XAFS measurements

Direct methods for studying the structure of molten salts are X-ray and neutron diffraction analyses, infrared and Raman spectroscopy, NMR (nuclear magnetic resonance) measurement, and also very recently, XAFS (X-ray Absorption Fine Structure) measurement in melts, were developed. Fiowever, the most frequently used direct methods are X-ray and XAFS measurements, Raman spectroscopy, and NMR measurements. Therefore these three methods of direct investigation will be briefly described here. 

Sometimes, molecular dynamics and Monte Carlo simulation also are regarded as direct methods. On the other hand, using these two methods, no direct measurement is made and thus they will not be discussed here. 

X-ray diffraction and XAFS (X-ray Absorption Fine Structure) measurements are powerful techniques to investigate the local atomic structure of solid and liquid phases and have been successfully applied to many types of materials such as solutions, catalysts, amorphous solids, etc. However, the X-ray diffraction and XAFS measurement of molten salts bring a number of common difficulties due to high temperature and their specific properties, as in the experimental work. 

Only a few X-ray diffraction and XAFS studies at high temperatures can be found in the literature mainly due to technical difficulties in experimental work. E.g. Okamoto et al. (1998, 1999) measured the X-ray diffraction of some molten rare earth and uranium trihalides. The obtained X-ray diffraction data were analyzed using molecular dynamics technique. This procedure is almost standard in the structural analysis of molten salt systems. 

The heating part with the quartz cell is placed inside the electric furnace provided with a water jacket. A kapton film 20 mm in diameter and 50 p,m in thickness is used as the window for the X-ray beam. The furnace can be moved in horizontal and vertical directions with respect to the beam path to adjust the sample position. The atmosphere in the furnace could be controlled with respect to composition, pressure and gas flow. A thermocouple is set in the bottom of the heater part. The device could be used up to approximately 1000°C. 

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XAFS measurements on the beautiful blue ion Cr(H20) + suggest tetragonal distortion with Cr —O = 1.99A (equatorial) and Cr —O = 2.30A (axial).Solvent proton relaxation measurements on Cr(II) in CHjOD indicate (a) two exchanging coordinated CHjOD,... 

Inui, M., Takeda, S., Maruyama, K., Shirakawa, Y., and Tamaki, S., XAFS measurements on molten silver-halides, /. Non-Cryst. Solids, 192-193, 351-354, 1995. 

Financial support by the Deutsche Forschungsgemeinschaft (Fr 1372/1-1, Fr 1372/1-2) and the Fonds der Chemischen Industrie is gratefully acknowledged. We thank HASYLYB for allocating beamtime and Dr. Markus Tischer for help during the XAFS measurements. 

XAFS measurements were performed at the HASYLAB synchrotron (beamline XI. 1) in Hamburg, Germany, and at the European Synchrotron Radiation Facility (beamline BM 29) in Grenoble, France. The measurements were done in transmission mode using ion chambers filled with a mixture of Ar and N2 to have a px of 20% in the first and a px of 80% in the second ion chamber. The monochromator was detuned to 50% of maximum intensity to avoid higher harmonics present in the X-ray beam. 

Palladium-gold forms a continuous range of solid solutions (Section 2.6) and supported bimetallic particles are easily made. XAFS measurements confirmed that co-adsorption of [Au(en)2]3+ (en = ethylenediamine) and [Pd(NH3)4]2+ cations at natural pH onto silica gave small bimetallic particles (<5nm) after reduction at 623 K, irrespective of the Pd Au ratio.181 The same result was obtained by controlling the pH at 10 (Pd Au = 1), but at pH 7 (Pd Au = 1.3), reduction at 573K gave particles with a gold-rich... 

In the second part of the review, specific examples of applications are presented, with the first section on XAFS measurements made under static conditions and the second on those made in a time-resolved manner under dynamic conditions. We end the review with an outlook to the future, where advances in time, spatial, and energy resolution are highlighted. 

