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1.
ChemSusChem ; 13(23): 6360-6369, 2020 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-32672415

RESUMO

A straightforward procedure has been developed to prepare a porous carbon material decorated with iron by direct pyrolysis of a mixture of a porous polymer and iron chloride. Characterization of the material with X-ray diffraction, X-ray absorption spectroscopy, and electron microscopy indicates the presence of iron carbide nanoparticles encapsulated inside the carbon matrix, and elemental mapping and cyanide poisoning experiments demonstrate the presence of atomic Fe centers, albeit in trace amounts, which are active sites for electrochemical CO2 reduction. The encapsulated iron carbide nanoparticles are found to boost the catalytic activity of atomic Fe sites in the outer carbon layers, rendering the material highly active and selective for CO2 reduction, although these atomic Fe sites are only present in trace amounts. The target material exhibits near-unity selectivity (98 %) for CO2 -to-CO conversion at a small overpotential (410 mV) in water. Furthermore, the material holds potential for practical application, as a current density over 30 mA cm-2 and a selectivity of 93 % can be achieved in a flow cell.

2.
Nanoscale ; 12(15): 8180-8187, 2020 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-32248213

RESUMO

A main goal of molecular electronics is to relate the performance of devices to the structure and electronic state of molecules. Among the variety of possibilities that organic, organometallic and coordination chemistries offer to tune the energy levels of molecular components, spin crossover phenomenon is a perfect candidate for elaboration of molecular switches. The reorganization of the electronic state population of the molecules associated to the spin crossover can indeed lead to a significant change in conductivity. However, molecular spin crossover is very sensitive to the environment and can disappear once the molecules are integrated into devices. Here, we show that the association of ultra-small 1.2 nm platinum nanoparticles with FeII triazole-based spin crossover coordination polymers leads to self-assemblies, extremely well organized at the sub-3 nm scale. The quasi-perfect alignment of nanoparticles observed by transmission electron microscopy, in addition to specific signature in infrared spectroscopy, demonstrates the coordination of the long-chain molecules with the nanoparticles. Spin crossover is confirmed in such assemblies by X-ray absorption spectroscopic measurements and shows unambiguous characteristics both in magnetic and charge transport measurements. Coordinating polymers are therefore ideal candidates for the elaboration of robust, well-organized, hybrid self-assemblies with metallic nanoparticles, while maintaining sensitive functional properties, such as spin crossover.

3.
J Synchrotron Radiat ; 27(Pt 1): 230-237, 2020 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-31868757

RESUMO

A microfluidic laboratory recently opened at Synchrotron SOLEIL, dedicated to in-house research and external users. Its purpose is to provide the equipment and expertise that allow the development of microfluidic systems adapted to the beamlines of SOLEIL as well as other light sources. Such systems can be used to continuously deliver a liquid sample under a photon beam, keep a solid sample in a liquid environment or provide a means to track a chemical reaction in a time-resolved manner. The laboratory provides all the amenities required for the design and preparation of soft-lithography microfluidic chips compatible with synchrotron-based experiments. Three examples of microfluidic systems that were used on SOLEIL beamlines are presented, which allow the use of X-ray techniques to study physical, chemical or biological phenomena.

4.
J Synchrotron Radiat ; 26(Pt 6): 1980-1985, 2019 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-31721743

RESUMO

Electrochemistry and electrocatalysis have been receiving increased attention recently due to their crucial contribution to electrical-to-chemical conversion systems. We describe here the development and operation of a new spectroelectrochemical transmission cell for time-resolved X-ray absorption spectroscopy of solutions. X-ray absorption spectra were recorded on the ROCK beamline of SOLEIL under constant and scanning potentials. Spectra were recorded at a frequency of 2 Hz during a cyclic voltammetry experiment performed on a 20 mM solution of FeIIICl3·6H2O at 20 mV s-1 scanning speed. Spectra with good signal-to-noise ratios were obtained when averaging ten spectra over 5 s, corresponding to a 100 mV potential range. A 90% conversion rate from Fe(III) to Fe(II) was spectroscopically demonstrated in cyclic voltammetry mode.

