EXAFS QEXAFS

In this study, a powerful combination of high resolution transmission electron microscopy (HRTEM), mass spectrometry (MS) and Quick EXAFS (QEXAFS) is used to study the reactions of [Pt2+(NH3)4](N03 )2 impregnated on Si02 during different pretreatment processes. MS is used to monitor which gases are produced during the pretreatment. QEXAFS is used to study the local structure of the Pt complex during the pretreatment. The timescale of... 

Figure 6.20 shows an example in which QEXAFS has been used in combination with XRD to study the temperature programmed reduction of copper oxide in a Cu/ZnO/Al203 catalyst for the synthesis of methanol [43,44]. Reduction to copper metal takes place in a narrow temperature window of 430-440 K, and is clearly revealed by both the EXAFS pattern and the appearance of the (111) reflection of metallic copper in the XRD spectra. Note that the QEXAFS detects the metallic copper at a slightly lower temperature than the XRD does, indicating that the first copper metal particles that form are too small to be detected by XRD, which requires a certain extent of long range order [43,44],... 

Abbreviations CCD, charge-coupled device DEXAFS, dispersive extended X-ray absorption fine structure EXAFS, extended X-ray absorption fine structure QEXAFS, quick extended X-ray absorption fine structure TPR, temperature-programmed reaction XRD, X-ray diffraction XAFS, X-ray absorption fine structure. 

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. 

Fig. 19. Combined QEXAFS and temperature-programmed sulfiding results of a Mo/ A1203 catalyst during sulfiding in a H2S/Ar gas mixture (a) Fourier transforms of the in situ EXAFS spectra above the Mo X-edge (b) variation in the H2S concentration in the gas outlet from the in situ EXAFS cell as simultaneously recorded by a mass spectrometer (61).

QEXAFS scans (30-90 seconds per scan) is suitable to study reactions like pretreatment processes. The whiteline area gives information concerning the oxidation state. The EXAFS region represents the geometrical structure of the Pt atom. The final metal particle size was obtained from the Pt-Pt first shell coordination number, H2 chemisorption26 as well as HRTEM. 

Three different pretreatments were performed and studied by both Quick EXAFS and MS. During each pretreatment the impregnated support was heated in one gas with a ramp of 2°C/min from room temperature to 400°C. The gases used were 1) either He (QEXAFS) or Ar (MS) (this sample is called Pt[Ar/He] ), 2) 02 ( Pt[02] ) and 3) H2( Pt[H2] ). 

Quick EXAFS measurements were performed at the HASYLAB synchrotron (station XI. 1) in Hamburg, Germany. The measurements were done in transmission mode using ion chambers filled with a mixture of Ar and N2 to have an absorption of 20% in the first and of 80% in the second ion chamber. The monochromator (a double Si-111 crystal) was detuned to 50% at maximum intensity to avoid the presence of higher harmonics in the X-ray beam. In QEXAFS mode, the monochromator is in continuous motion. 

On one hand, TR-XAFS investigations that require the best time resolution available (such as isothermal reactions or rapid decompositions with half lives of the order of one minute) may be performed at an energy-dispersive XAFS station, with full advantage taken of the time resolution in the sub-second range. On the other hand, TR-XAFS investigations of processes with half lives of the order of several minutes may be performed in the QEXAFS mode, with advantage taken of the increased EXAFS data quality for a detailed structural analysis. 

The general advantage of QEXAFS is the ability to collect an EXAFS spectmm (i.e., a co-addition of scans with a high signal-to-noise ratio) in seconds rather than tens of minutes or even hours. The technology that allows this increase in... 

Bonneviot L, Clause O, Che M, Manceau A, Dexpert H (1989b) EXAFS characterization of the adsorption sites of nickel ammine and ethyldiamine complexes on a silica surface. Catalysis Today 6 39-46 Bomebusch H, Clausen BS, Steffensen G, Liitzenkirchen-Hecht D, Frahm R (1999) A new approach for QEXAFS data acquisition. J Synchrotron Rad 6 209-211 Bostick BC, Fendorf S, Barnett MO, Jardine PM, Brooks SC (2002) Uranyl surface complexes formed on subsurface media from DOE facilities. Soil Sci Soc Am J (2002) 66 99-108 Bottero JY, Manceau A, Villieras F, Tchoubar D (1994) Structure and mechanisms of formation of iron oxide hydroxide (chloride) polymers. Langmuir 10 316-319 Bourg ACM, Joss S, Schindler PW, (1979) Ternary surface complexes 2. Complex formation in the system silica-Cu(II)-2,2 bipyridyl . Chimia 33 19-21... 

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. 

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