Diffraction and EXAFS

This chapter deals with the study of structural properties of catalysts and catalytic model surfaces by means of interference effects in scattered radiation. X-ray diffraction is one of the oldest and most frequently applied techniques in catalyst characterization. It is used to identify crystalline phases inside catalysts by means of lattice structural parameters, and to obtain an indication of particle size. Low energy electron diffraction is the surface sensitive analog of XRD, which, however, is only applicable to single crystal surfaces. LEED reveals the structure of surfaces and of ordered adsorbate layers. Both XRD and LEED depend on the constructive interference of radiation that is scattered by relatively large parts of the sample. As a consequence, these techniques require long-range order. 

Extended X-ray absorption fine structure (EXAFS) on the other hand, is due to the interference of electron waves between atoms and gives local structure informa- 

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I. V. Tomov, D. A. Oulianov, P. Chen, and P. M. Rentzepis, Ultrafast time-resolved transient structures of solids and liquids studied by means of X-ray diffraction and EXAFS. J. Phys. Chem. B 103(34), 7081-7091 (1999). 

The extent to which small particles of Pd and Pt show evidence of oxidation after exposure to air Is also highly variable. It Is difficult to confirm the evidence of X-ray diffraction and EXAFS (25) that most particles In the 15-20A size range consist entirely of oxide. We have found that such particles usually give single crystal patterns attributable to the metals. There Is, however, considerable evidence that, in the case of Pt on alumina, the Pt crystals have a well-defined epitaxial relationship with the crystallites (20-50A diameter) of the nominally "amorphous" alumina substrate. 

The solid curves below 7 A are calculated accurately for a model (Section 4) that fits the osmotic coefficient data. The curves above 7 A are merely schematic, showing in exaggerated form the oscillations that appear in gab at large r when the concentration is large, even for the models in Section 4. The dashed curve indicates the location and intensity of the peak in g+. (r) identified in aqueous NiCle in neutron diffraction and EXAFS studies, as reviewed in Section 5. 

A number of crystal structures are known, viz. (OEP)TiO, (TPP)TiO, (OEPMe2)Ti (43).147 For all these complexes titanium has four nitrogen donors and a double-bone oxygen, resulting in a classical square pyramid. The information obtained from X-diffraction and EXAFS is in excellent agreement. 

Iodosylbenzene 18 is a pale yellow, amorphous powder and essentially insoluble in all nonreactive media. X-Ray powder diffraction and EXAFS spectroscopy showed a zigzag polymeric structure of 18 with the two 1-0 distances (2.04 and 2.377 A) and the I-O-I bond angle (114°) (Fig. 4) [214]. A similar polymeric structure was determined for the imido analogue, (tosyliminoiodo)benzene PhINTs [214,215]. Polymeric iodosylbenzene is likely to be terminated by addition of water, i.e. HO(PhIO)nH [216]. 

X-ray diffraction, neutron diffraction, and EXAFS all give similar types of structural information that can be used to calculate a radial distribution function (RDF) that directly shows interatomic distances in the solution. 

Double salt problems are of practical importance in the chemical industry. However, only limited discussion has appeared concerning an elucidation of the mechanism of formation, because reliable information on the structure of ions in solution is not available. The development of solution X-ray diffraction and EXAFS techniques can throw new light on the problem to elucidate the mechanism of formation of double salts. As a case study we take a series of double salts M C1-MgCl2,nH20, where M1 denotes an alkali metal or an ammonium ion. 

Scattering, Powder Diffraction and EXAFS. Thus it is very important that the direction the science will take in the next 10 years is considered now. Now would be an excellent time to discuss the communities requirements with the scientific staff at Daresbury Laboratory. The purpose of the options presented here is to provide a flavour of what may be possible for polymer science on DIAMOND. It is not designed to direct your ideas in any way. 

Cd/Ca silicate hydrates prepared by coprecipitation and Cd-exchanged C-S-H, have been characterized by Si and Cd MAS NMR X-ray diffraction, and EXAFS. ... 

Similarly, a combination of powder XRD (X-Ray Diffraction) and EXAFS (Extended X-ray Absorption Fine Structure) was used for the structural characterization of the phases of titania and vanadia. A relatively rare, mixed phase was found, which may explain the high catalytic activity. 

MODERN TECHNIQUES OF STUDYING SOLVAHON AXD DIFFRACTION AND EXAFS 541 4nr g(i)... 

Modern techniques of studying solvation AXD diffraction and EXAFS... 

Low temperature measurements, use of several absorbing species in the same compound, comparison between seemingly contradictory X-Ray diffraction and EXAFS data allow to solve three dimensional structural problems related to physical properties. 

HEN/RED] Henderson, C. M. B., Redfem, S. A. T., Smith, R. 1., Knight, K. S., Chamock, J. M., Composition and temperature dependence of cation ordering in Ni-Mg olivine solid solutions a time-of-flight neutron powder diffraction and EXAFS study. Am. Mineral, 86, (2001), 1170-1187. Cited on pages 239,438. 

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