Naoki Toshima

Bimetallic nanoparticles—novel materials for chemical and physical applications

A new class of materials for catalysis have been intensively investigated, that is, ‘bimetallic nanoparticles’. Extensive studies of non-supported bimetallic nanoparticle dispersions, stabilized by polymers or ligands, started only about 10 years ago. Many preparative procedures have been proposed, and detailed characterizations have been carried out on bimetallic nanoparticles, thanks to the rapid improvement of analytical technology on surface and nanoscale materials. In this review, we focus on the preparation, characterization and application to catalysis of polymer- or ligand-stabilized bimetallic nanoparticles in dispersion, emphasizing our own work and introducing recent progress of this area.

Preparation of Colloidal Transition Metals in Polymers by Reduction with Alcohols or Ethers

Abstract Colloidal dispersions of rhodium, palladium, osmium, iridium, and platinum are prepared by refluxing the methanol-water solutions of rhodium(III) chloride, palladium(II) chloride, osmium(VIII) oxide, sodium chloroiridate, and chloroplatinic acid, respectively, in the presence of poly(vinyl alcohol) as a protective colloid. The preparations of colloidal dispersions of rhodium are successful in the presence of vinyl polymer with polar group such as poly(vinyl alcohol), polyvinylpyrrolidone, or poly(methyl vinyl ether). Polyethyleneimine, gelatin, polyethylene glycol), and dextran are ineffective as the protective colloid. Water-soluble primary alcohols such as methanol and ethanol, water-soluble secondary alcohols such as 2-propanol, and water-soluble diethers such as 1,4-dioxane are available as reductants for preparation of the colloidal dispersion of rhodium. The average diameters of metal particles in the colloidal dispersions of palladium, rhodium, platinum, iridium, and osmium in poly(vinyl a...

Polymer-protected palladium–platinum bimetallic clusters: preparation, catalytic properties and structural considerations

Refluxing mixed solutions of palladium(II) chloride and hexachloroplatinic(IV) acid in ethanol–water (1 : 1, v/v) in the presence of poly(N-vinyl-2-pyrrolidone) results in well dispersed and stable colloidal dispersions of polymer-protected palladium/platinum bimetallic clusters. The UV–VIS absorption spectra and transmission electron micrographs indicate that these bimetallic cluster particles are homogeneous in size, each particle containing both palladium and platinum atoms. Dispersions of these bimetallic clusters were used as catalysts for the selective hydrogenation of cycloocta-1,3-diene to cyclooctene at 30 °C under hydrogen gas at atmospheric pressure. The catalytic activity for the partial hydrogenation was found to depend on the metal composition of the particles. A bimetallic cluster with a mole ratio of Pd : Pt = 4 : 1 was the most active catalyst, twice as active as a typical colloidal palladium catalyst, for the diene hydrogenation. The dependence of the activity and the selectivity on the surface structure of the cluster particle strongly indicates that the cluster particle with the highest catalytic activity had a ‘Pd-surrounded Pt core’ structure.

Frequency modulation response of a liquid-crystal electro-optic device doped with nanoparticles

Palladium nanoparticles covered with liquid-crystal molecules were prepared by UV irradiation of an alcohol solution of palladium(II) acetate in the presence of liquid-crystal molecules. The prepared Pd nanoparticles have an average diameter of 2.5 nm. A twisted nematic (TN) liquid-crystal device (LCD) was fabricated by doping with Pd nanoparticles covered with another kind of nematic liquid-crystal molecules. In this device the sign of the dielectric anisotropy (Δe) of the liquid-crystal molecules, which cover Pd nanoparticles, is opposite to that of nematic liquid-crystal molecules, which work as the host of the device (Δe>0). The TN-LCD cell fabricated in this research exhibits a frequency modulation response to an applied alternative voltage wave form.

Platinum nanoparticle is a useful scavenger of superoxide anion and hydrogen peroxide

Bimetallic nanoparticles consisting of gold and platinum were prepared by a citrate reduction method and complementarily stabilized with pectin (CP-Au/Pt). The percent mole ratio of platinum was varied from 0 to 100%. The CP-Au/Pt were alloy-structured. They were well dispersed in water. The average diameter of platinum nanoparticles (CP-Pt) was 4.7 ± 1.5 nm. Hydrogen peroxide (H2O2) was quenched by CP-Au/Pt consisting of more than 50% platinum whereas superoxide anion radical () was quenched by any CP-Au/Pt. The CP-Au/Pt quenched these two reactive oxygen species in dose-dependent manners. The CP-Pt is the strongest quencher. The CP-Pt decomposed H2O2 and consequently generated O2 like catalase. The CP-Pt actually quenched which was verified by a superoxide dismutase (SOD) assay kit. This quenching activity against persisted like SOD. Taken together, CP-Pt may be a SOD/catalase mimetic which is useful for medical treatment of oxidative stress diseases.