5.
ACS Nano ; 13(10): 11372-11381, 2019 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-31584800

RESUMO

Unveiling the mechanism of electrocatalytic processes is fundamental for the search of more efficient and stable electrode materials for clean energy conversion devices. Although several in situ techniques are now available to track structural changes during electrocatalysis, especially of water oxidation, a direct observation, in real space, of morphological changes of nanostructured electrocatalysts is missing. Herein, we implement an in situ electrochemical Transmission Electron Microscopy (in situ EC-TEM) methodology for studying electrocatalysts of the oxygen evolution reaction (OER) during operation, by using model cobalt oxide Co3O4 nanoparticles. The observation conditions were optimized to mimic standard electrochemistry experiments in a regular electrochemical cell, allowing cyclic voltammetry and chronopotentiometry to be performed in similar conditions in situ and ex situ. This in situ EC-TEM method enables us to observe the chemical, morphological, and structural evolutions occurring in the initial nanoparticle-based electrode exposed to different aqueous electrolytes and under OER conditions. The results show that surface amorphization occurs, yielding a nanometric cobalt (oxyhydr)oxide-like phase during OER. This process is irreversible and occurs to an extent that has not been described before. Furthermore, we show that the pH and counterions of the electrolytes impact this restructuration, shedding light on the materials properties in neutral phosphate electrolytes. In addition to the structural changes followed in situ during the electrochemical measurements, this study demonstrates that it is possible to rely on in situ electrochemical TEM to reveal processes in electrocatalysts while preserving a good correlation with ex situ regular electrochemistry.

6.
Angew Chem Int Ed Engl ; 58(45): 16172-16176, 2019 Nov 04.
Artigo em Inglês | MEDLINE | ID: mdl-31496012

RESUMO

Conversion of CO2 into valuable molecules is a field of intensive investigation with the aim of developing scalable technologies for making fuels using renewable energy sources. While electrochemical reduction into CO and formate are approaching industrial maturity, a current challenge is obtaining more reduced products like methanol. However, literature on the matter is scarce, and even more for the use of molecular catalysts. Here, we demonstrate that cobalt phthalocyanine, a well-known catalyst for the electrochemical conversion of CO2 to CO, can also catalyze the reaction from CO2 or CO to methanol in aqueous electrolytes at ambient conditions of temperature and pressure. The studies identify formaldehyde as a key intermediate and an unexpected pH effect on selectivity. This paves the way for establishing a sequential process where CO2 is first converted to CO which is subsequently used as a reactant to produce methanol. Under ideal conditions, the reaction shows a global Faradaic efficiency of 19.5 % and chemical selectivity of 7.5 %.

7.
Nat Commun ; 10(1): 3602, 2019 Aug 09.
Artigo em Inglês | MEDLINE | ID: mdl-31399585

RESUMO

Molecular catalysts that combine high product selectivity and high current density for CO2 electrochemical reduction to CO or other chemical feedstocks are urgently needed. While earth-abundant metal-based molecular electrocatalysts with high selectivity for CO2 to CO conversion are known, they are characterized by current densities that are significantly lower than those obtained with solid-state metal materials. Here, we report that a cobalt phthalocyanine bearing a trimethyl ammonium group appended to the phthalocyanine macrocycle is capable of reducing CO2 to CO in water with high activity over a broad pH range from 4 to 14. In a flow cell configuration operating in basic conditions, CO production occurs with excellent selectivity (ca. 95%), and good stability with a maximum partial current density of 165 mA cm-2 (at -0.92 V vs. RHE), matching the most active noble metal-based nanocatalysts. These results represent state-of-the-art performance for electrolytic carbon dioxide reduction by a molecular catalyst.

8.
ChemSusChem ; 12(19): 4500-4505, 2019 Oct 08.
Artigo em Inglês | MEDLINE | ID: mdl-31432616

RESUMO

A Fe quaterpyridine complex was used as a molecular precursor for the electrochemical reduction of CO2 to CH4 in acetonitrile in the presence of triethanolamine. CH4 was produced with a faradaic yield of approximately 2.1 % at 25 °C and 1 atm pressure of CO2 as reactant. Controlled potential electrolysis coupled to ex situ X-ray photoelectron spectroscopy and X-ray absorption spectroscopy of the electrode surface revealed the formation of metallic iron covered by iron oxides as species responsible for catalysis.

9.
ACS Cent Sci ; 5(3): 558-568, 2019 Mar 27.
Artigo em Inglês | MEDLINE | ID: mdl-30937383

RESUMO

The oxygen evolution reaction (OER) is a key process that enables the storage of renewable energies in the form of chemical fuels. Here, we describe a catalyst that exhibits turnover frequencies higher than state-of-the-art catalysts that operate in alkaline solutions, including the benchmark nickel iron oxide. This new catalyst is easily prepared from readily available and industrially relevant nickel foam, and it is stable for many hours. Operando X-ray absorption spectroscopic data reveal that the catalyst is made of nanoclusters of γ-FeOOH covalently linked to a γ-NiOOH support. According to density functional theory (DFT) computations, this structure may allow a reaction path involving iron as the oxygen evolving center and a nearby terrace O site on the γ-NiOOH support oxide as a hydrogen acceptor.

