XANES region states

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. 

XAFS spectroscopy has been used frequently to determine the oxidation state of an element as a catalyst is subjected to treatments such as reduction, oxidation, or exposure to some reactant. It is the near edge of the X-ray absorption spectrum, the XANES region that is usually used for these measurements. As mentioned in the introduction, the first to recognize the value of XANES for this type of investigation was van Nord-strand (1960), and his report appeared well before the advent of synchrotron radiation sources. 

There are many other such examples, illustrated by the results of Figures 8 (Bare, S.R., unpublished data) and 9 (Ressler et al., 2002). The latter example is interesting in that instead of the edge position as a monitor of the average molybdenum oxidation state, the absolute energy position of a feature in the XANES region was used (indicated by an arrow in Figure 9). 

Fig. 13. Pictorial view of the final-state radial wave functions relevant for core transitions in a molecule. The core transitions take place in an effective molecular potential seen by the excited photoelectron. The final states in the continuum XANES region are quasi-bound multiplescattering resonances (MSR), also called shape resonances. Below the continuum threshold E0 transitions to unoccupied valence states appear. 0 is the energy of the core ionization potential (from ESCA). Ec is the energy where the wavelength of initially excited photoelectrons conforms to the interatomic distance. For E < E0, discrete transitions to unoccupied valence states. E0 < E < Ec, continuum XANES. For < Ec, the EXAFS theory breaks down. The dotted curves show the wave functions of the initially excited photoelectron. From Bianconi (30).

Information on the valence state of Pd can be obtained from the analysis of the X-ray absorption near-edge structure (XANES) region. The calculated ratio of Pd° and the total amount of Pd in the 0.4 wt%-Pd/H-USY are summarized in Figure 22.11. 

The computational analysis of the XANES region is very complex, and hence it is often used mainly for qualitative comparison to give information on both oxidation states and coordination numbers. It is particularly used in the analysis of transition metal oxidation states and coordination numbers in catalysts... 

In the XANES region of the X-ray absorption spectrum, two physical processes play a significant role. These are electronic transitions into unoccupied electronic states and multiple scattering (MS) resonances. 

The XANES region contains information on the electronic state of the absorbing atom as well as on its coordination geometry. In general, three types of XANES features can be distinguished Pre-edge peaks carrying information on coordi-... 

Current XAFS research in the physical and theoretical area is devoted mainly to a better understanding of the XANES region in order to gain better access to the high content of information. Experimental improvements focus on improving time and spatial resolution. For in situ studies, the trend is to combine additional methods (XRD, gas chromatography, differential scanning calorimetry [100]) with XAFS in order to yield more comprehensive information about the state of the sample or about its catalytic activity. 

XANES spectra provide local structure information and the oxidation state. In the XANES region, electrons that are excited from the core level (usually the K level) jump into unfilled bound states, nearly bound states (resonances) or continuum states. Due to multiple electron scattering, XANES spectra are harder to interpret than EXAFS spectra. 

Initially, the as-synthesized catalyst was characterized by EXAFS and XANES (region a), and then the reaction mixture comprising benzyl alcohol/cyclohexane (solvent) saturated with 02/mixture was introduced to the catalyst at 50 °C (region b). Palladium was found to be in a partially oxidized state and there was no catalytic activity for this material under this condition as evidenced from the IR spectrum of the effluent from the reactor. Then, the catalyst material was exposed to H2-saturated cyclohexane to reduce the palladium to metallic state (region C). After about 30 min of the exposure, aU the palladium atoms are found to be in the metallic state. This has been confirmed by both XANES and EXAFS data, as shown in Figure 12.10. Analysis of the EXAFS data showed evidence for the formation of palladium hydride. This leads to a more elongated EXAFS function and to a shift... 

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