XANES data analysis

For EXAFS and particularly for XANES, data analysis is complex. The oscillation frequency/bond distance dependence means that extensive use is made of Fourier transform analysis. Most applications to date have been in the EXAFS region. In order to acquire sufficiently strong signals in a reasonable time, use has to be made of high-intensity photon fluxes, which are available at synchrotron facilities. These provide a broad-band tuneable source of high-intensity radiation, but the reduced number of facilities limits widespread dissemination of the technique. Reflection (fluorescent detection) mode is usually preferred to transmission. Experiments can be conducted in any phase, and the probing of electrode surfaces in situ is an important application. 

XANES data analysis is considerably more complex than EXAFS analysis. 

Some topics have been omitted from this review. This holds for the structure and function of metal sites in metalloproteins and metalloenzymes in relation to enzymatic catalysis, for which the reader is referred to Cramer and Hodgson (68) and Doniach et al. (80). Also, chemisorption studies and the structure of adsorbate-covered surfaces are not considered in this review, which deals with XAES in transmission, thus characterizing bulk material. It is noted that even in the case of chemisorbed atoms XANES data analysis requires physically the definition of clusters of considerable size. On the other hand, the analysis is simplified for adsorbed molecules. Very pronounced near-edge effects (usually obtained by electron-stimulated Auger measurements) are observed for low-Z-atom(C, N, O, F>containing chemisorbed... 

In the XANES data analysis the first step is to remove the continuum absorption background due to transitions from levels of lower binding energy. This procedure is generally called pre-edge background subtraction. The relative absorption aCEj/a is usually plotted, where is the smooth atomic absorption above threshold determined by the usual EXAFS analysis. Fig. 2 shows two typical XANES spectra where the XANES are normalized to the atomic absorption. 

The adsorptions of H, O, and S04 on Pt/C electrocatalyst electrodes have been further investigated by O Grady and Ramaker by comparing the XANES data at the Pt L2 and L3 absorption edges. In their analysis, the difference spectrum, which they term AS for antibonding state, is obtained as follows ... 

In the first section will be presented XAS from the physical principles to data analysis and measurements. Then section 2 will be devoted to a discussion of a few examples to illustrate the power and limitations of XAS for gaining structural information. Examples are focused on EXAFS studies on nanocrystalline materials. Detailed reviews for applications on other fields of materials science or for presenting the complementary information available by the study of the X-ray Absorption Near Edge Structure (XANES) part of the X-ray absorption spectrum can be found in a number of books [3-5], A brief overview of the recent development of the technique regarding the use of X-ray microbeams available on the third generation light sources will be finally presented in the last section. 

XRD and LEED are laboratory techniques, although synchrotrons offer advantages for XRD. XAFS, on the other hand, is conducted almost exclusively at synchrotrons. This - and the fact that EXAFS data analysis is complicated and not always without ambiguity - has inhibited the widespread use of the technique in catalysis. XANES, however, is becoming increasingly popular, as it may routinely yield similar information as X-ray photoelectron spectroscopy (XPS), but under in-situ conditions. 

EXAFS spectra can, in principle, be well described on the basis of scattering theory (although data analysis is time-consuming and demand considerable expertise). XANES spectra can only be predicted on the basis of first-principle calculations. As noted above, this is generally not necessary because in practice XANES data are interpreted with the help of reference spectra. 

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. 

Furthermore, quantitative structural phase analysis, for instance, is important for investigations of solid catalysts, because one frequently has to deal with more than one phase in the active or precursor state of the catalyst. Principal component analysis (PCA) permits a quantitative determination of the number of primary components in a set of experimental XANES or EXAFS spectra. Primary components are those that are sufficient to reconstruct each experimental spectrum by suitable linear combination. Secondary components are those that contain only the noise. The objective of a PCA of a set of experimental spectra is to determine how many "components" (i.e., reference spectra) are required to reconstruct the spectra within the experimental error. Provided that, first, the number of "references" and, second, potential references have been identified, a linear combination fit can be attempted to quantify the amount of each reference in each experimental spectrum. If a PCA is performed prior to XANES data fitting, no assumptions have to be made as to the number of references and the type of reference compounds used, and the fits can be performed with considerably less ambiguity than otherwise. Details of PCA are available in the literature (Malinowski and Flowery, 1980 Ressler et al., 2000). Recently, this approach has been successfully extended to the analysis of EXAFS data measured for mixtures containing various phases (Frenkel et al., 2002). 

The data analysis reveals that the Co-S coordination number increases in the order Co Sg < Co/C < Co-Mo/C. This means that the cobalt atoms in the catalysts have a higher sulfur coordination than those in CogSg and furthermore, that this phenomenon is more pronounced when cobalt is present in a Co-Mo-S structure. The EXAFS results are substantiated by the XANES spectra. The 1s —> 3d transition of the Co/C catalyst suggests a somewhat higher percentage of octahedral cobalt as present in Co Sg whereas for the Co-Mo/C catalyst it indicates an octahedral-like sulfur coordination. 

The unique direct spectroscopic information obtainable from XANES data with the new analysis technique is highlighted here. Data have been taken at both the cathode (oxygen reduction) and at the anode (hydrogen or methanol oxidation). 

B. K. Teo, EXAFS Basic Principles and Data Analysis, Springer Verlag, Berlin (1986) DC Koningsberger, R Prins, X-ray Absorption Principles, Applications, Techniques of EXAFS, SEXAFS and XANES, John Wiley and Sons, New York (1988) P.A. Lee, P.H. Citrin, P. Eisenberger, B.M. Kincaid, Rev. Mod. Phys. 53, 769-806 (1981) T.M. Hayes, J.B. Boyce, Sol St. Phys. 37, 173-351 (1982) E.A. Stern, S.M. Heald, In Handbook of Synchrotron Radiation Vol. 1, Elsevier Science Publishers, Amsterdam, pp 955-1014, (1983) H. Winick, S. Doniach, Synchrotron Radiation Research, Plenum Press, New York (1980). 

Sayers, D. E., and Bunker, B. A., 1988, Data analysis in Koningsberger, D. C., and Prins, R., eds., X-ray Absorption Principles, Applicahons, Techniques ofEXAFS, SEXAFS, and XANES New York, John Wiley and Sons. 

Sayers DE, Bunker G (1988) EXAFS data analysis In X-ray absorption Principles, Applications, Techniques of EXAFS, SEXAFS and XANES. Koningsberger DC, Prins R (eds) John-Wiley Sons, New York... 

E. Stem, in X-Ray Absorption Principles, Applications, Techniques of EXAFS, SEX-AFS and XANES (D. C. Koningsberger and R. Prins, eds.), p. 3. Wiley, New York, 1988. B.-K. Teo, EXAFS Basic Principles and Data Analysis. Springer-Verlag, New York, 1986. 

The results from analysis of the EXAFS data (Fig. 8a) indieate that Pb is associated mainly with two phases Pb-carbonate (45%) and Pb bound to an Fe phase (55%). However, the discrepancy in the XANES fit (Fig. 8b) and the large energy shift in the XANES data for the standard of Pb adsorbed on goethite (Fig. 8c) provide ambiguous data regarding the actual presence of a Pb species associated to Fe-oxyhydroxides in this dust sample and warrant further investigation. A combination of /rXRF and xXRD was used for additional sample characterization, specifically to confirm the presence or absence of a Pb-Fe oxyhydroxide phase. 

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