Local structural information

EM provides local structural information about the samples in both real and reciprocal space, for example local structural information about the surface and the bulk of the sample at the atomic level, together with chemical, electronic and three-dimensional structural information are now routinely available. Some of these methods are described in this chapter. Electron-sample interactions and scattering are fundamental to EM. EM is a diffraction technique in which crystals diffract electrons in accordance with Bragg s law, nX = Idhki sin 9, where X is the... 

Nakano, M Kawamura, K. Ichikawa, Y. 2003. Local structural information of Cs in smectite hydrates by means of an EXAFS study and molecular dynamics simulations. Applied Clay Science, 23, 15-23. 

One final key feature of photoelectron diffraction which is not shared by LEED or SEXAFS is the ability to exploit so-called chemical shifts in photoelectron binding energies of atoms of the same element in different structural and electronic environments to obtain chemical state specificity in the local structural information. 

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,... 

Extended X-ray absorption fine structure (EXAFS) on the other hand, is due to the interference of electron waves between atoms, and provides local structure information that is limited to a few interatomic distances. Here, we talk about the distance and the number of nearest and next-nearest neighbors of atoms in the catalyst. The more uniform the environment is through the catalyst, the more meaningful is the EXAFS information. Related to this method is X-ray absorption near edge spectroscopy (XANES), which deals with the detailed shape of the absorption edge, and yields important information on the chemical state of the absorbing atom. Commonly, one uses nowadays the acronym XAFS to include both EXAFS and XANES. 

Consequently, it has been confirmed that the broad n peak features in the XAS of CB can provide local structure information, especially on the nonbenzenoid structure and/or hydrogenated cluster-edged carbon atoms. 

XAS can be used in several different ways to determine local structural information about catalysts in reactive atmospheres. This structural information may be static or dynamic it may be geometric or electronic. The depth of information that can be ascertained is often dependent upon the type of catalyst, for example, supported metal nanoclusters versus bulk or surface oxides. It may also be controlled by some property of the catalyst, for example, the concentration of the element in the catalyst that is being investigated. In this section a few examples are provided to highlight the importance and relevance of XAFS in catalyst characterization. The examples are focused on (1) structural information characterizing samples in reactive atmospheres, (2) transformation of one species to another, (3) oxidation state determination, (4) determination of supported metal cluster size and shape, and (5) electronic structure. These examples illustrate the type of information that can be learned about the catalyst from XAFS spectroscopy. 

EXAFS is an X-ray absorption technique which gives detailed local structure information, such as the type, number and distance of neighbouring atoms [36]. As in XPS, the basic process is the photoelectric effect a photon is absorbed by an atom or ion and an electron is emitted from an inner shell. 

A method that yields local structural information is EXAFS which utilizes the scattering of photoelectrons emitted from specific atomic core levels for determining the interatomic distances and counting the near neighbors (but does not determine the directions between the emitting atom and its neighbors). 

It has been shown for both the selective oxidation of butane (43) and the ammoxidation of methyl-substituted aromatics (55) that the strength of spin-spin exchange in -containing bulk catalysts is related to the catalytic performance. Moreover, results discussed in this section illustrate that the spin-spin exchange properties are already determined by the conditions of catalyst formation and can be assessed in a unique way by EPR spectroscopy. Thus, this technique provides local structural information even for amorphous constituents or disordered phases—information that is hardly accessible by other methods such as XRD. This point is significant because active sites in real catalysts are frequently not located in well-crystallized phases but instead in disordered or even amorphous phases. 

In contrast to NOEs, scalar coupling relies on localized direct interaction. They have therefore potentiality to define precise local structure information. They can also be used to obtain stereospecific assignments (e.g., for C J1 methylene protons) in combination with intra-residual and sequential NOEs.72... 

A structural surface characterization with high resolution is always restricted to a small fraction of the total surfiice area. The question of how representative the characterized fiaction is for the whole sample is thus crucial. The utilization of well-defined substrate electrodes facilitates the interpretation of local structural information to a large extent. Starting from well-defined substrate surfaces, composite model electrodes with defined mesoscopic structures can be prepared. Out of the various approaches two examples will be discussed, one describing the nanometer-scale mocUfication of Pt(l 11) surfaces by ruftieniuni, the other the modification of conductive surfaces by nanometer-sized particles. 

The utilization of well-defined surfaces facilitates the interpretation of local structural information, but the application of STM is not restricted to such model systems. When irregular surfaces are investigated with STM, the local character of the measurement... 

SAXD and WAXD have provided the lamellar and lateral lipid organization in the SC. However, these techniques require large quantities of the SC for sample preparation therefore, they cannot provide any information regarding the local structural information on the lipids. Electron diffraction can be used to obtain the... 

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. 

Difficult technique for liquids. Local structural info for solids. Amorphous powder electrodes Local structural information. Solid-state electrochemistry, e.g., batteries... 

Because there are more ex situ characterization techniques than this chapter could cover, here we only describe briefly the principles of some selected ex situ techniques. These characterization tools could provide average or collective chemical information, local chemical information with spatial resolution at micrometer and submicrometer ranges, and local structural information at nanometer and atomic scales for heterogeneous catalysts. Examples are selected that demonstrate the performances of these techniques, with a special focus on their applications in characterizing nanocatalysts for energy production and energy conversion. 

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