Hiroyuki Asakura

Ultrathin rhodium nanosheets

Despite significant advances in the fabrication and applications of graphene-like materials, it remains a challenge to prepare single-layered metallic materials, which have great potential applications in physics, chemistry and material science. Here we report the fabrication of poly(vinylpyrrolidone)-supported single-layered rhodium nanosheets using a facile solvothermal method. Atomic force microscope shows that the thickness of a rhodium nanosheet is <4 Å. Electron diffraction and X-ray absorption spectroscopy measurements suggest that the rhodium nanosheets are composed of planar single-atom-layered sheets of rhodium. Density functional theory studies reveal that the single-layered Rh nanosheet involves a δ-bonding framework, which stabilizes the single-layered structure together with the poly(vinylpyrrolidone) ligands. The poly(vinylpyrrolidone)-supported single-layered rhodium nanosheet represents a class of metallic two-dimensional structures that might inspire further fundamental advances in physics, chemistry and material science.

Highly efficient, NiAu-catalyzed hydrogenolysis of lignin into phenolic chemicals

A highly efficient, stable NiAu catalyst that exhibits unprecedented low temperature activity in lignin hydrogenolysis was for the first time developed, leading to the formation of 14 wt% aromatic monomers from organosolv lignin at 170 °C in pure water.

Stabilizing a Platinum1 Single‐Atom Catalyst on Supported Phosphomolybdic Acid without Compromising Hydrogenation Activity

In coordination chemistry, catalytically active metal complexes in a zero- or low-valent state often adopt four-coordinate square-planar or tetrahedral geometry. By applying this principle, we have developed a stable Pt1 single-atom catalyst with a high Pt loading (close to 1 wt %) on phosphomolybdic acid(PMA)-modified active carbon. This was achieved by anchoring Pt on the four-fold hollow sites on PMA. Each Pt atom is stabilized by four oxygen atoms in a distorted square-planar geometry, with Pt slightly protruding from the oxygen planar surface. Pt is positively charged, absorbs hydrogen easily, and exhibits excellent performance in the hydrogenation of nitrobenzene and cyclohexanone. It is likely that the system described here can be extended to a number of stable SACs with superior catalytic activities.

Development of Palladium Surface-Enriched Heteronuclear Au-Pd Nanoparticle Dehalogenation Catalysts in an Ionic Liquid

Heteronuclear Au-Pd nanoparticles were prepared and immobilized in the functionalized ionic liquid [C(2)OHmim][NTf(2)]. The structural and electronic properties of the nanoparticles were characterized by a range of techniques and the surface of the nanoparticles was found to be enriched in Pd. Moreover, the extent of Pd enrichment is easily controlled by varying the ratio of Au and Pd salts used in the synthesis. The heteronuclear nanoparticles were found to be effective catalysts in dehalogenation reactions with no activity observed for the pure Au nanoparticles and only limited activity for the pure Pd nanoparticles. The activity of the heteronuclear nanoparticles may be attributed to charge transfer from Pd to Au and consequently to more efficient reductive elimination.

Thermally stable single atom Pt/m-Al2O3 for selective hydrogenation and CO oxidation

Single-atom metal catalysts offer a promising way to utilize precious noble metal elements more effectively, provided that they are catalytically active and sufficiently stable. Herein, we report a synthetic strategy for Pt single-atom catalysts with outstanding stability in several reactions under demanding conditions. The Pt atoms are firmly anchored in the internal surface of mesoporous Al2O3, likely stabilized by coordinatively unsaturated pentahedral Al3+ centres. The catalyst keeps its structural integrity and excellent performance for the selective hydrogenation of 1,3-butadiene after exposure to a reductive atmosphere at 200 °C for 24 h. Compared to commercial Pt nanoparticle catalyst on Al2O3 and control samples, this system exhibits significantly enhanced stability and performance for n-hexane hydro-reforming at 550 °C for 48 h, although agglomeration of Pt single-atoms into clusters is observed after reaction. In CO oxidation, the Pt single-atom identity was fully maintained after 60 cycles between 100 and 400 °C over a one-month period.

