Defended Ph.D. Theses

Tom Vermeyen

• 20/12/2023
• 16.00 uur
• Locatie: Campus Drie Eiken, O.01
• Promotoren: Wouter Herrebout (UAntwerpen) & Patrick Bultinck (UGent)
• Departement Chemie

Abstract

Determination of the so-called Absolute Configuration (AC) of a chiral compound is an important analytical step in research areas such as medicine and agrochemistry.

The AC of a chiral compound can be determined with Vibrational Circular Dichroism (VCD), which measures the difference in absorbance of left and right circularly polarised infrared radiation. VCD spectra exhibit substantial chiral sensitivity and contain an abundance of conformational information.

Unfortunately, there are no general empirical rules capable of linking a VCD spectrum to a specific AC or predicting the influence of the conformations on the VCD spectrum.

Therefore, in each VCD application one has to rely on expensive (DFT) calculations for the conformers of all possible AC's.

In this thesis, the added value of Machine Learning (ML) is explored for the AC determination workflow with VCD.

The presented results demonstrate that ML models are capable of directly extracting the AC from the VCD spectrum after training these models on a set of structurally similar compounds.

Neural Networks can successfully predict the influence of conformations on the spectrum from their geometries, as long as these conformations correspond to the same AC.

Additionally, the potential of linear ML models is tested to determine the composition of terpene mixtures and natural oils.

Wencong Xu

• 11/12/2023
• 10.00 uur
• Locatie: Campus Drie Eiken, S.001
• ​Online Doctoraatsverdediging
• Promotoren: Annemie Bogaerts, Vladimir Galvita & Vera Meynen
• Departement Chemie

Abstract

Plasma is considered one of the promising technologies to solve greenhouse gas problems, as it can activate CO₂ and CH₄ at relatively low temperatures. Among the various types of plasmas, the gliding arc plasmatron (GAP) is promising, as it has a high level of non-equilibrium and high electron density. Nevertheless, the conversion of CO₂ and CH₄ in the GAP reactor is limited. Therefore, combining the GAP reactor with catalysts and making use of the heat produced by the plasma to provide thermal energy to the catalyst, forming a post-plasma catalytic (PPC) system, is hypothesized to improve its performance. Previous studies have been reported on the PPC system, combining catalysts with other types of gliding arc plasmas, such as two-dimensional (2D) gliding arc plasma. However, the improvement in the conversion or selectivity was limited. Adding extra heating with high temperature can be a solution for this, while this will also increase the energy cost. Therefore, in this PhD research, we investigate important aspects of the PPC concept towards the use of the heat produced by GAP plasma to heat the plasma bed, without additional energy input.​ 

Aiming at this, based on a literature study (chapter 1), Ni-loaded layered double hydroxide (LDH) derived catalyst with good thermal catalytic DRM performance were chosen as the catalyst material. Before applying the LDH as a support material, the rehydration property of calcined LDH in moist and liquid environment was studied as part of chapter 2. The data indicated that after high temperatures calcination (600-900 °C), the obtained layered double oxides (LDOs) can rehydrate into LDH, although, the rehydrated LDH were different from the original LDH. In chapter 3, different operating conditions, such as gas flow rate, gas compositions (e.g. CH₄/CO₂ ratio and nitrogen dilution), and addition of H₂O were studied to investigate optimal conditions for PPC DRM, identifying possible differences in temperature profiles and exhaust gas compositions that might influence the catalytic performance. Subsequently, the impact of different PPC configurations, making use of the heat and exhaust gas composition produced by the GAP plasma, is shown in Chapter 4. Experiments studying the impact of adjusting the catalyst bed distance to the post-plasma, the catalyst amount, the influence of external heating (below 250 °C) and the addition of H₂O are discussed. As only limited improvement in the performance was achieved, a new type of catalyst bed was designed and utilized, as described in chapter 5. This improved configuration can realize better heat and mass transfer by directly connecting to the GAP device. The performance was improved and became comparable to the traditional thermal catalytic DRM results obtained at 800 °C, although obtained by a fully electrically driven plasma.

