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Magic Numbers of Gold Clusters Stabilized by PVP
Hironori Tsunoyama and Tatsuya Tsukuda*
J. Am. Chem. Soc., 131, 18216-18217 (2009).
We applied matrix-assisted laser desorption/ionization (MALDI) for mass analysis of Au clusters stabilized by poly(vinyl-pyrrolidone) (PVP) and discovered a series of magic numbers for Au cluster size: 35 ± 1, 43 ± 1, 58 ± 1, 70 ± 3, 107 ± 4, 130 ± 1, and 150 ± 2. Magic numbers smaller than ~70 agree with those of free Au clusters and can be explained qualitatively by the electronic shell model. In contrast, magic numbers larger than ~100 are obviously different from those of the free clusters. We suggest that the deviation from the electronic shell model is due to perturbation of the electronic and/or geometric structures caused by interaction with PVP.
Size Determination of Gold Clusters by Polyacrylamide Gel Electrophoresis in a Large Cluster Region
Keisaku Kimura*, Nobuyuki Sugimoto, Seiichi Sato,
Hiroshi Yao, Yuichi Negishi, and Tatsuya Tsukuda
J. Phys. Chem. C, 113, 14076-14082 (2009).
Polyacrylamide gel electrophoresis (PAGE) is used to separate the component of the glutathione protected gold clusters up to Au160 core size and compare the size with the mobility data from PAGE. Both electrospray mass spectrometry and transmission electron microscopy were systematically applied to analyze Au-glutathione clusters thus isolated. We have found a linear relationship between the mobility of the gold cluster compounds to the logarithm of the number of gold atoms of the core. Better fitting to the logarithm of the number of thiolate molecules adsorbed on the gold core was found and was expressed by a simple equation that bears conformation of ligand and charge density of clusters. Hence the mobility of the metallic clusters was correlated to the component of the ligand of the cluster.
Preparation of ~1 nm Gold Clusters Confined within Mesoporous Silica and Microwave-Assisted Catalytic Application for Alcohol Oxidation
Yongmei Liu, Hironori Tsunoyama, Tomoki Akita, and
J. Phys. Chem. C, 113, 13457-13461 (2009).
A simple, effective method has been demonstrated to immobilize ~1 nm Au clusters within mesoporous silicas (SBA-15, MCF, HMS) using triphenylphosphine-protected Au11 (Au11:TPP) clusters as precursors, which were deposited on the silica surface in an organic medium. A unique feature of this method is the ability to disperse Au11:TPP homogeneously over a large surface area by optimizing the solvent-mediated interaction. The Au11:TPP-silica composite was then carefully calcined to remove the protecting ligands while suppressing the aggregation of the resulting Au clusters. For SBA-supported Au clusters, the absence of the surface plasmon band in reflectance spectrum indicated that contamination by Au NPs larger than 2 nm was negligibly small. The Au cluster size supported on SBA was estimated to be 0.8±0.3 nm by HAADF-STEM observations. The SBA-supported Au clusters exhibited catalytic activity for oxidation of various alcohols by H2O2 under microwave irradiation and were found to be reusable.
Effect of Electronic Structures of Au Clusters Stabilized by Poly(N-vinyl-2-pyrrolidone) on Aerobic Oxidation Catalysis
Hironori Tsunoyama, Nobuyuki Ichikuni, Hidehiro
Sakurai, Tatsuya Tsukuda*
J. Am. Chem. Soc., 131, 7086-7093 (2009).
Au clusters smaller than 1.5 nm and stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) showed higher activity for aerobic oxidation of alcohol than those of larger size or stabilized by poly(allylamine) (PAA). X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy of adsorbed CO, and X-ray absorption near edge structure measurements revealed that the catalytically active Au clusters are negatively charged by electron donation from PVP, and the catalytic activity is enhanced with increasing electron density on the Au core. Based on similar observations of Au cluster anions in the gas phase, we propose that electron transfer from the anionic Au cores of Au:PVP into the LUMO (π*) of O2 generates superoxo- or peroxo-like species, which plays a key role in the oxidation of alcohol. On the basis of these results, a simple principle is presented for the synthesis of Au oxidation catalysts stabilized by organic molecules.
Aerobic Oxygenation of Benzylic Ketones Promoted by Gold Nanocluster Catalyst
Hidehiro Sakurai*, Ikuyo Kamiya, Hiroaki Kitahara,
Hironori Tsunoyama, Tatsuya Tsukuda
SYNLETT, 2, 0245-0248 (2009).
Gold nanoclusters stabilized by poly(N-vinyl- 2-pyrrolidone) (Au:PVP) promote the oxidation of benzylic ketones, including auto-oxidation-type bond cleavage reactions and α-hydroxylation, under ambient conditions. The catalyst accelerates the formation of an α-peroxide intermediate, from which bond cleavage spontaneously proceeds in aqueous solvent to give the auto-oxidation products. In contrast, the α-hydroxylation product is obtained predominantly in DMSO solvent.