XAFS spectroscopies

The X-ray absorption fine structure (XAFS) acronym refers to the oscillatory structure observed in the absorption coefficient just above 

Historically, it was controversial to recognise the local nature of the XAFS techniques (see Lytle for a recent discussion)39 but now the theoretical description of the techniques is reasonably well-settled.36,37 The absorption of X-ray by matter is described in many text books.40 The treatment of the radiation as an electric field without practical spatial variation on a molecular/local scale and eliminating magnetic parts leads to the Fermi Golden Rule for the X-ray cross section  

The intensity of the absorption process is then proportional to the square of the transition matrix element connecting the initial ((pi) and final ( pf) states times a delta function which ensures that it satisfies the conservation of energy theorem. The elimination of the spatial dependence of the electric field corresponds to a series expansion of its /l7rz/x dependence up to the first term (linear dependence in r equation (1)) this yields the dipole approximation of the interaction energy between the atom electronic cloud and the X-ray radiation field. Better approximations will include quadrupole, octupole and so forth terms. However, except in few cases, some of them here detailed, the dipole approximation gives a quantitative analysis of the XANES shape and EXAFS oscillations. 

As a general rule, Eqs. (1) and (2) can be solved by using modern quatum-mechanical methods. Two general approaches are discerned. The first one involves ab-initio or DFT methods to calculate the initial and 

Another important factor is the Debye-Waller factor e-DW. This accounts for thermal and static disorder effects concerning the move-ment/position of atoms around their equilibrium/averaged position. A point to stress is that the nature of this term is different to the 

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A XAS experiment involves the irradiation of a sample with a tuneable source of monochromatic X-rays, usually from a synchrotron facility (high brilliance). Third-generation synchrotrons have sufficient intensity to observe XAFS spectra up to 100 keV. Nevertheless, laboratory-scale XAFS spectroscopy is of importance, despite the vast availability of synchrotron beam time [305]. 

Soils around the Pb-Zn Port Pirie smelter in the semi-arid region of South Australia were found to contain 2,220 mg/kg Pb (about 3 km away from the mine) (Cartwright et al., 1976). Lead concentrations decreased to 140 mg/kg at a distance of 16 km and were further reduced to 32 mg/kg at 33.5 km. In tailing piles of the Leadville mining area of Colorado, U.S., Pb concentrations ranged from 6,000-10,000 mg/kg (Brown et al., 1999). XAFS spectroscopy showed that 50% of the total Pb occurred as adsorbed... 

X-ray Diffraction, X-ray Photoelectron Spectroscopy, and XAFS Spectroscopy Study 741... 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The aim of this work was to apply combined temperature-programmed reduction (TPR)/x-ray absorption fine-structure (XAFS) spectroscopy to provide clear evidence regarding the manner in which common promoters (e.g., Cu and alkali, like K) operate during the activation of iron-based Fischer-Tropsch synthesis catalysts. In addition, it was of interest to compare results obtained by EXAFS with earlier ones obtained by Mossbauer spectroscopy to shed light on the possible types of iron carbides formed. To that end, model spectra were generated based on the existing crystallography literature for four carbide compounds of... 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

Peterson, M.L. Brown Jr., G.E. Parks, G.A. (1997) Quantitative determination of chromium valence in environmental samples using XAFS spectroscopy. In Voigt, J.A. Bunker, B.C. Casey,W.H. Wood,T.E. Crossey, L.J. (eds.) Aqueous chemistry and geochemistry of oxides, oxyhydroxides, and related materials. Materials Research Society, Pittsburgh... 

Wu, Z. and Farges, F. (1999). Anharmonicity around Th in crystalline oxide-type compounds An in-situ high temperature XAFS spectroscopy study to 1500°C. Physica B, 266, 282-9. 

This example illustrates the qualitative nature of information that can be gleaned from macroscopic uptake studies. Consideration of adsorption isotherms alone cannot provide mechanistic information about sorption reactions because such isotherms can be fit equally well with a variety of surface complexation models assuming different reaction stoichiometries. More quantitative, molecular-scale information about such reactions is needed if we are to develop a fundamental understanding of molecular processes at environmental interfaces. Over the past 20 years in situ XAFS spectroscopy studies have provided quantitative information on the products of sorption reactions at metal oxide-aqueous solution interfaces (e.g., [39,40,129-138]. One... 

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. 

In recent years, it has been realized that techniques based on X-ray absorption provide important additional possibilities for catalyst characterization. Techniques such as X-ray absorption fine structure (XAFS) spectroscopy have had a significant impact on catalyst research. For example, the application of these techniques has for the first time allowed structural descriptions of many catalysts which, because of the presence of microcrystalline structures (nanophase particles) or amorphous phases, cannot be elucidated by XRD. 