Maruan Bracci

• 24/11/2023
• 09.00 uur
• ​Online Doctoraatsverdediging
• Promotoren:  Sabine Van Doorslaer & Inés García-Rubio
• Dubbeldoctoraat met de Universiteit van Zaragoza
• Departement Chemie

Abstract

This work is focused on the in-depth spectroscopic analysis of horseradish peroxidase (HRP) and chloroperoxidase (CPO), proteins selected for their scientific and industrial relevance, with applications in biotechnology and pharmacology. The enzymatic cycle of both peroxidases goes via an oxidized intermediate state, called Compound I. Although abundantly studied, the exact description of the difference in the electronic structure of Compound I of various heme proteins remains challenging and forms the focus of this work in which this elusive state is being studied with Continuous-Wave and Pulsed Electron Paramagnetic Resonance (EPR), UV-Vis spectroscopy, and stopped-flow spectrophotometry. 

In order to realize this goal, different problems needed to be tackled. First of all, in an attempt to enhance CPO production, an innovative solid substrate method has been explored, promising potential for scalability. Furthermore, the author made crucial contributions to the developmental stages of three different rapid freeze quench apparatus used for trapping intermediate Compound I state from CPO immediately after the initiation of the catalytic cycle. Pulsed EPR techniques, such as HYSCORE and ENDOR, were utilized to investigate the resting state of both enzymes and their Compound I intermediate, providing insights into the electronic structure of their active centres and highlight their differences. To aid in the interpretation of HYSCORE spectra, a user-friendly graphical interface was developed, enabling researchers to manipulate experimental data and conduct simulations, even with minimal coding expertise. Moreover, a theoretical model of the electronic structure of Compound I was effectively utilized for the proteins under investigation in this study, highlighting the powerful interplay between theory and experiment. 

Gilles De Smet

• 26/10/2023
• 17.00 uur
• Locatie: Campus Middelheim, G.010
• Promotoren: Bert Maes & Gwilherm Evano
• Departement Chemie

Abstract

The increasing CO2 levels and associated climate change are drivers to move towards a circular chemical industry. Therefore, there is besides recyclable carbon an urgent need for means to access renewable carbon. Two main strategies have been put forward: (i) carbon capture and utilization (CCU) and (ii) use of biomass as feedstock. One particularly interesting biorenewable resource is lignin, part of lignocellulose biomass, giving access to phenolics, i.e. guaiacols, syringols, catechols and pyrogallols. These substrates are often referred to as platform molecules with the potential to serve as renewable feedstock for the production of commodity chemicals as an alternative to oil. However, lignin derived aromatics have a high oxygen content, which is in sharp contrast to benzene, toluene, xylene (BTX) obtained from current petroleum refining. Therefore, instead of an oxidative approach to introduce functional groups required for oil, a reductive approach is needed for bioaromatics. One way to allow a controlled decrease in oxygen content of lignin derived aromatics while maintaining aromaticity is chemoselective hydrodeoxygenation. A particular interesting subclass of HDO reactions is selective hydrodehydroxylation allowing removal of hydroxy groups in presence of methoxy groups. However, this transformation requires pre-activation of the hydroxy group by installment of a leaving group. Acetate is introduced as a renewable leaving group for selective hydrodeacetoxylation of aryl acetates using a homogeneous Ni0-NHC catalyst and pinacolborane reductant in green dimethyl carbonate solvent. Proof-of-concept for oil derived substrates was demonstrated using renewable 4-propylguaiacol obtained from pine wood via reductive catalytic fractionation (RCF). Furthermore, overcoming the limitations of air- and water-sensitive Ni0 catalysts, a heterogeneous Ni-catalyzed hydrodeoxygenation method was developed for methyl aryl carbonates and catechol carbonates. This methodology was also shown applicable to renewable 4-propylguaiacol and 4-propylcatechol obtained from RCF of pine wood. Next to aromatic substrates, hydrodeoxygenation of aliphatic hydroxy groups, pre-activated as methyl carbonates, using air-stable Ni0(cod)(dq) as catalyst giving the corresponding alkane products is also described.

Eline Biscop

• 20/10/2023
• 11.00 uur
• Locatie: Campus Drie Eiken, O.05
• ​Online Doctoraatsverdediging​
• Promotoren: Annemie Bogaerts, Evelien Smits & Abraham Lin
• Departement Chemie

