[Show abstract][Hide abstract] ABSTRACT: FULL TEXT: http://www.lehigh.edu/operando/Publications/2012%20Fe-Ni%20O2%20Evo.pdf
Mixed Fe–Ni oxide electrocatalysts for the oxygen evolution reaction in alkaline electrolytes were synthesized using three different approaches: evaporation induced self-assembly, hard templating, and dip-coating. For each synthesis method, a peak in oxygen evolution activity was observed near 10 mol % Fe content, where the mixed metal oxide was substantially more active than the parent metal oxide electrocatalysts. X-ray diffraction (XRD) analysis showed the formation of a mixed NiO/NiFe2O4 phase at low Fe concentrations, and formation of Fe2O3 at compositions above 25 mol % Fe. Raman vibrational spectroscopy confirmed the formation of NiFe2O4, and did not detect Fe2O3 in the electrocatalysts containing up to 20 mol % Fe. X-ray absorption near edge structure (XANES) showed the Fe in the mixed oxides to be predominantly in the +3 oxidation state. Extended X-ray absorption fine structure (EXAFS) showed changes in the Fe coordination shells under electrochemical oxygen evolution conditions. Temperature programmed reaction spectroscopy showed the mixed oxide surfaces also have superior oxidation activity for methanol oxidation, and that the reactivity of the mixed oxide surface is substantially different than that of the parent metal oxide surfaces. Overall, the NiFe2O4 phase is implicated in having a significant role in improving the oxygen evolution activity of the mixed metal oxide systems.
[Show abstract][Hide abstract] ABSTRACT: We measured the thermal expansion coefficient and Debye temperature of nanoporous gold (NPG) using the extended x-ray absorption fine structure technique. Reduction of the nearest-neighbor distances in NPG by ca. 0.01 Å compared to the bulk gold was attributed to the surface tension caused, in turn, by the finite-size effect of the NPG ligaments. We also demonstrated that the Debye temperature in NPG is 5% lower than in bulk gold. We interpreted these observations in the framework of a bimodal distribution of surface and bulk bonds with different values of Debye temperature. The surface bonds with low Debye temperature extend within ca. four layers of Au atoms located on the pore surface, in a good agreement with prior resistivity measurements and theoretical predictions.
[Show abstract][Hide abstract] ABSTRACT: Reported herein is the synthesis of the previously unknown [Ir(1,5-COD)(μ-H)](4) (where 1,5-COD = 1,5-cyclooctadiene), from commercially available [Ir(1,5-COD)Cl](2) and LiBEt(3)H in the presence of excess 1,5-COD in 78% initial, and 55% recrystallized, yield plus its unequivocal characterization via single-crystal X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy, electrospray/atmospheric pressure chemical ionization mass spectrometry (ESI-MS), and UV-vis, IR, and nuclear magnetic resonance (NMR) spectroscopies. The resultant product parallels--but the successful synthesis is different from, vide infra--that of the known and valuable Rh congener precatalyst and synthon, [Rh(1,5-COD)(μ-H)](4). Extensive characterization reveals that a black crystal of [Ir(1,5-COD)(μ-H)](4) is composed of a distorted tetrahedral, D(2d) symmetry Ir(4) core with two long [2.90728(17) and 2.91138(17) Å] and four short Ir-Ir [2.78680 (12)-2.78798(12) Å] bond distances. One 1,5-COD and two edge-bridging hydrides are bound to each Ir atom; the Ir-H-Ir span the shorter Ir-Ir bond distances. XAFS provides excellent agreement with the XRD-obtained Ir(4)-core structure, results which provide both considerable confidence in the XAFS methodology and set the stage for future XAFS in applications employing this Ir(4)H(4) and related tetranuclear clusters. The [Ir(1,5-COD)(μ-H)](4) complex is of interest for at least five reasons, as detailed in the Conclusions section.
