EXAFS investigations

We have found EXAFS to be a very effective method for obtaining structural information on bimetallic cluster catalysts (8,12-15,17) These types of catalysts, and bimetallic catalysts in general, have been the subject of extensive research in the EXXON laboratories since the 1960 s (18-25). In this paper we present a brief review of the results of some ofour EXAFS investigations on bimetallic cluster catalysts. 

The results of the EXAFS studies on osmium-copper clusters lead to conclusions similar to those derived for ruthenium-copper clusters. That is, an osmium-copper cluster Is viewed as a central core of osmium atoms with the copper present at the surface. The results of the EXAFS investigations have provided excellent support for the conclusions deduced earlier (21,23,24) from studies of the chemisorption and catalytic properties of the clusters. Although copper is immiscible with both ruthenium and osmium in the bulk, it exhibits significant interaction with either metal at an interface. 

Toshima N, Yonezawa T, Harada M, Asakuara K, Iwasawa Y (1990) The polymer-protected Pd-Pt bimetallic clusters having catalytic activity for selective hydrogenation of diene. Preparation and EXAFS investigation on the structure. Chem Lett 19 815-818... 

Difficulties may arise when a sample contains Ti ions in more than one type of location (the usual case). An intense peak representative of tetrahedral Ti (the majority species) can then also include contributions from minor quantities of Ti in 5- and 6-fold coordination (34). In particular, such species are observed if the samples are not fully dehydrated or contain larger amounts of Ti. EXAFS investigations of TS-1 (98,99) and TS-2 (81,100) indeed showed the presence of... 

Chisholm-Brause, C. J., G. E. Brown Jr., and G. A. Parks (1989), "EXAFS Investigation of Aqueous Cu(II) Adsorbed on Oxide Surfaces In-Situ", Physica S158, 646-648. 

Studies of intercalates have also been reported in recent years. In their EXAFS investigation of Btj in graphite, Heald Stern (1978) have found that while the intercalate retains its molecular structure, the Br-Br distance increases so as to match the periodicity of the graphite lattice. In the series of pseudo-stoichiometric alkali... 

EXAFS has been very useful in the study of catalysts, especially in investigating the nature of metal clusters on surfaces of the supported metal catalysts (Kulkarni et al, 1989 Sinfelt et al, 1984). A variety of systems has been examined already and there is still considerable scope for investigation in this area. Since EXAFS gives bond distances and coordination numbers and is absorber-selective, it is possible to study one metal at a time (Fig. 2.12). Thus, an EXAFS investigation of sulphided Co—Mo— Al20j and related catalysts has shown the nature of the reactive surface species (Kulkarni Rao, 1991). Cu/ZnO catalysts have revealed certain unusual features suggesting the complex nature of the species involved in methanol synthesis (Arunarkavalli et al, 1993). Time-resolved EXAFS is of considerable value for the study of catalysts (Sankar et al, 1992). 

HDS catalysts have been characterized extensively with a wide variety of tools, and several extensive reviews of the subject have been presented (85,88-91). Substantial effort has been aimed at relating catalytic activity and selectivity to microscopic properties such as catalyst composition, electronic structure, and geometric structure. EXAFS investigations of working catalysts have provided information about the composition, average local coordination, and interatomic distances of atoms in the catalyst clusters. It has been concluded that the active phase under operating conditions is MoS2-like particles with a dimension of 10—20 A (92-94). 

The molybdenum dispersion also depends on the phosphorus content of the catalyst. Atanasova et al. (68, 87) reported that the dispersion of molybdenum and nickel, measured by X-ray photoelectron spectroscopy (XPS), shows a steep increase due to the presence of phosphorus at low loadings. The dispersion of molybdenum in NiMoP/Al catalysts increases further as a result of calcination, whereas that of nickel decreases. In contrast, Sajkowski et al. (83) reported, on the basis of an extended X-ray absorption fine structure (EXAFS) investigation, that phosphorus does not affect the size of the polymolybdate species, Mangnus et al. (31) inferred that the stacking of molybdates does not increase as a result of the addition of phosphorus since the height of a temperature-programmed reduction (TPR) peak at 400°C due to the reduction of deposited multilayered molybdenum oxo-species was found to be independent of the phosphorus content. However, Chadwick el al. (60) concluded from XPS measurements that the dispersion of molybdenum decreases upon addition of phosphorus. 

Hydroxide, aqua, and hydrates. From the similar absorption spectra of Am in aqueous solution, AmCls, and in Lads, and the linear relationship between the decay rate of the americium fluorescence and the number of inner-sphere water molecules, it has been concluded that Am " is coordinated by nine inner-sphere water molecules. Similarly, the hydration number for the Cm ion has been estimated to be nine on the basis of fluorescence lifetimes. EXAFS studies of aqueous Am and Cm, however, have suggested coordination numbers closer to 10. " EXAFS investigation of Cf " " in aqueous solution indicates a coordination number of 8.5 ( 1.5), with a Cf—O distances of 2.41 0.02 A. This coordination number was confirmed for Am in the solid state by isolation of single crystals of the triflate salt of nonaqua complex, which contains a tricapped, trigonal prismatic cation that is isostructural with the analogous Pu" compound. ... 

Velu, S., Suzuki, K., Gopinath, C.S., Yoshida, H., and Hattori, T. XPS, XANES and EXAFS investigations of Cu0/Zn0/Al203/Zr02 mixed oxide catalysts. Physical Chemistry Chemical Physics, 2002, 4, 1990. 

Chiarello RP, Wogelius RA, Sturchio NC (1993) In situ synchrotron X-ray reflectivity measurements at the calcite-water interface. Geochim Cosmochim Acta 57 4103-4110 Chisholm-Brause CJ, Brown GE Jr, Parks GA (1989a) EXAFS investigation of aqueous Co(II) adsorbed on oxide surfaces in situ. Physica 158 646-648... 

Denecke MA, Reich T, Pompe S, Bubner M, Heise KH, Nitsche H, Allen PG Bucher JJ, Edelstein NM, Shuh DK, Czerwinski KR (1998b) EXAFS investigations of the interaction of humic acids and model compounds with uranyl cations in sohd complexes. Radiochim Acta 82 103-108 Dent AJ, Ramsay JDF, Swanton SW (1992) An EXAFS study of uranyl ions in solutions and sorbed onto sihca and montmorillonite clay colloids. J Colloid Interface Sci 150 45-60 d Espinose de la Caillerie J-B, Kermarec M, Clause (1995a) Impregnation of y-alumina with ( ) and ( ) ions at neutral pH Hydrotalcite-type coprecipitate formation and characterization J Am Chem Soc 117 11471-11481... 

Hennig C, Panak PJ, Reich T, Rossberg A, Raff J, Selenska-Pobell S, Matz W, Bucher JJ, Bernhard G, Nitsche H (2001) EXAFS investigation of uranium)VI) complexes formed at Bacillus cereus and Bacillus sphaericus surfaces. Radiochim Acta 89 625-631 Hessler JP, Seifert S, Winans RE, Fletcher TH (2001) Small-angle X-ray studies of soot inception and growth. Faraday Discuss 119 395-407... 

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