学術論文 2018

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., Just Accepted Manuscript

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., Just Accepted Manuscript

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, Just Accepted Manuscript

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.