EXAFS catalyst characterization

Rc-Pt [Re2Pl(CO)i2] 197 K-AI2O3 Catalyst characterization (IR. XPS. TPR, chemisorption) Catalyst characterization (EXAFS. chemisorption) and methylcyclohe.xane dehydrogenation 203 204... 

Temperature-Programmed EXAFS/ XANES Characterization of the Impact of Cu and Alkali Promoters to Iron-Based Catalysts on the Carbide Formation Rate... 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

At the present time much effort is being devoted to tailor-making of new nanomaterials with specific catalytic properties. In this quest for constantly decreasing the dimensions of the catalytically active components, one will unavoidably encounter materials that will be partly or completely X-ray amorphous. The present review has shown that the combined EXAFS/ XRD techniques are uniquely well suited for providing the necessary structural understanding. Thus, in view of the trend in catalyst technologies and advances in technique developments, the application of the combined techniques will no doubt play an increasing role in future catalyst characterization efforts. We now briefly discuss some likely applications and technique developments which involve the X-ray techniques discussed presently. 

The beauty of XANES for catalyst characterization is that it readily yields oxidation states and average compositions of supported particles, whereas the strong point of EXAFS is that it yields structural information on the atomic scale. 

Among the techniques ideally suited for in situ studies are infrared, Raman, and nuclear magnetic resonance (NMR) spectroscopies and extended x-ray absorption fine structure (EXAFS). While still relatively new, the scanning tunneling and atomic force microscopes are expected to play an increasingly important role in catalyst characterization. Both instruments permit visualization of a catalyst surface at the atomic level and hold the potential of showing how atoms and molecules interact with a surface. 

XRD in dedicated laboratory environments provides and will provide a wealth of useful information, because they allow operation with few time constraints and unsurpassed experimental flexibility. Synchrotron sources, on the other hand, provide unique opportunities to combine XRD with EXAFS spectroscopy, which together provide an enormous advantage as long as the conduct of the experiments is not constrained by beam time limitations. Bulk transformations under reaction conditions are typically slow, being characterized by time scales of hundreds of hours, and are therefore prohibitive for standard user operations at synchrotrons. It is also difficult to handle many catalytically relevant reactants safely at synchrotrons, so that XRD investigations are limited to a few reactants, in contrast to the situation in most catalyst characterization laboratories. 

XAS, and particularly its application to catalysis, has been the subject of several previous reviews and books. In 1988, Koningsberger and Prins published the book "X-ray absorption principles, applications, techniques of EXAFS, SEXAFS and XANES" (Koningsberger and Prins, 1988). In this monograph there is a thorough description of the technique together with a chapter on its application to catalysis. Iwasawa in 1996 published "XAFS for catalysts and surfaces" (Iwasawa, 1996), which focused solely on XAFS spectroscopy as applied to catalyst characterization. This volume includes a chapter by Bazin, Dexpert, and Lynch about measurements of catalysts in reactive atmospheres, and several other chapters allude to examples of such characterization. Recently a book entitled In situ Spectroscopy of Catalysts" (Weckhuysen, 2004) was published that contains three chapters focused on XAFS of catalysts in reactive atmospheres one on XANES, one on EXAFS, and one on time-resolved XAFS. 

The first chapter (Chapter 10) in the section on catalyst characterization summarizes the most common spectroscopic techniques used for the characterization of heterogeneous catalysts, such as XPS, Auger, EXAFS, etc. Temperature programmed techniques, which have found widespread application in heterogeneous catalysis both in catalyst characterization and the simulation of pretreatment procedures, are discussed in Chapter 11. A discussion of texture measurements, theory and application, concludes the section on the characterization of solid catalysts (Chapter 12). 

Table 3, EXAFS results characterizing fresh and used supported metal cluster catalysts...

Catalyst characterization indicates that Ru deposition in a high loading on different molecular sieves mainly occurs on the external surface of these. However, as XPS and EXAFS measurements showed, some differences are induced by the features of the molecular sieves even at these high metal loadings. Chlorine fixim the RuCl, precursor has a certain contribution to these differences. Subsequent treatment with tartaric acid seems to induce a further delocalization of the charge on Ru irrespective of the molecular sieve support. 

The few examples discussed here, added to the large number anyone can find in the literature, are definite proof of the usefulness X-ray spectroscopy has today in the field of catalyst characterization. We have restricted our illustrations to the EXAFS domain but it becomes clearer every day that the edge region is also of great interest XANES (X-ray Absorption Near Edge Structure) provides information on the electronic states taken by the active species during the reaction. As we very briefly reported, the number of empty d states can be followed accurately and relations with the different chemical pathways of the reaction may be established (see for example The whole will undoubtedly contribute to the development of time-resolved studies done under real reaction conditions, so that kinetic measurements will be one of the major uses of the technique in the near future. 

Further optimization of the catalyst prepared by spontaneous deposition of Pt and further catalyst characterization with electrochemical and EXAFS techniques. 

The aspects of catalyst characterization have been discussed in detail elsewhere, for example, see references [2-6], and the numerous methods, some of which are restricted to specific elements and isotopes, that can supply the above knowledge have been tabulated. However, a review should be useful here of only the methods that are most readily available to the general catalytic researcher, such as TEM (Transmission Electron Microscopy) including SEM (Scanning Electron Microscopy) and HREM (High Resolution Electron Microscopy), XRD (X-ray Diffraction), physisorption and chemisorption. Because of the unique information it can provide, EXAFS (Extended X-ray Absorption Fine Structure) is also discussed briefly. 

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

---