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Author: Rautenberg, Christine × Type: Text ×

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    Determining the location of Co2+ in zeolites by UV-Vis diffuse reflection spectroscopy : A critical view
    (Basel : MDPI, 2020) Bellmann, Andrea; Rautenberg, Christine; Bentrup, Ursula; Brückner, Angelika
    UV-Vis spectroscopy as well as in situ FTIR spectroscopy of pyridine and CO adsorption were applied to determine the nature of Co species in microporous, mesoporous, and mixed oxide materials like Co-ZSM-5, Co/Na-ZSM-5, Co/Al-SBA-15, and Co/Al2O3-SiO2. Because all sample types show comparable UV-Vis spectra with a characteristic band triplet, the former described UV-Vis band deconvolution method for determination and quantification of individual cationic sites in the zeolite appears doubtful. This is also confirmed by results of pyridine and CO adsorption revealing that all Co-zeolite samples contain two types of Co2+ species located at exchange positions as well as in oxide-like clusters independent of the Co content, while in Co/Al-SBA-15 and Co/Al2O3-SiO2 only Co2+ species in oxide-like clusters occur. Consequently, the measured UV-Vis spectra represent not exclusively isolated Co2+ species, and the characteristic triplet band is not only related to γ-, β-, and α-type Co2+ sites in the zeolite but also to those dispersed on the surface of different oxide supports. The study demonstrates that for proper characterization of the formed Co species, the use of complementary methods is required. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
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    Stability studies of ionic liquid [EMIm][NTf2] under short-term thermal exposure
    (London : RSC Publishing, 2016) Neise, Christin; Rautenberg, Christine; Bentrup, Ursula; Beck, Martin; Ahrenberg, Mathias; Schick, Christoph; Keßler, Olaf; Kragl, Udo
    Ionic liquids (ILs) as new media for synthesis and as functional fluids in technical applications are still of high interest. Cooling a steel component from an annealing temperature of nearly 850 °C down to room temperature in a liquid bath is a technically important process. The use of ionic liquids offers advantages avoiding film boiling of the quenching medium. However, such a high immersion temperature exceeds the thermal stability of the IL, for example such as [EMIm][NTf2]. To obtain information about formation of potential toxic decomposition products, potential fragments at varied states of decomposition of [EMIm][NTf2] were studied by various spectroscopic and gravimetric methods. For the first time it was possible to quantify fluorine-containing products via mass spectrometry coupled directly with thermogravimetric (TG) measurements. While chemical and spectroscopic analysis of thermally stressed ILs revealed no hints concerning changes of composition after quenching hot steel for several times, the mass-spectrometer (MS) coupled TG analysis gives information by comparing the decomposition behaviour of fresh and used ILs. A number of fragments were detected in low amounts confirming the proposed decomposition mechanism.