| 1. |
- Sauer, Christopher, 1993
(författare)
-
Green Aromatics: Catalytic Valorisation of bio-derived 2,5-dimethylfuran over Zeolites and Zeotypes
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis discusses the use of biomass as a potentially green feedstock for the chemical industry in the urgent shift away from fossil resources. I elaborate on reasons why we cannot afford to burn virgin biomass for energy production, among them a variety of ecosystem services that forests and other lands provide. In addition, the utilisation of biomass should be focused on products that sequester and lock away carbon for more extended periods, e.g. timber, materials and chemicals. In particular, biomass can be used as an alternative "carbon neutral" feedstock for the chemical industry, where we can preserve the already existing chemical complexity in the bio-based molecules. One example is the upgrading of furans to benzene, toluene and xylene (BTX) aromatics with the help of zeolite catalysis. These aromatics are important commodity chemicals, where the shift to a bio-based resource could make use of already existing knowledge, catalyst and production infrastructure. However, research is necessary to understand these new feedstock molecules and their interaction with the catalysts and to enable the design of applicable catalysts. In order to study the interaction of the furans, in particular 2,5-dimethylfuran (2,5-dmf), I describe and discuss the development of an analytical methodology that utilises infrared spectroscopy and mass spectrometry for the on-line identification and quantification of product molecules during catalytic reactions. This on-line analysis method is then applied to the catalytic conversion of 2,5-dmf to aromatics over a range of zeolite and zeotype catalysts. In-depth studies with ammonia as a probe molecule of the catalytic active acid sites, as well as temperature programmed experiments with ammonia and 2,5-dmf give insights into product distribution, selectivity changes and deactivation of the catalyst. For example, olefins and aromatics are initially preferred products, while with increasing time on stream, the isomerisation of 2,5-dmf becomes dominant. The incorporation of Ga into the zeotype framework, resulting in a Ga-Silicate, shows how targeted catalyst design can increase overall aromatics production. This catalyst is also suitable for selective isomerisation of 2,5-dmf to 2,4-dimethylfuran, which has a rare substitution pattern. Finally, itwas found that the most valuable of BTX, p -xylene, can be produced more selectively when 2,5-dmf is pre-adsorbed onto zeolite ZSM-5 and then released during a temperature programmed product desorption.
|
|
| 2. |
- Halldin Stenlid, Joakim, 1987-
(författare)
-
Computational Studies of Chemical Interactions: Molecules, Surfaces and Copper Corrosion
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The chemical bond – a corner stone in science and a prerequisite for life – is the focus of this thesis. Fundamental and applied aspects of chemical bonding are covered including the development of new computational methods for the characterization and rationalization of chemical interactions. The thesis also covers the study of corrosion of copper-based materials. The latter is motivated by the proposed use of copper as encapsulating material for spent nuclear fuel in Sweden.In close collaboration with experimental groups, state-of-the-art computational methods were employed for the study of chemistry at the atomic scale. First, oxidation of nanoparticulate copper was examined in anoxic aqueous media in order to better understand the copper-water thermodynamics in relation to the corrosion of copper material under oxygen free conditions. With a similar ambition, the water-cuprite interface was investigated with regards to its chemical composition and reactivity. This was compared to the behavior of methanol and hydrogen sulfide at the cuprite surface.An overall ambition during the development of computational methods for the analysis of chemical bonding was to bridge the gap between molecular and materials chemistry. Theory and results are thus presented and applied in both a molecular and a solid-state framework. A new property, the local electron attachment energy, for the characterization of a compound’s local electrophilicity was introduced. Together with the surface electrostatic potential, the new property predicts and rationalizes regioselectivity and trends of molecular reactions, and interactions on metal and oxide nanoparticles and extended surfaces.Detailed atomistic understanding of chemical processes is a prerequisite for the efficient development of chemistry. We therefore envisage that the results of this thesis will find widespread use in areas such as heterogeneous catalysis, drug discovery, and nanotechnology.
