Linear combination XANES

X-ray absorption near-edge structure (XANES) can be used to analyze the oxidation states of many elements and to quantify the components of different valences for a given element in the samples. In order to perform the analysis of oxidation state, we can obtain the information by comparing the absorption edge position of the absorbing atom with that of standard samples. Moreover, we can use linear combination XANES (LC-XANES) to quantify and determine oxidation states of a given element in the samples. ... 

Table 74 Linear combination fits of XANES spectra with Ce3+ and Ce4+ reference compounds. Note that complete surface reduction (bold) is complete at lower temperatures with increasing metal loading. Ceria BET SA = 125 m2/g428 ... 

In theory, to describe a XANES spectrum is not an easy task. The equations discussed earlier in this chapter for EXAFS are not valid at low k-values (i.e., energies close to that of the edge), and instead the X-ray absorption will have to be calculated from first principles, which is a specialism in itself. Fortunately, XANES spectra can usually be very well interpreted with the help of reference spectra of known compounds, and constructing linear combinations of references to fit the spectrum of the catalyst often works well to obtain quantitative information on composition. 

The relative amounts of Pd(0) and Pd(II) were estimated by using a linear combination fit of the spectrum of the catalyst reduced in H2 (taken as Pd(0)) and the as-prepared catalyst (taken as Pd(II)). The changes in the XANES spectrum as a function of the various reaction conditions are subtle, but there is a measurably greater amount of reduced palladium present when the alcohol is present compared to when there is only 02 present in the supercritical C02 solvent. 

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 final states may be mixed with other orbitals and can be used to determine the coordination environment of an element in a compound, its electron density and oxidation state. The intensity can be used to accurately determine relative oxidation state ratios. Furthermore, XANES can be used to determine relative amounts of species by linear combination of individual compounds. In recent years, the codes for XANES calculation (especially John Rehr s FEFF code), have significantly improved and it can be expected that theoretical models of compounds will be accurately determined by XANES measurement and calculation in the future. 

The time-dependent concentration of species (Fig. 8) can be determined by a simple analysis in terms of a linear combination of the XANES spectra of Cu2+, Cu1 +, and metallic copper (Fig. 6). Three different kinetic domains must be considered ... 

Figure 20. Se -edge XANES spectra of (A) Se-containing model compounds (a) Se042 (aq), (b) SeC>32 (aq), (c) selenomethionine (aq), (d) elemental (red) selenium (monoclinic) (B) (a) Se in Kesterson Reservoir soil at depth of 0 to 0.05 m (340 ppm Se), (b) Se in Kesterson Reservoir soil at depth of 0.05 to 0.15 m (40 ppm Se), (c) Se in mushroom (Agaricus bernardii) collected adjacent to the reservoir (500 ppm Se). Linear combination fits of the Se K-XANES of the three Kesterson samples resulted in the following components (a) 97% elemental Se + 3% aqueous selenite (b) 86% elemental selenium + 14% aqueous selenite (c) 71% selenomethionine + 11% aqueous selenite + 18% selenocystine. Data were taken on SSRL beam line 4-3. (after Pickering et al. 1995)...

The XAS data analysis program WinXAS v. 3.1 [19] was used for examining the time-resolved Co K absorption edge XANES data. The identification of the number of phases present during in situ reduction was done by a principal component analysis (PCA) of the experimental spectra [20]. Reference spectra were then used in a linear combination fitting procedure to determine the quantity of each phase present. The algorithm uses a least squares procedure to refine the sum of a given number of reference spectra to an experimental spectrum. 

In general, tiie linear combination of XANES and the principal component analysis give very good fit to the experimental data. Figure 7 shows a close... 

Figure 2. Manganese XANES spectra of model compounds used as a linear combination to fit unknown spectra of P-MnOj reacted with Co(II)EDTA .

Figure 4. XANES spectra of pyrolusite coated silica reacted with Co(II)EDTA (a) experimental curves and (b) corresponding first-derivative experimental curves (solid lines) and associated fits (dashed-line). Fitted functions were constructed using a linear combination of the standard spectra described in Table 1.

Determined by linear combination of the XANES spectra of the catalyst reduced in 5% H2/He and the as-prepared catalyst ( 10%). The experiments were performed in the order indicated in the table. Adapted with permission from Ref. [102]. 

EXAFS characterization of SO poisoned catalysts. The Pt/silica with different dispersion and Pt/alumina catalysts were prereduced and tested for catalytic oxidation of CO with 20 ppm SOj and then analyzed by XAS. The XANES and EXAFS analysis (in air). Table 17.6, following the reaction shows significant amounts of oxidized Pt and the presence of Pt-Pt, and Pt-0 bonds, with no Pt-S bonds. From the Fourier transform of the EXAFS, all samples have metallic Pt, thus this must be included in the XANES fit. For several samples, the height of the white line was larger than in spectra of the PP reference, and no fit could be obtained using only PP and PP. However, excellent fits were obtained with a linear combination of PP and Pt . Acceptable fits were also possible using PP, PP, and Pt , which were about 25% less metallic Pt. Although the absolute values of the various Pt oxidation states differ with the two fits, the trends are the same. 

Analysis software like e.g. the TXMwizard [21] enables stacking and alignment routines for a set of 2D images from which then single pixel XANES or element specific information can be extracted and further analyzed, e.g. by linear combination fitting of suitable standards or principal component analysis [21]. 

Besides the straightforward fingerprints studies, XANES can also be applied to quantitative speciation. This is because XAS is a local probe teehnique, which implies no long-range order in the sample is required. Therefore, if the absorbing atoms are present in the sample at two different sites, the XANES spectrum obtained from this material can be represented by the weighted addition of the spectra of suitable reference samples. For many systems, XANES analysis based on linear combinations of known spectra from model compounds is sufficient to estimate ratios of different species. More sophisticated linear algorithms, such as principle component analysis and factor analysis, can also be applied to XANES spectra. ... 

Figure 11.20 Experimental XANES spectra at the Ce Lm edge of pure nano-Ce02, and nano-CeOj incubating in abiotic DMEM during 24 h. ( ) nano-Ce02 at 0.6gL The experimental data were fitted using linear combination of XANES spectra of nano-Ce02 suspended in water and Ce " oxalate reference compound. The arrows highlight the decrease of the intensity of the peak at 5740 eV and the growth of the peak at 5729 eV attributing to the appearance of Ce after incubation in the DMEM. These C atoms are assumed to be on the surface of nano-...

LC-XANES Linear combination X-ray absorption near edge... 

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