Hiroaki Nitani

Carbon-supported AuPd bimetallic nanoparticles synthesized by high-energy electron beam irradiation for direct formic acid fuel cell

Nanoparticle catalysts of carbon-supported Pd (Pd/C) and carbon-supported AuPd (AuPd/C) for the direct formic acid fuel cell (DFAFC) anode were synthesized by the reduction of precursor ions in an aqueous solution irradiated with a high-energy electron beam. We obtained three kinds of nanoparticle catalysts: (1) Pd/C, (2) AuPd/C of the core–shell structure, and (3) AuPd/C of the alloy structure. The structures of AuPd nanoparticles were controlled by the addition of citric acid as a chelate agent, and sodium hydroxide as a pH controller. The structures of nanoparticle catalysts were characterized using transmission electron microscopy, inductively coupled plasma atomic emission spectrometry, the techniques of X-ray diffraction and X-ray absorption fine structure. The catalytic activity of the formic acid oxidation was evaluated using linear sweep voltammetry. The oxidation current value of AuPd/C was higher than that of Pd/C. This indicated that the addition of Au to Pd/C improved the oxidation activity of the DFAFC anode. In addition, the AuPd/C of the alloy structure had higher oxidation activity than the AuPd/C of the core–shell structure. The control of the AuPd mixing state was effective in enhancing the formic acid oxidation activity.

In situ observation of carrier transfer in the Mn-oxide/Nb:SrTiO3 photoelectrode by X-ray absorption spectroscopy

The Mn-oxide/Nb:SrTiO3 photoelectrode for oxygen evolution reaction was investigated by in situ Mn K-edge XAFS spectroscopy under UV irradiation. The oxidization of the Mn oxide was observed via photoexcited carrier transfer, which results in the positive potential shift of the Mn oxide cocatalyst toward oxygen evolution reaction.

Preparation of carbon-supported PtCo nanoparticle catalysts for the oxygen reduction reaction in polymer electrolyte fuel cells by an electron-beam irradiation reduction method

We prepared carbon-supported PtCo bimetallic nanoparticles (PtCo/C) as electrode catalysts for the oxygen reduction reaction (ORR) at the cathodes in polymer electrolyte membrane fuel cells (PEFCs) by an electron-beam irradiation reduction method (EBIRM). An EBIRM allows nanoparticles to be easily prepared by the reduction of precursor ions in an aqueous solution irradiated with a high-energy electron beam. The structures of PtCo/C were characterized by transmission electron microscopy, inductively coupled plasma atomic emission spectrometry, and the techniques of X-ray diffraction and X-ray absorption near edge structure. It found for the first time that both PtCo alloy and Co oxide were prepared simultaneously on the carbon support by an EBIRM. The catalytic activity and durability of PtCo/C were evaluated by linear-sweep voltammetry and cyclic voltammetry, respectively. The addition of Co to Pt/C not only enhanced the catalytic activity for the ORR but also improved the catalytic durability. As the Co concentration increased, both behaviors became pronounced. These improvements are explained by the effects of both PtCo alloy and Co oxide. We demonstrated that an EBIRM can not only synthesize the alloy and oxide simultaneously on the carbon support but also mass-produce the electrode catalysts for PEFC cathodes.

Radiochemical synthesis of silver nanoparticles onto textile fabrics and their antibacterial activity

This paper presents a new technique for synthesizing silver nanoparticles immobilized on textile fabrics using a radiochemical process. In this process, the irradiation of a high-energy electron beam on an aqueous solution containing silver ions induces a reducing reaction that forms metallic silver nanoparticles. Small Ag particles of about 2–4 nm were observed together with relatively large particles of more than 10 nm. These nanoparticles are firmly immobilized on the surface of a support textile fabric without the need for any binder or surfactant. The amount of silver nanoparticles immobilized was found to depend on the water content of the support textile fabric, suggesting that the silver ions are reduced not only by radiochemical species generated by the radiolysis of water, but also by radiochemical species generated in the irradiated support fabric itself. The silver nanoparticles that were immobilized on the support textile fabric exhibited an excellent antibacterial activity across a wide antibacterial spectrum, even after a durability test involving washing the fabric 100 times.

