EXAFS reviews

Comprehensive EXAFS reviews on catalysis have been recently published. These encompass studies concerning surface mediated electrochemical processes and catalytic solids under environmental conditions. The first set of studies examined the local structural changes in the metal occurring in tandem with redox processes under voltage control. " Recent work in this area examines the stability of bimetallic Pt-Ru electrodes and their performance in the hydrogen oxidation reaction in the presence of CO or direct methanol fuel cells. Studies reviewed involving supported metal catalysts concern, in the first place, the reduction process " while other studies also consider the oxidation process and reduction under inert gases. All of these studies make an attempt to understand the relevance of the metal support interface... 

We have found EXAFS to be a very effective method for obtaining structural information on bimetallic cluster catalysts (8,12-15,17) These types of catalysts, and bimetallic catalysts in general, have been the subject of extensive research in the EXXON laboratories since the 1960 s (18-25). In this paper we present a brief review of the results of some ofour EXAFS investigations on bimetallic cluster catalysts. 

Spin-state transitions have been studied by the application of numerous physical techniques such as the measurement of magnetic susceptibility, optical and vibrational spectroscopy, the Fe-Mbssbauer effect, EPR, NMR, and EXAFS spectroscopy, the measurement of heat capacity, and others. Most of these studies have been adequately reviewed. The somewhat older surveys [3, 19] cover the complete field of spin-state transitions. Several more recent review articles [20, 21, 22, 23, 24, 25] have been devoted exclusively to spin-state transitions in compounds of iron(II). Two reviews [26, 27] have considered inter alia the available theoretical models of spin-state transitions. Of particular interest is the determination of the X-ray crystal structures of spin transition compounds at two or more temperatures thus approaching the structures of the pure HS and LS electronic isomers. A recent survey [6] concentrates particularly on these studies. 

EXAFS has recently been reviewed [307] XAS and NEXAFS monographs are also available [308,309]. 

Determination of the structures of Grignard reagents continues to be of interest, and reviews on this subject have appeared.126,127 Most of the structure authentications are done on crystalline materials, although solution studies performed with extended X-ray absorption fine structure (EXAFS) spectroscopy are also available. The Grignard compounds MeMgBr and EtMgBr in BunzO were studied at room temperature and —85 °C with EXAFS. At both temperatures, dimers are observed (Mg-(/r-Br) = 2.5 A Mg-O = 2.0 A).128... 

XAS data comprises both absorption edge structure and extended x-ray absorption fine structure (EXAFS). The application of XAS to systems of chemical interest has been well reviewed (4 5). Briefly, the structure superimposed on the x-ray absorption edge results from the excitation of core-electrons into high-lying vacant orbitals (, ] ) and into continuum states (8 9). The shape and intensity of the edge structure can frequently be used to determine information about the symmetry of the absorbing site. For example, the ls+3d transition in first-row transition metals is dipole forbidden in a centrosymmetric environment. In a non-centrosymmetric environment the admixture of 3d and 4p orbitals can give intensity to this transition. This has been observed, for example, in a study of the iron-sulfur protein rubredoxin, where the iron is tetrahedrally coordinated to four sulfur atoms (6). 

The solid curves below 7 A are calculated accurately for a model (Section 4) that fits the osmotic coefficient data. The curves above 7 A are merely schematic, showing in exaggerated form the oscillations that appear in gab at large r when the concentration is large, even for the models in Section 4. The dashed curve indicates the location and intensity of the peak in g+. (r) identified in aqueous NiCle in neutron diffraction and EXAFS studies, as reviewed in Section 5. 

The details of the analysis of the XANES and EXAFS regions of the XAS spectra are beyond the scope of this review. However, as XAS is becoming a more routine tool for the study of fuel cell catalysts. 

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. 

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

There are a number of excellent sources of information on copper proteins notable among them is the three-volume series Copper Proteins and Copper Enzymes (Lontie, 1984). A review of the state of structural knowledge in 1985 (Adman, 1985) included only the small blue copper proteins. A brief review of extended X-ray absorption fine structure (EXAFS) work on some of these proteins appeared in 1987 (Hasnain and Garner, 1987). A number of new structures have been solved by X-ray diffraction, and the structures of azurin and plastocyanin have been extended to higher resolution. The new structures include two additional type I proteins (pseudoazurin and cucumber basic blue protein), the type III copper protein hemocyanin, and the multi-copper blue oxidase ascorbate oxidase. Results are now available on a copper-containing nitrite reductase and galactose oxidase. 

