Takao A. Yamamoto

Osaka University, Suika, Ōsaka, Japan

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Publications (74)163.68 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Catalytic activities of sonochemically prepared Au-core/Pd-shell-structured bimetallic nanoparticles (NPs) immobilised on TiO2 were evaluated. Comparing with the mixture of monometallic Au and Pd NPs on TiO2, core/shell-immobilised catalysts exhibited higher activities for the partial reduction of nitrobenzene (NB) to aniline (AN), suggesting that the synergistic effect originating from the core/shell structure enhanced the catalytic activities. In the case of high Au/Pd ratios, where the Pd-shell thickness was calculated to be 0.5 nm or lower, infrared spectroscopic measurements of adsorbed CO showed that the Au cores were successfully covered with Pd shells. It was found that a thin Pd shell of one layer or two layers of Pd atoms effectively catalysed the reduction of NB under ambient temperature, whereas the formation of AN was not confirmed on monometallic Au NP-immobilised catalysts.
    Journal of Experimental Nanoscience 02/2015; 10(3). · 1.04 Impact Factor
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    ABSTRACT: Carbon-supported Pt–SnO2 electrocatalysts with various Sn/Pt molar ratios were prepared by an electron beam irradiation method. These catalysts were composed of metallic Pt particles approximately 5 nm in diameter together with low crystalline SnO2. The contact between the Pt and SnO2 in these materials varied with the amount of dissolved oxygen in the precursor solutions and it was determined that intimate contact between the Pt and SnO2 significantly enhanced the catalytic activity of these materials during the ethanol oxidation reaction. The mechanism by which the contact varies is discussed based on the radiochemical reduction process.
    Radiation Physics and Chemistry 11/2014; · 1.19 Impact Factor
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    ABSTRACT: Synchrotron X-ray-induced reduction of Au ions in an aqueous solution with or without support materials is reported. To clarify the process of radiation-induced reduction of metal ions in aqueous solutions in the presence of carbon particles as support materials, in situ time-resolved XANES measurements of Au ions were performed under synchrotron X-ray irradiation. XANES spectra were obtained only when hydrophobic carbon particles were added to the precursor solution containing Au ions. Changes in the shape of the XANES spectra indicated a rapid reduction from ionic to metallic Au in the precursor solution owing to synchrotron X-ray irradiation. In addition, the effects of the wettability of the carbon particles on the deposited Au metallic spots were examined. The deposited Au metallic spots were different depending on the relationship of surface charges between metal precursors and support materials. Moreover, a Au film was obtained as a by-product only when hydrophilic carbon particles were added to the precursor solution containing the Au ions.
    Journal of Synchrotron Radiation 09/2014; · 2.19 Impact Factor
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    ABSTRACT: The effects of phosphorus (PH2O2−) and copper (Cu2+) additions to the aqueous precursor solution on the structure of Pt–Cu nanoparticles were investigated for a radiation-induced reduction method. Addition of PH2O2− in the precursor solution reduced the diffraction intensity of Pt or Pt–Cu crystallites due to smaller size and/or lower crystallinity. Both the diffraction intensity and the particle size (measured by an electron microscope) were minimized when Cu/Pt ratio was 0.05–0.25, which was attributed to the effects of copper and phosphorus to stabilize crystallites and particles through the negative heat of mixing. The concomitant increase in phosphorus content suggested that PH2O2− is partly reduced and taken into the Pt lattice. Further increase of copper content caused larger particles and decrease in phosphorus content. These trends were also consistent with electrochemical surface area and oxidation/reduction behavior of Pt surface. The radiation-induced reduction method is suited to produce small Pt–Cu particles uniformly distributed on carbon support, which are potentially served for heat treatment for improved oxygen reduction performance.
    Journal of Nanoparticle Research 01/2014; 16(3). · 2.28 Impact Factor
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    ABSTRACT: In order to understand the formation process of metal–oxide composite in an electron beam irradiation method in aqueous phase, the structure and composition of obtained solid were correlated to the synthesis parameters. Transition metal did not precipitate alone by the electron beam irradiation, but they did in the presence of platinum or support. Due to the relatively high reduction potential, copper underwent reduction to metallic state and readily precipitated by forming Pt–Cu alloy and/or copper oxide on solid surface. In the Pt–Cu/CeO2 system, the structure of Pt–Cu was ruled by two competing factors, growth of alloy nanoparticles promoted by sulfate ion and deposition of metal (alloy) on CeO2 support with their concomitant partial oxidation. CeO2 was suggested to immobilize the metals oxidatively before they coalesce. Iron barely formed alloy with Pt, but it directly precipitated on support as oxide without being reduced to metal due to its oxophilicity. Oxide was formed either via reduction to metallic state (for Pt and Cu) or through direct oxygenation or hydroxylation on solid (for Fe). Under the restriction of reduction potential, the size and composition of alloy nanoparticles and the content of oxide phase were drastically modified by support surface property and anion species in the solution.
