EXAFS catalysis

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60). 

A wide selection of metal reference foils and powder films of ideal thickness for tranmission EXAFS is available from The EXAFS Materials Company, Danville, CA, USA. The transmission method is well-suited for in situ measurements of materials under industrially relevant conditions of extreme temperature and controlled atmosphere. Specially designed reactors for catalysis experiments and easy-... 

Owing largely to research over the last twenty years, the sulfided C0-M0/AI2O3 system is one of the best-characterized industrial catalysts [H. Topsoe, B.S. Clausen and F.E. Massoth, Hydrotreating Catalysis (1996), Springer-Verlag, Berlin]. A combination of methods, such as Mbssbauer spectroscopy, EXAFS, XPS, and infrared spectroscopy, has led to a picture in which the active site of such a catalyst is known in almost atomic detail. 

The ruthenium-copper and osmium-copper systems represent extreme cases in view of the very limited miscibility of either ruthenium or osmium with copper. It may also be noted that the crystal structure of ruthenium or osmium is different from that of copper, the former metals possessing the hep structure and the latter the fee structure. A system which is less extreme in these respects is the rhodium-copper system, since the components both possess the face centered cubic structure and also exhibit at least some miscibility at conditions of interest in catalysis. Recent EXAFS results from our group on rhodium-copper clusters (14) are similar to the earlier results on ruthenium-copper ( ) and osmium-copper (12) clusters, in that the rhodium atoms are coordinated predominantly to other rhodium atoms while the copper atoms are coordinated extensively to both copper and rhodium atoms. Also, we conclude that the copper concentrates in the surface of rhodium-copper clusters, as in the case of the ruthenium-copper and osmium-copper clusters. 

Investigations utilizing EXAFS have the very important feature of yielding information in an environment of the kind actually encountered in catalysis. We have recently demonstrated the feasibility of making measurements while a catalytic reaction is actually occurring. One can anticipate that measurements of this type will receive Increased emphasis in the future. For studies of the structures of highly dispersed metal catalysts, EXAFS may well be the most generally applicable physical probe currently available. 

XRD and LEED are laboratory techniques, although synchrotrons offer advantages for X-ray diffraction. EXAFS, on the other hand, is usually done at synchrotrons. This, and the fact that EXAFS data analysis is complicated and not always without ambiguity, have inhibited the widespread use of the technique in catalysis. 

At the time that EXAFS was introduced in catalysis, around 1975, the technique was considered to be one of the most promising tools for investigating catalysts. These high expectations have not quite been fulfilled, mainly because data analysis in EXAFS is highly complicated and, unfortunately, not always possible without ambiguity. A number of successful applications, however, have proven that EXAFS applied with care on optimized catalysts can be a very powerful tool in catalysis [27,31,321. 

Spectroscopic developments have accelerated advances in the field of catalysis. This volume analyzes the impact on catalyst structure and reactivity of EXAFS, SIMS, MSssbauer, magic-angle spinning NMR (MASNMR), and electron-energy-loss vibrational spectroscopy. Many of these techniques are combined with other analytical tools such as thermal decomposition and temperature-programmed reactions. 

Therefore, the data indicate that Co-Mo-S can be considered as a M0S2 structure with Co atoms located in edge positions. As discussed below, these Co atoms play a direct role in the catalysis. Furthermore, it is generally accepted that the HDS reaction involves adsorption on sulfur vacancies. The low sulfur coordination number (large coordinative unsaturation) estimated from the Co EXAFS may, in fact, reflect that active sites (vacancies) are associated with the Co atoms. 

A researcher in the field of heterogeneous catalysis, alongside the important studies of catalysts chemical properties (i.e., properties at a molecular level), inevitably encounters problems determining the catalyst structure at a supramolecular (textural) level. A powerful combination of physical and chemical methods (numerous variants x-ray diffraction (XRD), IR, nuclear magnetic resonance (NMR), XPS, EXAFS, ESR, Raman of Moessbauer spectroscopy, etc. and achievements of modem analytical chemistry) may be used to study the catalysts chemical and phase molecular structure. At the same time, characterizations of texture as a fairytale Cinderella fulfill the routine and very frequently senseless work, usually limited (obviously in our modem transcription) with electron microscopy, formal estimation of a surface area by a BET method, and eventually with porosimetry without any thorough insight. 

However, the predse structure of the catalyst and the precise role of CeC>2 in the present case and of Bi is not completely clear. In general terms, several explanations for the rate and selectivity enhancements by the promoter are possible [80] (a) geometric blocking of a fradion of sites and generation of specific surface ensembles, viz. formation of an ordered alloy (b) neighboring atom participation (Fig. 11.4), although the partial oxidized state of the promoter (Bix+) of the model is not confirmed by surface studies (LEED, XPS, EXAFS) (c) occurrence of bi-fundional catalysis, assuming that O or OH radicals formed on the promoter participate in the oxidation. 

EXAFS data indicated that tin was only surrounded by four platinum atoms at the same distance of 0.276 nm (Scheme 2.40). This result clearly indicates that tin is located on the metal surface and not in the bulk. For example, in a bulk Pt3Sn alloy tin is surrounded by 12 platinum atoms while in a surface alloy on a platinum bulk it is surrounded by six platinum atoms only. Note that such tin adatoms are always obtained when low amounts of tin are deposited on the metal. This is probably because tetrabutyl tin coordinates first on the metal atoms of the faces, which are the most hydrogenolyzing, rather than corner or edges for which the alkyl ligands remain coordinated to the tin. This fact will be very important in catalysis since it explains selective poisoning of metal particles (see below). 

Titanium Alkoxides Silica-supported titanium(IV) alkoxides and Ti-silicalite are industrial epoxidation catalysts [53-56] and have been applied in deperoxidation reactions [57]. Computational and EXAFS data [53, 54] as well as spectroscopic investigations on the surface species [58] have indicated that the dominant active surface species is a four-coordinate trisUoxy complex [(=SiO)3TiOH] [59] whose coordination shell expands to six-coordinate during catalysis [60]. 

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

In none of the above cases has a reaction been performed whilst taking the EXAFS data. Hamill et al. [50] have investigated catalysis of the Heck reaction by palladium salts and complexes in room-temperature ionic liquids. On dissolution of palladium ethanoate in [BMIMj and N-butylpyridinium ([BP] ) hexafluorophos-phate and tetrafluoroborate ionic Hquids, and triethyl-hexyl ammonium bis(trifluo-romethanesulfonyl)imide, a gradual change from ethanoate coordination to the formation of palladium metal was observed in the Pd K-edge EXAFS, as shown in Figure 4.1-13. 

Molybdoenzymes other than the nitrogenases are usually termed oxomolybdoenzymes. This prefix relates to the nature of the catalysis effected, i.e. the net effect of the conversion (xanthine to uric acid, sulfite to sulfate, nitrate to nitrite, or aldehyde to carboxylate) corresponds to the transfer of one oxygen atom to or from the substrate. Furthermore, molybdenum X-edge EXAFS studies have established that this metal is coordinated to one or more terminal oxo groups in each enzyme studied by this technique.204... 

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