学術論文 2018

X-Ray Absorption Spectroscopy on Atomically Precise Metal Clusters

Seiji Yamazoe and Tatsuya Tsukuda
Bull. Chem. Soc. Jpn., Just Accepted Manuscript.

Metal clusters show novel and size-specific properties due to unique geometric and quantized electronic structures. State-of-the art synthetic methods allow us to control with atomic precision the size and compositions of clusters stabilized with polymers, protected by ligands, and immobilized on supports. The geometric structure is key information for understanding the origin of the specific and novel properties and to rationally designing their functions. Single-crystal X-ray diffraction analysis provides direct and atomic-level structural information on ligand-protected metal clusters that can be crystallized, but cannot be applied to polymer-stabilized and supported clusters even though their size and composition are precisely defined. X-ray absorption spectroscopy (XAS) is a versatile tool for determining the local structure and electronic state of a specific element within the clusters regardless of their environment. In addition to static structures, dynamic changes in electronic and geometric structures can be probed by a time-resolved measurement. Simultaneous measurement of XAS with other spectroscopies provides further insight into the reaction mechanism. This article summarizes our XAS studies on the size and atomic packing of metal clusters, location of dopant in the clusters, interfacial structures between the clusters and the surroundings, thermal properties of the clusters, and structural and electronic dynamics during the reactions.

Interconversions of Structural Isomers of [PdAu8(PPh3)8]2+ and [Au9(PPh3)8]3+ Revealed by Ion Mobility Mass Spectrometry

Keisuke Hirata, Papri Chakraborty, Abhijit Nag, Shinjiro Takano, Kiichirou Koyasu, Thalappil Pradeep and Tatsuya Tsukuda
J. Phys. Chem. C, Just Accepted Manuscript.

Collision cross sections (CCSs) of ligand-protected metal clusters were evaluated using ion mobility mass spectrometry. The targets used in this study were phosphine-protected clusters [PdAu8(PPh3)8]2+ and [Au9(PPh3)8]3+, for which the total structures have been resolved by single-crystal X-ray analysis. The arrival time distributions of [PdAu8(PPh3)8]2+ as a function of the He flow rate in a cell located just in front of a traveling wave ion mobility cell filled with N2 buffer gas, demonstrated that it got converted to another structural isomer having a smaller CCS, with the increase in the nominal collision energy. A similar phenomenon was observed for [Au9(PPh3)8]3+. These results were explained by the collisional excitation and cooling with the buffer gas inducing the conversion of the packing arrangement of the ligands rather than the atomic structure of the metallic core: the ligand layer was converted from disordered to the closely packed arrangement found in a single crystal during this process. This study showed that the ligand layer with a disordered arrangement in solution was retained during desolvation upon electrospray ionization and was annealed into the most stable closely packed arrangement by collisions.

Hydride-Mediated Controlled Growth of a Bimetallic (Pd@Au8)2+ Superatom to a Hydride-Doped (HPd@Au10)3+ Superatom

Shinjiro Takano, Haru Hirai, Satoru Muramatsu and Tatsuya Tsukuda
J. Am. Chem. Soc., 140, 12314-12317 (2018).
Selected as Supplementary Cover Image

A hydride (H)-doped bimetallic superatom (HPdAu8)+ was produced by reacting BH4 with an oblate (PdAu8)2+ superatom protected by PPh3. The H atom in (HPdAu8)+ survived during the sequential addition of Au(I)Cl to form an (HPdAu10)3+ superatom, in sharp contrast to the proton release from a H-doped pure gold superatom (HAu9)2+ in the growth process to (Au11)3+. Single-crystal X-ray diffraction analysis and density functional theory calculations on (HPdAu10)3+ showed that the interstitially doped H atom induced a notable deformation of the core.

Hydride-Doped Gold Superatom (Au9H)2+: Synthesis, Structure and Transformation

Shinjiro Takano, Haru Hirai, Satoru Muramatsu and Tatsuya Tsukuda
J. Am. Chem. Soc., 140, 8380-8383 (2018).

