Energy-resolution X-ray Absorption Spectroscopy Research Articles

Improvement of Oxygen Reduction Reaction Performance of Fe-N-C Catalyst

Fuel cells are gaining worldwide attention as an acceptable technology for achieving carbon neutrality. However, existing fuel cells use large amounts of precious metals, which limits their widespread adoption. Precious metal-free fuel cells have been studied in our laboratory. The oxidation-reduction reaction (ORR) at the cathode of the fuel cell must start at a higher potential and proceed smoothly in a four-electron reduction reaction without producing hydrogen peroxide1,2. We have synthesized iron-based complex catalysts using the method reported by Plamen Atanassov3 et al. Iron nitrate dihydrate is mixed in aqueous solution with glucose, 2-methylimidazole, and zinc nitrate hexahydrate, followed by hydrothermal synthesis at 200°C for 24 hours. It is then acid-treated with nitric acid to dissolve excess metallic iron, and the process is repeated twice at 950°C to complete the Fe-N-C catalyst. However, SEM and XRD analysis confirmed the presence of large amounts of metallic iron. In response to these results, the synthetic process of the Fe-N-C catalyst was repeatedly improved, and the prepared catalyst was subjected to in-situ XAFS (X-ray absorption fine structure) analysis using synchrotron radiation at SPring-8 (Fig. 1). The peak around 7110 eV is significantly different from that of metallic iron (Fe foil), indicating that iron complex, iron phthalocyanine (II), and it is like the peak of iron phthalocyanine (II), confirming that the prepared Fe-N-C catalyst forms a good complex. The performance of the Fe-N-C catalyst with the confirmed complexation was then investigated, and a cathode that exhibited ORR performance comparable to that of the Pt catalyst was successfully synthesized (Fig. 2). In the future, further improvement of the catalyst preparation process will be discussed to develop a catalyst with much better performance than Pt.Reference 1N. Yamamoto, D.Matsumura, Y. Hagihara, K.Tanaka, Y. Hasegawa, K. Ishii, H. Tanaka, “Investigation of hydrogen superoxide adsorption during ORR on Pt/C catalyst in acidic solution for PEFC by in-situ high energy resolution XAFS”, Jounal of Power Sources, 557, 15, (2023), 232508 2S. Kusano, D. Matsumura, K. Ishii, H. Tanaka, J. Mizuki, “Electrochemical Absorption on Pt Nanoparticles in alkaline Solution Observed Using in-situ High Energy Resolution X-ray Absorption Spectroscopy”, Nanomaterials, 9, 4, (2019), 642 3 R. Gokhale, L-K. Tsui, K. Roach, Y. Chen, M. M. Hossen, K. Artyushkova, F. Garzon, p. Atanassov, “Hydrothermal synthesis of platinum-Group-Metal-Free Catalysts: Structural Elucidation and Oxygen Reduction Catalysis”, ChemElectroChem, 5, 14, (2017), 1848-1853 Figure 1

Experimental and Theoretical Investigation of High-Resolution X-ray Absorption Spectroscopy (HR-XAS) at the Cu K-Edge for Cu2ZnSnSe4

Energy sustainability is critical for social activities in the human world. The quaternary compound Cu2ZnSnSe4 (CZTSe), as a promising candidate for thin-film solar cell absorption with medium-level thermoelectric performance, is of interest for the purpose of utilizing solar energy. The defect chemistry and atomic ordering in this particular compound also triggers interests in understanding its crystallographic structure as well as defects. Hereby, high energy resolution X-ray absorption spectroscopy is employed to investigate the electronic and geometric structural complexity in pristine and cobalt-doped Cu2ZnSnSe4. The occupational atomic sites of Cu are found to be mixed with the Zn atoms, forming CuZn anti-defects, which serve as a knob to tune local electronic structures. With proper doping, the band structure can be manipulated to improve the optical and thermoelectric properties of the CZTSe compounds.

