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Eingeladener Vortrag
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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.
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.
In view of an understanding of catalytic efficiency the electronic and magnetic state of iron in Fe/KL and the influence of Pt on these properties in Pt-Fe/KL were investigated by superconducting quantum interference device (SQUID) magnetization measurements and Mössbauer spectroscopy. After calcination in air, iron is present only as Fe3+ ions in both samples, mainly in a paramagnetic state. There is also a small antiferromagnetic contribution, probably due to iron oxide and hydroxide particles which precipitate on the outer surface of the zeolite crystallites during preparation. After reduction in H2, the behavior of both samples is predominantly superparamagnetic, but the presence of Pt increases both the saturation magnetization and the size of the residual hysteresis loop. At the same there remains a pronounced paramagnetic contribution which appears to be even slightly higher than before reduction. The interpretation is that alloy particles of metallic PtFe are formed on the outer surface. Since small clusters inside the zeolite crystallites are most active for catalysis the alloy formation seems to be not of relevance for an enhancement of sulfur resistance. It was also shown that during reduction a fraction of Fe2+ is formed. The coordination of the Fe2+ ions is clearly influenced by Pt and it seems very likely that Fe2+ ions are in close contact with Pt clusters. This interaction is probably the origin of the enhanced sulfur resistance and catalytic activity in the reforming reactions.
PdSn catalysts prepared by various methods and used for nitrate reduction were investigated by means of in situ 119mSn Mössbauer spectroscopy. Characterization by 119mSn Mössbauer spectroscopy revealed significant differences concerning the modification of the palladium particles supported on alumina. The controlled surface reaction preparation method (CSR) leads to formation of PdSn alloys with different tin contents besides small amounts of Sn(II) species and unalloyed metallic Sn. On the other hand the impregnation method leads to a small amount of PdSn alloys and higher contents of Sn(II) species. Sn(IV) species were found in both kinds of catalysts. The CSR prepared PdSn catalysts showed an improved activity and selectivity compared to the catalysts obtained by the impregnation method.
Hydrogenation of nitrate to nitrogen using Pd/Al2O3 catalysts promoted by a second metal offers a promising process for nitrate removal in drinking water treatment. This study was aimed to elucidate the nature and function of promoting tin species in PdSn/Al2O3 catalysts obtained in different preparation routes. On one hand, a parent Pd/Al2O3 catalyst was doped via impregnation with aqueous solutions of SnCl2 of different concentrations. On the other hand, the palladium surface of the same parent Pd/Al2O3 catalyst was modified via controlled surface reaction (CSR) with hexane solutions of Sn(C4H9)4. The structure of the different PdSn/Al2O3 catalysts was investigated by 119Sn-Mössbauer spectroscopy and by means of various chemisorption techniques (static and pulse chemisorption of H2 or CO, measurement of the differential heat of CO chemisorption, FTIR spectroscopy of CO chemisorption). Catalytic properties were studied in batch experiments under atmospheric pressure. Promoting of the Pd/Al2O3 catalyst by CSR resulted in catalysts with a significantly higher activity compared to PdSn/Al2O3 catalysts obtained via incipient wetness method. Obviously, Sn(II)-species being present in the latter in high portion inhibit the nitrate reduction on bimetallic PdSn ensembles.
CO chemisorption reflected a palladium site blocking by tin species in both kinds of catalysts and indirectly indicated the generation of palladiumtin ensembles. In case of the CSR preparation the palladium is alloyed by metallic tin.
The palladium surface is diluted by tin atoms, i.e. bimetallic PdSnx ensembles are generated which are able to adsorb and activate nitrate ions. There is an optimum of tin loading, i.e. the activity decreases and the undesired ammonium production in a site reaction increases when the surface becomes too tin rich. Sn(II) species, being preferentially present in catalysts obtained by SnCl2 impregnation also strongly modify the chemisorption properties of the palladium surface, but obviously these species inhibit the nitrate reduction on the bimetallic PdSn ensembles, which are also present in these catalysts.
Characterization of the microstructure of high-chromium cast irons using Mössbauer spectroscopy
(2001)
The finish grinding process for cement materials uses ball mills with differently sized grinding balls. Because the grinding takes place through the impact and friction of the balls, balls with a high wear resistance are desirable and the materials of choice are the high-chromium white cast irons. This article examines the behavior of these cast irons when subjected to various heat treatments. The emphasis is on the dependence of the microstructure properties on the chemical composition of the alloys and the heat treatment parameters. The content of retained austenite and that of carbon in the martensite phase were determined by the use of Mössbauer spectroscopy. The results verify that a higher proportion of retained austenite with low carbon content and a martensite with higher hardness produce a material with good fracture toughness and resistance to abrasive wear.