[Show abstract][Hide abstract] ABSTRACT: While a high efficiency of contaminant removal by nanoscale zerovalent iron (nZVI) has often been reported for several contaminants of great concern, including aqueous arsenic species, the transformations and translocation of contaminants at and within the nanoparticles are not clearly understood. By analysis using in situ time-dependent X-ray absorption spectroscopy (XAS) of the arsenic core level for nZVI in anoxic As(III) solutions, we have observed that As(III) species underwent two stages of transformation upon adsorption at the nZVI surface. The first stage corresponds to breaking of As-O bonds at the particle surface, and the second stage involves further reduction and diffusion of arsenic across the thin oxide layer enclosing the nanoparticles, which results in arsenic forming an intermetallic phase with the Fe(0) core. Extended X-ray absorption fine-structure (EXAFS) data from experiments conducted at different iron/arsenic ratios indicate that the reduced arsenic species tend to be enriched at the surface of the Fe(0) core region and had limited mobility into the interior of the metal core within the experimental time frame (up to 22 h). Therefore, there was an accumulation of partially reduced arsenic at the Fe(0)/oxide interface when a relatively large arsenic content was present in the solid phase. These results illuminate the role of intraparticle diffusion and reduction in affecting the chemical state and spatial distribution of arsenic in nZVI materials.
[Show abstract][Hide abstract] ABSTRACT: We are grateful to the help of R. Popovitz-Biro (HRTEM/EELS), Hilla Friedman (SEM/EDS), Y. Feldman (XRD), H. Cohen and T. Bendikov (XPS), S. R. Cohen (AFM), and Y. Tsverin (some of the conductivity measurements). R.T. gratefully acknowledges the support of ERC project INTIF 226639, the Israel Science Foundation, AddNano project 229284 of the FP7 (EU) program (late tribological measurements), the Harold Perlman Foundation, and the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging. R.T. is the Drake Family Chair in Nanotechnology and director of the Helen and Martin Kimmel Center for Nanoscale Science; A.I.F. and R.V. acknowledge support by the U.S. Department of Energy (DOE) Grant No. DE-FG02-03ER15476. Use of the NSLS was supported by the U.S. DOE Grant No. DE-AC02-98CH10886. Beamlines X18B and X19A at the NSLS are supported in part by the Synchrotron Catalysis Consortium, U. S. DOE Grant No DE-FG02-05ER15688.
[Show abstract][Hide abstract] ABSTRACT: An in situ electrochemical X-ray absorption spectroscopy (XAS) cell has been fabricated that enables high oxygen flux to the working electrode by utilizing a thin poly(dimethylsiloxane) (PDMS) window. This cell design enables in situ XAS investigations of the oxygen reduction reaction (ORR) at high operating current densities greater than 1 mA in an oxygen-purged environment. When the cell was used to study the ORR for a Pt on carbon electrocatalyst, the data revealed a progressive evolution of the electronic structure of the metal clusters that is both potential-dependent and strongly current-dependent. The trends establish a direct correlation to d-state occupancies that directly tracks the character of the Pt-O bonding present.
Journal of the American Chemical Society 12/2011; 134(1):197-200. DOI:10.1021/ja210465x · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Many metal clusters in the 1-nm size range are catalytically active, and their enhanced reactivity is often attributed to their size, structure, morphology, and details of alloying. Synchrotron sources provide a wide range of opportunities for studying catalysis. Among them, extended X-ray absorption fine-structure (EXAFS) spectroscopy is the premier method for investigating structure and composition of nanocatalysts. In this review, we summarize common methods of EXAFS analysis for geometric and compositional characterization of nanoparticles. We discuss several aspects of the experiments and analyses that are critical for reliably modeling EXAFS data. The most important are sample homogeneity, the width of the size and compositional distribution functions, and accounting for multiple-scattering contributions to EXAFS. We focus on the contribution of structural disorder and structural/compositional heterogeneity to the accuracy of three-dimensional modeling.