Selective EXAFS

A wide selection of metal reference foils and powder films of ideal thickness for tranmission EXAFS is available from The EXAFS Materials Company, Danville, CA, USA. The transmission method is well-suited for in situ measurements of materials under industrially relevant conditions of extreme temperature and controlled atmosphere. Specially designed reactors for catalysis experiments and easy-... 

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identify and quantitate crystalline phases in bulk materials and at surfaces and interfaces. While NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under smdy, NMR can also provide structural insights on a molecularlevel. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly usefrd in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often fail. 

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... 

A cationic molybdenum sulfide cluster [Mo3S4(H20)9] " with incomplete cubane-type structure and a cationic nickel-molybdenum mixed sulfide cluster [Mo3NiS4Cl(H20)9p " with complete cubane-type structure were introduced into zeolites NaY, HUSY and KL by ion exchange. Stoichiometry of the ion exchange was well established by elemental analyses. The UV-visible spectra and EXAFS analysis data exhibited that the structure of the molybdenum cluster remained virtually intact after ion exchange. MoNi/NaY catalyst prepared using the molybdenum-nickel sulfide cluster was found to be active and selective for benzothiophene hydrodesulfurization. 

The low Ti content (up to 3 wt % in Ti02) makes the extraction of vibrational, energetic, and geometric features specific to Ti04 moieties a difficult task as the experimental data are dominated by the features of the siliceous matrix. This is the reason why the structure of the local environment around Ti(IV) species inside TS-1 was only definitively assessed more than 10 years after the discovery of the material, when the atomic selectivity of X-ray absorption spectroscopies (both XANES and EXAFS) were used [58-60]. 

Linear absorption measurements can therefore give the first indication of possible alloy formation. Nevertheless, in systems containing transition metals (Pd-Ag, Co-Ni,. ..) such a simple technique is no longer effective as interband transitions completely mask the SPR peak, resulting in a structurless absorption, which hinders any unambiguous identification of the alloy. In such cases, one has to rely on structural techniques like TEM (selected-area electron diffraction, SAED and energy-dispersive X-ray spectroscopy, EDS) or EXAFS (extended X-ray absorption fine structure) to establish alloy formation. 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

Formation of polynuclear lead species with parameters close to isolated lead bromophenoxides during DPC synthesis was found by EXAFS of frozen active reaction mixtures (Pb-0 = 2.34 A, Pb Pb = 3.83 A). Noteworthy, in samples of final reaction mixtures, where catalyst was inactive, short Pb Pb distances were absent. These polynuclear compounds have been tested as lead sources in large-scale runs (small scale reactions were inconclusive due to heterogeneity of reaction mixtures because these compounds are less soluble than PbO). It was found that the use of lead bromophenoxides instead of PbO increases both Pd TON (by 25-35%), and reaction selectivity (from 65 - 67 % to 75 - 84 %). Activity of different lead bromophenoxides was about the same (within experimental error) but the best selectivity was observed for complex Pb602(0Ph)6Br2. Therefore, the gain in selectivity vs. loss due to additional preparation step should be analyzed for practical application. 

Toshima N, Yonezawa T, Harada M, Asakuara K, Iwasawa Y (1990) The polymer-protected Pd-Pt bimetallic clusters having catalytic activity for selective hydrogenation of diene. Preparation and EXAFS investigation on the structure. Chem Lett 19 815-818... 

In this chapter, I will try to present an introduction to these various techniques with emphasis on EXAFS and X-ray standing waves and their application to the study of electrochemical interfaces. Each technique will be treated from theoretical and experimental points of view, and selected examples from the literature will be employed to illustrate their application to the study of electrochemical interfaces. 

For the Ti(OiPr)4/silica system, the advantage of MCM-41 (a mesoporous silica) over an amorphous silica is not evident either in terms of activity or selectivity for the epoxidation of cyclohexene with H202 in tert-butyl-alcohol.148 Nevertheless, deactivation of the catalysts seems slower, although the selectivity of the recovered catalysts is also lower (allylic oxidation epoxidation = 1 1). Treatment of these solids with tartaric acid improves the properties of the Ti/silica system, but not of the Ti/MCM-41 system, although NMR,149 EXAFS,150 and IR151 data suggest that the same titanium species are present on both supports. 

An oxoiron(V) species such as 6 derived from 1 and peroxides is accessible in nonaqueous media (51). The reaction of the tetraphenylphosphonium salt of la with 2 to 5 equivalents of m-chloroperbenzoic acid (mCPBA) at — 60°C in n-butyronitrile produces within about 10 s a bis-iron(IV)- i-oxo dimer followed by an as - yet uncharacterized EPR - silent iron(IV) intermediate. After 15 min, the deep green oxoiron(V) species 6 forms with distinct absorption maxima at 445 nm (e — 5400 Mr1 cm-1) and 630 nm (s — 4200 M-1 cm-1). At —60°C, 6 decays by 10% in 90 min, but it is stable for at least one month at 77 K. Selected spectral data for the oxoiron(V) species are shown in Fig. 13. DFT calculations favor the low-spin (S = 1/2) configuration of the ground state. The calculated Fe-0 bond length of 1.60 A is in excellent agreement with the EXAFS results. The Fe atom is displaced out of the 4-N plane by 0.5 A. 

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. 

A similar conclusion was also reached by Sankar et al. (46), who used EXAFS/ DFT techniques. From the selective decrease in the EPR intensity of the A type superoxo species during the epoxidation of styrene and allyl alcohol (Fig. 52), Srinivas et al. (52) concluded that these types of oxo species are preferentially consumed during the reaction. 

Figure 6.18 Left first shell coordination numbers from EXAFS versus H/M values from selective hydrogen chemisorption for a number of supported Ni, Rh, Ir and Pt catalysts. Right, relative diameter of half-spherical metal particles as a function of H/M. The curves (right) correspond to the straight lines in the left part of the Figure (adapted from [41,42[).

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