10.
Protoplasma ; 256(4): 1025-1035, 2019 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-30834984

RESUMO

Sulfur (S) and phosphorus (P) are essential elements for plant growth and physiological functioning. Their deficiency can limit N2 fixation and nodule development in nodulated legumes. The location of S within nodule tissues could provide insights into S metabolism and its little-known relationship with N2 fixation. Determinate and indeterminate nodules were inoculated with specific rhizobia and grown hydroaeroponically under sufficient versus deficient P supplies. Cryogenic and freeze-dried thin sections of nodules at the flowering stage were mapped using synchrotron micro-X-ray fluorescence to determine the S distribution within the nodule tissues with a spatial resolution of 2 or 3 µm. A large accumulation of S was found in the middle cortex for both types of nodules. S was also found in all of the other tissues but with a significantly lower signal. In the middle cortex, P deficiency decreased the S maximum fluorescence intensity by 20% and 25% for the determinate and indeterminate nodules, respectively. In addition, Mg and Cl maps were also collected showing that Mg was mostly localized in the middle and inner cortex, forming a Mg-rich ring consisting of several cell layers for the determinate nodules compared with only one cell layer for the indeterminate nodules. Cl was mainly accumulated in the outer cortex. It is concluded that the accumulation of S in the middle cortex is consistent with its involvement in the ionic equilibrium of the nodule, and in the osmotic variation of the inner cortex cell-size, which would regulate nodule permeability to oxygen.


Assuntos
Nódulos Radiculares de Plantas/metabolismo , Espectrometria por Raios X/métodos , Enxofre/metabolismo , Vigna/metabolismo , Cloretos/metabolismo , Flores/metabolismo , Magnésio/metabolismo , Fixação de Nitrogênio , Fósforo/metabolismo , Síncrotrons
11.
New Phytol ; 220(4): 1185-1199, 2018 12.
Artigo em Inglês | MEDLINE | ID: mdl-29944179

RESUMO

Through a mutualistic relationship with woody plant roots, ectomycorrhizal fungi provide growth-limiting nutrients, including inorganic phosphate (Pi), to their host. Reciprocal trades occur at the Hartig net, which is the symbiotic interface of ectomycorrhizas where the two partners are symplasmically isolated. Fungal Pi must be exported to the symbiotic interface, but the proteins facilitating this transfer are unknown. In the present study, we combined transcriptomic, microscopy, whole plant physiology, X-ray fluorescence mapping, 32 P labeling and fungal genetic approaches to unravel the role of HcPT2, a fungal Pi transporter, during the Hebeloma cylindrosporum-Pinus pinaster ectomycorrhizal association. We localized HcPT2 in the extra-radical hyphae and the Hartig net and demonstrated its determinant role for both the establishment of ectomycorrhizas and Pi allocation towards P. pinaster. We showed that the host plant induces HcPT2 expression and that the artificial overexpression of HcPT2 is sufficient to significantly enhance Pi export towards the central cylinder. Together, our results reveal that HcPT2 plays an important role in ectomycorrhizal symbiosis, affecting both Pi influx in the mycelium and efflux towards roots under the control of P. pinaster.


Assuntos
Proteínas Fúngicas/metabolismo , Hebeloma/metabolismo , Proteínas de Membrana Transportadoras/metabolismo , Micorrizas/fisiologia , Simbiose , Proteínas Fúngicas/genética , Regulação Fúngica da Expressão Gênica , Hebeloma/genética , Hebeloma/crescimento & desenvolvimento , Proteínas de Membrana Transportadoras/genética , Modelos Biológicos , Micélio/metabolismo , Fosfatos/metabolismo , Radioisótopos de Fósforo , Pinus/microbiologia , Regulação para Cima/genética
12.
J Am Chem Soc ; 140(10): 3613-3618, 2018 03 14.
Artigo em Inglês | MEDLINE | ID: mdl-29393639

RESUMO

The sandwich-type polyoxometalate (POM) [(PW9O34)2Co4(H2O)2]10- was immobilized in the hexagonal channels of the Zr(IV) porphyrinic MOF-545 hybrid framework. The resulting composite was fully characterized by a panel of physicochemical techniques. Calculations allowed identifying the localization of the POM in the vicinity of the Zr6 clusters and porphyrin linkers constituting the MOF. The material exhibits a high photocatalytic activity and good stability for visible-light-driven water oxidation. It thus represents a rare example of an all-in-one fully noble metal-free supramolecular heterogeneous photocatalytic system, with the catalyst and the photosensitizer within the same porous solid material.

