Fischer, Michael
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Item-typ:Veröffentlichung, A computational study of the interaction of C2 hydrocarbons with CuBTCThe adsorptions of ethane, ethene, and ethyne over the coordinatively unsaturated sites (CUS) of copper (II) benzene-1,3,5-tricarboxylate (CuBTC) were studied by means of density functional theory (DFT), using both cluster or periodic models. Exchange-correlation functionals from different rungs of the Jacobs ladder of the DFT were used and energies were corrected for the basis superposition error either through extrapolation to the complete basis set limit or upon the consideration of the Counterpoise method. From the calculated data, it was found that the adsorbate to CUS distances decrease in the order ethane > ethene ≈ ethyne and that the strength of adsorption increase in the order ethane to ethyne to ethene. The energies of interactions of ethene and ethyne with the CUS of CuBTC are approximately the double of that calculated for ethane. The calculated adsorption energies and geometries are in very satisfactory agreement with the available experimental results. The results of topological analyses confirm that the unsaturated bonds of ethene and ethyne form open shell like bonds with the CUS while interaction with ethane have predominant closed shell character.Wissenschaftlicher ArtikelBand:17325 63 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Fluoride Anions in All-Silica Zeolites: Studying Preferred Fluoride Sites and Dynamic Disorder with Density Functional Theory CalculationsIn all-silica zeolites synthesized via the “fluoride route”, the fluoride anions are typically incorporated in small cages, forming [SiO4F]− trigonal bipyramids. While diffraction and NMR experiments can elucidate the fluoride location(s) and the occurrence/absence of dynamic disorder, they provide limited insights into the factors that determine equilibrium position and dynamic behavior. To develop a more thorough understanding, electronic structure calculations in the framework of dispersion-corrected density functional theory (DFT) were performed for five all-silica zeolites (NON, STF, IFR, STT, and CHA frameworks). DFT-based predictions of the energetically preferred fluoride location within a given cage were mostly in excellent agreement with experiment. Apart from the known tendency of fluoride anions to be located close to small rings, there are no easily generalizable crystal-chemical rules to predict the most probable fluoride sites. DFT-based molecular dynamics calculations were employed to predict and explain the dynamic behavior of the fluoride anions, which differs markedly among the different frameworks. The simulations showed that the distances from the fluoride anion to the next-nearest neighboring Si atoms are key to determining whether dynamic disorder can occur or not. Although longer-range interactions with the organic structure-directing agents tend to play a less decisive role, they can lead to a suppression of dynamic disorder in some cases. In addition to providing detailed understanding of the behavior of fluoride anions in as-synthesized all-silica zeolites, the findings of the present work could contribute to a further elucidation of their structure-directing role during zeolite synthesis.Wissenschaftlicher ArtikelBand:125Heft:1634 29 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Sea ice and the air-sea exchange of CO2(2013-02-12); ; ; The objective of this thesis was to improve the state of knowledge concerning the inorganic carbon dynamics, including the precipitation of CaCO3 in sea ice and its significance for the polar carbon cycle as well as to obtain improved estimates of the diurnal and long-term CO2 flux on small scales during the winter-spring-summer transition. To study the exchange of CO2 between sea ice and the atmosphere as well as the biogeochemical parameters and the spatial and temporal distribution of ikaite in sea ice during different seasons, three field studies were conducted in the Arctic and Antarctic. High pCO2 values in sea ice of up to 3900 μatm were observed. The resulting fluxes ranged between -3.41 and 9.5 mmol m-2 d-1. Ikaite was found mostly in the uppermost layers of sea ice. Based on the observations the efflux from Arctic sea ice to the atmosphere can be expected to result in a flux of several Tg C a-1. The findings in this thesis support the concept of the sea ice driven carbon pump.Dissertation430 179 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Crystal chemical characterization of mullite-type aluminum borate compoundsAl-rich aluminum borates were prepared by different synthesis routes using various Al/B ratios, characterized by diffraction methods, spectroscopy and prompt gamma activation analysis. The 11B NMR data show a small amount of BO4 species in all samples. The chemical analysis indicates a trend in the Al/B ratio instead of a fixed composition. Both methods indicate a solid solution