    Journal of Alloys and Compounds 11/2013; 577:125–130. · 2.73 Impact Factor
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    Journal of Materials Science 07/2013; 48(14):5047-5054. · 2.31 Impact Factor
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    ABSTRACT: Magnetic particles generate sonic waves when they are exposed to an AC magnetic field. We call this phenomenon sonic wave emission by simulated magnetic nanoparticles (SWESMAN). We found an extremely long signal delay between the SWESMAN signal and the excitation current: it took 2.9 ms for a sonic wave to propagate 10 mm in a polyacrylamide gel, which is approximately 500 times longer than that expected. X–Y mapping of the signal delays was performed and a linear relationship between the signal delay and the propagation distance was found. Thus, the three-dimensional position can be determined from these signal delays. This study represents the first step toward using SWESMAN in practical applications. The extremely long signal delay has the potential to give a high spatial resolution in such applications.
    Materials Letters 05/2013; 98:51–54. · 2.27 Impact Factor
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    ABSTRACT: The influence of surface properties of CeO2 support on the structure of Pt–Cu bimetallic system in an aqueous-phase synthesis process using electron beam irradiation and the resulting impact on PROX performance were investigated. The exposed hydroxyls on CeO2 surface with low hydrogen-bonding water led to cluster-like Pt–Cu phase with low crystallinity and high oxidation state. It was attributed to a strong interaction of CeO2 and Pt–Cu phase. In CO atmosphere, such oxidic or fine Pt–Cu phase was reduced to platinum and partially reduced CuOx, and CeO2 was concomitantly reduced to retain carbonate species on its surface. This in turn affected catalytic performance, i.e., a significant increase in O2 conversion was observed in an oxygen-rich PROX condition compared to a stoichiometric condition while selectivity was retained in higher level than monometallic Pt. The higher oxidation state of Pt in the excess O2 was suggested to be responsible for the drastic change in activity. The surface chemical property of CeO2 was shown to affect PROX activity through the structure and oxidation state of Pt–Cu species which originates from metal–ceria interaction in the synthesis stage.
    Chemical Engineering Journal 05/2013; 223:347–355. · 4.06 Impact Factor
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    ABSTRACT: Electrode catalysts composed of carbon-supported PtRu nanoparticles (PtRu/C) for use as a direct methanol fuel cell anode were synthesized by the reduction of precursor ions in an aqueous solution via irradiation with a high-energy electron beam. The effect of pH control in the precursor solution on the PtRu mixing state and the methanol oxidation activity was studied in order to enhance the catalytic activity for methanol oxidation. The PtRu/C structures were characterized by transmission electron microscopy, inductively coupled plasma atomic emission spectrometry, X-ray fluorescence spectrometry, and X-ray diffraction and X-ray absorption fine structure techniques. The methanol oxidation activity was evaluated by linear sweep voltammetry. The initial pH of the precursor solution has little influence on the average grain size for the metal particles (approximately 3.5 nm) on the carbon particle supports, but the dispersibility of the metal particles, PtRu mixing state, and methanol oxidation activity differed. The maintenance of a low pH in the precursor solution gave the best dispersibility of the PtRu nanoparticles supported on the surface of the carbon particles, whereas, a high pH gave the best PtRu mixing state and the highest oxidation current although a low dispersibility of the PtRu nanoparticles supported on the surface of the carbon particles was obtained. The PtRu mixing state strongly correlated with the methanol oxidation current. In addition, a high pH was more effective for PtRu mixing when using an electron beam irradiation reduction method, because the complexation reaction of the chelating agents was improved, which resulted in an enhancement of the catalytic activity for methanol oxidation.
    Journal of Nanoparticle Research 05/2013; 15(5). · 2.28 Impact Factor
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    ABSTRACT: In order to investigate the effect of transition metal addition to platinum with different support materials on preferential CO oxidation, structure and chemical properties of supported bimetallic catalysts prepared by electron beam irradiation method were correlated to the catalytic performance. On Al2O3, decoration of Pt by small amount of Co (Co/Pt ∼ 0.03) drastically increased CO and O2 conversions while addition of equimolar Cu to Pt increased them only above 100 °C, where the rate-controlling factor was suggested to change from oxygen transport to CO activation. On CeO2, either addition of Co or Cu to Pt had minor or negative effect on high O2 conversion inherent to high oxygen transport at Pt–CeO2 interface. On Pt–Cu/CeO2, however, metal-CuOx interface dominates the reaction characteristics to give improved selectivity, which is suitable for deep CO removal in excess O2/CO condition. The order of selectivity above 100 °C, Pt–CoOx > Pt(alloy)–CuOx > Pt–CeO2 interfaces, was derived from structural analysis and catalytic tests.
