X-Ray-Based Techniques

In situ or operando studies preferentially combine the identification of the structure and the catalytic performance. This is a demanding task and cannot be realized without compromises [31-34], For X-ray-based techniques and UY-vis, Raman, or IR spectroscopy, the reactor needs windows where the light can shine through or a design so that NMR/EPR studies can become possible. The principle is given in Figure 4.3.5. 

The scattering of the X-ray beam is directly proportional to the number of electrons an element has. In any X-ray-based technique, elements which are isoelectronic (have the same number of electrons) are indistinguishable to X-rays. This can cause problems in assigning sites in single crystal diffraction, and can lead to additional absences in the powder X-ray diffraction pattern. 

Radiography - An x-ray based technique used for detecting flaws in materials, for example, voids and cracks in metals. 

Of the many X-ray based techniques available, a very powerful approach for probing interfacial structures is based on the measurement of X-ray reflectivity. The X-ray reflectivity is simply defined as the ratio of the reflected and incident X-ray fluxes. In the simple case of the mirror-like reflection of X-rays from a surface or interface, i.e., specular reflectivity, the structure is measured along the surface normal direction. Lateral structures are probed by non-specular reflectivity. The measurement and interpretation of X-ray reflectivity data (i.e., the angular distribution of X-rays scattered elastically from a surface or interface) (Als-Nielsen 1987 Feidenhans l 1989 Robinson 1991 Robinson and Tweet 1992) are derived from the same theoretical foundation as X-ray crystallography, a technique used widely to study the structure of bulk (three-dimensional or 3D) materials (Warren 1990 Als-Nielsen and McMorrow 2001). The immense power of the crystallographic techniques developed over the past century can therefore be applied to determine nearly all aspects of interfacial structure. An important characteristic of X-ray reflectivity data is that they are not only sensitive to, but also specifically derived from interfacial structures. 

Fig. 9 A schematic diagram of time vj. photon energy in a combined QEXAFS-Scattering experiment. The vertical scale is dependent upon the element under investigation. On specialised beamlines it is feasible to carry out the XAFS scan in under a second. More usual is a scan time of up to 1 minute. The X-ray scattering data can be obtained in a few seconds. Non-X-ray based techniques can be carried out independent of the X-ray energy to which the sample is exposed.

Auxiliary techniques. Although we mainly discuss the combination of X-ray based techniques in this manuscript there are strong tendencies at present to incorporate other experimental probes in the experiments as well. Therefore we briefly mention some of the other techniques which can be combined with time-resolved XAS measurements. 

The combination of different experimental X-ray based techniques in a single time-resolved experiment became possible due to the evolution in X-ray beam quality which has been based upon the improvements in synchrotron radiation sources and the optical elements required harnessing these X-ray beams with respect to energy resolution, beam size, intensity and focal depth. The introduction of non-X-ray based techniques in the same experiment has the advantage of bridging the information that can be obtained in a home laboratory with the results only obtainable in a synchrotron radiation laboratory. At present a wealth of opportunities exists which regretfully has not been utilised as much as is possible. 

In the reflection mode, typically specular reflectance is measured on the electrode surface. It is anticipated that the variation of the surface structure (e.g., surface adsorption, phase transitions, etc.) will result in appreciable changes in the reflectivity properties. One can thus correlate the structural characterislics gleaned from spectroscopic measurements with electrochanical results. Figure 2.15 shows a cell assembly for internal reflection spectroelectrochemistry. Several spectroscopic techniques have been used, such as infrared, surface plasmon resonance, and X-ray based techniques (reflectivity, standing wave, etc.). Figure 2.16 depicts a cell setup for (A) infrared spectroelectrochemistry (IR-SEC) and (B) surface X-ray diffraction. 

X-ray-based techniques are frequently used to identify the normal bonding structure of glasses. From X-ray diffraction patterns, the radial distribution function can be calculated and finally the structural parameters like the coordination number of each atom, the bond length, and the bond angle can be inferred [1,41]. 

In the X-ray-based techniques. X-ray crystallography is the most powerful tool for the determination of macromolecular 3D structures at a resolution of 0.15-2 nm but the requirement for a single crystal will greatly limit its application to numerous biological samples. The application of X-ray crystallography for metalloproteomics is illustrated in Chapter 7 (Protein Crystallography for Metalloproteins) in this book. 

X rays are a short-wavelength form of electromagnetic radiation discovered by Wilhelm Roentgen in 1895. X-Ray-based techniques provided important tools for the theoretical physicist in the first half of this century, and since the early 1950s they have found an increasing use in the fields of materials characterization. Today, the analytical techniques based on X-ray diffraction and X-ray spectrometry, both of which were first conceived almost 70 years ago, play a vital role in the analysis and study of inorganic and organic solids. 

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