Wetting Induced Oxidation of Pt-based Nano Catalysts Revealed by In Situ High Energy Resolution X-ray Absorption Spectroscopy

In situ high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) was used to systematically evaluate interactions of H2O and O2 adsorbed on Pt and Pt3Co nanoparticle catalysts in different particle sizes. The systematic increase in oxidation due to adsorption of different species (H2O adsorption <O2 adsorption <O2 + H2O coadsorption) suggests that cooperative behavior between O2 and H2O adsorptions is responsible for the overpotential induced by hydrated species in fuel cells. From the alloying and particle size effects, it is found that both strength of O2/H2O adsorption and their cooperative effect upon coadsorption are responsible for the specific activity of Pt catalysts

Phase transition from fcc to bcc structure of the Cu-clusters during nanocrystallization of Fe85.2Si1B9P4Cu0.8 soft magnetic alloy

A role of Cu on the nanocrystallization of an Fe85.2Si1B9P4Cu0.8 alloy was investigated by X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM). The Cu K-edge XAFS results show that local structure around Cu is disordered for the as-quenched sample whereas it changes to fcc-like structure at 613 K. The fcc Cu-clusters are, however, thermodynamically unstable and begin to transform into bcc structure at 638 K. An explicit bcc structure is observed for the sample annealed at 693 K for 600 s in which TEM observation shows that precipitated bcc-Fe crystallites with ∼12 nm are homogeneously distributed. The bcc structure of the Cu-clusters transforms into the fcc-type again at 973 K, which can be explained by the TEM observations; Cu segregates at grain boundaries between bcc-Fe crystallites and Fe3(B,P) compounds. Combining the XAFS results with the TEM observations, the structure transition of the Cu-clusters from fcc to bcc is highly correlated with the preliminary precipitation of the bcc-Fe which takes place prior to the onset of the first crystallization temperature, Tx1 = 707 K. Thermodynamic analysis suggests that an interfacial energy density γ between an fcc-Cu cluster and bcc-Fe matrix dominates at a certain case over the structural energy between fcc and bcc Cu, ΔGfcc − bcc, which causes phase transition of the Cu clusters from fcc to bcc structure

Integration of X-ray absorption fine structure databases for data-driven materials science

With the aim of introducing data-driven science and establishing an infrastructure for making X-ray absorption fine structure (XAFS) spectra findable and reusable, we have integrated XAFS databases in Japan. This integrated database (MDR XAFS DB) enables cross searching of spectra from more than 2000 samples and more than 700 unique materials with machine-readable metadata. The introduction of a materials dictionary with approximately 6000 synonyms has improved the search performance and links with large external databases have been established. In order to compare spectra in the database, the energy calibration policies of each institution were compiled, and the energy calibration methods across institutions were shown. This clarified how to utilize the MDR XAFS DB as a knowledge base. The database created through this cross-institution initiative is a model case for the further development of databases for other methods and material informatics using them

Enhancement in Kinetics of the Oxygen Reduction Reaction on a Nitrogen-Doped Carbon Catalyst by Introduction of Iron via Electrochemical Methods

The iron (Fe) electrodeposition–electrochemical dissolution has been employed on nitrogen-doped carbon material (P-PI) prepared via multi-step pyrolysis of a polyimide precursor to achieve the introduction of Fe species, and its influence on the oxygen reduction reaction (ORR) is investigated by cyclic and rotating ring-disk electrode voltammetry in 0.5 M H₂SO₄. After the electrochemical treatment, the overpotential and H₂O₂ production percentage of ORR on the P-PI are decreased and the number of electrons transferred is increased in the meanwhile. In combination with the results of X-ray absorption fine structure spectra, the presence of Fe–N_<i>x</i> sites (Fe ions coordinated by nitrogen) is believed to be responsible for the improved ORR performance. Further kinetic analysis indicates that a two-electron reduction of O₂ is predominant on the untreated P-PI with coexistence of a direct four-electron transformation of O₂ to H₂O, while the introduction of Fe species leads to a larger increase in the rate constant for the four-electron reduction than that for the two-electron process, being in good agreement with the view that Fe–N_<i>x</i> sites are active for four-electron ORR