QEXAFS method

Wong J, Froeba M, Frahm R (1995) A critical assessment of the QEXAFS method. Rev Sci Instrum 66 1517-1520... 

To date, mainly two groups have been responsible for the development of the combined EXAFS/XRD methods, namely the group of Clausen, Topspe, Niemann, and co-workers, who developed the combined EXAFS/ XRD and QEXAFS/XRD techniques, and the group of Thomas, Greaves, and co-workers, who developed the DEXAFS/XRD method. In the following, some of the developments are discussed briefly. 

In addition to characterizing a catalytically active material under dynamic reaction conditions, the method has been used to characterize the structural evolution taking place during the preparation of active catalysts from precursor materials. Either a DXAFS spectrometer (Fiddy et al., 2002 Hatje et al., 1994 Sankar et al., 1992 Sankar et al., 1993 Shido et al., 2002 Thomas et al., 1994 Yamaguchi et al., 2000, 2002) or the QEXAFS mode at a conventional XAFS beam line (Cimini and Prins, 1997 Wienold et al., 2003) was employed. Because a controlled thermal treatment of a precursor material permits a better tailoring of the time... 

We have also seen that the combined QEXAFS/XRD method [34] has some unique advantages to provide the in situ structural insight. The recent improvements in time resolutions by several orders of magnitude will allow the investigation of new types of dynamic phenomena. 

Two philosophies have been developed for the fast registration of X-ray absorption spectra [73-76]. One of these can be characterized as a brute-force variant of the normal transmission experiment, where the energy of the primary beam is increased stepwise. It takes about 20-30 min to take a full spectrum, and much of this time is used to perform corrections in the adjustment of the beam and to calm down mechanical vibrations and instabilities, which occur after every movement of the monochromator crystal. The QEXAFS (Quick-Scanning EXAFS) method [75,76] uses a step-wise, rather than a continuous motion of the motors driving the movement of the crystals, so that such extra times are not necessary. If the counting times of the detectors are minimized as well, a full XAFS spectrum of sufficient quahty can be obtained in a few seconds. Spectra with only minor losses in quahty as compared to conventional step-by-step scanning can be obtained in a few minutes. 

Clausen, B., Grabek, L., Steffensen, G., et al. (1993). A Combined QEXAFS/XRD Method for On-Line In Situ Studies of Catalysts Examples of Dynamic Measurements of Cu-Based Methanol Catalysts, Catal. Lett., 20, pp. 23-36. 

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