|
|
| 3. |
- Liu, Yuan
(författare)
-
Theoretical Studies of Natural Gas Hydrates and H-bonded Clusters and Crystals
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis H-bonded systems (natural gas hydrates, water clusters, and crystal ice) are studied by density functional theory (DFT) computations.Natural gas hydrates (NGHs) play an important role in energy and environmental fields: NGHs are considered as a promising backup energy resource in the near-future due to their tremendous carbon content; improper exploration of NGHs could induce geological disasters and aggravate the greenhouse effect. In addition, many technologies based on gas hydrates are being applied and developed. The thermodynamic stabilities of various water cavities in different clathrate crystalline phases occupied by hydrocarbon gas molecules are studied by dispersion-corrected hybrid functionals. The Raman spectra of C-C and C-H stretching vibrations of hydrocarbon molecules in various water cavities in the solid state are derived. The trends of C-H stretching vibrational frequencies are found to follow the prediction by the “loose cage ─| tight cage” model. In addition, the trends and origins of 13C NMR chemical shifts of hydrocarbon molecules in various NGHs are presented. These theoretical results will enlarge the database of C-C and C-H stretching vibrational frequencies and 13C NMR parameters of hydrocarbon molecules in NGHs, and provide valuable information to help identify the types of clathrate phases and varieties of guest molecules included in NGHs samples taken from natural sites.The behavior of water clusters may help to understand the properties of its liquid and solid states. The thermodynamic stabilities and IR spectra of a small-, medium-, and large-sized water cluster are studied in this work. After full optimization of (H2O)20,54,100 using the hybrid functional B3LYP, the electronic energies, zero-point energies, internal energies, enthalpies, entropies, and Gibbs free energies of the water clusters are computed. The OH stretching vibrational IR spectra of (H2O)20,54,100 are also presented and split into sub-spectra for different H-bond types based on the specific contributions from each group. It is found that the OH stretching vibrational frequencies of water are sensitive to the conformations of the H-bonds and the vibrations of the H-bonds belonging to different types are located in separated regions in the IR spectra. Thus, the spectroscopic fingerprints will reflect the H-bond topology of the water molecules in a water cluster.Ice XI has been suggested to be involved in the process of planetary formation as a considerable electric field might be formed from the ferroelectric ice XI in space. IR and Raman spectroscopic technology can be directly used to identify the occurrence of ferroelectric ice XI in laboratory or extraterrestrial settings. Due to the difficulty for DFT to describe non-covalent systems, the performance of 16 different DFT methods applied on the ice Ih, VIII, IX, and XI crystal phases are assessed. Based on the computational accuracy and cost, the IR and Raman spectra of ice Ih and XI are derived and compared. The librational vibrations are found to be the identifier which can be used to distinguish ice Ih and ice XI in the universe. In addition, the existence only one kind of H-bond in ice Ih is demonstrated from the overlapping sub-spectra for different types of H-bonded pair configurations in 16 isomers of ice Ih.The region of water under negative pressure is an exotic land in lack of exploitation. Guest free clathrate hydrate (clathrate ice) of sII type has been recently confirmed experimentally at negative pressure. Does any other clathrate ice phase exist at negative pressure region? Since clathrate hydrate are isostructural with silica clathrate minerals and semiconductor clathrates, and crystal structure prediction by analogy with known structures and first-principles computations is an effective way to find new crystalline phases of solid materials, we are motived to look for new clathrate ice phases from silica or semiconductor clathrate materials based on first-principles computations. Borrowing the idea new clathrate frameworks of ZnO and SiC can be constructed by connecting their bubble clusters in different ways, new clathrate ice phases (sL, sL_I, sL_II, and sL_III) are generated by connecting the water bubble clusters according to different rules. Using the non-local dispersion-corrected vdW-DF2 functional, clathrate ice sL with ultralow density (0.6 g/cm3) is predicted by first-principles phase diagram computations to be stable under larger negative pressures than the sII phase. The phase diagram of water is thus extended into the lower negative pressure region.
|
|
| 4. |
- van den Bossche, Maxime, 1989
(författare)
-
Methane oxidation over palladium oxide. From electronic structure to catalytic conversion
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding how catalysts work down to the atomic level can provide ways to improve chemical processes on which our contemporary economy is heavily reliant. The oxidation of methane is one such example, which is important from an environmental point of view. Methane is a potent greenhouse gas and natural and biogas vehicles need efficient catalysts to prevent slip of uncombusted fuel into the exhaust. Commercial catalysts for methane oxidation are often based on palladium or platinum. Metallic palladium, however, is easily converted to palladium oxide when the engine is operated at oxygen rich conditions. In this thesis, various aspects of complete methane oxidation over PdO(101) are investigated with computational methods based on density functional theory (DFT). PdO(101) is the active surface for methane oxidation, and firstly, the reaction intermediates CO and H are studied in detail. Possible pathways for H2 adsorption, dissociation and eventual water formation are investigated, in connection to core-level spectroscopy experiments. Similarly, the adsorption configurations for carbon monoxide on clean and oxidized palladium are examined with a combination of DFT calculations, core-level and infrared spectroscopy. Secondly, a detailed kinetic model is constructed that describes the catalytic conversion of CH4 to CO2 and H2O over PdO(101). This is done in a first-principles microkinetics framework, where the kinetic parameters are obtained by applying density functional and transition state theory. The kinetic model provides a fundamental understanding of findings from reactor experiments, such as the rate limiting steps and poisoning behaviour, and shows qualitatively different behaviour of adsorbates on oxide as compared to metal surfaces. Lastly, limitations of the commonly used class of generalized gradient functionals are illustrated in the computation of several properties of adsorbates on metal oxide surfaces. These include core-level shifts and thermodynamic and reactive properties of adsorbates on the PdO(101) surface. Similarly, the description of several molecular and cooperative adsorption processes are also found to be sensitive to the applied exchange-correlation functional on the BaO(100), TiO2(110) and CeO2(111) surfaces.