Abstract

Cancer, the second leading global cause of death, remains a formidable challenge despite advances in conventional therapies. The emergence of treatment resistance underscores the need for innovative approaches in the fight against this horrible disease. Over the past two decades, non-thermal plasma (NTP) has garnered attention as a novel addition to conventional treatments. NTP, a partially ionized gas, contains reactive oxygen and nitrogen species (RONS) that can induce oxidative stress in cells, ultimately leading to cell death. While oxidative stress therapies hold promise, their pro-oxidant nature requires careful consideration in therapy development. Chapter 1 introduces NTP, while Chapter 2 explores its potential as a selective treatment for glioblastoma and melanoma. The results reveal that various experimental factors can influence outcomes, making selectivity assessment challenging. However, lower NTP doses show promise in selectively targeting cancer cells. Chapter 3 investigates the stability of long-lived RONS in clinically relevant solutions, emphasizing the importance of understanding NTP-derived RONS behavior for reliable clinical applications. Chapter 4 delves into cell death pathways activated by NTP, demonstrating its potential to induce multiple forms of cell death, reducing the risk of resistance development. Chapter 5 shifts focus to the drawbacks of oxidative stress-inducing therapies, revealing that high NTP doses can enhance metastatic potential, while low doses have minimal impact on cancer cell migration. This underscores the need to optimize NTP parameters for effective treatment with minimal side effects. In summary, the thesis emphasizes the importance of optimizing NTP treatment parameters. While high NTP doses show strong cytotoxic responses, low repetitive NTP treatments have clinical potential by selectively targeting cancer cells without increasing metastatic potential. However, this approach may be limited to superficial tumors, requiring supplementary surgical interventions for deeper tumors. Chapter 6 concludes by highlighting challenges and future prospects for NTP treatment in cancer therapy.

Ilenia Serra

• 18/10/2023
• 14.00 uur
• Locatie: Campus Drie Eiken, R.109
• Promotoren: Sabine Van Doorslaer & Inés García-Rubio
• Departement Chemie

Abstract

Chlorite dismutases (Clds) are heme b-containing oxidoreductases of bacterial origin which degrade chlorite to form chloride and molecular oxygen. This unique enzymatic activity was originally found in perchlorate- and chlorate-reducing bacteria, where Clds act as scavenging systems for the toxic chlorite produced during the respiration. In addition to their intriguing reactivity – the formation of an O-O bond was formerly described only for oxygenic organisms and in a methano-oxidizing bacterium – chlorite dismutases raised interest for their potential industrial applications, such as in the field of bioremediation, construction of biosensors and as a tool to improve aeration in bioreactors, to cite a few.

This work is principally focused on the study of the dimeric chlorite dismutase from Cyanothece sp. PCC7425 (CCld) to gain insights into different enzyme properties, including the role of key amino acids in the heme surroundings, modes of ligand binding and mechanistic details. The integration of different approaches, including site-directed mutagenesis, structure determination, activity and stability assays, was necessary to elucidate the investigated aspects. The project can be viewed as a combination of three main research questions: i) the role(s) of a conserved arginine residue situated at the distal side of the heme iron and the impact of its flexibility on different enzyme properties; ii) the mechanism of substrate binding and the influence of the arginine in this aspect; iii) the nature of the intermediate states formed during the catalytic cycle and their dependency on pH. Exploiting the paramagnetic nature of the ferric heme iron, either in the resting state of the enzyme, or upon ligand binding and in reaction with the substrate(s), electron paramagnetic resonance (EPR) spectroscopy was chosen in this work as a principal approach to address these questions. In addition to that, preliminary experiments of enzyme immobilization in different materials were performed, with the scope of expanding the chlorite dismutase potential for industrial usages.

Overall, this work aimed at gaining a comprehensive understanding of the molecular determinants of chlorite dismutases’ peculiar reactivity, using an integrative structural biology approach, where EPR spectroscopy proved to be a valuable technique to obtain information about the heme-containing active site of CCld.

Prakash Kumar Sahoo

• 02/10/2023
• 16.30 uur
• Locatie: Campus Drie Eiken, Aula R2
• Promotoren: Shoubnik Das & Bert Maes
• Departement Chemie

Abstract

The employment of costly, single-use catalysts, highly toxic reagents/reactants, and solvents is still a major problem within synthetic chemistry. Therefore, it becomes difficult for pharma industries to reduce the price as well as environmental toxicity. In this regard, ample attention needs to be given to the chemical community to replace them with inexpensive, abundant, benign, and reusable substitutes. The goal of this doctoral thesis was to contribute towards the sustainable development of novel synthetic methodologies for the synthesis and functionalization of pharmaceutical molecules and natural products using metal-free homogeneous and sustainable, single-atom-doped heterogeneous photocatalysis.

