Gilberto Vlaic

XAFS study of Ti-silicalite: structure of framework Ti(IV) in presence and in absence of reactive molecules (H2O, NH3)

X-ray absorption at the Ti K edge (both XANES and EXAFS) of a very pure Ti-silicalite containing a small fraction of Ti(IV) substituting Si(IV), has been performed in order to study the effect of the presence or absence of ligands such as H2O and NH3 on the Ti(IV) coordination sphere. In particular, the effect of an outgassing treatment at 400 K and of the interaction with NH3 has been studied and described in detail. It has been found that the Ti(FV) is fourfold coordinated in the samples outgassed at 400 K and expands its coordination sphere number under the action of adsorbates.

Carbon tetrachloride hydrodechlorination with organometallics-based platinum and palladium catalysts on MgO

Hydrodechlorination of carbon tetrachloride was performed at atmospheric pressure over Pd and Pt catalysts supported on various inorganic oxides (Al2O3, MgO, CeO2) and obtained from different organometallic precursors (Pd(acac)2, [Pd(C3H5)Cl]2, Pt(acac)2 and Pt(CH3CN)2Cl2. Generally, Pd-based catalysts show low conversion levels and high selectivity to C1–C7+ hydrocarbons with a distribution that fits a typical Schulz–Flory mechanism for alkyl polymerization. On the other hand, Pt catalysts show high and stable activity, the major products are CHCl3 and in lesser extent methane. Catalytic performances depend on the stability, under reaction conditions, of the metal phase, that could be related to the nature of chlorine species pre-adsorbed on catalysts. Owning to their superior catalytic performances, the structural modifications of Pt catalysts under reaction conditions were characterized by EXAFS spectroscopy.

A zinc-rich CHA-type aluminophosphate

Abstract Zinc(II)-substituted chabazite-like aluminophosphate ZnAPO-34 of excellent phase purity and a high amount of incorporated zinc was synthesized hydrothermally in the presence of tetraethylammonium hydroxide (TEAOH). The characterization of the product indicates that 20% of framework aluminum is replaced by zinc and shows that template is located at the center of the unit cell, in the large interstitial cavity of the chabazite framework. 27 Al and 31 P MAS n.m.r. results are consistent with the X-ray structural determination. Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopic analysis proves the isomorphous substitution of zinc for aluminum in the framework.

The CeO2-ZrO2 System: Redox Properties and Structural Relationships.

CeO 2 -ZrO 2 mixed oxides show improved oxygen storage property compared to CeO 2 due to the involvement of the bulk of the oxide in low temperature redox processes. In the present work we investigated the M-O local structure by means of EXAFS and Raman spectroscopy. Evidence is found that significant distortions of the oxygen sublattice are induced by introduction of Zr 4+ into the CeO 2 lattice. It is suggested that these distortions generate mobile oxygens which are responsible for the improved redox properties at moderate temperatures.

Multiple solutions in data fitting: a trap in EXAFS structural analysis and some ideas to avoid it

A possible source of error on interatomic distance determination in EXAFS multishell data analysis is described on the basis of fitting a simulated signal for a cluster of Rh atoms with an interaction of O atoms; a fit of an experimental signal is also presented. The origin of this type of mistake is briefly discussed.

EXAFS analysis of the L3 edge of Ce in CeO2: effects of multi-electron excitations and final-state mixed valence

Cerium oxide (IV) (CeO2) is extensively employed in heterogeneous catalysis, particularly as a promoter of noble metal action in three-way catalysts. For this reason there is a great scientific and economical interest in the development of any possible chemical or structural analysis technique that could provide information on these systems. EXAFS spectroscopy has revealed itself as a powerful technique for structural characterization of such catalysts. Unfortunately, good quality K-edge spectra of cerium are not yet easily obtainable because of the high photon energy required (>40 keV). On the other hand, at lower energies it is easy to collect very good spectra of the L3 edge (5.5 keV), but L3-edge spectra of cerium (IV) are characterized by the presence of two undesired additional phenomena that interfere with EXAFS analysis: final-state mixed-valence behaviour and intense multi-electron excitations. Here, a comparative analysis of the K, L3, L2 and L1 edges of Ce in CeO2 has been made and a procedure for obtaining structural parameters from L3-edge EXAFS, even in the presence of these features, has been developed. This procedure could allow further studies of catalytic compounds containing tetravalent cerium surrounded by oxygen ligands.

F‐test in EXAFS fitting of structural models

A. M i c h a l o w i c z * , K Provost * * , S. Larue l le ÷, A. M i m o u n i * a n d G. V la ic x The meaning and the way to estimate these quantities is discussed elsewhere (Lytle, S&C report, 1988 and present XAFS conference, 1998). We want to compare two fits noted AX2~ and AX22 with respectively the degrees of freedom v~ and v2. In order to avoid confusing notation we shall not use A%zv in the following formulas.

EXAFS characterization of oxaliplatin anticancer drug and its degradation in chloride media.

Oxaliplatin is a second-generation platinum-based anticancer drug. Its degradation is studied in solution, in the presence of chloride ions (in neutral or acidic media) in excess. In both cases the degradation product precipitates immediately. The EXAFS spectra of these products show that they are identical. EXAFS modeling and refinement of the first coordination sphere shows that two light atoms are replaced by two chloride ions. The complete refinement of the local structure is possible by studying the multiple-scattering signal. The results show that the main multiple-scattering contribution is due to the binding oxalato group and that the degradation product is [Cl(2)-(diaminocyclohexane)-Pt(II)].

In-situ EXAFS investigation of non-acidic CVD-based Pt/KL catalyst under oxidation-reduction cycles

The structural evolution of non-acidic chemical-vapour deposition (CVD)-based Pt/KL catalyst has been investigated by in-situ extended X-ray absorption fine structure (EXAFS) spectroscopy. After in-situ reduction (H2, 400 °C), EXAFS results suggested the presence of small metal particles which may be localized inside the KL-zeolite channels. After the reforming of methylcyclopentane at 500 °C (and 1 atm), no coke deposition was evidenced by EXAFS, in agreement with the non-acidic character of this catalyst.Severe O2/H2 regeneration cycles did not alter the structure and location of Pt particles inside the zeolite.

EXAFS Study of Ti, Fe and Ga Substituted Silicalites

The local environment of isomprphous substituted Ti, Fe and Ga metal atoms in silicalite (hereafter indicated as TS-1, Fe-S and Ga-S respectively) has been studied by means of EXAFS spectroscopy. Measurements have been carried out on the samples in presence of template and after template removal at increasing temperatures in order to follow the evolution of the local environment of the metal heteroatoms. Ti(IV) atoms in (TS-1) show a great stability, while Ga(III) and particularly Fe(III) atoms have propensity to migrate from framework into extraframework positions, forming small oxide particles trapped inside the MFI channels upon increasing the template burning temperature. The established stability scale is Ti(IV) > Ga(III) > Fe(III). These conclusions are qualitatively supported by UV-Vis, IR and Raman data.