Quick-EXAFS

Figure 6.20 Quick EXAFS and XRD measurements recorded during the temperature programmed reduction of copper in a Cu/Zn0/Al203 methanol synthesis catalyst. The disappearance and appearance of peaks with increasing temperature in the series of EXAFS spectra corresponds to the conversion of oxidic to metallic copper. The intensity of the relatively sharp peak around 9040 eV, indicative of Cu metal, clearly illustrates the kinetics of the reduction process, as does the intensity of the (111) reflection of Cu metal in the XRD spectra (adapted from Clausen 44J).

Figure 9.17 Quick EXAFS measurements show the effect of temperature-programmed sulfidation on the Mo K edge in NiMo/SiOj catalysts (adapted from Cattaneo et al. [60]).

Fig. 6.17 Quick EXAFS and XRD measurements recorded during the temperature-programmed reduction of copper in a Cu/ZnO/AbOs methanol synthesis catalyst. The disappearance and appearance of peaks with increasing temperature in the series of EXAFS spectra corresponds to the conversion...

Fig. 1. Quick EXAFS spectra of the sulfidation of a M0/AI2O3 catalyst, measured during continuous heating of the catalyst in 5% H2S in H2 from room temperature to 673 K at 5 K/min and holding at 673 K for 30 min.

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. 

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... 

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. 

M.K. Oudenhuijzen, P.J. Kooyman, B. Tappel, J.A. van Bokhoven and D C. Koningsberger, Understanding the Influence of the Pretreatment Procedure on Platinum Particle Size and Particle-size Distribution for SiC>2 Impregnated with Pt2+(NH3)4(NC>3 )2 A Combination of HRTEM, Mass Spectrometry and Quick EXAFS , J. Catal., 205 (2002), 135-146. 

Fottinger K, et al. Dynamic structure of a working methanol steam reforming catalyst in situ quick-EXAFS on Pd/ZnO nanoparticles. J Phys Chem Lett. 2011 2(5) 428—33. 

Because it is widely recognized that a significant amount of reactivity takes places rapidly during sorption processes, it has become necessary to employ new experimental techniques that can adequately measure these processes with high temporal sensitivity. Techniques such as rapid-scan infrared spectroscopy and quick-EXAFS have shown great promise in the early stages of their development and application in our field. It will be over the next decade(s) that these... 

Dent, A. I. (2002). Development of time-resolved XAFS insti umentation for quick EXAFS and energy-dispersive EXAFS measurements on catalyst systems. Top. Catal. 18, 27-35. 

There are various strategies for avoiding these effects, some of which are common to bulk and non-environmental EXAFS. For instance, if the beamline is capable of quick-EXAFS, then each scan will be taken so quickly that slow drifts will not cause artifacts (Gaillard et al. 2001). Some beamlines (MacDowell et al. 2001) create an image of the source on a set of slits, which in turn becomes a fixed virtual source. Attention to mechanics and temperature stability can pay off in terms of beam-position stability. If one is looking at a particle whose fluorescent yield or transmission is very different from that of its surroundings, then it pays to put the beam accurately on an extremum of yield or transmission. That way, small motions only cause second-order perturbations in the signal. This procedure also minimizes the effect of vibrations. 

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