Catalyst characterization fine structure

X-ray absorption spectroscopy combining x-ray absorption near edge fine structure (XANES) and extended x-ray absorption fine structure (EXAFS) was used to extensively characterize Pt on Cabosll catalysts. XANES Is the result of electron transitions to bound states of the absorbing atom and thereby maps the symmetry - selected empty manifold of electron states. It Is sensitive to the electronic configuration of the absorbing atom. When the photoelectron has sufficient kinetic energy to be ejected from the atom It can be backscattered by neighboring atoms. The quantum Interference of the Initial... 

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. 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

Due to the heterogeneity of the recently advanced solid-support catalyst for the hydroformylation, direct structural information on catalyst surface has been collected by extended X-ray absorption fine structure (EXAFS). Iwasawa is the first to directly characterize the structure of dimeric rhodium complexes supported on... 

In recent years, it has been realized that techniques based on X-ray absorption provide important additional possibilities for catalyst characterization. Techniques such as X-ray absorption fine structure (XAFS) spectroscopy have had a significant impact on catalyst research. For example, the application of these techniques has for the first time allowed structural descriptions of many catalysts which, because of the presence of microcrystalline structures (nanophase particles) or amorphous phases, cannot be elucidated by XRD. 

The other approach is to study real catalysts by using in-situ techniques such as infrared and Mossbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) and XRD, either under reaction conditions, or - as occurs more often -under a controlled environment after quenching the reaction. These in-situ techniques, however, are usually not sufficiently specific to yield the desired atom-byatom characterization of the surface, and often they determine the overall properties of the particles. The situation is represented schematically in Figure 1.8. 

Among the techniques ideally suited for in situ studies are infrared, Raman, and nuclear magnetic resonance (NMR) spectroscopies and extended x-ray absorption fine structure (EXAFS). While still relatively new, the scanning tunneling and atomic force microscopes are expected to play an increasingly important role in catalyst characterization. Both instruments permit visualization of a catalyst surface at the atomic level and hold the potential of showing how atoms and molecules interact with a surface. 

In addition to the structure in the dehydrated state, the structure of supported vanadia catalysts under redox reaction conditions is directly related to the catalytic performance. Vanadia catalysts are usually reduced to some extent during a redox reaction, and the reduced vanadia species have been proposed as the active sites [4, 19-24]. Therefore, information on the valence state and molecular structure of the reduced vanadia catalysts is of great interest. A number of techniques have been applied to investigate the reduction of supported vanadia catalysts, such as temperature programmed reduction (TPR) [25-27], X-ray photoelectron spectroscopy (XPS) [21], electron spin resonance (ESR) [22], UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [18, 28-32], X-ray absorption fine structure spectroscopy (XAFS) [11] and Raman spectroscopy [5, 26, 33-41]. Most of these techniques give information only on the oxidation state of vanadium species. Although Raman spectroscopy is a powerful tool for characterization of the molecular structure of supported vanadia [4, 29, 42], it has been very difficult to detect reduced supported... 

Catalysts and Related Surfaces, Characterized by X-ray Absorption Fine Structure, G.A. Somorjai, Ed., Baltzer, Basel, Switzerland, 1993, (Proceedings of a symposium held in Tokyo, Japan, 1992.)... 

The determination of the photoluminescence parameters (excitation frequency, emission frequency, Stokes shift, fine structure parameter, and lifetime) can lead to information which, at the simplest level, indicates the presence of an electronically excited state of a species, but which can be sufficiently detailed so as to lead to a clear identification and characterization of the photoluminescent sites(J6-44). Moreover, measurements of the variations in the intensity and positions of the bands as a function of time (time-resolved photoluminescence) provide valuable kinetic data representing the reactions occurring at the surface. Although most of the photoluminescence measurements have been carried out at low temperatures for specific reasons (see Section III.C.2), there is much evidence that some of the excited states involved are present even at higher temperatures and that they play an important role in catalytic and photocatalytic reactions. Therefore, it is clear that the information obtained by photoluminescence techniques is useful and important lor the design of new catalysts and photocatalysts. 

