EXAFS element-selectivity

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identify and quantitate crystalline phases in bulk materials and at surfaces and interfaces. While NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under smdy, NMR can also provide structural insights on a molecularlevel. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly usefrd in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often fail. 

A cationic molybdenum sulfide cluster [Mo3S4(H20)9] " with incomplete cubane-type structure and a cationic nickel-molybdenum mixed sulfide cluster [Mo3NiS4Cl(H20)9p " with complete cubane-type structure were introduced into zeolites NaY, HUSY and KL by ion exchange. Stoichiometry of the ion exchange was well established by elemental analyses. The UV-visible spectra and EXAFS analysis data exhibited that the structure of the molybdenum cluster remained virtually intact after ion exchange. MoNi/NaY catalyst prepared using the molybdenum-nickel sulfide cluster was found to be active and selective for benzothiophene hydrodesulfurization. 

Fig. 2. a) Required number of incoming x-ray photons to observe time-resolved EXAFS of transition metal compounds in H20 solution with a signal-to-noise ratio S/N = 1. No ligand or counterion contributions were included (see Fig. 1). Input parameters are /= 10%, %= 1 % (relative to the absorption edge jump of the selected element). The maxima of curves 2) in Fig. 1 for Fe and Ru correspond to the data points for these elements, b) Feasibility range for time-resolved x-ray absorption spectroscopy. The shaded region indicates the required x-ray dose per data point as a function of the fraction of activated species for the calculated EXAFS experiments on transition metal compounds shown in a). Curves (1) to (3) are extrapolated from experimental results (see section 3. for details) of time-resolved XANES.

XAFS spectroscopy provides element-specific information about the local chemistry and physical structure of the element under investigation. XANES provides information about the chemical state of the element, including the oxidation state, and sometimes the local geometry (via selection rules), and EXAFS provides quantitative information about the... 

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

A selective tool to study the local structure around an impurity is the Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Despite this fact, only the work of Emura et al. on Cu -doped NaCl [149] and NaBr [150] has, to our knowledge, employed this technique in the field of off-centre impurities in insulating materials. In contrast with magnetic resonance techniques, EXAES can also be used to explore non-paramagnetic impurities provided they can absorb X-rays, a fact that excludes tight elements tike Li or C. By contrast, EPR is better to work with low impurity concentrations below about 50 ppm and also when different oxidation states of the same element (for instance, Fe + and Fe" " [151]) are simultaneously present in a given sample. 

According to the structure and composition of materials and analysis requirements of the researcher, the following analysis techniques can be selected for the characterization of mesoporous materials XRD, TEM, adsorption-desorption (N2 or other gas), solid MAS NMR (29Si, 27Al, 13C, etc.), scanning electron microscopy (SEM), catalysis test, Fourier Transform infra-red (FT-IR), thermal analysis, UV-visible, and chemical analysis. IR, X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure XANES, extended X-ray absorption fine structure EXAFS and other spectral methods are commonly used to analyse metal elements such as Ti in the mesoporous material frameworks. 

The differences in Zn(II)-containing phase association determined by Juillot et al. (2002) vs. Manceau et al. (2000a) in smelter-contaminated soils show that in complex phase assemblages, EXAFS data alone may not be sufficient to uniquely identify the types of phases with which a contaminant is associated. In these cases, other complementary methods, such as selective chemical extraction and , are needed, and even then it may not be possible to uniquely identify all the phases that contain the contaminant element or to resolve differences in interpretation. 

The objective of this work is to determine the structure of the active Pt species for methane oxidation and to characterize the state of the surface on Cl-free and Cl-containing Pt/alumina catalysts. EXAFS spectroscopy probes the local structure around a selected element - important for a study of highly dispersed cartalysts for which long range order does not exist and x-ray diffraction may not be an appropriate technique. 

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