X-ray spectroscopy EXAFS

The techniques that have been most employed for investigating the electronic properties of small particles are photoemission (UPS, XPS), soft X-ray spectroscopy, EXAFS, photoionization mass spectrometry, and AES (23, 111, 240, 257d,e). While there is some controversy from theoretical work about the minimum particle size required to give bulk properties—from 10 (258) to several hundred atoms (259)—there seems to be a consensus that a cluster of about ISO atoms or more is required to observe a photoemission spectrum similar to that of the corresponding bulk metal (23, 260). When other properties are considered (ionization potential, density of states, valence bandwidth, etc.), the agreement is less satisfactory between the results obtained with different techniques (23). 

AAS = atomic absorption spectroscopy ATOF-MS = aerosol time-of-flight mass spectrometry 2,6-ndc = 2,6-naphthalenedicarboxylate bdc = 1,4-benzenedicar-boxylate bpdc = 1,3,5-benzenetricarboxylate bpydc = 2,2 -bipyridine-5,5 -dicarboxylate btb = 4,4, 4"-benzene-1,3,5-triylbenzoate btc = 1,3,5-benzenetricarboxylate 3D = three-dimensional dabco = l,4-diaza[2.2.2]bicyclo-octane EDX = energy-dispersive X-ray spectroscopy EXAFS = extended X-ray fine structure Fc = ferrocenyl ICP-AES = inductively coupled plasma atomic emission spectroscopy MOF = metal-organic framework PSD = postsynthetic deprotection PSM = Postsynthetic modification PXRD = Powder X-ray diffraction 1,4-ndc = 1,4-naphthalenedicarboxylate SALE = solvent-assisted linker exchange SBU = secondary building unit ... 

Linear absorption measurements can therefore give the first indication of possible alloy formation. Nevertheless, in systems containing transition metals (Pd-Ag, Co-Ni,. ..) such a simple technique is no longer effective as interband transitions completely mask the SPR peak, resulting in a structurless absorption, which hinders any unambiguous identification of the alloy. In such cases, one has to rely on structural techniques like TEM (selected-area electron diffraction, SAED and energy-dispersive X-ray spectroscopy, EDS) or EXAFS (extended X-ray absorption fine structure) to establish alloy formation. 

Dispersive EXAFS, 42 327, 328, 330-340 Dispersive X-ray spectroscopy, 34 204 Disproportionation, polyatomic adsorbates, BOC-MP, 37 115-117 Dissociation... 

Multlnuclear clusters of Fe(III) occur hound to the protein when Fe(II), at amounts less than or equal to that required to saturate the protein. Is allowed to oxidize JUi situ. The clusters, predicted from the results of EPR spectroscopy (35,38) and UV-difference spectroscopy (34), were observed and characterized by x-ray absorption (EXAFS) and MOssbauer spectroscopy (Figure 2 Ref. 39). Measurements were made with a complex of Fe(III) and the protein coats of apoferrltln after the binding of 10 Fe(II) atoms/molecule, the admission of air and equilibration for 24 hours. 

As noted earlier, metal ions in polar solvents will form complexes with the solvent molecules. X-Ray diffraction, EXAFS, and visible absorption spectroscopy show that nickel(II) ion in dilute aqueous solution is present as the green hexaaqua complex Ni(H20)62+, just as in solids such as NiS04-7H20, which is actually [Ni(H20)e]S04-H20. In the crystal, the extra water molecule is loosely associated with the sulfate ion independently of the nickel-aqua complex it is sometimes referred to as lattice water, as distinct from complexed water. 

Kirpichtchikova, T. A., Manceau, A., Spadini, L., Panfili, F., Marcus, M. A., andjacquet, T. (2006). Speciation and solubility of heavy metals in contaminated soil using X-ray microfluorescence, EXAFS spectroscopy, chemical extraction, and thermodynamic modeling. Geochim. Cosmochim. Acta 70, 2163-2190. 

EXAFS — Extended X-ray absorption fine structure spectroscopy. X-ray spectroscopy close and slightly above the X-ray absorption edge. See surface analytical methods. 

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. 

Experimental practice and selected results. Phys Rev 11 4825-4835 Lytle FW, Sayers DE, Stem EA (1982) The history and modem practice of EXAFS spectroscopy. In Bonnelle C, Mande (eds) Advances in X-ray Spectroscopy. Pergamon Press, New York, p 267-286 Lytle FW, Via GH, Sinfelt JH (1980) X-ray absorption spectroscopy catalyst applications. In Winick H, Doniach S (eds) Synchrotron Radiation Research. Plenum Press, New York, p 401-424 MacDowell AA, Celestre RS, Tamura N, Spolenak R, Valek BC, Brown WL, Bravman JC, Padmore HA, BattermanBW, Patel (2001) Submicron X-ray diffraction. Nucl Instrum Methods A 468 936-943 Macy JM (1994) Biochemistry of selenium metabolism by Thauera selenatis gen. nov. sp. nov. and use of the organism for bioremediation of selenium oxyanions in San Joaquin Valley drainage water. In Frankenberger WT Jr, Benson S (eds) Selenium in the Environment. Marcel Decker, Inc., New York, p 421-444... 

EXAFS is also well suited for the study of finely divided metal (or metal oxide or metal sulfide) clusters supported within the pore structure (see Chapter 6). These particles are readily observed by X-ray spectroscopy, even if they are disordered throughout the solid. Analysis can even determine the average particle size of such clusters, which is of vital importance in catalytic preparation. Typically, for example, platinum supported on zeolites (and other solid acids) is a highly effective catalyst in the reforming of hydrocarbons. 

---