|
|
| 5. |
- Feng, Yingxin, 1994
(författare)
-
Reaction kinetics of NH3-SCR over Cu-CHA from first principles
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ammonia-assisted selective catalytic reduction (NH3-SCR) is an advanced technology to reduce nitrogen oxide (NOx) emissions from lean-burn engines. NH3 is added to the exhaust gas and reacts selectively with NOx, forming N2 and H2O. Small-pore Cu-exchanged chabazite (Cu-CHA) is a widely used zeolite-based catalyst for NH3-SCR thanks to its high activity, high selectivity, and durability. However, one issue is the formation of small amounts of nitrous oxide (N2O), which is a strong greenhouse gas. Atomic level understanding is valuable to improve the performance of NH3-SCR catalyst and meet the increasingly stringent emission standards. The state of Cu-CHA during NH3-SCR reaction depends sensitively on the reaction temperature. At low temperatures, Cu-ions are solvated by NH3, forming Cu[(NH3)2]+ complexes, while framework-bound Cu dominates at high temperatures. Experimentally, a non-monotonic behavior in NO conversion is observed with increasing temperature, which reflects a change in the active site. In this thesis, NH3-SCR over Cu-CHA is investigated using density functional theory (DFT) calculations and kinetic simulations. In the low-temperature range (<250℃), a pair of Cu[(NH3)2]+ is required for O2 adsorption to form a Cu-peroxo complex, which is a key Cu-intermediate. The reaction intermediates HONO and H2NNO can be converted to N2 and H2O over Brønsted acid sites. H2NNO is found to be the origin of N2O formation when decomposing over Cu-peroxo species. At high-temperature (>350℃), the proposed reaction mechanism suggests that the reaction proceeds over single framework Cu sites and starts from the co-adsorption of O2 and NO. The main source of N2O formation at high temperatures is suggested to be ammonium nitrate decomposition. By combining the reaction mechanisms for high and low temperatures, the non-monotonic temperature-dependent reaction activity is successfully reproduced. The present work deepens the understanding of the reaction mechanism of NH3-SCR on Cu-CHA in a wide temperature range and provides theoretical support for further improvement of the catalyst performance.
|
|
| 6. |
- Fretz, Samuel Joseph, 1987
(författare)
-
Adding Utility to Carbon Materials: Introducing Dopants Using Highly Soluble Metal Salts and Functionalizing Surfaces via Bromomethylation
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon-based materials have received intense research interest over the past few decades due to their unique combination of properties including porosity, non-toxicity, chemical inertness, low density, and electrical conductivity, which has allowed them to find a wide array of applications including supercapacitors, batteries, CO2 capture, fuel cells, and catalysis. To expand their utility, a variety of techniques have been developed to enhance their reactivity and functionality. One such method is doping, wherein heteroatoms (i.e. non-carbon elements) are purposefully incorporated into the carbon structure with the goal of introducing new reactivity to the material. The first paper in this thesis focuses on using soluble Fe salts as dopants for iron/nitrogen-doped ordered mesoporous carbons (Fe-OMC). The anion was found to have a strong effect on the structure, Fe loading, and oxygen reduction reaction (ORR) activity of the Fe-OMC. High Fe loadings of above 3 wt% were obtained for one of the soluble salts, but their activity in polymer electrolyte membrane fuel cells (PEMFCs) did not increase appreciably compared to the standard chloride salt. Electron paramagnetic resonance (EPR) was used to gain insight into the structure and ORR activity of the various Fe species within each Fe-OMC. Another method for increasing the utility of carbon materials is grafting or surface functionalization, which consists of covalently attaching small, organic molecules to the carbon surface. In three papers of this thesis, we report a novel two-step method for the surface functionalization of high surface area carbon materials. The carbons are first subjected to the bromomethylation reaction then, in the second step, many nucleophiles can substitute bromide resulting in monolayer-functionalized surfaces that can be tailored for a specific application. Example nucleophiles include azide, amines, iodide, sulfite, and amide enolates. Several carbon materials efficiently and reproducibly undergo these reactions and the surface-bound groups are stable for months under ambient conditions. This two-step scheme has numerous advantages over other surface modification techniques for carbon including use of solution-phase reagents, minimal harm to the carbon framework, monolayer functionalization, and no carbon pretreatment steps. A total of 12 surface groups were synthesized, which demonstrates the synthetic flexibility of this two-step technique. Four of the twelve modified carbons were used as cathodes in lithium-sulfur (Li-S) batteries. When used with an electrolyte containing lithium nitrate (LiNO3), the functionalized cathodes show increased capacities by virtue of utilizing more S. When used with electrolytes lacking LiNO3, the surface groups attenuate the lithium polysulfide (LiPS) shuttle as measured by the much higher initial Coulombic efficiencies (ICEs) recorded for the functionalized cathodes relative to the unfunctionalized control. The observations with both electrolytes evidence strong interactions between the electroactive S and the surface groups. The higher binding energies (BEs) computed by density functional theory (DFT) support strong interactions between the surface groups and various sulfur species while cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) lend evidence for a significantly reduced LiPS shuttle on the functionalized carbon surfaces. Based on these results with Li-S batteries, we hope that this two-step method of introducing organic groups to carbon surfaces will find wide-spread use in many applications.
