トピックス
Al13−超原子の液相合成
2017.12.11
T. Kambe, N. Haruta, T. Imaoka, K. Yamamoto
Nature Commun. 2017, 8, 2046.
超原子、すなわちそれらが構成される元素とは異なる特性を模倣するクラスターは、調整可能な特性を持つ前例のない材料の構築ブロックとして機能する可能性がある。超原子の溶液相合成法の開発は、この研究分野の将来の進展に不可欠な成果となるであろう。ここでは、デンドリマーテンプレートを用いて溶液中でアルミニウムクラスター（最もよく知られたスーパ―アトムであるAl13−）を生成する方法を報告する。Al13−クラスターは、質量分析および走査透過型電子顕微鏡を使用して同定され、X線光電子分光法により結合エネルギーが測定される。Al13−の超原子安定性は、その酸化傾向を評価することで示される。さらに、溶液中でのAl13−の合成は電気化学測定を可能にし、その結果はAl13−の酸化を示唆している。この溶液相合成法は、クラスター科学の実験的発展において重要な役割を果たす。
Solution-phase synthesis of Al13− using a dendrimer template
Superatoms, clusters that mimic the properties of elements different to those of which they are composed, have the potential to serve as building blocks for unprecedented materials with tunable properties. The development of a method for the solution-phase synthesis of superatoms would be an indispensable achievement for the future progress of this research field. Here we report the fabrication of aluminum clusters in solution using a dendrimer template, producing Al13−, which is the most well-known superatom. The Al13− cluster is identified using mass spectrometry and scanning transmission electron microscopy, and X-ray photoelectron spectroscopy is used to measure the binding energies. The superatomic stability of Al13− is demonstrated by evaluating its tendency toward oxidation. In addition, the synthesis of Al13− in solution enables electrochemical measurements, the results of which suggest oxidation of Al13−. This solution-phase synthesis of Al13− superatoms has a significant role for the experimental development of cluster science.
Finding the Most Catalytically Active Platinum Clusters With Low Atomicity
2015.07.15
T. Imaoka, H. Kitazawa, W. Chun, K. Yamamoto
Angew. Chem. Int. Ed. 2015, 54, 9810-9815.
最も高い触媒活性を持つ白金クラスターの発見
サブナノメートルスケールでは、金属クラスター内のわずか一原子の差異により電子的および幾何学的配置に起因する触媒活性の変化を引き起こす可能性がある。本論文では、極めて小さい原子数、特に20未満の白金クラスターの構成原子数特異的触媒活性を報告している。クラスターの原子配置構造は、大きなナノ粒子で一般的な面心立方格子（fcc）とは全く異なる。そのような小さなクラスターについての電気化学的な検討からは、原子数が12〜20の範囲において、Pt19が最も触媒的に活性であることが明らかになった。
On a subnanometer scale, an only one-atom difference in a metal cluster may cause significant transitions in the catalytic activity due to the electronic and geometric configurations. We now report the atomicity-specific catalytic activity of platinum clusters with significantly small atomicity, especially below 20. The atomic coordination structure is completely different from that of the larger face-centered cubic (fcc) nanocrystals. Here, an electrochemical study on such small clusters, in which the atomicity ranged between 12 and 20, revealed Pt19 as the most catalytically active species. In combination with a theoretical study, a common structure that leads to a high catalytic performance is proposed.
Magic Number Pt13 and Misshapen Pt12 Clusters: Which One is the Better Catalyst?
2013.07.31
T. Imaoka, H. Kitazawa, W. Chun, S. Omura, K. Albrecht, K. Yamamoto
J. Am. Chem. Soc. 2013, 135, 13089-13095.
Magic Number Pt13 and Misshapen Pt12 Clusters: Which One is the Better Catalyst?