Activity and Durability of PtRuP Catalysts and Their Atomic Structures

PtRuP catalysts were synthesized by electroless plating method and chelate ligand was added to the synthetic solution to narrow down difference of reduction potentials in Pt and Ru ions. XRD and XAFS analyses revealed that the addition of the ligand promoted alloying of Pt and Ru atoms in the PtRuP catalyst. Methanol oxidation activity and durability were improved by the well-alloyed PtRuP catalyst, which supported bi-functional mechanism. INTRODUCTION Based on bi-functional mechanism, an explanation for improvement of CO tolerance in Pt catalyst by addition of Ru proposed by Watanabe, Pt and Ru atoms in the PtRu catalyst should be well alloyed and Ru atom should be in close vicinity to Pt one at surface of the catalyst. However, Pt core/Ru shell structure is likely to be formed in wet syntheses such as alcohol reduction and electroless plating, because Pt ion is preferentially reduced due to difference of standard reduction potentials in Pt and Ru ions. In PtRuP catalyst synthesized by the alcohol reduction method, the highest methanol oxidation activity was achieved with composition of Pt78Ru22. The composition largely deviates from Pt50Ru50 commonly known as the best composition for the PtRu catalyst. This deviation is coming from the difference of the standard reduction potentials. Bulk composition of Pt in the catalyst should be higher than 50 at.% in order to make surface composition of the catalyst into 50 to 50, because Pt enriched core is formed by the preferential reduction of Pt ion in the alcohol reduction method. In this study, PtRuP catalysts were synthesized by electroless plating method. Chelate ligand was added in the synthetic solution to narrow down the difference of the reduction potentials. Effects of the ligand addition on methanol oxidation activity and on durability of the catalysts are reported in terms of atomic structures of the PtRuP catalysts. EXPERIMENTALS PtRuP catalysts were synthesized by electroless plating method by using NaPH2O2 as a reducing agent as well as source of P. H2PtCl6, RuCl3 and NaPH2O2 were dissolved in ion exchanged water and PtRuP catalyst was deposited on carbon support at 358 K. Chelate ligand was added in the synthetic solution to narrow down the difference of the reduction potentials. Methanol oxidation activity of the catalysts was evaluated by linear sweep volammetry (LSV). Durability of the catalysts was tested by repeated cyclic voltammetry (0.2-1.1 V vs. NHE). Atomic structures of the PtRuP catalysts were analyzed by XRD and by X-ray absorption fine structure (XAFS) of Pt-LIII and Ru-K edges. RESULTS AND DISCUSSION LSV measurements showed that methanol oxidation activity was improved in PtRuP catalyst synthesized with chelate ligand. Figure 1 shows XRD patterns of PtRuP and commercialized Pt catalysts. Diffraction angle from (111) plane of PtRuP catalyst synthesized without chelate ligand is almost same as that of Pt catalyst. On the other hand, the diffraction shifted toward higher angle in PtRuP catalyst synthesized with chelate ligand. This sift toward higher diffraction angle suggests that alloying of Pt and Ru atoms in the PtRuP catalyst was advanced, because lattice spacing of Pt decreases by alloying with Ru which has smaller atomic radius than that of Pt. Results of XAFS analysis are summarized in Table 1. Here, indexes of PRu and PPt (PRu=NRu-Pt/(NRu-Ru+NRu-Pt), NRu-Pt is Pt coordination number and NRu-Ru is Ru coordination one viewing from Ru atom, PPt is viewing from Pt atom) were introduced to compare frequency of neighboring Pt atom to Ru one. PRu and PPt of PtRuP catalyst synthesized with chelate ligand are larger than those of PtRuP catalyst synthesized without ligand, which indicates alloying of Pt and Ru atoms was advanced in the PtRuP catalyst synthesized with chelate ligand. Figure 2 shows durability of PtRuP and commercialized PtRu catalysts. It was found that addition of chelate ligand improved durability of PtRuP catalyst. Compositional analysis of the repeated CV tested solution showed that dissolution amount of Ru was smaller in PtRuP catalyst synthesized with chelate ligand, which also arises from well alloyed structure of the PtRuP catalyst synthesized with chelate ligand. In conclusion, addition of chelate ligand narrowed down difference of reduction potentials of Pt and Ru ions and well alloyed PtRuP catalyst was synthesized by electroless plating method. Methanol oxidation activity and durability were improved by the well-alloyed PtRuP catalyst. REFERENCES (1) M. Watanabe and S. Motoo, J. Electroanal. Chem. Interfacial Electrochem., 60, 267 (1975). (2) Daimon and Y. Kurobe, Catalysis Today, 111, 182 (2006). Fig.1 XRD Patterns of Catalysts. 34 36 38 40 42 44 2θ (deg.) In te ns ity (a .u .) Pt (111) PtRuP with chelate ligand PtRuP without chelate ligand

X-ray-induced reduction of Au ions in an aqueous solution in the presence of support materials and in situ time-resolved XANES measurements.