The research described in this thesis can be divided into four chapters. In the first chapter, we have shown the development of a robust, cost- and energy-efficient strategy to introduce diverse nucleophiles, including alcohols, carboxylic acids, and amines, via the functionalization of benzylic C−H bonds of simple aromatic building blocks as well as complex drug molecules and natural products. The second chapter demonstrates a redox-neutral decarboxylative radical polar crossover process for the synthesis of linear aliphatic amines using 4CzIPN as an organo-photocatalyst. The synthetic utility of this method is further demonstrated by the late-stage functionalization of pharmaceuticals as well as the synthesis of drug compounds. In the third chapter, a Mn metal-doped g-c3n4 heterogeneous photocatalyst has been developed to achieve the vicinal dichlorination of alkenes using N-Chlorosuccinamide as a chlorinating agent. Noteworthily, both unactivated and activated alkenes provide good to excellent yields with mild reaction conditions. Interestingly, nine pharmaceutical compounds have been functionalized with our system to prove the synthetic applicability of our method. Finally, the fourth chapter demonstrates a Mn metal-doped heterogeneous photocatalytic method for the selective para-chlorination of alkenes using N-chlorosuccinamide as a chlorinating agent. Mostly electron-rich aromatics worked in this system.

Overall, the thesis contributes to the pharmaceutical industry by developing different methodologies that can selectively functionalize specific positions of complex pharmaceutical molecules with functional groups like amine, ether, ester, and chlorine in the presence of both homogeneous and single-metal-atom-doped heterogeneous photocatalysts using a mild photo-redox system.

Kaimin Zhang

• 21/09/2023
• 16.00 uur
• Locatie: Campus Drie Eiken, O.01
• ​Online Doctoraatsverdediging​
• Promotoren: Vera Meynen & Sammy Verbruggen
• Departement Chemie

Abstract

Grafting organophosphonic acids modification (PAs) on metal oxides has shown to be a flexible technology to tune the surface properties of metal oxides for various applications. Nevertheless, there are still puzzles that need to be addressed, such as the correlations between metal oxides properties (types of surface reactive sites) and the modification (modification degree), the correlations between metal oxides properties and the properties of modified surfaces. Moreover, the currently used liquid-phase method for the grafting has associated impeding effects of solvent on tailoring the modification degree, and also causes the formation of metal phosphonate side products. The solid-phase method can induce the unwanted changes in crystal phase of supports. Based on these questions, the three titania supports with divergent surface properties were selected as the metal oxides supports investigated, propylphosphonic acid (3PA) modification was carried out under the same synthesis conditions: four different concentration, two solvents (water or toluene), and one reaction time (4 h) and temperature (90 ℃). MeOH chemisorption was introduced to probe the surface (un)reactive sites for 3PA modification. On the other hand, MeOH chemisorption and inverse gas chromatography (IGC) were combined to characterize the changes in surface polarity and acidic properties induced by the modification. Next, a solid-phase method, manual grinding, was proposed to graft 3PA on titania, avoiding the impeding effects of solvent on improving modification degree and the formation of the titania phosphonate side products, as well as preserving the crystal phase. The results indicate that methanol chemisorption can qualitatively analysis the surface active sites that are consumed by 3PA modification, its chemisorption capacity shows the consistent trend with the maximum modification degree, hereby revealing the kinds of interactions that are important in controlling surface coverage. Titania supports is found to play an important role in changes in surface polarity and acidic properties by charactering the three modified titania samples at a similar modification degree using methanol chemisorption coupled with in-situ infrared and thermogravimetric-mass spectrometer, and IGC. Moreover, IGC provides additional information about the changes in binding modes. Furthermore, grafting 3PA modification on titania was achieved by manual grinding. Compared to the liquid-phase method, the maximum modification degree obtained by the manual grinding is 25 % higher while using 83.3 % or 75.0% lower amount of expensive 3PA and limiting the formation of titania phosphonate side products. Compared to the reactive milling method, the proposed manual grinding method preserves the crystal phases of titania.

Steven De Meyer

• 15/09/2023
• 16.00 uur
• Locatie: Campus Middelheim, A.143
• Promotor: Koen Janssens
• Departement Chemie

Abstract

Scientific research into cultural heritage has significantly grown in importance over the past decades. The growing popularity of macroscopic imaging techniques such as X-ray fluorescence or Fourier transform infrared now means that conservators and restorers have access to highly objective information on the chemical composition of a painting without the necessity for destructive sampling. As many works of art are heterogeneous on the macroscopic scale, it is clear that solely relying on microscopic samples does not provide sufficiently representative information on the condition of a painting and that macroscopic imaging techniques should be considered a crucial part of the analytical toolkit for conservation science.