EXAFS has also been used for investigating a number of ferrodoxins (for a review, see Teo and Shulman As these studies were carried out a few years ago when rigorous analysis methods were not developed, only 2-shell analyses were performed and therefore information is available on only the Fe—S and Fe—Fe distances. A more comprehensive analysis should provide further details of the cluster. For ferrodoxins containing [2 Fe—2 S ] and (4 Fe—4 S ), Fe—S and Fe—Fe distances have been determined to be 2.25 + 0.01 A and 2.70 0.03 A. The increase in bond distances upon reduction for these proteins is small (<0.02 A) and may just be considered outside the error limit. The small changes upon reduction are better defined compared to what may be possible from the crystallographic determination at 2 A which had suggested a possible increase of 0.06 A upon reduction... 

The analysis of the SEXAFS data is basically identical to the analysis of conventional EXAFS data We will simply recall the basic ideas that sustain the conventionally used Fourier analysis of the SEXAFS data, making reference to Fig. 2, which will be further discussed below. The reason for being brief is that excellent reviews are available in widely diffused journals that were written by the promoters of the technique and warrant exhaustivity on the subjects The (S)EXAFS signal is defined as ... 

An EXAFS experimental set-up has three primary components (i) a source of X-rays, (ii) a monochromator (and collimator) and (iii) a detector. Synchrotron radiation is being widely used for EXAFS, but where this facility is not available, a rotating anode source would be suitable. Progress in EXAFS instrumentation has been comprehensively reviewed in the AIP proceedings (1980). 

HDS catalysts have been characterized extensively with a wide variety of tools, and several extensive reviews of the subject have been presented (85,88-91). Substantial effort has been aimed at relating catalytic activity and selectivity to microscopic properties such as catalyst composition, electronic structure, and geometric structure. EXAFS investigations of working catalysts have provided information about the composition, average local coordination, and interatomic distances of atoms in the catalyst clusters. It has been concluded that the active phase under operating conditions is MoS2-like particles with a dimension of 10—20 A (92-94). 

X-ray and neutron diffraction methods and EXAFS spectroscopy are very useful in getting structural information of solvated ions. These methods, combined with molecular dynamics and Monte Carlo simulations, have been used extensively to study the structures of hydrated ions in water. Detailed results can be found in the review by Ohtaki and Radnai [17]. The structural study of solvated ions in lion-aqueous solvents has not been as extensive, partly because the low solubility of electrolytes in 11011-aqueous solvents limits the use of X-ray and neutron diffraction methods that need electrolyte of -1 M. However, this situation has been improved by EXAFS (applicable at -0.1 M), at least for ions of the elements with large atomic numbers, and the amount of data on ion-coordinating atom distances and solvation numbers for ions in non-aqueous solvents are growing [15 a, 18]. For example, according to the X-ray diffraction method, the lithium ion in for-mamide (FA) has, on average, 5.4 FA molecules as nearest neighbors with an... 

Although combined EXAFS/XRD has been found to be very useful in studies of dynamic phenomena in catalysts, only a limited number of experiments have been reported so far. The difficulty in designing and constructing in situ cells that can be used both in XRD and in X-ray absorption spectroscopy is probably the most important limitation hindering more widespread use. Here, we briefly review some of the work performed with the combined EXAFS/XRD techniques. 

At the present time much effort is being devoted to tailor-making of new nanomaterials with specific catalytic properties. In this quest for constantly decreasing the dimensions of the catalytically active components, one will unavoidably encounter materials that will be partly or completely X-ray amorphous. The present review has shown that the combined EXAFS/ XRD techniques are uniquely well suited for providing the necessary structural understanding. Thus, in view of the trend in catalyst technologies and advances in technique developments, the application of the combined techniques will no doubt play an increasing role in future catalyst characterization efforts. We now briefly discuss some likely applications and technique developments which involve the X-ray techniques discussed presently. 

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