Situ X-Ray Absorption Studies

As we have seen, EXAFS studies almost invariably will be made using synchrotron radiation as the X-ray source. The use of such a central facility inevitably means that, as a user, one has little control over the choice of much of the experimental equipment. Typically, the beam line will already be equipped with an appropriate monochromator, the software to drive it, as well as detectors and data acquisition software. In addition, all the safety features will be in place. One does, however, have to decide on an appropriate experimental configuration and the type of detector to be used. 

For nonelectrochemical, solid-state EXAFS studies of the heavier nuclei, the most generally used experimental configuration is transmission, i.e., the incident and transmitted photon fluxes are measured with ionization chambers. 

A major difficulty with this configuration arises from the fact that all nuclei of a particular type in the beam will contribute to the X-ray absorption spectrum. Thus, for a system where there is a thin surface layer on a bulk substrate (e.g., an iron oxide on iron) the observed spectrum will arise from 

FIGURE 10. A schematic diagram of the apparatus for EXAFS studies of electrode surfaces using fluorescence detection. 

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Hayes, KF. Papelis, C. Leckie, J.O. (1988) Modeling ionic strength Effects on anion adsorption at hydrous oxide/solution interfaces. J. Colloid Interface Sd. 78 717—726 Hayes, KF. Roe, A.L. Brown, G.E. Hodgson, KO. Leckie, J.O. Parks, G.A. (1987) In-situ X-ray absorption study of surface complexes Selenium oxyanions on a-FeOOH. Sdence 238 783-786... 

Ruitenbeek, M., Van Dillen, A.J., de Groot, E.M.F., Wachs, I.E., Geus, J.W. and Koningsberger, D.C. (2000) The structure of vanadium oxide spedes on. gamma-alumina an in situ X-ray absorption study during catalytic oxidation. Topics in Catalysis, 10 (3 ) 241-54. 

K. O., and Leckie, J. O. (1991). In situ x-ray absorption study of lead ion surface complexes at the goethite/water interface. Langmuir 7, 367-373. 

Figure 2 gives a scheme illustrating some applications in geochemistry and technology where surface reactivity (kinetics of dissolution, catalytic activity, photochemical activity) depends on surface structure, expecially on surface coordination. It has been shown by various spectroscopic techniques [electron-spin resonance (ESR), electron double-resonance spectroscopy (ENDOR), electron-spin echo modulation (e.g., see Motschi, 1987), Fourier transform infrared spectroscopy (Zeltner et al., 1986), and in situ X-ray absorption studies of surface complexes (EXAFS) (Hayes et al., 1987 Brown, 1989)] that inner-sphere... 

Mukerjee S, McBreen J (1996) Hydrogen electrocatalysis by carbon supported Pt and Pt alloys - an in situ x-ray absorption study. J Electrochem Soc 143(7) 2285-2294... 

McBreen J, Mukeijee S. In-situ X-ray-absorption studies of a Pt-Ru electrocatalyst. J Electrochem Soc 1995 142 3399-d04. 

Wu S, Lipkowski J, Tyiiszczak T and Hitchcock A P 1997 Eariy stages of copper eiectrocrystaiiization eiectrochemicai and in situ x-ray absorption fine structure studies of coadsorption of copper and chioride at the Au(111) eiectrode surface J.Phys. Chem. B 101 10 310-22... 

There have been many recent studies in support of this mechanistic approach. Stepwise reductive formation of Ag3+ and Ag4+ clusters has been followed using spectroscopic methods by Henglein [33], Reduction of copper (II) to colloidal Cu protected by cationic surfactants (NR4+) through the intermediate Cu+ prior to nucleation of the particles [36] as monitored by in situ x-ray absorption spectroscopy is another example. The seed-mediated synthesis also serves as evidence in support of this mechanism [38-41],... 

Brown Jr., G. E., G. A. Parks, and C. J. Chisholm-Brause (1989), "In-Situ X-Ray Absorption Spectroscopic Studies of Ions at Oxide-Water Interfaces", Chimia43, 248-256. 

We next discuss x-ray absorption studies. To put matters in context, it is useful to understand that conventional studies using Auger electron spectroscopy (AES) and x-ray photoemission spectroscopy (XPS) can be carried out only ex situ in high vacuum after electrochemical treatment since the techniques involve electron detection. X-ray absorption spectroscopy can, in contrast, be used for valence and structural environment studies. As x-rays only are involved, they can be carried out in situ in an electrochemical or similar cell. 

C. J. (1989) In situ X-ray absorption spectroscopic studies of ions at oxide-water interfaces. Chimia 43 248-256... 

Early stages of copper electrodeposition and coadsorption of chloride on the Au(lll) electrode surface have been studied by Wu et al. [390] applying electrochemical methods and in situ X-ray absorption measurements. The results indicate a large degree of static disorder and exclude the presence of high-symmetry structures. Krznaric and Goricnik [391]... 

Grunwaldt J-D, Baiker A. Axial variation of the oxidation state of Pt-Rh/Al203 during partial methane oxidation in a fixed-bed reactor an in situ X-ray absorption spectroscopy study. Catal Lett. 2005 99 5. 

Ressler T, Hagelstein M, Hatje U, Metz W. In situ X-ray absorption spectroscopy studies on chemical oscillations in the C0/02 system on supported Pd catalysts. J Phys Chem B. 1997 101 6680. 

Grunwaldt J-D, Ramin M, Rohr M, Michailowski A, Patzke GR, Baiker A. High pressure in situ x-ray absorption spectroscopy cell for studying simultaneously the liquid phase and the solid/liquid interface. Rev Sci Instrum. 2005 76 054104. 

The use of synchrotron based in situ x-ray absorption spectroscopy (XAS) for the study of catalysis, both heterogeneous and electrocatalysis has matured over the last decade with simultaneous efforts in the United Sates, European Union and Japan. Some recent exemplification of the state of the art can be obtained in the following references, " and an extensive database of literature on its application to catalysis can be accessed electronically (www.exafs.chem.msu.su/ papers). Detailed aspects on application of the technique and methodology used for data analysis has been recently published. " ... 

A. Tadjeddine, D. Guay, M. Ladouceur, G. Tourillon, Electronic and structural characterization of underpotentially deposited submonolayers and monolayer of copper on gold (111) studied by in situ X-ray-absorption spectroscopy. Phys. Rev. Lett. 66, 2235-2238, 1991. 

N. Alonso-Vante, M. Fieber-Erdmann, P. Borthen, H. Strehblow, E. Holub-Krappe, An in situ grazing incidence X-ray absorption study of ultra thin RU < Se3,cluster-like electrocatalyst layers. Electrochim. Acta, 2000, 45, 4227 236. 

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