Extended X-ray Absorption Fine Spectroscopy Research Articles

Single-Atom Ruthenium Catalytic Sites for Acetylene Hydrochlorination.

The high cost of noble metal catalysts has been a major factor limiting their industrial applications. It is thus of strong interest to develop catalysts with minimum metal loading. Here, we designed and prepared a single-atom ruthenium catalyst through a cascade anchoring strategy to maximize the efficiency of Ru atoms for acetylene hydrochlorination. The single-atom catalyst supported on commercial activated carbon (AC) exhibits excellent catalytic activity with acetylene conversion of 95.4% at an acetylene gas hourly space velocity (GHSV) of 720 h-1 and almost no deactivation during a 600 h catalyst lifetime test. In conjunction with a series of experimental characterizations of the catalyst, including aberration-corrected scanning transmission electron microscopy (Ac-STEM), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine spectroscopy (EXAFS), density functional theory (DFT) study shows that RuN4 sites are likely responsible for acetylene hydrochlorination catalytic activity. This work provides a strategy to design efficient single-atom catalysts for acetylene hydrochlorination and helps us to gain deeper understanding of single-atom catalytic mechanisms.

Induced Chirality in Confined Space on Halogen Gold Complexes

The solubilization of HAuCl4 in toluene within optically active reverse micelles and lamellar structures formed by (1R,2S)-Dodecyl(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide (DMEB) has allowed us to evidence the complex phenomenology accompanying the confinement of Au salt within these nanostructures. Together with a chloride/bromide exchange process occurring in the first coordination sphere of an Au ion, UV–vis and electronic circular dichroism (ECD) spectra reveal the appearance of an induced dichroic signal attributable to Au complexes entrapped in the hydrophilic domain of the DMEB chiral nanostructures. Interestingly, change of the effective oxidation state and coordination geometry of the gold ion confined in DMEB nanostructures has been inferred by an analysis of the Au and Br X-ray Absorption Fine Spectroscopy (X-ray Absorbition Near Edge Spectroscopy and Extended X-ray Absorption Fine Spectroscopy) signals. Remarkably, bromine, not covalently bonded to the organic part, acts as the vector that transfers the chirality information on the DMEB to the gold complexes. Structural information on the HAuCl4 solubilized in DMEB reverse micelles dispersed in toluene, obtained by SAXS, indicate the formation of elongated clusters consisting of Au complexes stabilized by DMEB, confirming that the chirality transfer occurs in the micellar core of the DMEB and persists in the solid composites obtained by slow evaporation of the solvent.

Studies on KIT-6 Supported Cobalt Catalyst for Fischer–Tropsch Synthesis

KIT-6 molecular sieve was used as a support to prepare cobalt catalyst for Fischer–Tropsch synthesis (FTS) using an incipient wetness impregnation method to produce cobalt loadings of 15 and 25 wt%. The catalysts were characterized by BET surface area, X-ray diffraction, scanning transmission election microscopy (STEM), extended X-ray absorption fine spectroscopy and X-ray absorption near edge spectroscopy. The catalytic properties for FTS were evaluated using a 1L CSTR reactor. XRD, pore size distribution, and STEM analysis indicate that the KIT-6 mesostructure remains stable during and after cobalt impregnation and tends to form smaller cobalt particles, probably located inside the mesopores. The mesoporous KIT-6 exhibited a slightly higher cobalt dispersion compared to amorphous SiO2 supported catalyst. With the higher Co loading (25 wt%) on KIT-6, partial structural collapse was observed after the FTS reaction. Compared to an amorphous SiO2 supported cobalt catalyst, KIT-6 supported cobalt catalyst displayed higher methane selectivity at a similar Co loading, likely due to diffusion effects. KIT-6 molecular sieve was used as a support to prepare cobalt catalyst for Fischer–Tropsch synthesis (FTS) using an incipient wetness impregnation method to produce cobalt loadings of 15 and 25 wt%. The catalysts were characterized by BET surface area, X-ray diffraction (XRD), scanning transmission election microscopy (STEM), extended X-ray absorption fine spectroscopy (EXAFS) and X-ray absorption near edge spectroscopy (XANES). The catalytic properties for FTS were evaluated using a 1L CSTR reactor. The mesoporous KIT-6 exhibited a slightly higher cobalt dispersion compared to amorphous SiO2 supported catalyst. With the higher Co loading (25 wt%) on KIT-6, partial structural collapse was observed after the FTS reaction. XRD, pore size distribution, and STEM analysis indicate that the KIT-6 mesostructure remains stable during and after cobalt impregnation and tends to form smaller cobalt particles, probably located inside the mesopores. Compared to an amorphous SiO2 supported cobalt catalyst, KIT-6 supported cobalt catalyst displayed higher methane selectivity at a similar Co loading, likely due to diffusion effects.

Architecture of Bimetallic PtxCo1−x Electrocatalysts for Oxygen Reduction Reaction As Investigated by X-ray Absorption Spectroscopy

Carbon-supported PtxCo1−x alloy nanoparticles are prepared via a modified Watanabe’s process for oxygen reduction reaction (ORR). The relationship between the variations in alloying extent and Pt d-band vacancies in Pt−Co/C catalysts, which are tunable by changing the Pt and Co composition, are systematically studied. All of the catalysts are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV), and XAS. TEM images indicate that the dispersion of the metal nanoparticles on the carbon support is uniform. The XAS technique containing both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine spectroscopy (EXAFS) is utilized to extract the Pt d-band vacancies and alloying extent of Pt and Co in PtxCo1−x nanoparticles, respectively. Rotating disk electrode measurements of PtxCo1−x nanoparticle catalysts with various Pt:Co atomic compositions (3:1, 1:1, and 1:3) reveal that Pt1Co1/C nanocatalyst showed enhanced ORR activity. It i...