In situ time-resolved XANES measurements of Au ions in an aqueous solution in the presence of support materials were performed under synchrotron X-ray irradiation. The synchrotron X-ray-induced reduction of Au ions leads to the formation of Au nanoparticles on the carbon particles, acrylic cell or polyimide window. The deposited Au metallic spots were affected by the wettability of carbon particles.

Multiple-wavelength neutron holography with pulsed neutrons

Multiple-wavelength neutron holography demonstrated a reconstruction of highly precise 3D atomic images around dopants. Local structures around impurities in solids provide important information for understanding the mechanisms of material functions, because most of them are controlled by dopants. For this purpose, the x-ray absorption fine structure method, which provides radial distribution functions around specific elements, is most widely used. However, a similar method using neutron techniques has not yet been developed. If one can establish a method of local structural analysis with neutrons, then a new frontier of materials science can be explored owing to the specific nature of neutron scattering—that is, its high sensitivity to light elements and magnetic moments. Multiple-wavelength neutron holography using the time-of-flight technique with pulsed neutrons has great potential to realize this. We demonstrated multiple-wavelength neutron holography using a Eu-doped CaF2 single crystal and obtained a clear three-dimensional atomic image around trivalent Eu substituted for divalent Ca, revealing an interesting feature of the local structure that allows it to maintain charge neutrality. The new holography technique is expected to provide new information on local structures using the neutron technique.

In situ observation of reduction kinetics and 2D mapping of chemical state for heterogeneous reduction in iron-ore sinters

Iron-ore sinters constitute the major component of the iron-bearing burden in blast furnaces, and the mechanism of their reduction is one of the key processes in iron making. The heterogeneous reduction of sintered oxides was investigated by the combination of X-ray fluorescence and absorption fine structure, X-ray diffraction, and computed tomography. Two - dimensional mapping of the chemical states (CSs) was performed. The iron CSs FeIII, FeII, and Fe0 exhibited a heterogeneous distribution in a reduced sinter. The reduction started near micro pores, at iron-oxide grains rather than calcium-ferrite ones. The heterogeneous reduction among grains in a sinter may cause the formation of micro cracks. These results provide fundamental insights into heterogeneous reduction schemes for iron-ore sinters.

Invariant nature of substituted element in metal-hexacyanoferrate

The chemical substitution of a transition metal (M) is an effective method to improve the functionality of materials. In order to design the highly functional materials, we first have to know the local structure and electronic state around the substituted element. Here, we systematically investigated the local structure and electronic state of the host (Mh) and guest (Mg) transition metals in metal-hexacyanoferrate (M-HCF), Nax(Mh, Mg)[Fe(CN)6]y (1.40 < x < 1.60 and 0.85 < y < 0.90), by means of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) analyses. The EXAFS and XANES analyses revealed that the local structure and electronic state around Mg are essentially the same as those in the pure compound, i.e, Mg-HCF. Such an invariant nature of Mg in M-HCF is in sharp contrast with that in layered oxide, in which the Mg valence changes so that local Mg-O distance (dM-Og) approaches the Mh-O distance (dM-Oh).

Newly designed double surface bimorph mirror for BL-15A of the photon factory

BL-15A is a new x-ray undulator beamline at the Photon Factory. It will be dedicated to two independent research activities, simultaneous XAFS/XRF/XRD experiments, and SAXS/WAXS/GI-SAXS studies. In order to supply a choice of micro-focus, low-divergence and collimated beams, a double surface bimorph mirror was recently developed. To achieve further mirror surface optimization, the pencil beam scanning method was applied for “in-situ” beam inspection and the Inverse Matrix method was used for determination of optimal voltages on the piezoelectric actuators. The corrected beam profiles at every focal spot gave good agreement with the theoretical values and the resultant beam performance is promising for both techniques. Quick and stable switching between highly focused and intense collimated beams was established using this new mirror with the simple motorized stages.