Curve-fit analysis

Such a function exhibits peaks (Fig. 9C) that correspond to interatomic distances but are shifted to smaller values (recall the distance correction mentioned above). This finding was a major breakthrough in the analysis of EXAFS data since it allowed ready visualization. However, because of the shift to shorter distances and the effects of truncation, such an approach is generally not employed for accurate distance determination. This approach, however, allows for the use of Fourier filtering techniques which make possible the isolation of individual coordination shells (the dashed line in Fig. 9C represents a Fourier filtering window that isolates the first coordination shell). After Fourier filtering, the data is back-transformed to k space (Fig. 9D), where it is fitted for amplitude and phase. The basic principle behind the curve-fitting analysis is to employ a parameterized function that will model the... 

By Fourier transforming the EXAFS oscillations, a radial structure function is obtained (2U). The peaks in the Fourier transform correspond to the different coordination shells and the position of these peaks gives the absorber-scatterer distances, but shifted to lower values due to the effect of the phase shift. The height of the peaks is related to the coordination number and to thermal (Debye-Waller smearing), as well as static disorder, and for systems, which contain only one kind of atoms at a given distance, the Fourier transform method may give reliable information on the local environment. However, for more accurate determinations of the coordination number N and the bond distance R, a more sophisticated curve-fitting analysis is required. 

Vanadium K-edge XANES measured in transmission mode at 15 K showed that the attached V complex (2) maintained its square pyramidal conformation with a V=0 bond (Figure 2.2a). Curve-fitting analysis of V K-edge EXAFS Fourier transforms (Figure 2.2b) provided local structure information on the supported V complex (2) with an unsaturated conformation, which differs from that of the V-monomer precursor (1). The EXAFS curve-fitting was performed in the R-space with two shells short V=0 and long V—O bonds. A V=0 bond was observed at 0.157 0.001 nm,... 

Structural analysis using both curve-fitting and multivariate statistical methods can be employed to ATR-FTIR spectra. However, because disordered and helix bands may not be well resolved, band assignments in curve-fitting analysis can be difficult. PLS analysis, on the other hand, can determine the secondary structure of proteins from H2O ATR-FTIR spectra with higher accuracy because it is not necessary to assign bands using this method. 

Results of Curve-Fitting Analysis of Rh K-Edge EXAFS Data Obtained at 300 K for NaY-Entrapped Rh Cluster Samples"... 

Figure 22.6 Coordination numbers determined by curve fitting analysis plotted as a function of temperature measured in TPRs. ( ) Pd-Pd (metal), (A) Pd-O, ( ) Pd Osurfacei-...

The changes in the CNs in the second TPO experiment are summarized in Figure 22.7 (2nd). The profile of the second TPO was different from that of the first TPO in that the oxidation of Pd clusters progressed without an increase in the CN (Pd—Pd). As a result, the removal of the Pd clusters from the zeolite framework and their oxidation took place simultaneously in the second TPO. The curve-fitting analysis of the EXAFS data obtained in the third TPR run is included in Figure 22.6 (3rd). The changes in the CNs were very similar to those of the second run. In other words, the formation of Pd4 clusters was observed after the disappearance of the... 

The CNs of the Pd—O(N) and Pd—Pd bonds were determined by curve-fitting analysis the data are summarized in Figure 22.10. 

Figure 22.10 CNsof Pd—O(N) and nearest-neighboring Pd —Pd bonds determined by curve-fitting analysis of 0.4 wt%-Pd/H-USY plotted as a function of time measured in an 8% H2 flow ( ) Pd —Pd (metal) (O) Pd—O or Pd —N.

Table 1 Structural parameters estimated by the curve-fitting analysis for Pt Lm-edge EXAFS of coat R catalysts.

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