13.
Chem Commun (Camb) ; 54(14): 1694-1697, 2018 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-29303524

RESUMO

The ligand 2,2'-bipyridine (bpy) can support metal centers in low formal oxidation states by delocalization of electron density into its π-system. We show that, in a model rhodium complex supported by the pentamethylcyclopentadienyl ligand (Cp*), the analogous dimethyldipyridylmethane ligand (Me2dpma) enforces a bpy-like coordination environment but disrupts the inter-ring conjugation responsible for charge delocalization upon metal reduction. As a result, reduction proceeds in discrete one-electron steps (Rh(iii) to Rh(ii) to Rh(i)), contrasting with the 2e- chemistry engendered by bpy. Upon reduction to Rh(i), the Me2dpma ligand rearranges to activate strong π-backbonding via facial coordination of one pyridine motif. Structural and spectroscopic studies confirm stabilization of the Rh(i) center in this compound, revealing a mode of metal-ligand cooperation that represents a useful counterpoint to charge delocalization in conjugated poly(pyridyl) ligands.

14.
Inorg Chem ; 56(22): 14227-14236, 2017 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-29090915

RESUMO

Lanthanide oxysulfide nanoparticles have recently attracted interest in view of their potential applications, such as lighting devices and MRI contrast agents, which requires a good stability in air and a controlled surface. In order to address these issues, in this work, air-sensitive Ce2O2S nanoparticles of hexagonal shape were successfully prepared and characterized under inert conditions. Bimetallic Gd2(1-y)Ce2yO2S nanoparticles of similar shape and size were also synthesized for the whole composition range (y from 0 to 1). X-ray diffraction structural data are found to follow Vegard's law up to y = 0.4, which is attributed to the loss of stability in air of Ce-rich nanocrystals beyond this threshold. This picture is supported by X-ray absorption spectra taken at the S K-edge and Ce L3-edge that show the partial oxidation of sulfide species and of CeIII to CeIV in the presence of air or water. A complementary near-ambient-pressure X-ray photoelectron spectroscopy study shows that at least two types of oxidized sulfur species form on the nanoparticle surface. Even in Gd2O2S nanoparticles that are generally considered to be air-stable, we found that sulfide ions are partially oxidized to sulfate in air. These results unveil the physicochemical mechanisms responsible for the surface reactivity of lanthanide oxysulfides nanoparticles in air.

15.
Adv Sci (Weinh) ; 4(10): 1700088, 2017 10.
Artigo em Inglês | MEDLINE | ID: mdl-29051853

RESUMO

Many organisms form minerals from precursor phases that crystallize under strict biological control. The dynamic intracellular processes of formation, transport, and deposition of these precursor phases are challenging to identify. An unusual situation is recently revealed for the calcifying alga Emiliania huxleyi, as the cells contain a compartment filled with a concentrated Ca and P phase but the final calcite crystals, which are nucleated in a different compartment, are P-free. Thus, the connection of the Ca-P-rich pool to the mineralization process remains unclear. Here, pulse-chase experiments are used with Sr to label the Ca-P-rich phase in E. huxleyi cells, and cryo X-ray absorption spectroscopy and analytical transmission electron microscopy to follow the Sr within cells. It is found that Sr is first found in the Ca-P-rich phase and then becomes incorporated into the calcite. This demonstrates that the calcium used by the cells to build calcite originates from the Ca-P-rich pool.

16.
Struct Dyn ; 4(5): 054307, 2017 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-28944255

RESUMO

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6-15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions.

17.
Artigo em Inglês | MEDLINE | ID: mdl-29515287

RESUMO

This review focuses on the use of X-ray absorption and emission spectroscopy techniques using hard X-rays to study electrocatalysts under in situ/operando conditions. We describe the importance and the versatility of methods in the study of electrodes in contact with the electrolytes, when being cycled through the catalytic potentials during the progress of the oxygen-evolution, oxygen reduction and hydrogen evolution reactions. The catalytic oxygen evolution reaction is illustrated with examples using Co, Ni and Mn oxides, and Mo and Co sulfides are used as an example for the hydrogen evolution reaction. A bimetallic, bifunctional oxygen evolving and oxygen reducing Ni/Mn oxide is also presented. The various advantages and constraints in the use of these techniques and the future outlook are discussed.

