Search the repository

A+ | A- | | SLO | ENG

Abstract: We present the results of our analysis of Gaia19dke, an extraordinary microlensing event in the Cygnus constellation that the Gaia satellite discovered. This event featured a strong microlensing parallax effect, resulting in multiple light curve peaks. We conducted extensive photometric, spectroscopic, and high-resolution imaging follow-up observations to determine the mass and nature of the invisible lensing object. Using the Milky Way priors on the density and proper motion of lenses, we found that the lens is likely to be located at a distance of DL=3.3(+2.1, -1.7) kpc, and has a mass of ML=0.50(+0.82, -0.27) M☉. Based on its low luminosity and mass, we propose that the lens in the Gaia19dke event is either a main sequence star or an isolated white dwarf. Due to its brightness, longevity, and lack of blending, Gaia19dke is a target for which the forthcoming Gaia Data Release 4 data will help to constrain the parameters of the lens.
Keywords: gravitational lensing, micro stars, fundamental parameters, white dwarfs, astrophysics, solar astrophysics, stellar astrophysics, astrophysics of galaxies, instrumentation for astrophysics, methods for astrophysics
Published in RUNG: 13.01.2025; Views: 179; Downloads: 1
Link to file
This document has many files! More...

Abstract: In recent decades, there has been a growing interest in producing biofuels and biochemicals from renewable sources. Furfural stands as one of the ligno(hemi)cellulosic biomass derived platform chemical, which can be transformed into numerous value-added products. The goal of this PhD was to systematically study hydrotreatment reactions of furfural under varying operating conditions and to gain insights into the reaction mechanism and kinetics. An extensive experimental and computational study of hydrogenation, hydrodeoxygenation, oligomerisation and etherification of furfural in a three-phase batch reactor was performed. The goals were divided into three consecutive objectives. In the first part, hydrotreatment of furfural over Pd/C catalyst under various reaction conditions, including the solvent selection (solventless conditions, tetrahydrofuran, isopropanol), atmosphere (nitrogen, hydrogen), temperature (100–200 °C), pressure (25–75 bar) and stirring speed, was studied. A reaction pathway network and a micro-kinetic model were developed, incorporating thermodynamics (hydrogen solubility), mass transfer, adsorption, desorption, and surface reactions. These phenomena and their contribution to the surface coverages, TOF’s and global reaction rates were studied. The hydrogen presence on the catalyst surface was found to influence the main reaction pathway, leading to ring, aldehyde group or full hydrogenation. In the second part, various monometallic catalysts (Pd/C, Pt/C, Re/C, Ru/C, Rh/C, Ni/C, Cu/C) were tested at 100 -200 °C with 60 bar of hydrogen and tetrahydrofuran as solvent. A generalized reaction pathway network was developed. H2 temperature-programmed reduction (H2-TPR) and CO temperature-programmed desorption (CO-TPD) were conducted, and a regression analysis of the results was subsequently performed by numerical modelling and optimisation. The obtained adsorption and desorption kinetic parameters for active metallic sites were further used in a generalized micro-kinetic model, applicable to all tested catalysts. Pd/C exhibited high activity and non-selective hydrogenation of furfural, while other catalysts showed selective aldehyde group hydrogenation followed by deoxygenation, consistent with density functional theory (DFT) calculations. Ru/C uniquely produced 2 methyltetrahydrofuran and ring-opening products at 200 °C. In silico optimization of reaction conditions for promising catalysts ((Pd/C, Pt/C, Re/C, Ni/C) aimed to maximize the yield of the target product. In the third part, the influence of support on catalytic activity was studied. Hydrotreatment of furfural over Pd/Al2O3, Pd/SiO2, Ru/Al2O3, Ru/SiO2, Ni/Al2O3, and Ni/SiO2 was performed between 150 - 200 °C, using 60 bar of hydrogen and tetrahydrofuran as solvent. The strength and rate of adsorption and desorption to/from acidic, metallic and interface site structures were determined, using H2-TPR, CO-TPD and NH3-TPD and subsequent regression analysis of the results by numerical modelling and optimisation. The resulting parameters were sequentially used in the generalized micro-kinetic model to quantify the contribution of the active metal (Ni, Pd, or Ru), support (Al2O3 or SiO2), interphase sites and their relationship on catalyst activity and selectivity. Evaluation of morphological and structural characteristics, adsorption/desorption and intrinsic reaction kinetics has indicated that the coverage of acidic sites (on alumina or silica) facilitated yielding ring hydrogenation and inhibited deoxygenation, decarbonylation and cyclic compound opening. The rates for aromatics or aldehyde functional groups were, nonetheless, affected in a different order. The used and developed methods and findings of this PhD offer useful guidelines for transforming furfural into high-value chemicals through catalytic hydrotreatment, with significant implications for future research and industrial applications.
Keywords: lignocellulosic biomass, furfural, catalytic hydrogenation, micro-kinetic mass transfer model, reaction kinetics, first-principle methods, furfuryl alcohol, tetrahydrofurfuryl alcoholv, dissertations
Published in RUNG: 08.11.2024; Views: 496; Downloads: 8
Full text (9,22 MB)

Keywords: numerical methods | black hole physics | hydrodynamics | relativistic processes
Published in RUNG: 22.02.2024; Views: 1546; Downloads: 10
Full text (17,18 MB)
This document has many files! More...

Keywords: numerical methods, black hole physics, hydrodynamics, relativistic processes
Published in RUNG: 02.10.2023; Views: 1887; Downloads: 5
Link to file
This document has many files! More...

Keywords: Optimization, Network/graphs, Linear programming, Decomposition methods
Published in RUNG: 14.02.2023; Views: 1758; Downloads: 0
This document has many files! More...