X-ray absorption fine structure spectroscop

Foster A. L., Breit G. N., Whitney J. W., Welch A. H., Yount J. C., Alam M. K., Islam M. N., Islam M. S., Karim M., and Manwar A. (2000) X-ray absorption fine structure spectroscopic investigation of arsenic species in soil and aquifer sediments from Brahmanbaria, Bangladesh. 4th Annual Arsenic Conference, San Diego, June 18-22, 2000. 

Hahner, G., A. Marti, N.D. Spencer, and W.R. Caseri. 1996. Orientation and electronic structure of methylene blue on mica A near edge x-ray absorption fine structure spectroscopic study. J. Chem. Phys. 104 7749-7757. 

GejfASj,Si jf j, glasses (filled symbols) with x y = 1 1. Reprinted with permission from Sen S. and Aitken B. G., Atomic structure and chemical order in Ge-As selenide and sulfoselenide glasses an x-ray absorption fine structure spectroscopic study, Phys. Rev. B, 66, 134204-10 (2002). Copyright (2002) by the American Ph5rsical Society. 

EXAFS Extended X-ray absorption fine structure spectroscopy. A spectroscopic technique which can determine interatomic distances very precisely. 

In general, several spectroscopic techniques have been applied to the study of NO, removal. X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) are currently used to determine the surface composition of the catalysts, with the aim to identify the cationic active sites, as well as their coordinative environment. 

Final justification for using terms such as inner- or outer-sphere awaits direct spectroscopic confirmation. Electron Spin Resonance, Mossbauer, and Fourier Transform Infrared-Cylindrical Internal Reflection Spectroscopic techniques are being used to establish the structure of surface complexes (see, e.g., McBride, Ambe et al., and Zeltner et al., this volume). The potential for using EXAFS (extended x-ray absorption fine structure) to establish the type of surface complex for Pb + adsorbing onto goethite is currently being undertaken in our laboratory. 

The task in using these simple expressions (7) and (8) lies in finding the number of ligands n, the force constants Xt./o> and values for the bond length difference Ar. The values of n and Arare obtained from X-ray crystallographic or extended X-ray absorption fine structure data. The force constants /, and fs are obtained from available vibrational spectroscopic data using the equation. 

The pHPZC of ferric hydroxide surfaces is about 8 [127], so aqueous Pb2+ should be electrostatically repelled from these surfaces at pH values less than 8. However, as seen in Figure 7.6(a), the Pb2+ present in this aqueous solution is sorbed essentially completely to ferric hydroxide surfaces at pH 6. This behavior suggests that Pb2+ forms direct chemical bonds to these surfaces in order to overcome the repulsive electrostatic forces below the pHpzc of ferric hydroxide. This conclusion based on macroscopic uptake data has been confirmed by direct spectroscopic observation using X-ray absorption fine structure (XAFS) spectroscopy under in situ conditions (i.e., with aqueous solution in contact with a-FeOOH surfaces at ambient temperature and pressure) [133,134]. These studies showed that the aquated Pb(II) ion forms dominantly inner-sphere, bidentate complexes on a-FeOOH surfaces. 

Many important heterogeneous catalytic reactions occur at the interface between a solid catalyst and liquid or liquid-gas reactants. Notwithstanding the importance of solid-catalyzed reactions in the presence of liquid reactants, relatively little attention has been paid to spectroscopic methods that allow researchers to follow the processes occurring at the solid-liquid interface during reaction. This lack can be explained in part by the fact that there are only a few techniques that give access to information about solid-liquid interfaces, the most prominent of them being attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray absorption fine structure (XAFS) spectroscopy. 

The spectroscopic techniques described in this section include IR, Raman, and UV-visible spectroscopy, nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Techniques based on particle scattering, transitions in the nucleus, and radioisotope techniques that produce radiation that is a measure of the chemical environment are described in Sections IV.B and C. Some of these techniques, such as IR and UV-visible spectroscopy, have been applied to studies of catalysts for more than 30 years, whereas others, such as EXAFS, are relatively new to catalytic studies. 

Tepljakov AV, Kong MJ, Bent SF (1997) Vibrational Spectroscopic Studies of Diels-Alder Reactions with the Si(100)-2 X 1 Surface as a Dienophile, J. Am. Chem. Soc. 119 11100-11101 Tepljakov AV, Kong MJ, Bent SF (1998) Diels-Alder reactions of butadienes with the Si(100)-2 X 1 surface as a dienophile Vibrational spectroscopy, thermal desorption and near edge x-ray absorption fine structure studies, J. Chem. Phys. 108 4599-4606... 

In recent years a variety of spectroscopic and other techniques have been employed to investigate and monitor hydrosilylation reactions. The techniques include multinuclear NMR, transmission electron microscopy, extended X-ray absorption fine structure (EXAFS), etc. Results from these experiments indicate that depending on the precatalyst, colloids and/or mononuclear complexes take part as catalytic intermediates. 

In cases where there is a low concentration of cation of interest, if the cations are highly disordered in the zeolite framework, or if good crystalline samples are unavailable, atom specific or environment-specific spectroscopic probes may be preferable to determine local structures about the cation in the zeolite. NMR (4), IR ( 5, 6) ESR (7-10), optical (9,10), MSssbauer effect (11-15), and x-ray absorption studies (2,16,17,18) have been used to determine cation microenvironments. In particular, it has been shown that EXAFS (Extended X-ray Absorption Fine Structure) of the cation can often be used to give direct structure information about cation environments in zeolites, but EXAFS techniques, while giving radial distances and relative coordination numbers, are insensitive to site symmetry and cannot, in general, give both coordination numbers and relative site populations. Clearly it is desirable to use complementary spectroscopic techniques to fully elucidate the microenvironments in dilute, polycrystalline zeolite systems. 

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