|
|
| 7. |
- Karlsson, Rasmus, 1987-
(författare)
-
Theoretical and Experimental Studies of Electrode and Electrolyte Processes in Industrial Electrosynthesis
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Heterogeneous electrocatalysis is the usage of solid materials to decrease the amount of energy needed to produce chemicals using electricity. It is of core importance for modern life, as it enables production of chemicals, such as chlorine gas and sodium chlorate, needed for e.g. materials and pharmaceuticals production. Furthermore, as the need to make a transition to usage of renewable energy sources is growing, the importance for electrocatalysis used for electrolytic production of clean fuels, such as hydrogen, is rising. In this thesis, work aimed at understanding and improving electrocatalysts used for these purposes is presented.A main part of the work has been focused on the selectivity between chlorine gas, or sodium chlorate formation, and parasitic oxygen evolution. An activation of anode surface Ti cations by nearby Ru cations is suggested as a reason for the high chlorine selectivity of the “dimensionally stable anode” (DSA), the standard anode used in industrial chlorine and sodium chlorate production. Furthermore, theoretical methods have been used to screen for dopants that can be used to improve the activity and selectivity of DSA, and several promising candidates have been found. Moreover, the connection between the rate of chlorate formation and the rate of parasitic oxygen evolution, as well as the possible catalytic effects of electrolyte contaminants on parasitic oxygen evolution in the chlorate process, have been studied experimentally.Additionally, the properties of a Co-doped DSA have been studied, and it is found that the doping makes the electrode more active for hydrogen evolution. Finally, the hydrogen evolution reaction on both RuO2 and the noble-metal-free electrocatalyst material MoS2 has been studied using a combination of experimental and theoretically calculated X-ray photoelectron chemical shifts. In this way, insight into structural changes accompanying hydrogen evolution on these materials is obtained.
|
|
| 8. |
- Maurina Morais, Eduardo, 1989
(författare)
-
Exploring new protic ionic liquids: From synthesis to fundamental properties
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ionic liquid community frequently leverages the selling point that more than a million new ionic liquids could conceivably be created. Nonetheless, the number of commercially available compounds is orders of magnitude lower. This highlights the fact that only a small number of all possible ionic liquids are actively being researched, a reality particularly noticeable in the niche field of protic ionic liquids. In such a scenario, research focusing on the development of even a small number of viable alternatives to the popular alkylammonium- and imidazolium-based cations could potentially have a big impact, by paving the way for the synthesis of new families of ionic liquids. However, for these new alternatives to be widely used by the community, they must be easy to synthesize and have desirable properties. In this thesis, I discuss the challenges that I have encountered and the lessons that I have learned while trying to explore the chemical space of protic ionic liquids. This exploration started with the development of a procedure for the synthesis of pure and dry protic ionic liquids, which was used to make new triazolium-based protic ionic liquids. Additionally, this first work highlights the importance of using air-free techniques to analyze these hygroscopic compounds. Later, these insights were used to develop a new setup for the determination of ionic conductivity in ionic liquids. The latter was used in conjunction with pulsed-field gradient nuclear magnetic resonance diffusion experiments and density functional theory experiments to understand the differences in transport properties between triazolium- and imidazolium-based protic ionic liquids. Finally, we once again turned our attention to the imidazolium cation and explored how simple modifications to its electronic structure, by means of functionalization with electron-withdrawing groups, can enhance its acidity, and how that affects the properties of these nitro- and cyano-functionalized protic ionic liquids. This thesis aims to highlight the importance of developing new methods for the synthesis and analysis of protic ionic liquids, as well as to explore how computational modeling can be used to rationalize the observed differences in the physicochemical properties of these compounds.
