XANES structures states

Figure 8. Empirical correlations of Fe porphyrins XANES structure with spin state. A) The "Ligand Field Indicator Region" identified by Chance et al. (Reproduced with permission from reference 44. Copyright 1986, Journal of Biological Chemistry) B) Spin state sensitive bands identified by Oyanagi et al. Redrawn from data in Reference 45.

The speciation of trace elements in solid phases determines their mobility and toxicity. Spectroscopic techniques such as XANES and XAFS, can be used to determine directly the oxidation and structural state of elements in coal combustion byproducts. For example, Huggins et al. (2000) used these synchrotron techniques to determine that Cr and As occur predominately in the less toxic oxidation states Cr(IIl) and As(V) in CCBs. In addition, they found As, Cd, Cr, Ni, and Zn were present primarily as oxidized species (i.e., as oxides, sulphates, arsenates, etc.) in unweathered CCBs. 

The characteristics of reforming catalysts make them one of the most frequent cases where EXAFS is used. The very small metallic particles cannot be detected in transmission electron microscopy the long range order required for XRD analysis is absent, and the low metal contents make XPS analysis difficult. The observation of XANES structures at Pi and Re thresholds can be used to determine the electronic state. Some examples of reference compounds are shown in Figure 11.10. In particular a sharp peak (called a white line for historical reasons) is visible at the edge with an intensity related to the oxidisation state of the element. This peak in the absorption coefficient is a consequence of the existence of the empty electron states close to zero binding energy. 

The extended fine structure (EXAFS) was used to determine bond distances, coordination number and disorder. The near edge (XANES) was used as an Indication of electronic state. Significant results Include, 1) a reversible change of shape of clean supported metal clusters as a function of temperature, 2) supported Pt clusters have more disorder or strain compared to the bulk metal, and 3) a clear determination of the bonds between the catalytic metal atoms and the oxygen atoms of the support. 

X-ray absorption spectroscopy combining x-ray absorption near edge fine structure (XANES) and extended x-ray absorption fine structure (EXAFS) was used to extensively characterize Pt on Cabosll catalysts. XANES Is the result of electron transitions to bound states of the absorbing atom and thereby maps the symmetry - selected empty manifold of electron states. It Is sensitive to the electronic configuration of the absorbing atom. When the photoelectron has sufficient kinetic energy to be ejected from the atom It can be backscattered by neighboring atoms. The quantum Interference of the Initial... 

Final detennination of the structure was made by proposing a structural model with Cu sitting in threefold hollow sites and O atoms on atop sites with respect to the Cu atoms (Fig. 27.16). A program, FEFFIT, was used to analyze the data (Stem et al., 1995). This calculates the phase and amplitude parameters for the various backscatters. The EXAFS for the parallel polarization could be fitted six Cu-Cu interactions at a bond distance of 2.67 A and three Cu-Pt interactions at 2.6 A. For the perpendicular polarization, the data could be fitted one Cu-0 interaction at 1.96 A and three Cu-Pt interactions at 2.6 A. The Cu-Pt bond length is shorter than the sum of the metallic radii of Cu and Pt, which is 2.66 A. This indicates a Cu oxidation state different from zero, which agrees with the XANES results. 

Controlled decomposition of pre-formed [(COD)Pt(CH3)2] in the presence of triorganoaluminium led to the preparation of the first Pt cluster (size 0.75 + 0.1 nm). The one-shell structure and the metallic state were confirmed by XPS and XANES [352]. 

XPS has typically been regarded primarily as a surface characterization technique. Indeed, angle-resolved XPS studies can be very informative in revealing the surface structure of solids, as demonstrated for the oxidation of Hf(Sio.sAso.5)As. However, with proper sample preparation, the electronic structure of the bulk solid can be obtained. A useful adjunct to XPS is X-ray absorption spectroscopy, which probes the bulk of the solid. If trends in the XPS BEs parallel those in absorption energies, then we can be reasonably confident that they represent the intrinsic properties of the solid. Features in XANES spectra such as pre-edge and absorption edge intensities can also provide qualitative information about the occupation of electronic states. 

Fig. 1. X-ray absorption near-edge structure (XANES) of reference compounds with various As valence states and mine tailings samples. The As K-edge excitation potential for arsenic in the ground state (As0) is at 11868 eV. The As K-edge excitation potential increases with increasing valence state.

X-ray absorption near edge structure (XANES) The X-ray absorption spectrum, as for EXAFS, may also show detailed structure below the absorption edge. This arises from excitation of core electrons to high level vacant orbitals, and can be used to estimate the oxidation state of the metal ion. 

In solid state physics, the sensitivity of the EELS spectrum to the density of unoccupied states, reflected in the near-edge fine structure, makes it possible to study bonding, local coordination and local electronic properties of materials. One recent trend in ATEM is to compare ELNES data quantitatively with the results of band structure calculations. Furthermore, the ELNES data can directly be compared to X-ray absorption near edge structures (XANES) or to data obtained with other spectroscopic techniques. However, TEM offers by far the highest spatial resolution in the study of the densities of states (DOS). 

XAS has been successfully employed in the characterization of a number of catalysts used in low temperature fuel cells. Analysis of the XANES region has enabled determination of the oxidation state of metal atoms in the catalyst or, in the case of Pt, the d band vacancy per atom, while analysis of the EXAFS has proved to be a valuable structural tool. However, the principal advantage of XAS is that it can be used in situ, in a flooded half-cell or true fuel cell environment. While the number of publications has been limited thus far, the increased availability of synchrotron radiation sources, improvements in beam lines brought about by the development of third generation sources, and the development of more readily used analysis software should increase the accessibility of the method. It is hoped that this review will enable the nonexpert to understand both the power and limitations of XAS in characterizing fuel cell electrocatalysts. 

Briefly, XANES is associated with the excitation process of a core electron to bound and quasibound states, where the bound states interacting with the continuum are located below the ionization threshold (vacuum level) and the quasibound states interacting with the continuum are located above or near the threshold. Thus, XANES contains information about the electronic state of the x-ray absorbing atom and the local surrounding structure. However, as stated above, unhke EXAES, since the excitation process essentially involves multielectron and multiple scattering interactions, interpretation of XANES data is substantially more complicated than that of EXAFS data. 

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