Quick X-ray Absorption Fine Structure Research Articles

Valve turning towards on-cycle in cobalt-catalyzed Negishi-type cross-coupling

Ligands and additives are often utilized to stabilize low-valent catalytic metal species experimentally, while their role in suppressing metal deposition has been less studied. Herein, an on-cycle mechanism is reported for CoCl2bpy2 catalyzed Negishi-type cross-coupling. A full catalytic cycle of this kind of reaction was elucidated by multiple spectroscopic studies. The solvent and ligand were found to be essential for the generation of catalytic active Co(I) species, among which acetonitrile and bipyridine ligand are resistant to the disproportionation events of Co(I). Investigations, based on Quick-X-Ray Absorption Fine Structure (Q-XAFS) spectroscopy, Electron Paramagnetic Resonance (EPR), IR allied with DFT calculations, allow comprehensive mechanistic insights that establish the structural information of the catalytic active cobalt species along with the whole catalytic Co(I)/Co(III) cycle. Moreover, the acetonitrile and bipyridine system can be further extended to the acylation, allylation, and benzylation of aryl zinc reagents, which present a broad substrate scope with a catalytic amount of Co salt. Overall, this work provides a basic mechanistic perspective for designing cobalt-catalyzed cross-coupling reactions.

Time-resolved study of thermal decomposition process of (NH4)2[PtCl6]: Intermediates and Pt nucleation

Evolution in crystal, electronic and local atomic structures of Pt in ammonium hexachloroplatinate in the course of thermal decomposition in inert and reducing atmospheres have been studied by Powder X-Ray Diffraction (PXRD) and Quick X-ray Absorption Fine Structure (QXAFS) at Pt L3-edge for deeper understanding the thermally-induced solid state reaction and the formation of metallic nanoparticles. A three-step thermal decomposition mechanism of (NH4)2[PtCl6] in the inert atmosphere with the intermediate products Pt(NH3)2Cl2 and PtCl2 has been found instead one-[G.Meyer, A.Möller, J. Less. Common. Met. 170 (1991) 327–331] and two-step one [Q.Kong, F.Baudelet, J.Han, S.Chagnot, L.Barthe, J.Headspith, R. Goldsbrough, F.E.Picca, O.Spalla, Sci. Rep. 2 (2012) 1018–1025] considered early. In the reducing atmosphere, the thermal decomposition is a two-step process with the formation of the intermediate PtCl2. The best approach to determining the number of thermal decomposition steps turned out to be the express-analysis of QXAFS spectra offered in the papers, based on the simultaneous presentation of the most important parameters extracted from X-ray Absorption Near Edge Structure (XANES) and Fourier transformed Extended XAFS (EXAFS). This express-analysis was tested by comparison with results of various approaches such as conventional EXAFS fitting, linear combination fit (LCF), Multivariate Curve Resolution Alternating Least Squares method (MCR ALS).

Simultaneous Operando Time-Resolved XAFS–XRD Measurements of a Pt/C Cathode Catalyst in Polymer Electrolyte Fuel Cell under Transient Potential Operations

We have succeeded in simultaneous operando time-resolved quick X-ray absorption fine structure (QXAFS)–X-ray diffraction (XRD) measurements at each acquisition time of 20 ms for a Pt/C cathode catalyst in a polymer electrolyte fuel cell (PEFC), while measuring the current/charge of the PEFC during the transient voltage cyclic processes (0.4 VRHE → 1.4 VRHE → 0.4 VRHE) under H2(anode)–N2(cathode). The rate constants for Pt–O bond formation/dissociation, Pt charging/discharging, Pt–Pt bond dissociation/reformation, and decrease/increase of Pt metallic-phase core size under the transient potential operations were determined by the combined time-resolved QXAFS–XRD technique. The present study provides a new insight into the transient-response reaction mechanism and structural transformation in the Pt surface layer and bulk, which are relevant to the origin of PEFC activity and durability as key issues for the development of next-generation PEFCs.

In Situ Quick X-ray Absorption Fine Structure and Small-Angle X-ray Scattering Study of Metal Nanoparticle Growth in Water-in-Oil Microemulsions during Photoreduction

Formation of metal nanoparticles (silver (Ag) and palladium (Pd) nanoparticles) in water/alkane/sodium bis(2-ethylhexyl) sulfosuccinate (AOT) water-in-oil (w/o) microemulsions consisting of AOT, water, and organic solvent (n-heptane, n-octane, n-decane, and isooctane) by photoreduction was monitored by means of a combination of in situ quick X-ray absorption fine structure (QXAFS) and small-angle X-ray scattering (SAXS) measurements. The stages of reduction–nucleation and the association process (aggregative particle growth and Ostwald ripening) of metal atoms to produce metal nanoparticles were discriminated in the course of the photoreduction time. In situ QXAFS results indicated that the aggregative growth follows a sigmoidal profile described both by the solid-state kinetic model, specifically the Avrami–Erofe’ev model, and by the chemical-mechanism-based kinetic Finke–Watzky model. The SAXS results clearly showed that the formation of Ag and Pd nanoparticles preferentially follows a reduction–nucleat...

