nanoparticles of the present invention, that is, the advantages brought about by the topological features of their shapes, were also examined based on their activity for the methanol oxidation reaction. From the N 2 adsorption isotherm, the surface area of PtB was determined to be about 20 m 2 · g −1 . Even when electrocatalytic activity was normalized by surface area, the activity of mesoporous Pd @ Pt nanoparticles was still almost twice that of platinum black. From this result, it can be interpreted that the large activity observed here results in an increase in active sites. In general, defect sites and atomic steps enhance the dissociation of water and methanol molecules (Non-Patent Documents 58-60). As can be seen from FIG. 3 (d), the mesoporous structure not only provides a large surface area, but also provides rich atomic steps due to the concave surface topology. This is considered to have promoted dissociation of C—H bond and OH bond of methanol decomposition.
結論として、本発明では一様なサイズのメソ細孔を有しているコロイド状のメソポーラスＰｄ＠Ｐｔナノ粒子を簡単に液相で合成する方法が与えられる。本方法で重要な点は、界面活性剤ミセルを組み立てることにある。強酸性の媒体中で金属種を緩慢に還元することにより、Ｆ１２７ミセルはメソポアを形成するためのテンプレートとして直接的に働くことができる。Ｐｄ種の方が還元される傾向が大きいことから、Ｐｄ種の還元が優先的に起こり、その結果としてコア−シェル構造のＰｄ＠Ｐｔナノコロイドが形成される。この新規な方法により、全金属メソポーラスナノ粒子を１ステップで合成することができるようになる。 In conclusion, the present invention provides a simple method for synthesizing colloidal mesoporous Pd @ Pt nanoparticles having uniformly sized mesopores in the liquid phase. The important point in this method is to assemble the surfactant micelles. By slow reduction of metal species in a strongly acidic medium, F127 micelles can act directly as a template for forming mesopores. Since Pd species tend to be reduced, reduction of Pd species occurs preferentially, resulting in the formation of core-shell structured Pd @ Pt nanocolloids. This novel method makes it possible to synthesize all-metal mesoporous nanoparticles in one step.
以上詳細に説明したように、本発明における１ステップ合成は、金属ナノ粒子合成のための従来の方法（例えば、種を成長させてサイト固有のエッチングを行う方法）でも、またメソポーラス金属を合成するための方法（例えば、ソフトテンプレート法（soft-templating method）あるいはハードテンプレート法）でも達成できなかったことである。凹面状表面を有するメソポーラス金属ナノ粒子は、大きな表面積だけではなく触媒反応のための豊富な活性サイトをも提供する。低濃度の界面活性剤溶液中での全湿式プロセスに基づいた本方法は、他の金属や合金系にも広く適用することが可能であるとともに、工業的な大量生産のための有望な方法として使用することができる。 As described above in detail, the one-step synthesis in the present invention is a conventional method for synthesizing metal nanoparticles (for example, a method of growing seeds and performing site-specific etching), and also synthesizing mesoporous metal. It has not been possible to achieve the above-described methods (for example, soft-templating method or hard template method). Mesoporous metal nanoparticles having a concave surface provide not only a large surface area but also abundant active sites for catalytic reactions. The method based on the all-wet process in low concentration surfactant solution can be widely applied to other metal and alloy systems, and as a promising method for industrial mass production. Can be used.
Claims (19)
前記コアは第１の金属を前記第１の金属と異なる第２の金属よりも多く含み、
前記シェルは前記第２の金属を前記第１の金属よりも多く含む、
請求項１から４の何れかに記載のメソポーラス金属ナノ粒子。 A core and a shell around the core;
The core includes more first metal than a second metal different from the first metal;
The shell includes more of the second metal than the first metal;
The mesoporous metal nanoparticle according to any one of claims 1 to 4.
金属イオンを含む金属塩と、
前記金属イオンを還元する還元剤と
を含む溶液中で前記金属イオンを還元する、
メソポーラス金属ナノ粒子の製造方法。 A surfactant with a critical micelle concentration or higher,
A metal salt containing a metal ion;
Reducing the metal ion in a solution containing a reducing agent that reduces the metal ion;
Method for producing mesoporous metal nanoparticles.
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