Over the past several years a number of instrumental techniques have been applied to the characterization of catalysts.12,33 Ultraviolet, infrared,25.36 a d raman22 spectroscopic data have provided information concerning the nature of substrates adsorbed on a catalyst surface as well as the composition and stmcture of many oxide catalysts. Extended X-ray absorption fine structure (EXAFS) is... 

Fung AS, KeUey MJ, Koningsberger DC, Gates BC (1997) y-Al O -Supported Re-Pt cluster catalyst prepared from [Re PtlCOljJ Characterization by extended X-ray absorption fine structure spectroscopy and catalysis of methylcyclohexane dehydrogenation. J Am Chem Soc 119 5877... 

XPS and FTIR have been used for surface characterization in order to prove the formation of carbonaceous material on the NaP-modified MnOx catalyst surface. The results obtained offer clear evidence about formation of hydrocarbon deposits on the catalyst surface during the methane treatment. XPS showed a carbon line at 285 eV, whose intensity increased with a longer treatment in methane, this suggests that the amount of the carbon deposit increased. Intensity of Auger Na(KLL) line at ca. 264 eV was used as reference, since both ESCA and AFM studies showed that the surface of NaP particles remained uncovered by coke for a longer time than the manganese oxide surface The fine structure of the XPS line of C Is... 

Characterization of the catalysts was made using high-resolution transmission electron microscopy (TEM) (Hitachi H-9000), X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and Fourier transform-infrared spectroscopy (FT-IR). 

The method of accounting for the overlapping of iridium Lm EXAFS in the analysis of data on the fine structure associated with the platinum Zm edge of platinum-iridium catalysts has been tested in EXAFS experiments on a physical mixture of platinum and iridium and on a bulk alloy of the two elements (48). Figure 4.29 shows the effect of the value assumed for Rw on the quality of fit to the experimental data for xf "(A0 that can be obtained by Eqs. 4.13, 4.16, and 4.17. The quality of fit is characterized by the normalized fitting error, which is defined in this case as the ratio of the standard deviation of fit for a given value of R to the minimum standard deviation obtained over the range of distances examined. 

Spectroscopy also provides structural information about supported metals. The EXAFS (extended X-ray absorption fine structure) technique helps to define average crystallite structures, and infrared spectroscopy provides structural characterization of chemisorbed species. The challenges of determining structures of supported metals are great because of the nonuniformities of the metal crystallites in almost all catalysts. 

The most important objective in the characterization of oxide surfaces requires a depth of knowledge similar to that available in homogeneous catalysis. Recently, the characterization of oxide surfaces at the atomic and molecular level has received a great boost from the development of a variety of sophisticated techniques, including scanning tuimeling microscopy (STM)[7-8] and X-ray absorption fine structure (XAFS)[9-10], which provide valuable support for the mostly empirical approach to catalyst design. 

It is not always easy to characterize the electronic and crystallographic structures of very small aggregates. Their size (a few nanometers) is due to the fact that as many atoms as possible must be active and therefore must be at the surface. Moreover, the analysis has to be done in situ, under the true reaction conditions, in order to build a physical model for the role of the catalyst. Then, many experimental techniques have been used, including most recently electron microscopy and X-ray absorption. We focus our attention here on the EXAFS (Extended X-ray Absorption Fine Structure) technique and its possibilities for the study of supported metal catalysts. Most of the examples come from a collaboration between LURE and some public CNRS laboratories (Strasbourg, Meudon) and a private one (IFF — Rueil Malmaison). We begin with some generalities about the technique and the type of catalysts studied, then move to several examples of application. 

X-ray absorption is a powerful technique to obtain local electronic and structural properties and has been widely used to characterize catalysts (Yokoyama 1995). This technique involves the determination of the electronic state through X-ray absorption near edge structure (XANES) analysis and of the local structure through the extended X-ray absorption fine structure (EXAFS). The data can be collected not only in a static but also in dynamic state (in-situ conditions) allowing one to study the catalyst during pretreatment and catalytic reaction. 

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