Anomalous Structural Change of Layered Perovskite Manganites La2-2x Sr1+2xMn2O7

The temperature-dependent structural changes were investigated by using the X-ray diffraction and the Ex- tended X-ray absorption fine spectroscopy (EXAFS) for layered perovskite manganites La2-2xSr1+2xMn2O7 (x=0.33 and 0.4). The anomalous changes of Mn-O bond length and Jahn-Teller (J-T) distortion of MnO6 octahedron were observed, corresponding to ferromagnetic transition temperature (~ 120 K). Around the temperature of 2D short-range magnetic or- dering (~ 235 K), there occurs an anomaly of the Jahn-Teller distortions of MnO6. These results indicate that magnetic properties of perovskite manganites La2-2xSr1+2xMn2O7 depend heavily on their microstructure change. The orbital state occupancy is involved in the observed Mn-O bond change.

Unusual Heme Iron-Lipid Acyl Chain Coordination in Escherichia coli Flavohemoglobin

Escherichia coli flavohemoglobin is endowed with the notable property of binding specifically unsaturated and/or cyclopropanated fatty acids both as free acids or incorporated into a phospholipid molecule. Unsaturated or cyclopropanated fatty acid binding to the ferric heme results in a spectral change observed in the visible absorption, resonance Raman, extended x-ray absorption fine spectroscopy (EXAFS), and x-ray absorption near edge spectroscopy (XANES) spectra. Resonance Raman spectra, measured on the flavohemoglobin heme domain, demonstrate that the lipid (linoleic acid or total lipid extracts)-induced spectral signals correspond to a transition from a five-coordinated (typical of the ligand-free protein) to a hexacoordinated, high spin heme iron. EXAFS and XANES measurements have been carried out both on the lipid-free and on the lipid-bound protein to assign the nature of ligand in the sixth coordination position of the ferric heme iron. EXAFS data analysis is consistent with the presence of a couple of atoms in the sixth coordination position at 2.7 Å in the lipid-bound derivative (bonding interaction), whereas a contribution at 3.54 Å (nonbonding interaction) can be singled out in the lipid-free protein. This last contribution is assigned to the CD1 carbon atoms of the distal LeuE11, in full agreement with crystallographic data on the lipid-free protein at 1.6 Å resolution obtained in the present work. Thus, the contributions at 2.7 Å distance from the heme iron are assigned to a couple of carbon atoms of the lipid acyl chain, possibly corresponding to the unsaturated carbons of the linoleic acid.

EXAFS study of noble gases implanted in highly stressed amorphous carbon films

In this work we report on the study of the local environment of Ar and Kr atoms implanted into an amorphous carbon (a-C) matrix. By intentionally changing the a-C deposition conditions we were able to trap noble gas (NG) atoms under different internal pressure (intrinsic stress) ranging from 1 up to 12 GPa. This enables us to investigate the effects of the internal pressure on the implanted NG atoms subjected to the highly strained environment of the carbon matrix. For this purpose, extended X-ray absorption fine spectroscopy (EXAFS) was performed on the K absorption edge of the implanted Ar (3.2 keV) and Kr (14.3 keV) atoms as a function of the intrinsic stress of the carbon matrix. The analysis of X-ray near edge spectroscopy (XANES) shows an evident increase of the white line for both gases as the stress increases. This result is indicative of clustering of the implanted NG atoms. The analysis of the EXAFS results for Ar atom indicates an increase of the first neighbor distance from 2.4 to about 2.8 Å with increasing compressive stress. The latter figure is comparable to the Ar–Ar (Kr–Kr) inter-atomic distance of compressed solid Ar (Kr). This finding gives further evidence for the formation of small NG clusters in a highly strained a-C matrix.

Relationship between short-range order and ease of nucleation in Na 2Ca 2Si 3O 9, CaSiO 3 and PbSiO 3 glasses

A relationship between the short-range order around the modifier cations and the crystal nucleation tendency in silicate glasses is demonstrated. New extended X-ray absorption fine spectroscopy (EXAFS) results on the local structure around calcium and lead atoms were obtained and analyzed for both vitreous and crystalline samples. Three different silicate systems were studied: wollastonite (CaSiO 3) and soda-lime–silica (Na 2Ca 2Si 3O 9), for which volume nucleation is easily observed and lead metasilicate (PbSiO 3) for which nucleation occurs only on the sample surfaces in typical laboratory conditions. In the glasses that have a high nucleation tendency (Na 2Ca 2Si 3O 9 and CaSiO 3), the local structures of these modifier cations are similar to their short-range order in the isochemical crystalline phases, whereas the local structure in the glass that presents a low nucleation tendency (PbSiO 3) is quite different from that of its isochemical crystal phase.

EXAFS and Raman spectroscopy study of binary indium fluoride glasses

Extended X-ray absorption fine spectroscopy (EXAFS) and Raman scattering studies of InF3-BaF2 and InF3-SrF2 binary glasses are reported. For all compositions, the local structure of the glasses is built with InF6 units. For all glasses studied, the indium neighbour's number and the In-F mean bond length are equal to the values of the InF3 crystalline phase (6 and 0.205 nm, respectively).