Structural information from XAFS

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. 

Although the experiments referred to here demonstrate the wealth of kinetics and structural data that can be obtained from TR-XAFS data, application of XAFS spectroscopy combined with complementary techniques provides unique and even more detailed information. This statement refers to the most elegant way of using XAFS spectroscopy simultaneously with other methods (e.g., XRD Clausen, 1998 Clausen et al., 1993 Dent et al., 1995 Thomas et al., 1995), and it also refers to XAFS experiments complemented by experiments carried out under similar experimental conditions (e.g., laboratory techniques such as XRD, Raman spectroscopy, TG/DTA). More often than not, a detailed XAFS analysis is possible only when all additional data (characterizing phases, metal valences, and structure) representing the catalyst are available. Furthermore, the analysis of TR-XAFS data should aim at extracting as much information from the XANES part and the EXAFS part of a XAFS spectrum as possible. 

TABLE 3.1. Structural Information Derived from XAFS Analysis for Ni Sorption on Various Sorbonents and for Known Ni Hydoxides ... 

Research on zeolites and mesoporous materials depends critically on the availability of characterization techniques that provide information on their electronic and structural properties. Many techniques (e.g. XRD, NMR, XAFS, UV-V1S, IR, Raman) provide information about bulk properties whereas surface sensitive techniques (e.g. XPS, SIMS, LEIS) will provide information from the surface of the particles of porous materials. For modern research spatially resolved information is indispensable, in particular with the advent of complex hierarchical materials that combine micropores and mesopores. For the latter sake, electron microscopy is of growing importance for the study of molecular sieves as is also apparent from the number of papers published on this topic over the last ten years (Fig. 1). Please note that the almost four-fold increase in papers over the last ten years about electron microscopy on molecular sieves far outnumbers the relative increase of the total number of papers on molecular sieves (increase by factor 1.4). 

Often, the unique and unusual molecular structure in SCF solutions profoundly affects the reactivity in these systems. Thus, the elucidation of this structure is one of the first requirements for developing a predictive capability for reaction rates and pathways. A powerful spectroscopic technique that has only recently been used for in situ characterization of the molecular structure in supercritical fluid solutions is X-ray absorption fine structure (XAFS). XAFS provides detailed structural information about the number of nearest-neighbor atoms, bond distances, and bond strengths (from the Debye-Waller factor). The application of XAFS to a wide range of SCF solutions provides another powerful tool to explore the detailed structure of SCFs. 

An example of the type of information that can be derived from XAFS is shown in Figure 3.3-4 which presents a radial structure plot (RSP) for an... 

Table II. Structural Information Derived From XAFS Analysis Using EXCURVE ...

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

The physical technicpies for the structural characterization of such nanomaterials largely differ from canventional ones (such as X-ray diffraction). In the present example, indirect information is collected from X-ray absorption fine structure (XAFS) spectroscopy, conversion electron MoGbauer spectroscopy, and also X-ray photoelectron spectroscopy. 

X-Ray absorption fine structure (XAFS) is a powerfiil technique for the study of active sites, but certain limitations have to be taken into account. It is an averaging method information arising from all kinds of species formed by the element of... 

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