|
|
| 9. |
- White, Jai, 1991-
(författare)
-
From Facets to Flow: The Electrooxidation of Glycerol on Pd-based catalysts
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Glycerol, a by-product of biodiesel refineries, has uses in industries such as cosmetics, food, and pharmaceuticals. However, its usage is small compared to the amount of glycerol produced from biodiesel production. Therefore, there is an opportunity to use glycerol, an important platform chemical, as a cheap feedstock for the synthesis of valuable chemicals. These chemicals can be formed in aqueous media through the glycerol electrooxidation reaction (GEOR) on the anode with hydrogen gas concurrently generated on the cathode. This thesis focuses on the GEOR in alkaline media on Pd and PdNi catalysts. The works compiled here evaluate the GEOR using electrochemical methods such as cyclic voltammetry, galvanostatic polarisation curves, chronoamperometry and chronopotentiometry. Pd and PdNi catalysts were fabricated through chemical synthesis, and electrodeposition onto Ni substrates. Singularly oriented Pd crystal facets were studied, showing those approximating Pd (111) as the most active. Similarly faceted bimetallic PdNi nanoparticles proved significantly more active than pure Pd. Effects of mass transport, studied for Pd/NiRDE and PdNi/NiRDE, indicated performance effects linked to diffusion and underutilisation of thicker catalyst layers. In aerated solutions, industrially relevant current densities were achieved on PdNi/Nifoam in concentrated electrolytes at elevated temperatures for extended periods. The analysis of glycerol oxidation products, formed during steady state measurements, was done using high performance liquid chromatography. The two major products were consistently shown to be glycerate and lactate. This work, covering many aspects of the GEOR, shows that Pd-based catalysts have potential for future industrial application.
|
|
| 10. |
- Asfaw, Habtom Desta
(författare)
-
Multifunctional Carbon Foams by Emulsion Templating : Synthesis, Microstructure, and 3D Li-ion Microbatteries
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon foams are among the existing electrode designs proposed for use in 3D Li-ion microbatteries. For such electrodes to find applications in practical microbatteries, however, their void sizes, specific surface areas and pore volumes need be optimized. This thesis concerns the synthesis of highly porous carbon foams and their multifunctional applications in 3D microbatteries. The carbon foams are derived from polymers that are obtained by polymerizing high internal phase water-in-oil emulsions (HIPEs).In general, the carbonization of the sulfonated polymers yielded hierarchically porous structures with void sizes ranging from 2 to 35 µm and a BET specific surface area as high as 630 m2 g-1. Thermogravimetric and spectroscopic evidence indicated that the sulfonic acid groups, introduced during sulfonation, transformed above 250 oC to thioether (-C-S-) crosslinks which were responsible for the thermal stability and charring tendency of the polymer precursors. Depending on the preparation of the HIPEs, the specific surface areas and void-size distributions were observed to vary considerably. In addition, the pyrolysis temperature could also affect the microstructures, the degree of graphitization, and the surface chemistry of the carbon foams.Various potential applications were explored for the bespoke carbon foams. First, their use as freestanding active materials in 3D microbatteries was studied. The carbon foams obtained at 700 to 1500 oC suffered from significant irreversible capacity loss during the initial discharge. In an effort to alleviate this drawback, the pyrolysis temperature was raised to 2200 oC. The resulting carbon foams were observed to deliver high, stable areal capacities over several cycles. Secondly, the possibility of using these structures as 3D current collectors for various active materials was investigated in-depth. As a proof-of-concept demonstration, positive active materials like polyaniline and LiFePO4 were deposited on the 3D architectures by means of electrodeposition and sol-gel approach, respectively. In both cases, the composite electrodes exhibited reasonably high cyclability and rate performance at different current densities. The syntheses of niobium and molybdenum oxides and their potential application as electrodes in microbatteries were also studied. In such applications, the carbon foams served dual purposes as 3D scaffolds and as reducing reactants in the carbothermal reduction process. Finally, a facile method of coating carbon substrates with oxide nanosheets was developed. The approach involved the exfoliation of crystalline VO2 to prepare dispersions of hydrated V2O5, which were subsequently cast onto CNT paper to form oxide films of different thicknesses.
|
|
