Near-edge Spectroscopy Research Articles

Intrazeolite chemistry of nickel(0) complexes and Ni(0,II) clusters studied by EXAFS, solid-state NMR and FT-IR spectroscopy

Adsorption, thermal decomposition, and reactivity of NI(CO)/sub 4/ in the cage system of dehydrated zeolite Y has been studied with EXAFS and near-edge spectroscopies, in particular monitoring changes in oxidation states and Ni ligation. Ni(CO)/sub 4/ is adsorbed as an intact molecule in cation-free zeolite Y, LiY, and NaY. Symmetry changes of the molecule in LiY and NaY are assigned to the formation of cation--OC-Ni bridges. Thermal treatment of the Ni(CO)/sub 4/NaY adduct leads to loss of CO concomitant with the formation of a bimodal Ni phase. A major part of the Ni forms small clusters (approximately 0.5 to 1.5 nm), in addition to larger crystallites (5-30 nm), sticking at the outer surface of the zeolite crystals. Oxidation of Ni(CO)/sub 4/NaY to NiONaY with molecular oxygen occurs almost to completion below 295 K. The small NiO particles are stable to sintering up to 670 K. Nickel-phosphineNaY adducts have been prepared by direct adsorption from solution (Ni(CO)/sub 3/PMe/sub 3/, Ni(CO)/sub 2/(PMe/sub 3/)/sub 2/), and by synthesis inside the NaY supercage (Ni(CO)/sub 3/PMe/sub 3/, Ni(CO)/sub 3/(PPh/sub 2/CHMe/sub 2/), Ni(CO)/sub 3/P(tau-Bu)/sub 3/, Ni(PMe/sub 3/)/sub x/). The NaYNi(CO)/sub 3/(PPh/sub 2/CHMe/sub 2/) adduct is a ship-in-a bottle complex, i.e., the Ni complex is too largemore » to exit the zeolite supercage. The substitution of carbonyl ligands by phosphines was monitored in the near-edge and EXAFS modulations as well as with /sup 31/P MAS NMR. EXAFS is an excellent method to monitor ligand coordination and provides information that is complimentary to IR spectroscopy.« less

Structural investigations of TiO 2SiO 2 glassy and glass-ceramic materials prepared by the sol-gel method

In the present paper we will show by both extended X-ray absorption fine structure (EXAFS) and near-edge (XANES) spectroscopies that the coordination of Ti 4+ ions is only tetrahedral in TiO 2SiO 2 very homogeneous glasses obtained by the sol-gel method. The coordination of Ti 4+ is also tetrahedral in the essicated gels. An “intermediate” ordered phase, not yet well defined, has been observed at the beginning of the crystallization process. Moreover, the thermal treatment causes a progressive ordering of the T d tetrahedral units going from the essicated gels to the more and more dense glasses. The presence of six-fold Ti-sites has been clearly observed by both EXAFS and near-edge spectroscopies only in a partially crystallized glass containing the highest TiO 2 content (19 wt%), at 1200°C. On the contrary, in all the glasses having the lowest TiO 2 content (4.5 wt%) a four-fold Ti coordination has been observed until 1200°C. The crystalline phase, which appears at high temperatures, is anatase having a crystalline size in the range 50–100 Å for thermal treatments in the range 1000–1200°C for the sample richer in TiO 2. An appearance of crystallinity at higher temperatures for lower TiO 2 concentrations has been observed by X-ray diffraction (XRD) analysis, in agreement with other authors.