Popping of Graphite Oxide: Application in Preparing Metal Nanoparticle Catalysts

A popcorn-like transformation of graphite oxide (GO) is reported and used to synthesize metal nanoparticle catalysts. The popping step is unique and essential, not only generating a high-surface-area support but also partially decomposing the metal precursors to form well-separated metal oxide nuclei, which would further evolve into highly dispersed and uniform-sized nanoparticles in the subsequent reduction.

Incarceration of (PdO)nand PdnClusters by Cage-Templated Synthesis of Hollow Silica Nanoparticles

Metal nanoclusters have recently attracted considerable interest because of their distinct chemical and physical properties, such as catalytic activities, optical properties, and magnetic behavior, which are different from those of both bulk materials and mononuclear metal species. When the number of metal centers (n) is small, the properties of the metal clusters (Mn) change dramatically with increasing n value. Therefore, intense efforts have been made to synthesize small-numbered, well-defined clusters with nondistributed n values. Gas-phase preparation by metal vaporization is a promising physical method to prepare smallnumbered Mn clusters (typically in the range of n = 10– 10000). Solution synthesis through the reduction of metal ion precursors within templates is also an efficient method to give Mn clusters of similar size. [5] With all of these methods it is still difficult to strictly control the number of metal centers particularly when n is around 10 or less. Furthermore, the resulting synthesized clusters are often chemically and physically labile and are quite prone to oxidation or fusing into larger clusters. The most controllable methods to synthesize metal clusters with small n values (n = 4–60) have been reported utilizing the voids of dendrimers, but this method is hampered by the tedious dendrimer synthesis. Herein, we report a unique approach to incarcerated metal clusters with strictly controlled n values (ca. 12) within hollow silica nanoparticles. Our method utilizes a Pd12L24 spherical complex as a template for the hollow silica synthesis. After sol-gel condensation around the sphere, the incarcerated Pd12L24 core is calcinated to give (PdO)n oxide clusters and subsequently reduced to Pdn metal clusters (Figure 1). X-ray absorption fine structure (XAFS) analysis indicated that the

Local Structure and La L1 and L3-Edge XANES Spectra of Lanthanum Complex Oxides

La L1 and L3-edge X-ray absorption near-edge structure (XANES) of various La oxides were classified according to the local configuration of La. We found a correlation between both of the areas of the pre-edge peaks of the La L1-edge XANES spectra and the full width at half-maximum of white line of La L3-edge XANES spectra and the local configuration of La. Theoretical calculation of the XANES spectra and local density of states reveals the difference of La L1 and L3-edge XANES spectra of various La compounds is related to the p-d hybridization of the unoccupied band and broadening of the d band of La induced by the difference of local configuration. In addition, simplified bond angle analysis parameters defined by the angles of the La atom and the two adjacent oxygen atoms are correlated to the pre-edge peak intensity of the La L1-edge XANES spectra. These results indicate that quantitative analysis of La L1 and L3-edge XANES spectra could be an indicator of the local structure of La materials.

In situ time-resolved DXAFS study of Rh nanoparticle formation mechanism in ethylene glycol at elevated temperature

A combination of in situ time-resolved DXAFS and ICP-MS techniques reveals that the formation process of Rh nanoparticles (NPs) from rhodium trichloride trihydrate (RhCl(3)·3H(2)O) in ethylene glycol with polyvinylpyrrolidone (PVP) at elevated temperature is a first-order reaction, which indicates that uniform size Rh NPs appear consecutively and these Rh NPs do not aggregate with each other.

Rational Design of a Molecular Nanocatalyst‐Stabilizer that Enhances both Catalytic Activity and Nanoparticle Stability

Keywords: catalyst stability ; catalytic activity ; hydrogenation ; nanoparticles ; stabilizers Reference EPFL-ARTICLE-184053doi:10.1002/cctc.201200552View record in Web of Science Record created on 2013-02-27, modified on 2017-12-03