Colloidal silver catalysts for oxidation of ethylene

Abstract Colloidal silver catalysts protected by poly( N -vinyl-2-pyrrolidone) (PVP) were prepared by an ethanol reduction method of silver perchlorate in the presence of PVP. Transmission electron micrograph of the obtained colloids revealed fine particles (av. diameter 3.1 nm) with uniform size and narrow size distribution. Oxidation of ethylene catalyzed by PVP-protected silver colloids was performed in ethanol/water (1/1, v/v) at 90–95°C under 1 atm of ethylene/oxygen (2/1). Products were analyzed with gas chromatography, being identified to be ethylene oxide. The silver catalysts thus prepared had the higher catalytic activity than a commercial silver catalyst for oxidation of ethylene. Addition of alkali metal ions remarkably increased the catalytic activity of the colloidal silver catalyst.

Oxidation of ethylene catalyzed by colloidal dispersions of poly(sodium acrylate)-protected silver nanoclusters

Abstract Silver nanoclusters protected by poly(sodium acrylate) (PAA) were prepared by UV irradiation of an alcohol–water solution of silver perchlorate in the presence of PAA. Colloidal dispersions of PAA-protected Ag nanoclusters (PAA–Ag nanoclusters) were very stable at room temperature for months and composed of fine particles with an average diameter of 4.2 nm. X-Ray photoelectron spectroscopy data have suggested the 0-valency of Ag in PAA–Ag nanoclusters. Oxidation of ethylene, catalyzed by PAA–Ag nanoclusters, was performed in glycol under 1 atm. of ethylene/oxygen (2/1 v/v). Products were identified as ethylene oxide by analysis with gas chromatography. Ag nanoclusters thus prepared had higher catalytic activity than a commercial silver catalyst. The oxidation rate catalyzed by the PAA–Ag nanoclusters remarkably increased with increasing reaction temperature. PAA-protected Ag nanoclusters had much higher activity than poly(N-vinyl-2-pyrrolidone)-protected ones at high temperature. Addition of both cesium and rhenium ions eminently increased the catalytic activity of PAA–Ag nanoclusters.

Thermal transporting properties of electrically conductive polyaniline films as organic thermoelectric materials

Thermal transporting properties of electrically conductive polyaniline films were first investigated in wide range of temperatures above room temperature as organic thermoelectric materials. Thermal conductivities of various protonic acid-doped polyaniline films were measured by combination of a laser flash method and a differential scanning calorimeter in relation with electrical conductivity and a kind of dopant. The thermal conductivities thus measured are in the range of conventional organic polymers, indicating that the doped polyaniline films have extremely low thermal conductivities among electrically conductive materials, and have correlation with neither electrical conductivity, nor a kind of dopant. Consequently the polyaniline film, which shows very high electrical conductivity, has comparable thermoelectric figure-of-merit (ZT) with feasible inorganic thermoelectric materials such as iron silicide.

Preparation of Colloidal Rhodium in Poly(vinyl Alcohol) by Reduction with Methanol

Abstract Refluxing of a solution of poly(vinyl alcohol) and rhodium(III) chloride in methanol-water gives a colloidal dispersion of rhodium which is an effective catalyst for hydrogenation of cyclohexene in methanol at 30°C under atmospheric hydrogen pressure. Formaldehyde is produced quantitatively with the reduction of rhodium(III) chloride to metallic rhodium. The rhodium particles in the colloidal dispersion are found to consist of two kinds of particles, about 8 and 40 A in diameter by electron microscopy. The sizes of the small (8 A) and large (40 A) particles are almost constant during the course of refluxing. The number of small particles, which is the great majority of particles at the early stage of refluxing, gradually decreases; concurrently the number of large particle increases on prolonged refluxing. An absorption peak appears at 260 nm at the early stage of refluxing. The presence of the 260 nm peak, which indicates the coordination of poly(vinyl alcohol) to rhodium(III) ion, is indispensa...

Conductive polymers as a new type of thermoelectric material

Thermoelectric properties were investigated for the films of electrically conductive doped polyanilines. The thermoelectric performance, evaluated by thermoelectric figure-of-merit (ZT = T (S 2 σ) / κ), of various protonic acid-doped polyaniline bulk films was found to depend on the electrical conductivity a of the film. Thus, the higher the electrical conductivity, the higher the figure-of-merit is, because the thermal conductivity κ of polyaniline films does not depend on the electrical conductivity. Among the conductive bulk films of polyaniline, the highest figure-of-merit (ZT = 1 X 10 -4 ) was observed for (±)-10-camphorsulfonic acid (CSA)-doped polyaniline in an emeraldine form ( σ = 188 S cm -1 ) at room temperature. The multilayered film, composed of electrically insulating emeraldine base layers and electrically conducting CSA-doped emeraldine salt layers, exhibited 6 times higher ZT at 300 K than that of a bulk film of CAS-doped polyaniline, showing the highest ZT value of 1.1 X 10 -2 at 423 K. Stretching of the CAS-doped polyaniline film also increased the figure-of-merit of doped polyaniline films along the direction of the stretching.