XAFS techniques

The TPR-XAFS technique confirmed that doping Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) remarkably promotes the carburization rate relative to the undoped catalyst. The EXAFS results suggest that either the Hagg or e-carbides were formed during the reduction process over the cementite form. A correlation is observed between the a-value of the product distribution and the carburization rate. 

X-ray absorption fine structure (XAFS) technique, 74 464-465 X-ray absorption near edge structure (XANES), 24 72... 

Historically, it was controversial to recognise the local nature of the XAFS techniques (see Lytle for a recent discussion)39 but now the theoretical description of the techniques is reasonably well-settled.36,37 The absorption of X-ray by matter is described in many text books.40 The treatment of the radiation as an electric field without practical spatial variation on a molecular/local scale and eliminating magnetic parts leads to the Fermi Golden Rule for the X-ray cross section ... 

Hexacarbonyl molybdenum Mo(CO)6 was successfully used to prepare intrazeolite molybdenum sulfide clusters in the cavities of NaY (CVD technique) [4,5,7,8]. The decomposition and sulfidation of Mo(CO)e encaged in NaY were extensively studied by Okamoto et al. [7-11] by means of temperature programmed decomposition (TPDE), XPS, and XAFS techniques. It has been claimed that the structure of molybdenum sulfides is described as molybdenum dinuclear sulfide clusters M02S4. de Bont et al. [12] supported the formation of molybdenum sulfide dimer species. The extremely high dispersion of molybdenum sulfide clusters prepared fi"om Mo(CO)6 was also suggested by an NO adsorption capacity much hi er than those of other conventional catalyst systems such as M0S2/AI2O3 [9]. 

Soon after the correct physical explanation for XAFS had been given and when more experimental stations with improving technical stability had been constructed at synchrotrons, the first applications of XAFS techniques to zeolites were reported. In fact, there are several good reasons to use the information content of XAFS in the characterization of zeolite-type solids (zeotypes). 

In the case of Ag, probably one of the promoter cations most widely studied, UV-vis, and XAFS techniques were able to indicate that isolated or highly dispersed Ag(I) cations yielded highly active and selective SCR catalysts. This was also a conclusion indirectly extracted by the fact that intermediate loadings of Ag were in fact adequate in order to optimize activity and that addition of Cs also produces further stabilization of Ag(I) cations under reaction conditions. However, the exact nature of the active phase was not revealed until recently when a XAFS (XANES and EXAFS) study was able to provide some further evidence. The XANES spectra of two active catalysts containing 1.5 and 4.5 wt%, and of Ag(I) containing reference samples are shown in Fig. 10.8(a). 

L. M. Colyer, G. N. Greaves, S. W. Carr, K. K. Fox, A. J. Dent, andR. H. Jones, fo sift/shirty of ceramic formation from Co +, Cu + and Zn + exchanged zeolite-A using combined XRD/XAFS techniques, Nucl. Instr. MethodsB91, 107-110 (1995). 

Iwadate Y, Suzuki K, Onda N et al (2006) Local structure of molten LaQ3 analyzed by X-ray diffraction and La- LIII absorption- edge XAFS technique. J Alloy Comp 412 248—252... 

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