DEXAFS

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. 

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. 10. Apparatus for combined DEXAFS/XRD in situ experiments. [Adapted with permission from Nature, Ref. 53. Copyright 1991 Macmillan Magazines Limited.]...

Formation, interaction as well as reaction of chloro- and bromobenzene on Pd zeolites have been studied in situ in a continuous flow reactor at the Pd and Br K-edges using the ROMO II and DEXAFS stations at beamline X at HASYLAB. The experimental set-up of this beamline as well as the reactor and gas flow set-up is described elsewhere [5],... 

As it seemed impossible to carry out in situ DEXAFS studies at the Cl-K edge due to its low energy, we applied bromobenzene in exchange for chlorobenzene. The uptake of bromine during combustion of bromobenzene with air and interrupted by 5 and 3 min conducting pure air over the catalysts (Fig. 7) monitored from the viewpoint of the halogen gives no indication for a steady state. 

DEXAFS has, for example, been used to follow the dehydration of (Ni +, Na+)-loaden zeolite Y [84]. In another study, the reactions of partially Cu-exchanged Na-zeolite Y with pure H2 or with a mixture of H2/H2O have been investigated [85]. Pure hydrogen leads to the reduction of Cu + to Cu+,but not to metallic copper due to the strong interaction of the Cu(I) cations with the aliuninosilicate framework. After admission of additional water vapor, however, the Cu(I) cations become mobile and are reduced to Cu(0), forming small metallic copper clusters. 

Recently X-ray photoelectron methods such as EXAFS, XANES and DEXAFS have been applied together with IR spectroscopy to zeolites, giving information on the oxidation state of the cations, the coordination sphere and its dimensions as well as the changes of both during reactions in time-resolved experiments [113,139,140]. 

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