EXAFS studies

X-ray absorption studies of systems of electrochemical interest have been made by several groups of workers. The principal field of interest has been passive layers, in particular those on iron and its alloys, and to a lesser extent those on nickel. While early work in this area was performed ex situ, generally 

FIGURE 19. In situ difiractograms for Ag electrode in 0.1 M Pb(C104)2, 0.5 M NaC104, 1 mM HCIO4 (a) after 50 hours at -400 mV, (b) difference diffractogram (that at -400 mV minus that at -100 mV) after 100 hours of acquisition while modulating the electrode potential at 10 Hz. 

FIGURE 20. In situ FeK edge absorption spectrum for Fe/13% Cr film passivated in 0.1 M NaNOj. 

Kordesch and Hoffman have used a novel cell design that incorporates an emersed electrode and permits the use of electron yield techniques, remaining essentially in situ. 

FIGURE 22. ReflEXAFS results at an angle of incidence of 3.4mrad for nickel electrode passivated in 1JV H2SO4 (a) Ni, (b) Ni electrode in situ, (c) electrode after passivation, (d) NiO (after Ref. 44). 

These have been quite extensive and results have been published for plastocyanin (51), azurin (52), stellacyanin (53), umecyanin (54), and rusticyanin (55) in both oxidation states. Fitting procedures are not sensitive to the inclusion of the Cu—S(Met) bond, and if the latter does contribute the best fit the bond distance obtained is not always reliable. In the case of azurin, the axial carbonyl group is not included in the fit. No information is obtained from EXAFS concerning bond angles. Table IV is a listing of data reported. It is noted that (average) Cu—N(His) bonds are persistently shorter than those obtained from 

Bond Distances (A) Obtained from EXAFS Studies  

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It is an extremely moisture-sensitive dark brown solid (m.p. 54°C) bond lengths have been obtained from an EXAFS study [19]. 

Fe-S and Fe-Fe distances in [2Fe-2S], [3Fe-4S], and [4Fe-4S] clusters are all very similar 2.3 and 2.7 A, respectively. In the [6Fe-6S] prismane model cluster, however, there is an additional Fe-Fe distance at 3.7 A (Fig. 2). If a [6Fe-6S] cluster were present in the Fepr protein, then this longer Fe-Fe distance should he visible with extended X-ray absorption fine structure (EXAFS). As a consequence, EXAFS studies were carried out at the CCLRC Synchrotron Radiation facility in Daresbury, UK. The two Fepr proteins (those of D. vulgaris and D. desulfuricans), as well as a synthetic [6Fe-6S] cluster, were subjected to an EXAFS study. Low-temperature EXAFS... 

The discovery of a new heterodinuclear active site in [NiFe] hydro-genases opens the way for the proposal of catalytic cycles based on the available spectroscopic data on the different active site redox states, namely EXAFS studies that reveal that the Ni-edge energy upon reduction of the enzyme supports an increase in the charge density of the nickel (191). 

The Cu/ZnO system is very dynamic. The morphology of the Cu particles responds immediately to a change in reduction potential of the gas mixture above it. EXAFS studies suggest that the change in morphology is associated vith the extent that the metal particles vet the underlying support [B.S. Clausen, J. Schiotz, L. Gr4-bffik, C.V. Ovesen, K.W. Jacobsen, J.K. Norskov and H. Topsoe, Top. Catal. 1 (1994)... 

Alonso-Vante N, Malakhov IV, Nikitenko SG, Savinova ER, Kochubey DI (2002) The structure analysis of the active centers of Ru-containing electrocatalysts for the oxygen reduction. An in situ EXAFS study. Electrochim Acta 47 3807-3814... 

The results of the EXAFS studies on osmium-copper clusters lead to conclusions similar to those derived for ruthenium-copper clusters. That is, an osmium-copper cluster Is viewed as a central core of osmium atoms with the copper present at the surface. The results of the EXAFS investigations have provided excellent support for the conclusions deduced earlier (21,23,24) from studies of the chemisorption and catalytic properties of the clusters. Although copper is immiscible with both ruthenium and osmium in the bulk, it exhibits significant interaction with either metal at an interface. 

Extended X-ray absorption fine structure (EXAFS) studies have been very useful for obtaining structural information on bimetallic cluster catalysts. The application to bimetallic systems is a particularly good one for illustrating the various factors which have an influence on EXAFS. Moreover, the applicability of EXAFS to this area has been very timely, in view of the enormous interest in bimetallic systems in both catalytic science and technology. 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

Summarizing, the in situ UV-Vis, XANES, and EXAFS studies of Bonino et al. [49] and of Prestipino et al. [50] on hydrated and anhydrous peroxo/hy-droperoxo complexes on crystalhne microporous and amorphous meso-porous titanosilicates have shown, for the first time, the equilibriiun between r] side-on and end-on complexes. The amount of water is the key factor in the equilibrium displacement. In this regard please note that, owing to the hydrophobic character of TS-1, substrates such as olefins are the dominant species in the channels. This fact assures a relatively local low concentration of water, which in turn guarantees a sufficient presence of the active end-on... 

Recently characterization of bimetallic nanoparticles by EXAFS were extensively reported [122-124,176], Structural transformation of bimetallic Pd/Pt nanoparticles, which were prepared by a sequential loading of H2PtClg onto the Pd loaded catalyst, was investigated with EXAFS at high temperatures [176], The results of EXAFS at Pd K and Pt L-III edges showed that Pt was surface-enriched or anchored on the Pd metal core with an increase of the Pt content. The structure of the obtained bimetallic Pd/Pt nanoparticles seemed to be retained upon heating up to 1273 K under ambient condition [176], Pt/ Au bimetallic nanoparticles can be prepared by polyol method and stabilized by PVP [122], XANES and EXAFS studies were also performed on the samples and their results supported the idea of a Pt-core/Au-shell structure with the elements segregated from each other [122],... 

Gracia FJ, Guerrero S, Wolf EE, Miller JT, Kropf A. 2005. Kinetics, operando ETIR, and controlled atmosphere EXAFS study of the effect of sulfur on Pt-supported catalysts during CO oxidation. J Catal 233 372-387. 

An EXAFS study of 3-NiO(OH) indicates that the Ni sites experience Jahn-Teller distortion, with four Ni—O bonds at 1.87 A and two at 2.03 A in the //-phase containing Nim and Ni this is absent.179... 

In order to simplify the discussion, the EXAFS studies on electrochemical systems reported to date will be divided into the categories listed below ... 

Figure 14. In situ cell for performing EXAFS studies on passivated iron films. (From Ref. 72, with permission.)...

Miller, J.T., Marshall, C.L., and Kropf, A.J. 2001. (Co)MoS2/alumina hydrotreating catalysts An EXAFS study of the chemisorption and partial oxidation with 02. J. Catal. 202 89-99. 

Ronning, M., Nicholson, D.G., and Holmen, A. 2001. In situ EXAFS study of the bimetallic interaction in a rhenium-promoted alumina-supported cobalt Fischer-Tropsch catalyst. Catal. Lett. 72 141 -6. 

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