Pre-edge peak

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]. 

That observed for TS-1 is not peculiar for TS-1 only and can be observed on other titanosilicates like Ti-MSA, a mesoporous amorphous material that has Ti(Vl) centers exposed on the surface of the pores [124,125]. In this case, easier experiments could be performed by Prestipino et al. [50] as the peroxo/hydroperoxo complexes can be formed by dosing f-butyl hydroperoxide (which does not enter the 10-membered rings of TS-1). The XANES spectrum of Ti-MSA in vacuum is typical for almost r -like Ti(IV) centers (the intensity of the Ai T2 pre-edge peak being only 0.69, as compared with 0.91 for TS-1). Upon dosing the t-butyl hydroperoxide in decane solution on Ti-MSA, a spectrum similar to that obtained on TS-1 contacted with anhydrous H2O2 is observed on both XANES and EXAFS regions [50]. When the... 

This trend in metal charge can also be confirmed in the XANES spectra for the CrAsi-yPy and I eAsi VI series. The Cr K-edge XANES spectra in CrAsi vPv resemble the Mn spectrum seen earlier for MnP (Fig. 19), with the pre-edge peak... 

Fig. 5. XANES spectrum of a typical TS-1 sample in vacuum. Inset intensity of the pre-edge peak (spectra normalized to the edge jump) for samples with various Ti contents. Because the height of the edge jump is proportional to the Ti content, the intensity of the normalized pre-edge is invariant (within experimental uncertainty) with Ti concentration [Reprinted from Ricchiardi et al (41) with permission. Copyright (2001) American Chemical Society].

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. 

The tetrahedral structure of these surface alkyl complexes on MCM-41(5oo) has been highlighted by XANES a sharp, intense pre-edge peak at 4969.6 0.3 eV is characteristic of an electronic transition of titanium, from the Is energetic level to molecular orbitals mixing 3d and 4p of Ti with the orbitals of the Ugands, in a complex where titanium is in a tetrahedral symmetry [28-31]. The same argument can be applied for species obtained from alcoholysis of 2a and 2b, especially using tert-butanol. 

Re species are in a dimeric form with a direct Re-Re bond. Re Lj-edge XANES in part (ii) of Figure 10.9a did not show the pre-edge peak attributed to tetrahedral conformation of Re and the edge posihon shifted to lower energy. These results indicate that the NH3 treatment reduced the Re monomers accompanied with dimerization. Negligible catalytic achvity at this stage demonstrates that small Re clusters such as dimers do not act as achve species for direct phenol synthesis from benzene and O2. 

The pre-edge peak, which arises from a ls- 3d transition, is formally forbidden. However, geometries which lack inversion centers allow for orbital mixing which break down the selection rules governing transition probability. V in is in a distorted octahedral... 

In summary, the intensity of a metal K-pre-edge peak is low and dominantly reflects a distortion of the metal site from centrosymmetric that allows metal 4p mixing into the valence 3d orbitals. 

Figure 11.3 XANES spectra of pure Cr203 (a) and K2Cr04-4H20 (b). Note the characteristic pre-edge peak in the latter representing the 100% Crvi endmember, which is virtually absent in the 0% Crvl endmember in (a). The data were collected at the Hamburg Synchrotron Laboratory (DESY-HASYLAB).

Figure 17 Plot of the isomer shifts versus Fe oxidation state of a series of low-spin Fe(lll) Fe(IV), Fe(V), and Fe(VI) complexes with cyclam-related ligands. A plot of the Fe X-ray absorption spectroscopy (XAS) pre-edge peak energies of some complexes versus Fe oxidation state is also shown (open squares). The lines with dashes and dots represent least squares fits. (From J. F. Berry etal. (2006) Science 312 1937-1941. Reprinted with permission from AAAS)...

The pre-edge intensity very often increases only upon removal of any adsorbed water vapor [64]. The water vapor acts as a ligand, which changes the intensity of the pre-edge peak. This is crucial for the understanding of how the oxidation catalysis works, since TS-1 is mostly used in aqueous solutions with hydrogen peroxide. It has been suggested that EXAFS can show the presence of titanium peroxo species on TS-1 [65]. 

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