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Eingeladener Vortrag
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Single-crystals of chromium doped with 10 16 119Sn ions per cm2 and energies ranging between 45 and 190 keV were investigated with conversion electron Mössbauer spectroscopy (CEMS), Auger electron spectroscopy (AES), grazing angle X-ray diffraction (GAXRD), and scanning electron microscopy (SEM). The Mössbauer spectra were found to be significantly different than those of the bulk. Both the maximum and the average hf. field values were higher by a factor of ˜3 than the corresponding quantities for the bulk. The overall shape of the spectra is also different. GAXRD study has revealed that the near-surface zone of the investigated samples has a nanocrystalline structure with the average size of grains ranging between ˜20 and ˜40 nm. The latter seems to be the main reason for the observed enhancement of the spin-density and its distortion.
Fe-ZSM-5 Catalysts for the Selective Reduction of NO by Isobutane - The Problem of the Active Sites
(2002)
Relations between the structure of FeZSM-5 catalysts prepared by interaction of FeCl3 with HZSM-5 and their catalytic behavior in the SCR of NO by isobutane were investigated by combining results of catalytic studies (1000 ppm NO, 1000 ppm isobutane, and 2% O2 in He, 30,000 h-1) and of physicochemical characterization (XRD, EXAFS, Mössbauer spectroscopy, TPR, IR, XPS). By variation of the preparation conditions (method of FeCl3 introductionchemical vapor deposition or solid-state ion exchange, washing intensity, calcination regime) and of the matrix (HZSM-5 with normal and high defect density), FeZSM-5 materials with strongly varying properties (aggregation degree of the Fe phase, acidity) were obtained. Significant discrepancies between conclusions derived from EXAFS, TPR, and Mössbauer spectroscopy were ascribed to a preference of Mössbauer spectroscopy for the detection of aggregated phases and a high defectivity of Fe oxide clusters formed upon calcination. The critical step for the preparation of a highly disperse Fe phase is extensive washing after Fe introduction. The catalytic behavior of overexchanged FeZSM-5 materials prepared by interaction of FeCl3 with HZSM-5 of normal defect density was not influenced by aggregation of a significant part of the Fe phase. The comparison of their catalytic properties with those of a Fe2O3/HZSM-5 mechanical mixture and of catalysts prepared by aqueous ion exchange or by CVD of FeCl3 into HZSM-5 of high defect density implies that the particular activity of overexchanged FeZSM-5 arises from minority sites. In these, the Fe ions are probably isolated. Other Fe sites of low nuclearity appear to add to the activity. The poor SCR performance (at a given ability for isobutane activation) of FeZSM-5 prepared with a defective ZSM-5 matrix may indicate that the Fe sites which provi de the particular activity of overexchanged FeZSM-5 require the cooperation of acidic sites.
The precursor [FeIII(L)Cl] (LH2 = N,N'-bis(2'-hydroxy-3'-X-benzyliden)-1,6-diamino-3-N-hexane) is a high-spin (S = 5/2) complex (with X = -CH3, -O-CH3). This precursor is combined with the bridging unit [SnIV(CN)4] to yield star-shaped pentanuclear clusters, [(L-X-FeIII)4Sn(CN)4]Cl4 57Fe-Mössbauer, 119mSn-Mössbauer, and ESR spectroscopy are used to study our samples. For X = -CH3 the 57Fe-Mössbauer data show a multiple spin transition between iron(III) in the high-spin and low-spin state. Changing the functional group from X = -CH3 to X = -O-CH3 turns the switchability off.
Polyvalent Ions - Their Behaviour in the Glass Melt and their Influence on Glass Crystallisation
(1998)
Self-assembly of Fe(II) and the ditopic ligand 1,4-bis(2,2':6',2"-terpyridine-4'-yl)benzene results in equilibrium structures in solutions, so-called metallosupramolecular coordination polyelectrolytes (MEPEs). It is exceedingly difficult to characterize such macromolecular assemblies, because of the dynamic nature. Therefore, hardly any structural information is available for this type of material. Here, we show that from dilute solutions, where small aggregates predominate, it is possible to grow nanoscopic crystals at an interface. A near atomic resolution structure of MEPE is obtained by investigating the nanoscopic crystals with electron diffraction in combination with molecular modeling. The analysis reveals a primitive monoclinic unit cell (P21/c space group, a = 10.4 Å, b = 10.7 Å, c = 34.0 Å, α = γ = 90°, β = 95°, ρ = 1.26 g/cm³, and Z = 4). The MEPE forms linear rods, which are organized into sheets. Four sheets intersect the unit cell, while adjacent sheets are rotated by 90° with respect to each other. The pseudooctahedral coordination geometry of the Fe(II) centers is confirmed by Mössbauer spectroscopy. The combination of diffraction and molecular modeling presented here may be of general utility to address problems in structural materials science.
The mainstream magnetic iron oxide particles used as contrast media for magnetic resonance (MR) imaging are composed of a magnetic iron oxide core surrounded by a dextran or carboxydextran coat. The core size ranges from 2 nm to less than 10 nm, and the hydrodynamic diameter ranges from 20 nm to about 120 nm. The coat prevents aggregation and sedimentation of the particles in aqueous solutions, achieves high biological tolerance, and prevents toxic side effects.
Two kinds of particles are considered: (i) large particles (>30 nm), called superparamagnetic iron oxide particles (SPIOs) for liver imaging; (ii) smaller particles (<30 nm hydrodynamic diameter), called ultrasmall SPIOs (USPIOs), e.g. for MR angiography.
To characterize the particles, Mössbauer spectra are presented for the two particle ensembles. These spectra allow insight into the magnetic coupling, the valency of the iron ions and a rough estimate of the core size to be deduced. On the basis of the concentration dependence of the MR signal intensities, two applications are discussed together with two representative clinical examples.
Metallosupramolecular coordination polyelectrolytes (MEPEs) based on rigid and flexible ditopic bis-terpyridine ligands and Fe(II) ions are investigated by Mössbauer spectroscopy. We demonstrate the influence of mechanical stress induced by grinding on the structure of MEPE as well as the ability of MEPE to self-repair through recrystallisation.
In this paper the factors which are responsible for precision and reliability of the experimental data are discussed. The detectors of different types are compared with respect to the precision of the final results. It is shown that the use of resonance detectors gives the best results in most cases of the Sn-119m Mössbauer spectroscopy. The estimation of spectral line distortion caused by a short distance between source and detector is presented. Different procedures of velocity scale calibration are discussed. It has to be underlined that Kuprijanov's method of the velocity scale calibration gives the most reliable and fast results.