Characterization XANES

Abstract Molecular spectroscopy is one of the most important means to characterize the various species in solid, hquid and gaseous elemental sulfur. In this chapter the vibrational, UV-Vis and mass spectra of sulfur molecules with between 2 and 20 atoms are critically reviewed together with the spectra of liquid sulfur and of solid allotropes including polymeric and high-pressure phases. In particular, low temperature Raman spectroscopy is a suitable technique to identify single species in mixtures. In mass spectra cluster cations with up to 56 atoms have been observed but fragmentation processes cause serious difficulties. The UV-Vis spectra of S4 are reassigned. The modern XANES spectroscopy has just started to be applied to sulfur allotropes and other sulfur compounds. 

The XANES spectra of Se, Ss, Sio, S12, S14 [222] and polymeric sulfur [223, 224] are all very similar as far as the peak positions are concerned but the relative intensities of the two peaks differ considerably. The spectra are characterized by an absorption line at 2471.7 eV (so-called white line) and a broad absorption in the region 2477-2480 eV [222] see Fig. 35 (energies calibrated to the white line of ZnS04 defined as 2481.4 eV). Since the spectra of the components of a mixture are additive, quantitative analyses are possible, even for mixtures of samples as similar as Ss and polymeric sulfur, for instance [224]. The interpretation of the spectra is, however, still somewhat controversial see [225]. 

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

X-ray absorption spectroscopy combining x-ray absorption near edge fine structure (XANES) and extended x-ray absorption fine structure (EXAFS) was used to extensively characterize Pt on Cabosll catalysts. XANES Is the result of electron transitions to bound states of the absorbing atom and thereby maps the symmetry - selected empty manifold of electron states. It Is sensitive to the electronic configuration of the absorbing atom. When the photoelectron has sufficient kinetic energy to be ejected from the atom It can be backscattered by neighboring atoms. The quantum Interference of the Initial... 

If a vacant site is occupied by another Nb atom, such that it is a dimer, new catalysts may be designed. The Nb dimer catalyst(2) was prepared by reaction of [Nb(ri -C5H5)H-p-(T, Ti -CsH4)]2 with a Si02 at 313 K, followed by treatment with 02 at 773 K. A proposed structure(2) was characterized by EXAFS, x-ray absorption near-edge structure(XANES), FT-IR, UV-vis, and XPS, which shows Nb-Nb (coordination number 0.9) and Nb-Si(2.3)... 

The quantification of the extra-framework titanium species in titanium silicalites of MFI structure, TS-1, was performed using either XANES at the Ti K-edge or XPS Ti (2p) photolines. In addition, two different framework sites, [Ti(OH)(OSi)3] and [Ti(OSi)4], were characterized in dehydrated samples using Diffuse Reflectance UV-visible, multiple scattering analysis of EXAFS, H and Si NMR spectroscopies. 

Nowadays there is a general consensus that the Ti(IV) atoms are incorporated as isolated centers into the framework and are substituting Si atoms in the tetrahedral positions forming [Ti04] units. The model of isomorphous substitution has been put forward on the basis of several independent characterization techniques, namely X-ray [21-23] or neutron [24-26] diffraction studies, IR (Raman) [52-57], UV-Vis [38,54,58], EXAFS, and XANES [52, 58-62] spectroscopies. 

XANES spectroscopy shows that a narrow and intense pre-edge peak at 4967 eV, due to the Is 3pd electronic transition involving Ti atoms in tetrahedral coordination, is present in well-manufactured TS-1 (Fig. 2c). Conversely this electronic transition of Ti(IV) species in Ti02 (anatase or rutile) is characterized by a very low intensity due to the small pd hybridization in octahedral symmetry. Indeed the transitions l2g are symmetrically forbidden in the case of octahedral coordination of Ti (IV), but the transition Ai T2 is allowed in the case of tetrahedral coordination of Ti(IV), as in the case of [Ti04] units [52,58-61,63,68]. 

Recently characterization of bimetallic nanoparticles by EXAFS were extensively reported [122-124,176], Structural transformation of bimetallic Pd/Pt nanoparticles, which were prepared by a sequential loading of H2PtClg onto the Pd loaded catalyst, was investigated with EXAFS at high temperatures [176], The results of EXAFS at Pd K and Pt L-III edges showed that Pt was surface-enriched or anchored on the Pd metal core with an increase of the Pt content. The structure of the obtained bimetallic Pd/Pt nanoparticles seemed to be retained upon heating up to 1273 K under ambient condition [176], Pt/ Au bimetallic nanoparticles can be prepared by polyol method and stabilized by PVP [122], XANES and EXAFS studies were also performed on the samples and their results supported the idea of a Pt-core/Au-shell structure with the elements segregated from each other [122],... 

Zn and N K-edge XANES have been used to distinguish the coordination geometries in (1,2-ethanediylidene)-bis(5 -methylhydrazonecarbodithionate) zinc complexes (109). The technique distinguished between tetrahedral species, square pyramidal dimers, and square pyramidal monomers, formed when pyridine was present. These studies were in conjunction with spectroscopic characterization and X-ray single-crystal data where possible.53 The results demonstrated the value of this technique when single crystals could not be obtained. 

XPS has typically been regarded primarily as a surface characterization technique. Indeed, angle-resolved XPS studies can be very informative in revealing the surface structure of solids, as demonstrated for the oxidation of Hf(Sio.sAso.5)As. However, with proper sample preparation, the electronic structure of the bulk solid can be obtained. A useful adjunct to XPS is X-ray absorption spectroscopy, which probes the bulk of the solid. If trends in the XPS BEs parallel those in absorption energies, then we can be reasonably confident that they represent the intrinsic properties of the solid. Features in XANES spectra such as pre-edge and absorption edge intensities can also provide qualitative information about the occupation of electronic states. 

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

Micro-scale characterization (p-XRF, p-XANES, p-XRD, petrography, and EPMA) was able to confirm As forms identified by bulk XRD and in all cases identify additional As forms. Grain-scale mineralogical analysis unambiguously identified a minimum of two and typically four or more As-bearing phases in each tailings sample (Fig. 2, Table 2). 

Bolis et al (43) reported volumetric data characterizing NH3 adsorption on TS-1 that demonstrate that the number of NH3 molecules adsorbed per Ti atom under saturation conditions was close to two, suggesting that virtually all Ti atoms are involved in the adsorption and have completed a 6-fold coordination Ti(NH3)204. The reduction of the tetrahedral symmetry of Ti4+ ions in the silicalite framework upon adsorption of NH3 or H20 is also documented by a blue shift of the Ti-sensitive stretching band at 960 cm-1 (43,45,134), by a decrease of the intensity of the XANES pre-edge peak at 4967 eV (41,43,134), and by the extinction of the resonance Raman enhancement of the 1125 cm-1 band in UV-Raman spectra (39,41). As an example, spectra in Figs. 15 and 16 show the effect of adsorbed water on the UV-visible (Fig. 15), XANES (Fig. 16a), and UV-Raman (Fig. 16b) spectra of TS-1. 

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