Doping of a hydride (H) into an oblate-shaped gold cluster [Au9(PPh3)8]3+ was observed for the first time by mass spectrometry and NMR spectroscopy. Density functional theory calculations for the product [Au9H(PPh3)8]2+ demonstrated that the (Au9H)2+ core can be viewed as a nearly spherical superatom with a closed electronic shell. The hydride-doped superatom (Au9H)2+ was successfully converted to the well-known superatom Au113+, providing a new atomically precise synthesis of Au clusters via bottom-up approach.

An Au25(SR)18 Cluster with a Face-Centered Cubic Core

Tsubasa Omoda, Shinjiro Takano, Seiji Yamazoe, Kiichirou Koyasu, Yuichi Negishi and Tatsuya Tsukuda
J. Phys. Chem. C, 122, 24, 13199-13204 (2018).

A representative thiolate (RS)-protected gold cluster, Au25(SR)18, shows a fingerprint-like characteristic spectral profile regardless of the R-groups, reflecting the common motif of the structural backbone made of Au and S: an icosahedral Au13 core fully protected by six staple units of Au2(SR)3. On the other hand, we reported in 2006 that an Au25(SPG)18 cluster (PGSH = N-(2-mercaptopropionyl)glycine) exhibited an optical absorption spectrum significantly different from that of the conventional Au25(SR)18, suggesting the formation of a non-icosahedral Au core. Here, we investigated the structure of Au25(SPG)18 by UV-Vis spectroscopy, extended X-ray absorption fine structure analysis and density functional theory calculations. Spectroscopic results indicated that Au25(SPG)18 has a face-centered cubic (FCC) Au core. We proposed a model structure formulated as Au15(SPG)4[Au2(SPG)3]2[Au3(SPG)4]2 in which an Au15(SPG)4 core with an FCC motif is protected by two types of staples with different lengths, Au2(SPG)3 and Au3(SPG)4. The formation of an FCC-based Au core is attributed to bulkiness around the α-carbon of the PGS ligand.

Collision-Induced Dissociation of Undecagold Clusters Protected by Mixed Ligands [Au11(PPh3)8X2]+ (X = Cl, C ≡ CPh)

Ryohei Tomihara, Keisuke Hirata, Hiroki Yamamoto, Shinjiro Takano, Kiichirou Koyasu and Tatsuya Tsukuda
ACS Omega, 3, 6, 6237-6242 (2018).

We herein investigated collision-induced dissociation (CID) processes of undecagold clusters protected by mixed ligands [Au11(PPh3)8X2]+ (X = Cl, C ≡ CPh) using mass spectrometry and density functional theory calculations. The results showed that the CID produced fragment ions [Aux(PPh3)yXz]+ with a formal electron count of 8 via sequential loss of the PPh3 ligands and AuX(PPh3) units in a competitive manner, indicating that the CID channels are governed by the electronic stability of the fragments. Interestingly, the branching fraction of the loss of the AuX(PPh3) units was significantly smaller for X = C ≡ CPh than that for X = Cl. We ascribed the effect of X on the branching fractions of dissociations of PPh3 and AuX(PPh3) to the steric difference.

Prominent hydrogenation catalysis of a PVP-stabilized Au34 superatom provided by doping a single Rh atom

Shingo Hasegawa, Shinjiro Takano, Seiji Yamazoe and Tatsuya Tsukuda
Chem. Commun., 54, 5915-5918 (2018).

A single rhodium atom was precisely doped into a gold cluster Au34 stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) as revealed by mass spectrometry. The Rh-doped Au:PVP exhibited remarkable catalytic activity for hydrogenation reactions of olefins, which was much higher than that of recently reported Pd-doped Au:PVP.

Gold Ultrathin Nanorods with Controlled Aspect Ratios and Surface Modifications: Formation Mechanism and Localized Surface Plasmon Resonance

Ryo Takahata, Seiji Yamazoe, Kiichirou Koyasu, Kohei Imura and Tatsuya Tsukuda
J. Am. Chem. Soc., 140, 21, 6640-6647 (2018).
Selected as Supplementary Cover Image

We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5-20 nm while keeping the diameter constant (~2 nm) by changing the relative concentration of OA and Au(I). It is proposed based on the time-resolved optical spectroscopy that AuUNRs are formed via the formation of small (< 2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is redshifted with the AR. The NIR bands of AuUNRs with AR < 5 were redshifted upon the ligand exchange from OA to thiolates, in sharp contrast to the blueshift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR< 5are not plasmonic in nature, but are associated with a single electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ~2 nm significantly affects the optical response. The redshift of the LSPR band can be ascribed to the increase in effective mass of electrons in AuUNRs.