Al5−xB1+xO9 where Al is substituted by B in the range of 1–3%. The structure of B-rich Al4B2O9 (C2/m, a=1488 pm, b=553 pm, c=1502 pm, ß=90.6°), was re-investigated by electron diffraction methods, showing that structural details vary within a crystallite. In most of the domains the atoms are orderly distributed, showing no signal for the postulated channel oxygen atom O5. The absence of O5 is supported by density functional theory calculations. Other domains show a probable disordered configuration of O5 and O10, indicated by diffuse scattering along the b direction.Wissenschaftlicher Artikel303 733 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Water adsorption in SAPO-34: elucidating the role of local heterogeneities and defects using dispersion-corrected DFT calculationsThe chabazite-type silicoaluminophosphate SAPO-34 is a promising adsorbent for applications in thermal energy storage using water adsorption–desorption cycles. In order to develop a microscopic understanding of the impact of local heterogeneities and defects on the water adsorption properties{,} the interaction of different models of SAPO-34 with water was studied using dispersion-corrected density-functional theory (DFT-D) calculations. In addition to SAPO-34 with isolated silicon atoms{,} the calculations considered models incorporating two types of heterogeneities (silicon islands{,} aluminosilicate domains){,} and two defect-containing (partially and fully desilicated) systems. DFT-D optimisations were performed for systems with small amounts of adsorbed water{,} in which all H2O molecules can interact with framework protons{,} and systems with large amounts of adsorbed water (30 H2O molecules per unit cell). At low loadings{,} the host–guest interaction energy calculated for SAPO-34 with isolated Si atoms amounts to approximately −90 kJ mol−1. While the presence of local heterogeneities leads to the creation of some adsorption sites that are energetically slightly more favourable{,} the interaction strength is drastically reduced in systems with defects. At high water loadings{,} energies in the range of −70 kJ mol−1 are obtained for all models. The DFT-D interaction energies are in good agreement with experimentally measured heats of water adsorption. A detailed analysis of the equilibrium structures was used to gain insights into the binding modes at low coverages{,} and to assess the extent of framework deprotonation and changes in the coordination environment of aluminium atoms at high water loadings.Wissenschaftlicher ArtikelBand:17Heft:38108 149 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Benchmarking DFT-GGA calculations for the structure optimisation of neutral-framework zeotypesStructure optimisations in the framework of plane-wave density functional theory (DFT) were performed for a set of reference structures of neutral-framework zeotypes and related compounds. The reference set comprised eight all-silica zeolites, four aluminophosphate zeotypes, and two dense polymorphs of SiO2 (α-quartz) and AlPO4 (α-berlinite). The optimisations considered a total of five GGA-type exchange–correlation functionals (GGA = generalised gradient approximation). Along with the very popular PBE functional, which is well-known to overestimate the lattice dimensions, two GGA functionals designed for solids (WC and PBEsol) and two variants of PBE including a pairwise dispersion correction (PBE-D2 and PBE-TS) were included. A detailed analysis of the agreement between DFT-optimised structures and experimental crystal structure data (obtained for calcined systems) showed that the inclusion of a dispersion correction greatly improves the prediction of the lattice parameters, with PBE-TS performing particularly well. On the other hand, WC and PBEsol give T–O bond lengths (T = tetrahedral sites) that are in better agreement with experimental data. The accurate reproduction of the T–O–T angles was found to be particularly challenging, as functionals without dispersion correction tend to overestimate these angles, whereas dispersion-corrected variants underestimate them. For all-silica zeolites, the present results were compared to those of a previous DFT study using the hybrid B3LYP-D2 functional and to results of molecular mechanics calculations employing two popular force fields, with none of these methods performing better than PBE-TS or PBE-D2. In order to better understand some of the shortcomings of the functionals considered, additional results for two outliers that were removed from the set of reference structures were analysed. Finally, the ability to reproduce the relative stability was assessed for those SiO2 frameworks for which experimental enthalpies of transition are available. Here, PBE-D2 outperformed PBE-TS, which showed a systematic tendency to overestimate the energy difference (relative to α-quartz). On the basis of the present work, PBE-TS can be recommended as a reasonable default choice for structure optimisations of neutral-framework