EXAFS, time-resolved

Dispersive EXAFS Time resolved structural studies Wiggler, or tapered undulator... 

Time-resolved X-ray absorption is a very different class of experiments [5-7]. Chemical reactions are triggered by an ultrafast laser pulse, but the laser-induced change in geometry is observed by absorption rather than diffraction. This technique permits one to monitor local rather than global changes in the system. What one measures in practice is the extended X-ray absorption fine structure (EXAFS), and the X-ray extended nearedge strucmre (XANES). 

I. V. Tomov, D. A. Oulianov, P. Chen, and P. M. Rentzepis, Ultrafast time-resolved transient structures of solids and liquids studied by means of X-ray diffraction and EXAFS. J. Phys. Chem. B 103(34), 7081-7091 (1999). 

XANES spectroscopy is also the basis of chemically sensitive X-ray imaging, as well as qualitative and quantitative microspectroscopy [306], ptXANES is attractive for chemical analysis, with its spatial resolution down to 10 ptm. Variations on the theme are surface EXAFS (SEXAFS), grazing incidence XAS and in situ time-resolved XAS investigations. Grazing angle XAFS can be used for the study of ultrathin multilayer systems. 

EXAFS has been very useful in the study of catalysts, especially in investigating the nature of metal clusters on surfaces of the supported metal catalysts (Kulkarni et al, 1989 Sinfelt et al, 1984). A variety of systems has been examined already and there is still considerable scope for investigation in this area. Since EXAFS gives bond distances and coordination numbers and is absorber-selective, it is possible to study one metal at a time (Fig. 2.12). Thus, an EXAFS investigation of sulphided Co—Mo— Al20j and related catalysts has shown the nature of the reactive surface species (Kulkarni Rao, 1991). Cu/ZnO catalysts have revealed certain unusual features suggesting the complex nature of the species involved in methanol synthesis (Arunarkavalli et al, 1993). Time-resolved EXAFS is of considerable value for the study of catalysts (Sankar et al, 1992). 

Several years ago, it was demonstrated that time-resolved EXAFS measurements could be performed by use of the energy-dispersive method (41),... 

Small particles of binary alloys have been investigated in detail in static EXAFS experiments, but if information about the dynamic behavior of the alloy composition and the segregation phenomena is desired, time-resolved combined EXAFS/XRD studies are necessary. Figure 18 shows the atomic structure of a small binary particle of a Ni-Au alloy as predicted from Monte Carlo simulations (60). Ni and Au do not form a miscible alloy in the bulk but can form a stable alloy at the surface. The structural and chemical changes that occur when such particles are exposed to different... 

Fig. 2. a) Required number of incoming x-ray photons to observe time-resolved EXAFS of transition metal compounds in H20 solution with a signal-to-noise ratio S/N = 1. No ligand or counterion contributions were included (see Fig. 1). Input parameters are /= 10%, %= 1 % (relative to the absorption edge jump of the selected element). The maxima of curves 2) in Fig. 1 for Fe and Ru correspond to the data points for these elements, b) Feasibility range for time-resolved x-ray absorption spectroscopy. The shaded region indicates the required x-ray dose per data point as a function of the fraction of activated species for the calculated EXAFS experiments on transition metal compounds shown in a). Curves (1) to (3) are extrapolated from experimental results (see section 3. for details) of time-resolved XANES.

Figure 5. Selected time-resolved a) UV-Vis spectra and b) Zn K-edge EXAFS k3%(k) spectra recorded during the formation of ZnO quantum dot nanoparticles by heating of a precursor solution at 70°C in presence of LiOH. The ageing time ranges from 1 to 40 minutes. At the top of b) is reported for comparison purpose the EXAFS spectrum of a crystalline ZnO reference.

During these temperature and gas treatments, processes like reduction, oxidation and sintering take place. In Chapter 2 it is clarified what processes are responsible for the final metal particle size and particle size distribution. This is done using a combination time resolved extended X-ray absorption fine structure spectroscopy (quick EXAFS) and mass spectrometry. 

Newton MA, Jyoti B, Dent AJ, Fiddy SG, Evans J. Synchronous, time resolved, diffuse reflectance FT-IR, energy dispersive EXAFS (EDE) and mass spectrometric investigation of the behaviour of Rh catalysts during NO reduction by CO. Chem Commun. 2004 2382. 

Ferri D, Kumar MS, Wirz R, et al. First steps in combining modulation excitation spectroscopy with synchronous dispersive EXAFS/DRIFTS/mass spectrometry for in situ time resolved study of heterogeneous catalysts. Phys Chem Chem Phys. 2010 12 5634. 

XAS, and particularly its application to catalysis, has been the subject of several previous reviews and books. In 1988, Koningsberger and Prins published the book "X-ray absorption principles, applications, techniques of EXAFS, SEXAFS and XANES" (Koningsberger and Prins, 1988). In this monograph there is a thorough description of the technique together with a chapter on its application to catalysis. Iwasawa in 1996 published "XAFS for catalysts and surfaces" (Iwasawa, 1996), which focused solely on XAFS spectroscopy as applied to catalyst characterization. This volume includes a chapter by Bazin, Dexpert, and Lynch about measurements of catalysts in reactive atmospheres, and several other chapters allude to examples of such characterization. Recently a book entitled In situ Spectroscopy of Catalysts" (Weckhuysen, 2004) was published that contains three chapters focused on XAFS of catalysts in reactive atmospheres one on XANES, one on EXAFS, and one on time-resolved XAFS. 

These investigations were expanded to include time-resolved EXAFS experiments (Fierro-Gonzalez and Gates, 2005). The experimental conditions for the catalysis in this paper are similar to those in the earlier work. However, the substantially higher initial CO conversion of 60% was now measured, which declined to 40% within 20 min, whereas in the earlier work the initial conversion was 40%, which declined to 5% after 15 min. No explanation was given for this discrepancy, but it may be evidence for some potential problems with sample packing or gas flow issues in the cell. 

Time-Resolved EXAFS Measurement of the Stepwise Clustering Process of Pd Clusters at Room Temperature [33]... 

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