Huggins, F.E. and Huffman, G.P. (1996) Modes of occurrence of trace elements in coal from XAFS spectroscopy. International Journal of Coal Geology, 32(1-4), 31-53. 

Huggins, F.E., Huffman, G.P., Kolker, A. et al. (2002) Combined application of XAFS spectroscopy and sequential leaching for determination of arsenic speciation in coal. Energy and Fuels, 16(5), 1167-172. 

Huggins, F.E., Shah, N., Zhao, J. et al. (1993) Nondestructive determination of trace element speciation in coal and coal ash by XAFS spectroscopy. Energy and Fuels, 7(4), 482-89. 

X-ray absorption fine structure (XAFS) spectroscopy An X-ray absorption spectroscopy (XAS) method. XAFS provides information on the physical and chemical properties of matter on an atomic scale. XAFS may include X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. 

A XAFS spectroscopy study of the Pt-H antibonding state shaperesonance and Pt-H EXAFS... 

From the experimental X-ray absorption data it can be concluded that the variation in both the L2 and L3 whiteline intensity correlates with the support alkalinity. This variation cannot be due to particle size effects. Charge transfer and the presence of a gasphase adsorbate are also excluded as possible causes. Two causes for the observed changes in the whiteline intensity are left an electron rearrangement induced by the support, and atoms of the support material viewed as adsorbates. These two causes and the influence of the support on the part of the valence band below the Fermi level (undetectable with XAFS spectroscopy) will be discussed in the next sections using the results of the DFT and ab initio FMS calculations. 

M.K. Oudenhuijzen, J.H. Bitter and D C. Koningsberger, The Nature of the Pt-H bonding for strongly and weakly bonded hydrogen on Platinum. A XAFS spectroscopy study of the Pt-H antibonding shaperesonance and Pt-H EXAFS , J. Phys. Chem. B, 105 (2001), 4616-4622. 

Extending the equipment, the authors (Beale et al., 2005) recently added energy dispersive X-ray absorption spectroscopy (XAS). Raman and UV-vis spectra are recorded by illuminating opposite sides of a catalyst bed in a vertical tubular reactor and detecting the scattered and reflected light as described above. XAS is performed in the same horizontal plane but in transmission and with the beam orthogonal to the incident radiation of the other two methods. Example spectra were recorded for samples at 823 K. A combination of UV-vis (fiber optics) and XAFS spectroscopy for investigation of solids has also been described by Jentoft et al. (2004), who reported UV-vis measurements of samples at 773 K. 

The additional insight from UV-vis spectra also proved useful in an investigation of the homogeneous oxidation of benzyl alcohol to benzal-dehyde in which copper complexes were employed as catalysts (Mesu et al., 2005 Tinnemans et al., 2006). It was shown that the synchrotron radiation used for XAFS spectroscopy affected the reacting solutions besides a thermal effect, reduction of copper was induced. This effect was investigated in detail for a number of ligands (Mesu et al., 2006). 

Clearly there are many methods that can be used to probe a catalyst s structure under reaction conditions, as documented in other chapters in this series (Advances in Catalysis, Volumes 50 and 51, and this volume). In this chapter we focus on X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, sometimes simply referred to as X-ray absorption fine structure (XAFS) spectroscopy. In this review the term XAFS will be used generically, but EXAFS and XANES will be used when the information is specifically related to the extended or near edge structure, respectively. 

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... 

A major reason why XAFS spectroscopy has become a critically useful probe of catalyst structure is the fact that it is easily adapted to characterization of samples in reactive atmospheres. The X-ray photons are sufficiently penetrating that absorption by the reaction medium is minimal. Moreover, the use of X-ray- transparent windows on the catalytic reaction cell allows the structure of the catalyst to be probed at reaction temperature and pressure. For example, the catalyst may be in a reaction cell, with feed flowing over it, and normal online analytical tools (gas chromatography, residual gas analysis, Fourier transform (FT) infrared spectroscopy, or others) can be used to monitor the products while at the same time the interaction of the X-rays with the catalyst can be used to determine critical information about the electronic and geometric structure of the catalyst. 

In one of the first papers on the application of XAFS spectroscopy to catalysis (Lytle et al., 1974) is the statement "... these results demonstrate that the EXAFS technique can be a powerful tool for studying catalysis in order to determine the precise structural relationship between catalytically active sites and the surrounding atoms." It is exactly this precise structural relationship that is the critical kind of information needed if true structure-property relationships are to be developed. 

The result of all of these advances has allowed in situ XAFS spectroscopy to become a workhorse technique in catalyst characterization science, and also provided opportunities for the application to new areas. Indeed, the evolution of X-ray spectroscopy has not ended—recently there have been several significant new advances that are discussed briefly at the end of this chapter. 

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