Kiichirou Koyasu

Nonscalable Oxidation Catalysis of Gold Clusters

Small, negatively charged gold clusters isolated in vacuum can oxidize CO via electron-transfer-mediated activation of O2. This suggests that Au clusters can act as aerobic oxidation catalysts in the real world when their structure parameters satisfy given required conditions. However, there is a technical challenge for the development of Au cluster oxidation catalysts; the structural parameters of the Au clusters, such as size and composition, must be precisely controlled because the intrinsic chemical properties of the clusters are strongly dependent on these parameters. This Account describes our efforts to achieve precision synthesis of small (diameter <2 nm) Au clusters, stabilized by polymers and immobilized on supports, for a variety of catalytic applications. Since we aim to develop Au cluster catalysts by taking full advantage of their intrinsic, size-specific chemical nature, we chose chemically inert materials for the stabilizers and supports. We began by preparing small Au clusters weakly stabilized by polyvinylpyrrolidone (PVP) to test the hypothesis that small Au clusters in the real world will also show size-specific oxidation catalysis. The size of Au:PVP was controlled using a microfluidic device and monitored by mass spectrometry. We found that only Au clusters smaller than a certain critical size show a variety of aerobic oxidation reactions and proposed that the reactions proceed via catalytic activation of O2 by negatively charged Au clusters. We also developed a method to precisely control the size and composition of supported Au clusters using ligand-protected Au and Au-based bimetallic clusters as precursors. These small Au clusters immobilized on mesoporous silica, hydroxyapatite, and carbon nanotubes acted as oxidation catalysts. We have demonstrated for the first time an optimal Au cluster size for the oxidation of cyclohexane and a remarkable improvement in the oxidation catalysis of Au25 clusters by single-atom Pd doping. The non-scalable catalysis of Au clusters that we reported here points to the possibility that novel catalysis beyond that expected from bulk counterparts can be developed simply by reducing the catalyst size to the sub-2 nm regime.

Geometric and electronic structures of metal (M)-doped silicon clusters (M=Ti, Hf, Mo and W)

Abstract We have studied geometric and electronic structures of metal (M) atom doped silicon (Si) clusters, MSin (M=Ti, Hf, Mo and W), using mass spectrometry, a chemical-probe method and photoelectron spectroscopy. In the mass spectra for all of the mixed cluster anions, MSin−, both MSi15− and MSi16− were abundantly produced compared to neighbors. Together with the result of the adsorption reactivity and photoelectron spectroscopy, it has been revealed that one metal atom can be encapsulated inside a Sin cage at n⩾15.

Anion photoelectron spectroscopy of transition metal- and lanthanide metal-silicon clusters: MSin− (n=6–20)

The electronic properties of silicon clusters containing a transition or lanthanide metal atom from group 3, 4, or 5, MSi(n), (M=Sc, Ti, V, Y, Zr, Nb, Lu, Tb, Ho, Hf, and Ta) were investigated by anion photoelectron spectroscopy at 213 nm. In the case of the group 3 elements Sc, Y, Lu, Tb, and Ho, the threshold energy of electron detachment exhibits local maxima at n=10 and 16, while in case of the group 4 elements Ti, Zr, and Hf, the threshold energy exhibits a local minimum at n=16, associated with the presence of a small bump in the spectrum. These electronic characteristics of MSi(n) are closely related to a cooperative effect between their geometric and electronic structures, which is discussed, together with the results of experiments that probe their geometric stability via their reactivity to H(2)O adsorption, and with theoretical calculations.

Photoelectron spectroscopy of palladium-doped gold cluster anions; AunPd− (n=1–4)

Abstract Palladium-doped gold clusters, Au n Pd − (n=1–4) , were investigated using anion photoelectron spectroscopy at 4.66 eV photon energy. Electron affinities (EAs) and vertical detachment energies (VDEs) are determined, and the electronic structures of Pd-doped Au n clusters are compared to those of pure Au n clusters. A peak shape analysis reveals electronic and geometric similarity between Au n − and Au n −1 Pd − clusters and it is found that (1) an electron promotion occurs from 4d to 5s orbital in the Pd atom, and that (2) the bond of Au–Pd is formed through σ orbital between 6s of Au and 5s of Pd.

Experimental and theoretical characterization of MSi16(-), MGe16(-), MSn16(-), and MPb16(-) (M = Ti, Zr, and Hf): the role of cage aromaticity.