Grunwaldt et al. (2003b) reported XAFS measurements recorded during palladium-catalyzed alcohol oxidation in supercritical CO2. A commercial shell-impregnated catalyst consisting of 0.5 wt% Pd on alumina was used for benzyl alcohol oxidation (to benzaldehyde) in supercritical CO2 with pure O2 as oxidant. The conditions were 353 K and 150 bar. The results are summarized in Table 8. The authors reported only Pd XANES data, not EXAFS data, and thus the analysis is limited to information about the average oxidation state of the palladium. 

Time-resolved X-ray absorption spectra of an activated H5[PV2Moio04o] oxidation catalyst were recorded to determine correlations between the dynamic structure and the catalytic selectivity of the material (Ressler and Timpe, 2007). In addition to experiments carried out under steady-state conditions, time-resolved XAFS measurements at the Mo K-edge were performed under changing reaction conditions (with a time resolution of 30 s per spectrum) (Figure 52). Therefore, the gas-phase composition was isothermally switched from a reducing atmosphere (propene) to an... 

The TR-XAFS measurements were also performed with rubidium cations which have the same electronic configuration as that of Br" and the K-edge energies are also similar to each other. The measurements were performed on two solution surfaces, one at 0.1 mol dm" and the other at 5 mmol dm". To the lower concentration sample, sodium dodecyisulfate (SDS) was added, which accumulates the cation at the surface. These samples and conditions were chosen in order to check the conclusions given above. The results are included in Table 2. 

A special quartz cell for XAFS measurements of molten salts was developed by Okamoto et al. (2002). The cell having a sand-glass form is shown in Figure 10.1. The solid sample is placed in the upper container and the cell is heated. When the sample melts, the fused sample runs down through the narrow measurement part of the cell, where the... 

Figure 10.1. Schematic arrangement of the inner compartment of the furnace for XAFS measurement using a quartz cell, according to Okamoto et al. (2002).

The influence of absorption by the quartz cell was examined by measurements of blank cells and some molten salt samples. In the blank test, the displacement of the absorption baseline was not observed. The lowest energy, which could be used in this system, was estimated to be lOkeV. Thereby, the XAFS spectra of some molten halides could be successfully obtained. The results showed that a small difference was observed among their results. It was concluded that the developed device is also suitable for the XAFS measurement of hygroscopic molten salts. 

Results of XAFS measurements are compiled in Table 1 with the results of the estimates of the TEM measurements (for details on TEM results see ref. [ 13 ]). The catalysts are named according to the stabilizing polymer and particle size determined by TEM. Except for the PVP stabilized catalyst reduced with methanol, an excellent agreement exists between the size of the metal particles estimated from TEM and EXAFS. The methanol reduced sample shows the smallest particle size when estimated from EXAFS, while it appears to be much larger in TEM. At present we would like to speculate that this is due to the formation of agglomerates (diameter 3.5 nm) formed from very small particles (diameter 1.2 nm). Consequendy, the eatalyst is named Rh/PVP/3.5-a to indicate the difference with the sol containing primary particles of 3.5 nm in diameter. 

Partial support of this work by the Onderzoeks Stimulerings Ponds of the University of Twente, is gratefully acknowledged. The XAFS measurements were carried out at the National Synchrotron Light Source (Beamline X23A2), Brookhaven National Laboratory, which is supported by the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. 

Anderson AJ, Mayanovic RA, Chou I-M (1998b) Micro-beam XAFS measurements of zinc complexes in highly saline fluid inclusions to 400°C before and after experimental re-equilibration at high hydrogen pressure. Mineral Mag 62A 55-56... 

Mayanovic RA, Anderson AJ, Bajt S (1997) Microbeam XAFS studies on fluid inclusions at high temperatures. J de Physique IV 7 (Colloque C2, X-ray Absorption Fine Structure, Vol. 2) 1029-1030 Mayanovic RA, Anderson AJ, Bassett WA, Chou, I-M (1999) XAFS measurements on zinc chloride aqueous solutions from ambient to supercritical conditions using the diamond anvil cell. J Synchrotron Rad 6 195-197... 

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