Size-dependent Polymorphism in Aluminum Carbide Cluster Anions AlnC2: Formation of Acetylide-Containing Structures

Kazuyuki Tsuruoka, Kiichirou Koyasu, Shinichi Hirabayashi, Masahiko Ichihashi, and Tatsuya Tsukuda
J. Phys. Chem. C, 122, 15, 8341-8347 (2018).

Aluminum carbide clusters anions AlnC2 (n = 5–13) were observed as the most dominant products in gas-phase reactions of laser-ablated Aln with organic molecules, such as methanol, ethanol, pentane, acetonitrile or acetone. Density functional theory calculations predicted two possible isomeric structures for AlnC2: isomers in which two carbons are dissociated (type D) as in the case of the bulk aluminum carbide and novel isomers in which two carbons form an acetylide-like C2 unit. The latter isomers are further categorized into three types depending on the location of the C2 unit: the C2 unit is encapsulated within the Al cage (type I), contained in the surface of Al clusters (type S), or attached to the surface of Al clusters (type O). Size-dependent behavior of the adiabatic electron affinities of AlnC2 determined by photoelectron spectroscopy was explained in terms of polymorphism as a function of size (n): type I for n = 5–8, type D for n = 9–11, type D or O for n = 12, and type O for n = 13. The tendency in which the position of the C2 unit was shifted from inside to outside with the increase in n was ascribed to the balance between the stabilizations gained by forming the Al—C bonds and the Al—Al bonds. The smaller AlnC2 clusters (n = 5–8) prefer to surround the acetylide-like C2 unit with the Al atoms so as to maximize the number of the Al—C bonds, while larger ones (n = 12 and 13) prefer to attach the C2 unit onto the surface of the Al clusters so as to maximize the number of the Al–Al bonds.

Au25-Loaded BaLa4Ti4O15 Water-Splitting Photocatalyst with Enhanced Activity and Durability Produced Using New Chromium Oxide Shell Formation Method

Wataru Kurashige, Rina Kumazawa, Daiki Ishii, Rui Hayashi, Yoshiki Niihori, Sakiat Hossain, Lakshmi V. Nair, Tomoaki Takayama, Akihide Iwase, Seiji Yamazoe, Tatsuya Tsukuda, Akihiko Kudo, and Yuichi Negishi
J. Phys. Chem. C, 122, 13669-13681 (2018).

We report herein remarkable improvement of activity and stability of an Au25–loaded BaLa4Ti4O15 water-splitting photocatalyst. We first examined the influence of refining the gold cocatalyst on the individual reactions over the BaLa4Ti4O15 photocatalyst in this water-splitting system. The results revealed that refining the gold cocatalyst accelerates not only the hydrogen generation reaction, but also oxygen photoreduction reaction, which suppresses the H2 generation via photoreduction of protons. This finding suggests that photocatalytic activity will be enhanced if the O2 photoreduction reaction can be selectively suppressed by covering Au25 with a Cr2O3 shell which is impermeable to O2 but permeable to H+. Then, we developed new method for the formation of the Cr2O3 shell onto Au25. Our method utilizes the strong metal–support interaction between them. Water-splitting photoactivity of Au25–BaLa4Ti4O15 was improved by 19 times under an optimized coverage of the Cr2O3 shell. The Cr2O3 shell also elongated the lifetime of the photocatalysts by preventing the agglomeration of Au25 cocatalysts.

Efficient One-Pot Synthesis and pH-Dependent Tuning of Photoluminescence and Stability of Au18(SC2H4CO2H)14 Cluster

Ramakrishna Itteboina, U. Divya Madhuri, Partha Ghosal, Monica Kannan, Tapan Kumar Sau*, Tatsuya Tsukuda, and Shweta Bhardwaj
J. Phys. Chem. A, 122, 1228-1234 (2018).