zeotypes. While future benchmarking work could address a wider range of functionals and dispersion correction schemes, it needs to be considered that the limited availability of low-temperature crystal structure data limits the accuracy with which the deviations between computation and experiment can be assessed for this group of materials.Wissenschaftlicher ArtikelBand:135114 733 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Accurate structures and energetics of neutral-framework zeotypes from dispersion-corrected DFT calculationsDensity-functional theory (DFT) calculations incorporating a pairwise dispersion correction were employed to optimize the structures of various neutral-framework compounds with zeolite topologies. The calculations used the PBE functional for solids (PBEsol) in combination with two different dispersion correction schemes, the D2 correction devised by Grimme and the TS correction of Tkatchenko and Scheffler. In the first part of the study, a benchmarking of the DFT-optimized structures against experimental crystal structure data was carried out, considering a total of 14 structures (8 all-silica zeolites, 4 aluminophosphate zeotypes, and 2 dense phases). Both PBEsol-D2 and PBEsol-TS showed an excellent performance, improving significantly over the best-performing approach identified in a previous study (PBE-TS). The temperature dependence of lattice parameters and bond lengths was assessed for those zeotypes where the available experimental data permitted such an analysis. In most instances, the agreement between DFT and experiment improved when the experimental data were corrected for the effects of thermal motion and when low-temperature structure data rather than room-temperature structure data were used as a reference. In the second part, a benchmarking against experimental enthalpies of transition (with respect to α-quartz) was carried out for 16 all-silica zeolites. Excellent agreement was obtained with the PBEsol-D2 functional, with the overall error being in the same range as the experimental uncertainty. Altogether, PBEsol-D2 can be recommended as a computationally efficient DFT approach that simultaneously delivers accurate structures and energetics of neutral-framework zeotypes.Wissenschaftlicher ArtikelBand:146Heft:1798 88 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, What Is Being Measured with P-Bearing NMR Probe Molecules Adsorbed on Zeolites?(American Chemical Society, 2021-08-19); 24 38 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Thermal behavior of mullite between 4 K and 1320 KThe evolution of metric parameters of 2:1 and 3:2 mullites have been measured between 4 K and 1320 K using neutron and X‐ray powder diffraction. Negative thermal expansion was observed at low temperature for the a‐cell parameter and consequently for the cell‐volume, which is more pronounced for 2:1 mullite than those for 3:2 mullite. Each parameter is simulated using Grüneisen first‐order approximation for the zero pressure equation of state at 0 K, where the vibrational energy was calculated using microscopic approach. While the b‐ and c‐cell parameters require only one Debye term, a second Debye spectrum with negative Grüneisen parameter was required to fit the a‐cell parameter as well as the cell volume. At 4 K, 300 K and 1320 K the model, respectively, calculates the volume thermal expansion coefficients of 0.09x10−6 K−1, 9x10−6 K−1, and 17.3x10−6 K−1 for 2:1 mullite, and 0.09x10−6 K−1, 8.7x10−6 K−1, and 17.3x10−6 K−1 for 3:2 mullite. Temperature‐dependent Raman spectra and phonon density of states hint for the possible microscopic sources of the cell contraction at low temperature. A simple polynomial approach is presented to calculate the elastic stiffness coefficients of the 3:2 mullite, which are not available from experiments.Wissenschaftlicher Artikel292 546 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Investigating Structure and Dynamics of Zeolites and Zeotypes with Density Functional Theory Calculations(2022-10-04)This document contains the main part of my Habilitation thesis ("Habilitationsschrift"), submitted to the Faculty of Geosciences of the University of Bremen in October 2022. A general introduction covering structures and applications of zeolites is followed by three chapters summarising the key results of a total of 15 research articles that were published between 2015 and 2022. The titles of the individual chapters are: - Benchmarking DFT Calculations against Zeolite Structure Data (and Other Quantities) - Structure and Dynamics of Fluoride-Containing Zeolites and Zeotypes - Interaction of (Silico)Aluminophosphates with Adsorbed Water Molecules The submitted version of the thesis contained an Appendix with all 15 articles, which is not included in this public version for copyright reasons. All articles are available from the publishers' websites, with the current Appendix providing full bibliographic information including URLs.Habilitation209 85