18.
Opt Express ; 24(20): 22469-22480, 2016 Oct 03.
Artigo em Inglês | MEDLINE | ID: mdl-27828320

RESUMO

X-ray free electron lasers (XFELs) enable unprecedented new ways to study the electronic structure and dynamics of transition metal systems. L-edge absorption spectroscopy is a powerful technique for such studies and the feasibility of this method at XFELs for solutions and solids has been demonstrated. However, the required x-ray bandwidth is an order of magnitude narrower than that of self-amplified spontaneous emission (SASE), and additional monochromatization is needed. Here we compare L-edge x-ray absorption spectroscopy (XAS) of a prototypical transition metal system based on monochromatizing the SASE radiation of the linac coherent light source (LCLS) with a new technique based on self-seeding of LCLS. We demonstrate how L-edge XAS can be performed using the self-seeding scheme without the need of an additional beam line monochromator. We show how the spectral shape and pulse energy depend on the undulator setup and how this affects the x-ray spectroscopy measurements.

19.
Proc Natl Acad Sci U S A ; 112(17): 5319-24, 2015 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-25852147

RESUMO

The structural and electronic properties of a series of manganese complexes with terminal oxido ligands are described. The complexes span three different oxidation states at the manganese center (III-V), have similar molecular structures, and contain intramolecular hydrogen-bonding networks surrounding the Mn-oxo unit. Structural studies using X-ray absorption methods indicated that each complex is mononuclear and that oxidation occurs at the manganese centers, which is also supported by electron paramagnetic resonance (EPR) studies. This gives a high-spin Mn(V)-oxo complex and not a Mn(IV)-oxy radical as the most oxidized species. In addition, the EPR findings demonstrated that the Fermi contact term could experimentally substantiate the oxidation states at the manganese centers and the covalency in the metal-ligand bonding. Oxygen-17-labeled samples were used to determine spin density within the Mn-oxo unit, with the greatest delocalization occurring within the Mn(V)-oxo species (0.45 spins on the oxido ligand). The experimental results coupled with density functional theory studies show a large amount of covalency within the Mn-oxo bonds. Finally, these results are examined within the context of possible mechanisms associated with photosynthetic water oxidation; specifically, the possible identity of the proposed high valent Mn-oxo species that is postulated to form during turnover is discussed.


Assuntos
Manganês/química , Oxigênio/química , Complexo de Proteína do Fotossistema II/química , Absorciometria de Fóton , Espectroscopia de Ressonância de Spin Eletrônica , Oxirredução , Água/química
20.
Phys Chem Chem Phys ; 17(14): 8901-12, 2015 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-25747045

RESUMO

Multielectron catalytic reactions, such as water oxidation, nitrogen reduction, or hydrogen production in enzymes and inorganic catalysts often involve multimetallic clusters. In these systems, the reaction takes place between metals or metals and ligands to facilitate charge transfer, bond formation/breaking, substrate binding, and release of products. In this study, we present a method to detect X-ray emission signals from multiple elements simultaneously, which allows for the study of charge transfer and the sequential chemistry occurring between elements. Kß X-ray emission spectroscopy (XES) probes charge and spin states of metals as well as their ligand environment. A wavelength-dispersive spectrometer based on the von Hamos geometry was used to disperse Kß signals of multiple elements onto a position detector, enabling an XES spectrum to be measured in a single-shot mode. This overcomes the scanning needs of the scanning spectrometers, providing data free from temporal and normalization errors and therefore ideal to follow sequential chemistry at multiple sites. We have applied this method to study MnOx-based bifunctional electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). In particular, we investigated the effects of adding a secondary element, Ni, to form MnNiOx and its impact on the chemical states and catalytic activity, by tracking the redox characteristics of each element upon sweeping the electrode potential. The detection scheme we describe here is general and can be applied to time-resolved studies of materials consisting of multiple elements, to follow the dynamics of catalytic and electron transfer reactions.


Assuntos
Eletroquímica , Elétrons , Metais/química , Oxigênio/química , Espectrometria por Raios X/métodos , Catálise , Oxirredução , Água/química
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