Developing efficient ways to control the nanocluster properties and synthesize atomically-precise metal nanoclusters are the foremost goals in the field of metal nanocluster research. In this article, we demonstrate that the direct synthesis of atomically-precise, hydrophilic metal nanoclusters as well as tuning of their properties can be achieved by an appropriate selection of reactants, binding ligand, and their proportions. Thus a facile, single-step method has been developed for the direct synthesis of Au18(SC2H4CO2H)14 nanocluster in an aqueous medium under ambient conditions. The synthesis does not require any pH or temperature control and post-synthesis size-separation step. The use of a hydrophilic, bifunctional short carbon-chain capping ligand, HSC2H4CO2H, allows tuning of cluster properties such as the photoluminescence and stability in an aqueous medium via the variation of pH of the cluster solution. By using a phase transfer catalyst, the nanoclusters can also be transferred into toluene solvent which further enhances the nanocluster photoluminescence. The formation, composition, and purity of the product clusters have been characterized by using a number of methods such as the polyacrylamide gel electrophoresis (PAGE), UV-visible and FTIR spectroscopies, transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). Gold nanoclusters with properties such as water solubility, water-to-organic phase-transfer ability and tunable stability and photoluminescence are promising for various studies and applications. The work reveals a few principles that can be helpful in the development of a general toolbox for the rational design of size-selective synthesis and properties tuning of the metal nanoclusters.

Dynamic Behavior of Rh Species of Rh/Al2O3 Model Catalyst During Three-Way Catalytic Reaction — An Operando XAS Study

Hiroyuki Asakura, Saburo Hosokawa, Toshiaki Ina, Kazuo Kato, Kiyofumi Nitta, Kei Uera, Tomoya Uruga, Hiroki Miura, Tetsuya Shishido, Jun-ya Ohyama, Atsushi Satsuma, Katsutoshi Sato, Akira Yamamoto, Satoshi Hinokuma, Hiroshi Yoshida, Masato Machida, Seiji Yamazoe, Tatsuya Tsukuda, Kentaro Teramura, and Tsunehiro Tanaka*
J. Am. Chem. Soc. 140, 176-184 (2018).

The dynamic behavior of Rh species in 1 wt% Rh/Al2O3 catalyst during the three-way catalytic reaction was examined using a micro gas chromatograph, a NOx meter, a quadrupole mass spectrometer, and time-resolved quick X-ray absorption spectroscopy (XAS) measurements at a public beamline for XAS, BL01B1 at SPring-8, operando. The combined data suggest different surface rearrangement behavior, random reduction processes, and autocatalytic oxidation processes of Rh species when the gas is switched from a reductive to an oxidative atmosphere and vice versa. This study demonstrates an implementation of a powerful operando XAS system for heterogeneous catalytic reactions and its importance for understanding the dynamic behavior of active metal species of catalysts.

Doping a Single Palladium Atom into Gold Superatoms Stabilized by PVP: Emergence of Hydrogenation Catalysis

Shun Hayashi, Ryo Ishida, Shingo Hasegawa, Seiji Yamazoe, and Tatsuya Tsukuda*
Topics in Catalysis, 61, 136-141 (2018).

It is known that small gold clusters (average diameter: ~1.2 nm) stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) exhibit size-specific catalysis in aerobic oxidation reactions. A recent matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) study of Au:PVP revealed that Au clusters with the magic sizes of 34 and 43 were preferentially produced. Here, we reported how the doping of palladium (Pd) into Au:PVP affects the catalytic performance. MALDI-MS analysis of Pd-doped Au:PVP showed that a single Pd atom was selectively doped by co-reduction of Au and Pd precursor ions and that PdAu33 and PdAu43 were produced as the dominant species. Extended X-ray absorption fine structure (EXAFS) analysis indicated that a Pd atom was located at the exposed surface of the Au:PVP clusters. It was found that single Pd atom doping enhanced the catalytic activity for aerobic oxidation of benzyl alcohol and provided hydrogenation catalysis in a chemoselective manner to the C=C bonds over the C=O bonds.