A relationship between the size of metal particles and their catalytic activity has been established over a nanometer scale (2–10 nm). However, application on a subnanometer scale (0.5–2 nm) is difficult, a possible reason being that the activity no longer relies on the size but rather the geometric structure as a cluster (or superatomic) compound. We now report that the catalytic activity for the oxygen reduction reaction (ORR) significantly increased when only one atom was removed from a magic number cluster composed of 13-platinum atoms (Pt13). The synthesis with an atomic-level precision was successfully achieved by using a dendrimer ligand as the macromolecular template strictly defining the number of metal atoms. It was quite surprising that the Pt12 cluster exhibited more than 2-fold catalytic activity compared with that of the Pt13 cluster. ESI-TOF-mass and EXAFS analyses provided information about the structures. These analyses suggested that the Pt12 has a deformed coordination, while the Pt13 has a well-known icosahedral atomic coordination as part of the stable cluster series. Theoretical analyses based on density functional theory (DFT) also supported this idea. The present results suggest potential activity of the metastable clusters although they have been “missing” species in conventional statistical synthesis.
Formation of a Pt12 Cluster by Single-Atom Control That Leads to Enhanced Reactivity: Hydrogenation of Unreactive Olefins
2013.05.31
M. Takahashi, T. Imaoka, Y. Hongo, K. Yamamoto
Angew. Chem. Int. Ed. 2013, 52, 7419-7421.
Formation of a Pt12 Cluster by Single-Atom Control That Leads to Enhanced Reactivity: Hydrogenation of Unreactive Olefins
A platinum subnanocluster catalyst composed of 12 atoms was synthesized using a phenylazomethine dendrimer, which can assemble twelve PtCl4 units by stepwise complexation, followed by reduction to Pt0. Unreactive olefins that were not activated by conventional 2 nm Pt nanoparticles were successfully hydrogenated by the subnanocluster. EWG=electron-withdrawing group.
A Uniform Bimetallic Rhodium/Iron Nanoparticle Catalyst for the Hydrogenation of Olefins and Nitroarenes
2011.06.01
I. Nakamula, Y. Yamanoi, T. Imaoka, K. Yamamoto, H. Nishihara
Angew. Chem. Int. Ed. 2011, 50, 5830-5833.
A Uniform Bimetallic Rhodium/Iron Nanoparticle Catalyst for the Hydrogenation of Olefins and Nitroarenes
Mix and more than match: Relative to the catalytic activity of pure Rh nanoparticles in a dendrimer cage, Rh/Fe bimetallic nanoparticles in dendrimers have improved catalytic activity towards the hydrogenation of olefins, and unlike Wilkinson catalyst could catalyze nitroarene hydrogenation (see scheme, G4=4th generation dendrimer).
Size-specific catalytic activity of platinum clusters enhances oxygen reduction reactions
2009.07.20
K. Yamamoto, T. Imaoka, W. Chun, O. Enoki, H. Katoh, M. Takenaga, A. Sonoi
Nature Chem. 2009, 1, 397-402.
Colloidal platinum nanoparticles with diameters of 2–5 nm on carbon supports are currently regarded as the best catalysts for the oxygen reduction reaction. However, the particle size is limited by the conventional preparation methods that are used to synthesize small platinum particles; the inherent activity of ultrasmall nanoparticles has not yet been revealed. We present a practical synthesis for ultrafine subnanometre platinum clusters using a spherical macromolecular template with no disorder in molecular weight or structure. The template, a phenylazomethine dendrimer, offers control of the number of metal complexes in an assembly through stepwise complexation, allowing the complexes to accumulate in discrete nano-cages. Subsequent reduction of Pt(IV) chloride to Pt(0) results in the formation of platinum clusters composed of a defined number of atoms. As a result of exceptionally small particle size, the clusters exhibit very high catalytic activity for the four-electron reduction of oxygen molecules.
Quantum size effect in TiO2 nanoparticles prepared by finely controlled metal assembly on dendrimer templates
2008.02.03
N. Satoh, T. Nakashima, K. Kamikura, K. Yamamoto
Nature Nanotechnol. 2008, 3, 106-111.
The use of dendrimer templates to make metal-based nanoparticles of controlled size has attracted much interest. These highly branched macromolecules have well-defined structures that enable them to bind metal ions to generate precursors that can be converted into nanoparticles. We describe the sub-nanometre size control of both anatase and rutile forms of TiO2 particles with phenylazomethine dendrimers, leading to samples with very narrow size distributions. Such fine tuning is possible because both the number and location of metal ions can be precisely controlled in these templates. Quantum size effects are observed in the particles, and the energy gap between the conduction and valence bands exhibits a blueshift with decreasing particle size and is dependent on the crystal form of the material. The dependency of the bandgap energy on these factors is explained using a semi-empirical effective mass approximation.