Gold speciation in hydrothermal fluids revealed by in situ high energy resolution X-ray absorption spectroscopy

Abstract Gold mobilization, transfer, and concentration in the Earth’s crust are controlled by hydrothermal sulfur- and chloride-bearing fluids. Yet the exact chemical identity, structure, and stability of Au-bearing species and, in particular, the respective contributions of the sulfide (HS−) and trisulfur ion (S3⋅−) ligands to Au transport lack direct in situ evidence. Here we employed high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) on aqueous sulfate/sulfide/S3⋅−-bearing solutions at typical hydrothermal temperatures and pressures (T = 350 °C, P = 600 bar) to reveal differences in dissolved Au spectral signatures indicative of contrasting fluid-phase Au speciation as a function of acidity and redox conditions. Combined with in situ Au solubility measurements and quantum-chemical and thermodynamic modeling, our spectroscopic data provide direct evidence for the Au(HS)S3− and Au(HS)2− complexes predominant at acidic-to-neutral and alkaline conditions, respectively. Our findings thus directly confirm a recent speciation scheme for Au in aqueous S-bearing fluids established using less direct methods, and highlight an important role of the trisulfur ion in gold mobilization and concentration in hydrothermal-magmatic deposits associated with subduction zones. More generally, our results show that HERFD-XAS enables the identification of structural and coordination features in metal complexes virtually unresolvable using classical XAS techniques. By avoiding limitations of less direct techniques, our integrated high-resolution spectroscopic approach opens perspectives for studies of the speciation and solubility of gold and other metals in high T-P fluids, and potentially silicate melts, inaccessible to direct observation in nature.

Electrochemical Adsorption on Pt Nanoparticles in Alkaline Solution Observed Using In Situ High Energy Resolution X-ray Absorption Spectroscopy

The oxygen reduction reaction (ORR) on Pt/C in alkaline solution was studied by in situ high energy resolution X-ray absorption spectroscopy. To discuss the X-ray absorption near-edge structure (XANES), this paper introduced the rate of change of the Δμ (RCD), which is an analysis method that is sensitive to surface adsorption. The surface adsorptions as hydrogen (below 0.34 V), superoxide anion (from 0.34 V to 0.74 V), hydroxyl species (from 0.44 V to 0.74 V), atomic oxygen (above 0.74 V), and α-PtO2 (above 0.94 V) were distinguished. It is clarified that the catalytic activity in an alkaline solution is enhanced by the stability of atomic oxygen and the low stability of superoxide anion/peroxide adsorption on the platinum surface.

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.

High energy resolution x-ray absorption spectroscopy study of uranium in varying valence states

A high energy resolution x-ray absorption near-edge structure (XANES) spectroscopy study on ${\text{U}}^{4+}$ $({\text{UO}}_{2})$, ${\text{U}}^{5+}$ $([{\text{UO}}_{2}{\text{Py}}_{5}][{\text{KI}}_{2}{\text{Py}}_{2}])$, and ${\text{U}}^{6+}$ $[{\text{UO}}_{2}{({\text{NO}}_{3})}_{2}{({\text{H}}_{2}\text{O})}_{6}]$ demonstrates the potential of this experimental technique for qualitative/semiqualitative and quantitative actinide speciation investigations. We observe a pre-edge feature with quadrupole nature in a $\text{U}\text{ }{L}_{3}$ edge partial fluorescence yield-XANES spectrum. This feature is a tool for characterizing the participation of $5f$ orbitals in U-O bonding. The feature origin is explained by performing calculations with the finite difference method near-edge structure code based on the multiple-scattering theory and the finite difference method.

High Energy Resolution X-ray Absorption Spectroscopy of Environmentally Relevant Lead(II) Compounds

The determination of the chemical environment of Pb in natural samples is a challenge of great importance in environmental and health physics. We report a high energy resolution fluorescence detection (HERFD) X-ray absorption near-edge spectroscopy (XANES) study at the Pb L(3) and L(1) absorption edges to determine the chemical environment of Pb in a series of model and environmentally relevant compounds. HERFD spectroscopy can reveal increased spectral detail due to an apparent reduction in the core hole lifetime broadening. HERFD spectra of model Pb(II) compounds were compared to FEFF 8.4 multiple scattering calculations with reduced peak broadening parameters, and density of state (DOS) simulations, to determine the origins of the spectral features. A pre-edge in the L(3) XANES is revealed which is shown to arise from hybridization between the Pb p and d states. HERFD spectra of Pb(II)-containing environmentally relevant solutions were compared to model spectra and calculations. The results presented in this paper show that the chemical environment of Pb can be identified from spectral features resolved in HERFD spectroscopy at the Pb L(3) edge. The technique provides information that is complementary to conventional extended X-ray absorption fine structure (EXAFS) spectroscopy.