X-ray Absorption Fine Structure Spectroscopy Study Research Articles

Operand XAFS Study of Hydrazine Oxidation Catalysts for Use in Anion Exchange Membrane Fuel Cells

In recent research, direct hydrazine hydrate fuel cells (DHFCs) have been considered to be one of the promising candidates to breakthrough resource problem using anion exchange membrane (AEM) with non-platinum group metal (non-PGM) catalysts for both anode and cathode. However, the catalytic activity and selectivity of anode catalyst of DHFCs are required to improve for popularization of DHFCs. In DHFC, electrochemical reactions take place in the anode and the cathode as shown in following equations (1) and (2), respectively.Electrochemical reaction of DHFCAnode: N2H4 + 4OH− → N2 + 4H2O + 4e− (1)Cathode: O2 + 2H2O + 4e− → 4OH− (2)Overall: N2H4 + O2 → N2 + 2H2O (3)Chemical reaction (decomposition) in anodeN2H4 → N2 + 2H2 (4)3N2H4 → N2 + 4NH3 (5)In the electrooxidation reaction of hydrazine, the most desirable reaction is the four-electron reaction given by equation (1). Co exhibits a higher rate of chemical reaction (decomposition) than that of Ni. The reaction selectivity of anode catalyst of DHFC affects to the enlargement of the fuel cell system, because generated ammonia is required to decompose harmless gas to discharge to the environment.In this study, carbon supported Ni, Co, and Ni0.87Zn0.13 catalysts (Ni/C, Co/C, and Ni0.87Zn0.13/C) as an anode catalyst of DHFC were synthesized by impregnation/freeze-drying procedure. Operand X-ray absorption fine structure spectroscopy (XAFS) study was performed to understand the potential dependence of local atomic and electronic structure of hydrazine oxidation catalyst in an alkaline electrolyte. The potential dependence of local atomic and electronic structure of prepared catalysts in 1M KOH and 1M KOH + 0.1M hydrazine hydrate (HH) was investigated using operand XAFS at SPring-8 in Japan.Fig. 1 shows the schematic view of the operand XAFS measurement system. In this study a half-cell was used. Working electrodes were prepared by the same method for MEA fabrication. Ni, Co, Ni0.87Zn0.13 catalysts were coated on carbon paper (CP, GDL 10AA, SGL Technologies GmbH). Hg/HgO (XR440, Radiometer) and Pt coil were used as reference and counter electrode, respectively. Fig. 2 shows X-ray absorption near edge structure (XANES) spectra for Ni-K and Co-K edge in prepared catalysts coated on CP in alkaline 1M KOH + 0.1M hydrazine hydrate (HH), together with reference materials. Metallic structure of Ni0.87Zn0.13/C compared with Ni/C was supposed due to electron donor from Zn to Ni, and this atomic structure of Ni0.87Zn0.13/C was stable during electrochemical reaction in alkaline solution. In 1M KOH + 0.1M HH, Co-K edge of Co/C was closely with Co-foil except for potential of 0.402 V due to reduction by hydrazine hydrate against oxidized Co. From the operand XAFS results, high catalytic activity of Ni0.87Zn0.13/C compared with Ni/C was caused that Zn has a larger ionization tendency than Ni so that Ni in Ni0.87Zn0.13/C hardly gets rusty due to the sacrifice anticorrosive action. Ammonia was generated in DHFC by the surface of metallic Co reduced by hydrazine.AcknowledgementsThis work was performed under the Shared Use Program of Japan Atomic Energy Agency (JAEA) Facilities. Authors are grateful to Dr. Y. Nishihata and Dr. D. Matsumura of JAEA for the synchrotron XAFS measurement of beam number of 2013B3616 at BL14B1 of SPring-8. We would like to thank Professor Dr. P. Atanassov and Associate Professor Dr. A. Serov of Univ. of New Mexico for providing hydrazine oxidation catalysts. We are grateful to Professor Dr. N. Mizuno and Associate Professor Dr. K. Yamaguchi of The Tokyo Univ. for their advice on catalyst research.