    International Journal of Hydrogen Energy 04/2013; 38(11):4456–4465. · 2.93 Impact Factor
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    ABSTRACT: This paper describes a novel synthesis method of stabilizer-free Pt/TiO2 composite nanoparticles using electron beam irradiation. The chemical compositions were analyzed by inductively coupled plasma-atomic emission spectroscopy. The microstructures of the samples were observed by using transmission electron microscope. Pt nanoparticles with the sizes of 2–4 nm were deposited on TiO2 without any use of stabilizers. The concentrations of Pt ions and 2-propanol notably affected the size and shape of Pt nanoparticles. Their reactions of preferential CO oxidation were measured in temperature region from 60 to 140 °C. The Pt/TiO2 catalyst with spherical Pt nanoparticles exhibited a 67% of CO conversion rate and 100% of selectivity at a low temperature of 60 °C.
    Materials Research Bulletin 04/2013; 48(4):1347–1351. · 1.97 Impact Factor
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    ABSTRACT: The structure of bimetallic nanoparticles has a great impact on catalytic performance. By radiation-induced reduction of metal ions in the aqueous phase, supported bimetallic Pt–Cu catalysts were synthesized with various Cu loadings, support materials, and copper sources. These parameters had a great impact on the structural properties of Pt–Cu nanoparticles. On carbon black, Pt–Cu alloy nanoparticles were readily formed with little oxide phase. On CeO2 support, Pt–Cu was highly oxidized for the low-Cu region (Cu/Pt < 1), and Pt–Cu alloy nanoparticles were formed together with CuOx for the high-Cu region (Cu/Pt > 1). The structure of CeO2-supported catalysts obtained from two different copper sources (CuSO4 and CuCl2) suggested that such a drastic change in oxidation state is the result of two competing effects, CeO2 to oxygenate metals and the sulfate ion to stabilize them in the metallic state. The reaction characteristics in preferential CO oxidation reflected the Pt–Cu structure (oxidation state) determined by those parameters in the synthesis stage. The low-Cu oxidic samples showed excellent light-off performance, which was attributed to high oxygen transport from CeO2 to metals through the metal–CeO2 interface. The high-Cu samples comprising a Pt–Cu alloy and CuOx showed excellent selectivity, which was attributed to the metal–CuOx interface predominating over metal–CeO2. The latter exhibited 100% CO conversion in a wide temperature range in excess O2 conditions.
    The Journal of Physical Chemistry C 03/2013; 117(11):5742–5751. · 4.84 Impact Factor
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    ABSTRACT: Electrode catalysts composed of carbon supported PtRu nanoparticles (PtRu/C) synthesized by radiochemical process were annealed to control the PtRu substructure to enhance catalytic activity. The substructure of the PtRu nanoparticles synthesized by using high-energy electron beam under acidic condition was Pt-rich core/Ru-rich shell type, reflecting the redox potentials of each precursor ions. The material characterization techniques revealed that the reductive annealing led to the mixing of PtRu both in the core and on the surface. The sample with annealing temperature of 300 o C for 5 hour showed the highest methanol oxidation current, 2.3 times higher than that obtained with before annealing.
    Materials Research Society Symposium Proceedings 2013; 03/2013
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    ABSTRACT: Carbon supported Pt-SnO 2 electrocatalysts with different Sn/Pt molar ratios were prepared by an electron beam irradiation method. Dissolved gas conditions in the vials irradiated with electron beam were controlled to air or Ar. The results of the material analyses showed that both Pt and SnO 2 were immobilized onto carbon support in all catalysts. Bubbling Ar to the precursor solution led to steady change of metal contents in response to the precursor concentrations. The ethanol oxidation activity plotted against Sn/Pt ratio behaved differently with dissolved gas condition of the vial. This difference is discussed with supposed existing state of SnO 2 in connection with the reduction process of Pt and Sn.
    Materials Research Society Symposium 2013; 03/2013
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    Journal of Materials Science 03/2013; 48(5):2142-2150. · 2.31 Impact Factor
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    ABSTRACT: La0.666Sr0.373Mn0.943Cu0.018O3 spheres (average diameter: 2.4 mm) were used as magnetic hyperthermia mediators. They were prepared by the polymerized complex method and alginate gelation. A starch paste containing nine such spheres (0.1034 g) was maintained at a constant temperature when it was exposed to an alternating current magnetic field (frequency: 1 MHz; magnetic field strength: 20–55 Oe-rms). After exposure to a magnetic field with a field strength of 55 Oe-rms for 60 min, a temperature of T60=73.5 °C was obtained. T60 for field strengths of 40 and 55 Oe-rms differed by only 2.6 °C. La0.666Sr0.373Mn0.943Cu0.018O3 spheres satisfy the following important requirements for heating mediators: their amount can be controlled in tumors, their temperature can be controlled without monitoring, and their heating ability is independent of the field strength and direction. Thus, La0.666Sr0.373Mn0.943Cu0.018O3 spheres are promising heating mediators for inducing magnetic hyperthermia.
