Publikationsserver der RWTH Aachen University
Entwicklung einer neuen Methode zur impedanzspektroskopischen Erfassung der temperaturprogrammierten Desorption von Ammoniak in H-Form Zeolithen
Abstract
dc:descriptionThis PhD thesis deals with the development of a measuring system, that allows determining, in analogy to the well-known temperature-programmed desorption measurement of NH3 (NH3-TPD), the acidic properties of zeolites (H-ZSM-5 with SiO2/Al2O3 ratios from 30 to 1000). This new method, denoted IS-TPD, is based on a combination of NH3-TPD with impedance spectroscopy measurements (IS). The IS-TPD new method, with a zeolite sensor, is used for the NH3 detection instead of the usual one based on gas chromatography, whose functionality is based on the impedance spectroscopic measurement of the proton conductivity. With these developments it was possible to refinement the known mechanistic picture to the adsorption behaviour of NH3 in H-form-zeolites, which finally led to a new understanding of the functional principle of NH3-Sensors based on H-form-zeolites. Both impedance and NH3-TPD measurements were accomplished to provide a basis for the methodical development of the IS-TPD. The results of these measurements as well as the detailed picture of a model for the ammonia supported proton transport, which were developed in the so far completely independently regarded methods, provided a consistent picture of the supported proton transport mechanism in zeolites. Hence, the transition of a Grotthus-like, vehicle and hopping mechanisms, respectively, depending on the loading of the sample with NH3, was derived from the impedance measurements. These are compatible with the interpretation of the NH3-TPD profiles. This again forms the basis for the following combination of the two methods for the development of a new IS-TPD measuring system. Therein, the zeolite samples were coated in form of a thick-film on a directly heated inter-digital capacitor (idc) chip, with which the sample quantity was reduced of approximately 80 mg by the NH3-TPD to 0,5 mg. The outcome of this was a clearly smaller time of contact with same flow rate. This was manifested in a shift of the desorption maxima around 60°C (for the low temperature (LT) peak) and around 90°C (for the high temperature (HT) peak) and is to be explained by decreased readsorption. Furthermore it was found that the quantity of desorbed NH3 became smaller for each Brønsted acid site with decreasingly SiO2/Al2O3 ratios. This disagreed in first glance to the assumption that the samples were saturated with NH3. Hence, a new model picture for the functionality of the zeolite sensors was derived, which included the fact that the saturation measured by means of IS corresponds presumanly just to a saturation within surface-near range, which in this work was denoted “electrical saturation”. Accordingly, the samples will already achieve a constant impedance value in ammonia, if the conductivity within near-surface by far exceeds the volume saturation. In this case a further volume saturation obviously does not lead to a further increase of the conductivity. For model verification in the new IS-TPD measuring system the zeolites H-Y and H-beta were investigated, which showed similar behaviour than H-ZSM-5. Thus the work represents a new experimental basis for the characterisation of Brønsted acid sites in solids.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2007
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
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- Rodríguez-González, Luz
- Contributors dc:contributor
-
- Simon, Ulrich
Subjects
dc:subject × 11Rights
dc:rights- Statement dc:rights
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- info:eu-repo/semantics/openAccess
- Language dc:language
- ger