QEXAFS

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],... 

Physical adsorption, theory of, 4 211 PhysisorpUon, 26 356-358 Piezo-QEXAFS, 42 330-344 Pillared clays, 33 345-346... 

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. 

Fig. 8. Piezo-QEXAFS spectra at the Cu K-edge recorded during reduction of a Cu/ Zn0/Al203 methanol synthesis catalyst. The recording time was 50 ms/scan, and only every 40th scan is shown (51).

The fact that much shorter collection times are possible by using DEXAFS, QEXAFS, or piezo-QEXAFS means that many advanced dynamic phenomena can now be studied and one does not have to be concerned with problems in connection with sample changes during the measurements. 

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. 9. Schematic drawing of the apparatus for combined QEXAFS/XRD in situ experiments [adapted from Clausen et al. (53)].

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).

The technique is also referred to as NEXAFS, although this acronym appears to be used increasingly for X-ray absorption at low energies and in light elements [36, 48]. Like QEXAFS, XANES spectra can be recorded in fast mode, to monitor the kinetics of solid-state reactions in real time [49]. 

MoS2 is observed after sulfidation at temperatures above 300 °C. These results are easily reconciled with those from XPS and RBS described before, be it that the temperatures at which the changes occur are higher in the QEXAFS study, which is in fact expected for a temperature-programmed measurement. In addition, the QEXAFS were made on high-surface area catalysts, where diffusion limitations are likely, whereas the XPS studies concern planar, thin films, in which virtually all material is in contact with the gas phase. 

The sulfidation mechanism was investigated by temperature-programmed sulfidation, as the oxidic catalyst was heated in a flow of H2S and ff2, and the consumption of IH S and ff2 and the evolution of H20 were measured continuously (13). ft was found that IH S is taken up and H20 given off, even at room temperature, indicating a sulfur-oxygen exchange reaction. This conclusion was confirmed by quick extended X-ray absorption fine structure (QEXAFS) studies (Fig. f, phase 2), which also demonstrated... 

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... 

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. 

The QEXAFS scans were performed at the Pt L3 edge (11564 eV) and were taken from 11500 - 12000 eV with steps of 1 eV. Each 60 seconds 1 scan was taken. 

Table 2 Fit parameters3 of QEXAFS spectra during heating in H2 and variances of fits. (R-space fit, 2.5 < k < 9.5 A 1,1.3 < R <3.2 A, k2 weighted).

The QEXAFS results are given in Figure 8 and Table 3. Heating the sample in He first shows a small increase in the whiteline from 180°C to 200°C, accompanied by a decrease in the Pt-N coordination number. A sharp decrease from 1.7 to 1.4 in the whiteline intensity starts at 200°C and continues up to approximately 260°C, after which the whiteline continues to... 

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