Silicon (Si), germanium (Ge), tin (Sn), and lead (Pb) clusters mixed with a group-4 transition metal atom [M = titanium (Ti), zirconium (Zr), and hafnium (Hf)] were generated by a dual-laser vaporization method, and their properties were analyzed by means of time-of-flight mass spectroscopy and anion photoelectron spectroscopy together with theoretical calculations. In the mass spectra, mixed neutral clusters of MSi(16), MGe(16), and MSn(16) were produced specifically, but the yield of MPb(16) was low. The anion photoelectron spectra revealed that MSi(16), MGe(16), and MSn(16) neutrals have large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of 1.5-1.9 eV compared to those of MPb(16) (0.8-0.9 eV), implying that MSi(16), MGe(16), and MSn(16) are evidently electronically stable clusters. Cage aromaticity appears to be an important determinant of the electronic stability of these clusters: Calculations of nucleus-independent chemical shifts (NICSs) show that Si(16)(4-), Ge(16)(4-), and Sn(16)(4-) have aromatic characters with negative NICS values, while Pb(16)(4-) has an antiaromatic character with a positive NICS value.

Anion photoelectron spectroscopy of germanium and tin clusters containing a transition- or lanthanide-metal atom; MGen− (n = 8–20) and MSnn− (n = 15–17) (M = Sc–V, Y–Nb, and Lu–Ta)

The electronic properties of germanium and tin clusters containing a transition- or lanthanide-metal atom from group 3, 4, or 5, MGe(n) (M = Sc, Ti, V, Y, Zr, Nb, Lu, Hf, and Ta) and MSn(n) (M = Sc, Ti, Y. Zr, and Hf), were investigated by anion photoelectron spectroscopy at 213 nm. In the case of the group 3 elements Sc, Y, and Lu, the threshold energy of electron detachment of MGe(n)(-) exhibits local maxima at n = 10 and 16, while in the case of the group 4 elements Ti, Zr, and Hf, it exhibits a local minimum only at n = 16, associated with the presence of a small bump in the spectrum. A similar behavior is observed for MSn(n)(-) around n = 16, and these electronic characteristics of MGe(n) and MSn(n) are closely related to those of MSi(n). Compared to MSi(n), however, the larger cavity size of a Ge(n) cage allows metal atom encapsulation at a smaller size n. A cooperative effect between the electronic and geometric structures of clusters with a large cavity of Ge(16) or Sn(16) is discussed together with the results of experiments that probe their geometric stability via their reactivity to H(2)O adsorption.

Slow-Reduction Synthesis of a Thiolate-Protected One-Dimensional Gold Cluster Showing an Intense Near-Infrared Absorption

Slow reduction of Au ions in the presence of 4-(2-mercaptoethyl)benzoic acid (4-MEBA) gave Au76(4-MEBA)44 clusters that exhibited a strong (3 × 10(5) M(-1) cm(-1)) near-infrared absorption band at 1340 nm. Powder X-ray diffraction studies indicated that the Au core has a one-dimensional fcc structure that is elongated along the {100} direction.

Surface Plasmon Resonance in Gold Ultrathin Nanorods and Nanowires

We synthesized and measured optical extinction spectra of Au ultrathin (diameter: ∼1.6 nm) nanowires (UNWs) and nanorods (UNRs) with controlled lengths in the range 20-400 nm. The Au UNWs and UNRs exhibited a broad band in the IR region whose peak position was red-shifted with the length. Polarized extinction spectroscopy for the aligned Au UNWs indicated that the IR band is assigned to the longitudinal mode of the surface plasmon resonance.

Anion photoelectron spectroscopy of VnOm− (n=4–15;m=0–2)

The anion photoelectron (PE) spectra of small mass-selected vanadium oxide clusters VnOm− (n=4–15; m=0–2) are measured at a fixed photon energy of 4.66 eV with the aid of a magnetic bottle photoelectron spectrometer. Cluster anions are generated in a pulsed laser vaporization cluster source. The electronic structure of VnOm− clusters is investigated as a function of size n and composition m with special regard to the increasing oxidation state. The addition of one or two oxygen atoms to the vanadium cluster core induces a change of the electronic structure in the near-threshold binding energy region below 2 eV. Main spectral features are contributed from the transition metal d-derived orbitals, whereas the oxygen 2p contribution induces a hybridization between vanadium and oxygen frontier orbitals in the entire series of the investigated clusters n=4–15. Generally, electron affinities and vertical detachment energies increase with increasing cluster size revealing size-dependent discontinuities. Furthermo...

Structural transition of zinc oxide cluster cations: Smallest tube like structure at (ZnO)6+

Zinc oxide cluster cations have been analyzed by ion mobility spectrometry using a home-made drift cell combined with a time-of-flight reflectron mass spectrometer. Structural changes from cyclic to tube like structures were observed around n = 8, corresponding to predictions by theoretical calculations. The structures were assigned by comparing with the arrival time simulation using MOBCAL software. We have also observed ion-injection energy dependence of the structures of (ZnO)n(+). The smallest tube structure of (ZnO)6(+) has predominantly been observed at an injection energy of 200 eV. The extraordinary stability of the compact structure at this size has been observed for the first time.