    Journal of Magnetism and Magnetic Materials 03/2013; 329:49–52. · 2.00 Impact Factor
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    ABSTRACT: The magnetic properties and the magnetocaloric effect (MCE) of La0.7−xPrxCa0.3MnO3 perovskite materials (0 ⩽ x ⩽ 0.69) were investigated. Magnetic entropy changes (ΔSM) were calculated both from magnetization data using Maxwell’s relation and specific heat data. With increasing x, the Curie temperature TC decreases from 240 to 119 K. The maximum value of ΔSM, the adiabatic temperature change (ΔTad), and the relative cooling power (RCP) induced by demagnetization from 1 to 0 T were 6.8 J/kg K, 2.5 K, and 54 J/kg at 225 K, for x = 0.1. These parameters were about 200%, 80%, and 65% of the comparable Gd parameters, indicating that these materials are promising candidates as magnetic refrigerants in the sub-room temperature range (200–250 K).
    Journal of Alloys and Compounds 02/2013; 551:195–199. · 2.73 Impact Factor
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    ABSTRACT: Pt–Cu supported on CeO2 and mechanically mixed with CeO2 were synthesized using an electron beam irradiation method to probe the active metal–oxide interfaces for catalytic CO preferential oxidation. The lack of activity for the mechanical mixture of Pt with CeO2 showed the metal–CeO2 interface is critical for monometallic Pt. The comparable activity for the CeO2-supported Pt–Cu and mechanical mixture of Pt–Cu with CeO2 suggested platinum–copper contact as a new active site for bimetallic Pt–Cu. A non-linear increase of activity along the Cu content in catalyst and the Cu–O bonds detected in XANES spectra in the reaction condition at 100 °C suggested the presence of CuOx on the Pt–Cu alloy surface as strong chemisorption sites for oxygen. Graphical Abstract
    Catalysis Letters 01/2013; · 2.29 Impact Factor
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    ABSTRACT: This paper describes a novel synthesis method of polymer-free PtCu/C nanoparticles using irradiation of a high-energy electron beam. The method is a simple and quick one-pot process in aqueous solution system. The compositions were analyzed by inductively coupled plasma-atomic emission spectroscopy. The microstructures of the samples were observed by using transmission electron microscope. PtCu nanoparticles with 2-nm size were deposited on carbon supports without any use of polymer stabilizers. X-ray diffraction analysis showed that the random alloy formations of PtCu were occurred without any heat treatment. Radiolytic synthesis was effectively applied to the synthesis of polymer-free PtCu/C. We pointed out a difference in alloying behaviors between PtCu and PtRu systems which was reported in a previous study.
    Materials Letters 09/2012; 82:33–35. · 2.27 Impact Factor
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    ABSTRACT: A nanoparticle catalyst of PtRuAu/C was synthesized by including an Au precursor in the radiolytic process for preparing a PtRu/C catalyst. Their methanol oxidation activity and electrochemical durability were measured by linear sweep voltammetry before and after potential cycling treatment. PtRuAu/C had a significantly higher durability than PtRu/C while maintaining a comparable high activity. The morphology and substructure of the nanoparticles were investigated by energy-dispersive x-ray spectroscopy, x-ray diffraction, and x-ray absorption fine structure spectroscopy. Metallic nanoparticles with diameters of about 2 nm were obtained; they probably had Pt-core/PtRu-shell structures. Transmission electron microscopy observations after potential cycling revealed that 2-nm-diameter nanoparticles containing Au did not coarsen, whereas nanoparticles without Au coarsened significantly to 3.7 nm. Some crystal defaults were observed in the coarsened particles, implying that the coarsening was caused by Ostwald ripening. The Au addition to catalyst particles consisting of PtRu inhibits coarsening and consequently improves the electrochemical durability.
    Journal of Materials Research. 04/2012; 27(07).

Publication Stats

535 Citations
163.68 Total Impact Points

Institutions

  • 1998–2014
    • Osaka University
      • • Graduate School of Engineering
      • • School of Pharmaceutical Sciences
      Suika, Ōsaka, Japan
  • 2011
    • Doshisha University
      Kioto, Kyōto, Japan
  • 2005
    • Osaka Prefectural College Of Technology
      Ōsaka, Ōsaka, Japan