Reduction process of Cr(VI) by Fe(II) and humic acid analyzed using high time resolution XAFS analysis

The initial reduction behavior of Cr(VI) to Cr(III) has not been clearly understood due to its rapid reduction reaction. In order to study the reduction process of Cr(VI) in detail, we applied quick X-ray absorption fine structure (QXAFS) analysis to observe how Cr(VI) was reduced to Cr(III) by Fe(II) and humic acid (HA) with time. The Cr(VI) concentration was analyzed every 60s, and the plots of ln(Cr(VI)/Cr(VI)0) versus time were used to evaluate the reduction process based on their linearity. Reduction by Fe(II) showed a linear relation, whereas reduction by HA showed a nonlinear relation. With combined Fe(II) and HA, the linearity was unlike those of Fe(II) and HA individually. The reduction rate was not constant. The structure of Fe(II) produced by HA during the Cr(VI) reduction was investigated by using Mössbauer spectroscopy, which showed that Fe(II) produced by HA reduction of Fe(III) had the same structure as the initial Fe(II). HA can reduce Fe(III) back to Fe(II), and reproduced Fe(II) reduces Cr(VI). For Cr(VI) reduction by combined Fe(II) and HA, each reductant contribute differently: Fe(II) directly contributes to the Cr(VI) reaction, whereas HA reduces both Cr(VI) and Fe(III).

Analysis on Reaction and Degradation Mechanism for Mixed LiMn2O4-LiNi1-x-YCoxMnyO2 Composite Cathode in Lithium Ion Battery

A mixed LiMn2O4 (LMO)-LiNi1-x-yCoxMnyO2 (NCM) composite cathode has been used in batteries for Electric Vehicles (EVs) or Plug-in Hybrid Electric Vehicles (PHEVs) because of its safety, long-life and high capacity lengthening a cruising distance. However, there were few reports for the reaction of mixed composite cathodes and the detailed reaction and degradation mechanisms have not been clarified yet. In this study, the power fading mechanism of the LMO-NMC mixed composite cathode was studied with analysis using synchrotron radiation.The mixed composite cathodes were prepared by disassembling “initial” and “after degradation” (storage under 45° C and SOC100% for 900 days) lithium ion batteries for EVs. Laminate type cells were assembled with these cathodes, Li foil anode, and electrolyte (1M LiPF6 ethyl carbonate/dimethyl carbonate/ethyl methyl carbonate=3/3/4) for synchrotron radiation measurement. Time resolve X-ray diffraction (TRXRD) and quick X-ray absorption fine structure (QXAFS) measurements during discharge at low (0.83 mA cm-2) and high rate (7.50 mA cm-2) were performed at beamline 28XU of SPring-8 (Hyogo, Japan) in order to clarify the reaction and degradation (power fading) mechanisms for LMO and NCM. In addition, transmission electron microscope-electron diffraction (TEM-ED) and electron energy-loss spectroscopy (TEM-EELS) were performed to check the morphology of cathode particle.In low rate discharge experiment, the capacity for “initial” and “after degradation” electrodes was almost the same. However, in high rate discharge, the capacity remarkably faded for the “after degradation” electrode. In TRXRD and QXAFS results at low rate discharge, it was clarified that LMO discharged first and then NCM. In addition, the combination of the TRXRD and QXAFS measurements successfully allowed us to evaluate the reaction ratio of LMO to NCM in the mixed composite cathode. As a result, it was clarified that the reaction ratio of LMO was decreased at high rate discharge for the “after degradation” electrodes. In XRD profiles (Fig. 1), the LMO (531) diffraction peak was split into multiple peaks for the “after degradation” electrode at high rate discharge and the peak shift was observed continuously during discharge. These results suggested that the power fading in the mixed composite cathode was due to the decrease of reaction ratio of LMO to NCM by reaction distribution. Furthermore, as a result of TEM observation for the cross-section of an electrode, some amorphous layer on the surface of LMO particle was observed for the “after degradation” electrode. This suggested that Li intercalation/deintercalation at the surface of the LMO particle was disturbed by this amorphous layer and reaction distribution was generated in LMO.To summarize the results, the combination of the TRXRD and QXAFS measurements successfully allowed us to evaluate the reaction ratio of LMO to NCM in the mixed composite cathode. The power fading of the mixed composite cathode on storage under high temperature conditions was due to the decrease of reaction ratio of LMO to NCM caused by the reaction distribution.This work was supported by Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project from New Energy and Industrial Technology Development Organization (NEDO) in Japan.

