Tatsuya Tsukuda

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.

Electron microscopy of gold nanoparticles at atomic resolution

Detailed structure of a gold nanoparticle Adding only a few atoms or changing the capping ligand can dramatically change the structure of individual metal nanoparticles. Azubel et al. used aberration-corrected transmission electron microscopy to derive a three-dimensional reconstruction of water-soluble gold nanoparticles. Small-angle x-ray scattering and other techniques have also corroborated this model. They used this to determine the atomic structure, which compared favorably with density functional theory calculations, without assuming any a priori structural knowledge or the use of model fitting. Science, this issue p. 909 The atomic structure of a 68–gold atom nanoparticle is determined without prior structural knowledge or model fitting. Structure determination of gold nanoparticles (AuNPs) is necessary for understanding their physical and chemical properties, but only one AuNP larger than 1 nanometer in diameter [a 102–gold atom NP (Au102NP)] has been solved to atomic resolution. Whereas the Au102NP structure was determined by x-ray crystallography, other large AuNPs have proved refractory to this approach. Here, we report the structure determination of a Au68NP at atomic resolution by aberration-corrected transmission electron microscopy, performed with the use of a minimal electron dose, an approach that should prove applicable to metal NPs in general. The structure of the Au68NP was supported by small-angle x-ray scattering and by comparison of observed infrared absorption spectra with calculations by density functional theory.

Hierarchy of bond stiffnesses within icosahedral-based gold clusters protected by thiolates

Unique thermal properties of metal clusters are believed to originate from the hierarchy of the bonding. However, an atomic-level understanding of how the bond stiffnesses are affected by the atomic packing of a metal cluster and the interfacial structure with the surrounding environment has not been attained to date. Here we elucidate the hierarchy in the bond stiffness in thiolate-protected, icosahedral-based gold clusters Au25(SC2H4Ph)18, Au38(SC2H4Ph)24 and Au144(SC2H4Ph)60 by analysing Au L3-edge extended X-ray absorption fine structure data. The Au–Au bonds have different stiffnesses depending on their lengths. The long Au–Au bonds, which are more flexible than those in the bulk metal, are located at the icosahedral-based gold core surface. The short Au–Au bonds, which are stiffer than those in the bulk metal, are mainly distributed along the radial direction and form a cyclic structural backbone with the rigid Au–SR oligomers.

Au25 Clusters Containing Unoxidized Tellurolates in the Ligand Shell

We report herein the synthesis and characterization of Au25 clusters containing tellurolates (TePh) in the ligand shell ([Au25(TePh)n(SC8H17)18-n](-); n = 1-18). [Au25(TePh)n(SC8H17)18-n](-) clusters were synthesized by reacting [Au25(SC8H17)18](-) with diphenyl ditelluride ((PhTe)2) in solution. Characterization of the products by mass spectrometry and X-ray absorption fine structure analysis revealed that the tellurolates in [Au25(TePh)n(SC8H17)18-n](-), unlike those in tellurolate-protected gold nanoparticles, were not oxidized. Various experiments on the products and theoretical calculations on related clusters revealed that protection by the tellurolates distorts (expands) the central Au13 core and decreases the HOMO-LUMO gap of the Au25 clusters.

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.

Chemically Modified Gold Superatoms and Superatomic Molecules

Clusters of gold atoms can be viewed as superatoms, in which valence electrons confined in the particles occupy atomic-like, discrete electronic levels. Chemical modification of the gold superatoms and their aggregated molecules (superatomic molecules) with organic ligands is a promising approach for their application as the building units of new functional materials. This account surveys the present status of the rapidly growing field of gold superatoms and superatomic molecules protected by thiolates and phosphines. The major aim of this article is to provide a simple picture for the structure, stability and bonding scheme of chemically modified superatoms and superatomic molecules for the development of a new class of hierarchical materials.

Tuning the electronic structure of thiolate-protected 25-atom clusters by co-substitution with metals having different preferential sites

Trimetallic Au24-xAgxPd and tetrametallic Au24-x-yAgxCuyPd clusters were synthesized by the subsequential metal exchange reactions of dodecanethiolate-protected Au24Pd clusters. EXAFS measurements revealed that Pd, Ag, and Cu dopants preferentially occupy the center and edge sites of the core, and staple sites, respectively. Spectroscopic and theoretical studies demonstrated that the synergistic effects of multiple substitutions on the electronic structures are additive in nature.

Controlled Synthesis of Carbon‐Supported Gold Clusters for Rational Catalyst Design

The development of novel catalysts based on metal clusters requires a rational design principle as well as atomically precise synthetic methods. Toward this goal, we have developed a method to precisely and independently control the size, composition, and surface modification of heterogeneous gold clusters by calcination of the ligand-protected Au clusters on carbon supports. We studied the effects of these structural parameters using benzyl alcohol oxidation as a test reaction. Unexpectedly, Au144 and Au∼330 on hierarchically porous carbon exhibited significantly higher turnover frequency than Au25 and Au38 . This size dependence is ascribed to the difference in the geometric structures of the Au clusters; Au144 and Au∼330 have an icosahedral-based structure whereas Au25 and Au38 have a face-centered cubic structure. Doping of a single Pd atom into Au25 supported on carbon nanotubes remarkably enhanced the catalytic activity. The doping effect is explained in terms of the accelerated formation of the carbocation intermediate due to electron transfer from Pd to Au, since the doped Pd is buried within the Au clusters and is located at the interface between the supports. Residual thiolates on Au25 affected both the activity and selectivity; selective oxidation to benzaldehyde was achieved at optimized coverage. Non-formation of benzoic acid is due to the suppression of oxidation activity by electron withdrawal by thiolates and non-formation of benzyl benzoate is due to the site-isolation effect by thiolates. These results will provide useful information for the rational design of gold-cluster-based catalysts with desired performance.

Hydrogen-induced structural transformation of AuCu nanoalloys probed by synchrotron X-ray diffraction techniques

In situ X-ray diffraction measurements reveal that the transformation of a AuCu nanoalloy from a face-centered-cubic to an L10 structure is accelerated under a hydrogen atmosphere. The structural transformation rate for the AuCu nanoalloy under hydrogen above 433 K was found to be 100 times faster than that in a vacuum, which is the first quantitative observation of hydrogen-induced ordering of nanoalloys.