XAFS spectroscopy study of Eu(III) sorption on γ-Alumina: effect of pH

LIII edge X-ray Absorption Fine Structure (XAFS) spectroscopic study of Eu(III) sorbed on γ-alumina from aqueous solutions of different pH (values ranging from 6 to 8) has been carried out at XAFS beam line of Elettra Synchrotron facility, Italy, in transmission mode. Extended X-ray Absorption Fine Structure spectra of reference compounds, namely, Eu2O3, Eu(OH)3 and Eu-aquo complex in solution, were also measured. The data were analyzed using the IFEFFIT suite of code. XAFS spectra of the sorption samples is dominated by the Eu–O near neighbor co-ordination at distance 2.4 ± 0.1 A. 8–9 oxygen atoms, coming from both coordinating water molecule and oxygen atoms from alumina surface, surround the Eu(III) in the surface complex. Next near neighbor atoms in all the sorption samples consist of Al at distance ~3.6 and 3.8 A, which on comparison with literature data indicates towards Eu(III) bidentate binding to apical oxygen of two different alumina octahedra on γ-alumina surface.

X-Ray Absorption Studies of Zn 2+ Binding Sites in Bacterial, Avian, and Bovine Cytochrome bc1 Complexes

Binding of Zn 2+ has been shown previously to inhibit the ubiquinol cytochrome c oxidoreductase (cyt bc 1 complex). X-ray diffraction data in Zn-treated crystals of the avian cyt bc 1 complex identified two binding sites located close to the catalytic Q o site of the enzyme. One of them (Zn01) might interfere with the egress of protons from the Q o site to the aqueous phase. Using Zn K-edge x-ray absorption fine-structure spectroscopy, we report here on the local structure of Zn 2+ bound stoichiometrically to noncrystallized cyt bc 1 complexes. We performed a comparative x-ray absorption fine-structure spectroscopy study by examining avian, bovine, and bacterial enzymes. A large number of putative clusters, built by combining information from first-shell analysis and metalloprotein databases, were fitted to the experimental spectra by using ab initio simulations. This procedure led us to identify the binding clusters with high levels of confidence. In both the avian and bovine enzyme, a tetrahedral ligand cluster formed by two His, one Lys, and one carboxylic residue was found, and this ligand attribution fit the crystallographic Zn01 location of the avian enzyme. In the chicken enzyme, the ligands were the His 121, His 268, Lys 270, and Asp 253 residues, and in the homologous bovine enzyme they were the His 121, His 267, Lys 269, and Asp 254 residues. Zn 2+ bound to the bacterial cyt bc 1 complex exhibited quite different spectral features, consistent with a coordination number of 6. The best-fit octahedral cluster was formed by one His, two carboxylic acids, one Gln or Asn residue, and two water molecules. It was interesting that by aligning the crystallographic structures of the bacterial and avian enzymes, this group of residues was found located in the region homologous to that of the Zn01 site. This cluster included the His 276, Asp 278, Glu 295, and Asn 279 residues of the cyt b subunit. The conserved location of the Zn 2+ binding sites at the entrance of the putative proton release pathways, and the presence of His residues point to a common mechanism of inhibition. As previously shown for the photosynthetic bacterial reaction center, zinc would compete with protons for binding to the His residues, thus impairing their function as proton donors/acceptors.

Correlation between Size-Dependent Luminescent Properties and Local Structure around Eu3+ Ions in YBO3:Eu Nanocrystals: An XAFS Study

X-ray absorption fine structure spectroscopy (XAFS), a powerful tool for the study of the local environment of a particular element, especially the luminescent center, was introduced to investigate the correlation between luminescent properties and local structure around Eu3+ ions in YBO3:Eu nanocrystals, which was prepared by a more facile sol−gel pyrolysis process in our previous work and which exhibited size-dependent chromaticity, that is, a relatively better chromaticity could be obtained in the smaller-sized samples. The YBO3:Eu phosphors were studied at the Eu LIII edge (6977 eV), and XAFS results showed that the higher levels of disorder in the nanocrystals were responsible for the relatively better chromaticity. By analyzing recent studies on the nanosized materials, we considered that the high levels of disorder in the YBO3:Eu nanocrystals resulted from the low levels of crystallization.