Chromium(III) Oxidation by Three Poorly-Crystalline Manganese(IV) Oxides. 1. Chromium(III)-Oxidizing Capacity

The Cr(III)-oxidizing capacity of three layered poorly crystalline Mn(IV)O(2) phases, i.e. δ-MnO(2), Random Stacked Birnessite (RSB), and Acid Birnessite (AB), was determined in real-time and in situ, using Quick X-ray Absorption Fine Structure Spectroscopy (Q-XAFS). The results obtained with this technique, which allows the measurement of the total amount of Cr(VI) produced in the system, indicated that the Cr(III) oxidation reaction had ceased between 30 min and 1 h under most experimental conditions. However, this cessation was not observed with a traditional batch technique, which only allows the measurement of Cr(VI) present in solution and thus neglects the amount of Cr(VI) that may be sorbed to Mn(IV)O(2). This study also demonstrated that the Mn(IV)O(2) phase oxidizing the highest amount of Cr(III), which is positively charged in solution, was the mineral featuring the most negatively charged surface. Also, the results indicated that the presence of Mn(II) and/or Mn(III) impurities inside the Mn(IV)O(2) structure could enhance the mineral's capacity to oxidize Cr(III). The information provided in this study will be useful in predicting the capabilities of naturally occurring Mn oxide minerals, which are similar to the three synthetic Mn(IV)O(2) investigated, to oxidize Cr(III) to toxic and mobile Cr(VI) in the soil of contaminated sites.

Quick X-ray Absorption Fine Structure Studies on the Activation Process of Ni2P Supported on K-USY

Ni2P/K-USY (K-promoted ultrastable Y-type zeolite) is an efficient catalyst for hydrotreatment of petroleum feedstocks. The structural changes of the passivated catalyst during activation were foll...

Kinetics of Chromium(III) Oxidation by Manganese(IV) Oxides Using Quick Scanning X-ray Absorption Fine Structure Spectroscopy (Q-XAFS)

The initial kinetics of Cr(III) oxidation on mineral surfaces is poorly understood, yet a significant portion of the oxidation process occurs during the first seconds of reaction. In this study, the initial rates of Cr(III) oxidation on hydrous manganese oxide (HMO) were measured at three different pH values (pH 2.5, 3, and 3.5), using a quick X-ray absorption fine structure spectroscopy (Q-XAFS) batch method. The calculated rate constants were 0.201, 0.242, and 0.322 s(-1) at pH 2.5, 3, and 3.5, respectively. These values were independent of both [Cr(III)] and [Mn(II)] and mixing speed, suggesting that the reaction was "chemically" controlled and not dependent upon diffusion at the time period the rate parameters were measured. A second-order overall rate was found at three pH values. This represents the first study to determine the chemical kinetics of Cr(III) oxidation on Mn-oxides. The results have important implications for the determination of rapid, environmentally important reactions that cannot be measured with traditional batch and flow techniques. An understanding of these reactions is critical to predicting the fate of contaminants in aquatic and terrestrial environments.

Combined in situ analysis of Ni2P/MCM-41 under hydrodesulfurization conditions – Simultaneous observation of QXAFS and FTIR –

Supported Ni2P catalysts are efficient for the hydrodesulfurization (HDS) of liquid fuels. In situ extended x-ray absorption fine-structure (EXAFS) analysis of the Ni2P catalyst under realistic HDS conditions revealed the formation of Ni-S bonds in the catalyst system. In order to identify the origin of these bonds, a quick x-ray absorption fine structure (QXAFS) system was set up, which enabled measurement of one Ni K-edge EXAFS spectrum in 10 sec. In addition, a Fourier transform infrared (FTIR) unit was incorporated to the QXAFS system, which allowed simultaneous measurements of XAFS and IR spectra on the same sample. The catalyst (12.2 wt% Ni2P/MCM-41) was activated under H2 at 723 K to regenerate an active phosphide phase. After activation, a reaction gas mixture (thiophene/He/H2 = 0.1/1.9/98, at a total flow rate of 102 ml/min) was introduced. Under the reaction conditions, there was a change in the pre-edge area of XANES, which corresponded to the evolution of Ni-S bonds in EXAFS. The simultaneous measurements of XANES and IR revealed that this Ni-S could be attributed to the formation of a Ni phosphosulfide species rather than bonding between Ni and S of adsorbed thiophene. The combined in situ techniques proved to be an effective tool for the characterization of working catalysts.

Stepwise Growth of Pd Clusters in USY Zeolite at Room Temperature Analyzed by QXAFS

The stepwise growth of Pd in a USY zeolite was followed by in situ quick X-ray absorption fine structure (QXAFS) spectroscopy involving repeated alternating exposures to H2 and O2 flows at room temperature. During the first reduction of 0.4 wt % Pd/USY with 8% H2, Pd clusters with a coordination number (CN) of 5.1 formed within 15 min, probably in the supercage of USY. The growth followed first-order kinetics with respect to the concentration of Pd2+ and the CN of Pd−Pd. The clusters were stable up to 443 K. The clusters were partially oxidized in less than 2 min on exposure to 8% O2, and then quickly reduced with 8% H2 (second reduction) to afford larger Pd clusters with CN = 6.7. The clusters continued to increase in size in a stepwise fashion on further alternate exposures to O2 and H2. The cluster growth proceeded gradually when 0.8 wt % Pd was loaded on H-USY. In contrast, stable Pd clusters were not obtained when NH4-USY or high-silica USY were used as supports, suggesting that the clusters were stabilized on the H+ ions present in H-USY. This demonstrates the potential of QXAFS in following the clustering process of Pd, and indicates the possibility for fine-tuning the sizes of Pd clusters simply by changing the number of O2 and H2 exposure cycles.