The goal of this research has been to investigate the added value of reflection-mode macroscopic X-ray powder diffraction (MA-XRPD) for scientific and art historical investigations of cultural heritage artefacts. A prototype scanner was developed after careful consideration of the individual components. This mobile instrument allows for the analysis of flat objects such as oil paintings with reflection MA-XRPD. In this manner images can be obtained that show the distribution of crystalline components present at the surface of the stratigraphy. These crystalline materials can originate from different sources and include original pigments and non-original pigments.

MA-XRPD offers novel insights into original pigments such as ultramarine, copper sulfates and lead white. In Girl with a Pearl Earring by Vermeer the MA-XRPD instrument was used to prove that Vermeer used multiple subtypes of lead white to achieve subtle optical effects while in the painting The Night Watch by Rembrandt the lead white composition was studied in detail; multiple rare lead-based compounds were identified that could be linked to the usage of specific driers in the oil paint. By investigating the presence of degradation products, MA-XRPD can be used to assess the conservation state of an artwork. Secondary alteration products were identified in paintings by Nellius and Mignon, explaining why the paintings had visually deteriorated over time. MA-XRPD registered the presence of lead arsenates which were formed from the original yellow arsenic-based orpiment. By combining microscopic and macroscopic analysis, a chemical degradation pathway for the conversion of the unstable orpiment pigment was proposed. In this manner, MA-XRPD can also be used to provide highly valuable information for conservators and restorers by pinpointing areas that have undergone degradation and to guide sampling campaigns.

Leonora Podvorica

• 30/08/2023
• 09.00-11.00 uur
• Locatie: Universiteit van Turijn
• ​Online Doctoraatsverdediging
• Promotoren: Sabine Van Doorslaer (Universiteit Antwerpen) & Mario Chiesa (Universiteit Turijn)
• Departement Chemie

Abstract

This doctoral thesis investigates the nature and behavior of paramagnetic Ti(III) species in industrial Ziegler-Natta catalysts (ZNCs) used for the polymerization of polyethylene and polypropylene and is the result of a collaborative effort between the EPR laboratories of the University of Torino, the University of Antwerp and the Giulio Natta R&D Centre of Basell Poliolefine Italia S.r.l., carried out as part of the H2020 MSCA-EJD PARACAT project (grant agreement No 813209). The study focuses on the use of advanced Electron Paramagnetic Resonance (EPR) spectroscopy to examine the role of Lewis bases on the active sites in the (pre-)catalysts. EPR allows to monitor paramagnetic species, such as Ti(III) sites and organic radicals formed during catalysis. The research involves a systematic bottom-up analysis of the paramagnetic species in the ZN-materials starting from the MgCl2 support to pre-catalysts, activated catalysts, and pre-polymerized samples. The presence of radical species distributed in the MgCl2 support was found to be related to the presence of the Lewis base. The addition of different Lewis bases to a MgCl2/TiCl4 system was found to have also a direct effect on the distribution and coordination geometry of Ti(III) species on the surface of MgCl2, which in turn affects the activity and selectivity of the final catalyst. The nature of the Ti(III) species was found to be dependent on whether the activation process was performed in liquid or gas phase with a suitable co-catalyst. In the pre-polymerized samples, a dilution of the paramagnetic species was observed with the increase in polymer weight, but no change in the species was found. The current research provides insights into the behavior and activity of the ZNCs and contributes to the understanding of the catalytic mechanism.

Omar Biondo

• 03/07/2023
• 13.30 uur
• Locatie: Atlas Building, room 0.710 TU/e campus, Eindhoven (NL)
• Online Doctoraatsverdediging
• Promotoren: Annemie Bogaerts & Gerard van Rooij
• Departement Chemie

Abstract

Plasma-based CO2 conversion is worldwide gaining increasing interest. The aim of this work is to find potential pathways to improve the energy efficiency of plasma-based CO2 conversion beyond what is feasible for thermal chemistry. To do so, we use a combination of modeling and experiments to better understand the underlying mechanisms of CO2 conversion, ranging from non-thermal to thermal equilibrium conditions. Zero-dimensional (0D) chemical kinetics modelling, describing the detailed plasma chemistry, is developed to explore the vibrational kinetics of CO2, as the latter is known to play a crucial role in the energy efficient CO2 conversion. The 0D model is successfully validated against pulsed CO2 glow discharge experiments, enabling the reconstruction of the complex dynamics underlying gas heating in a pure CO2 discharge, paving the way towards the study of gas heating in more complex gas mixtures, such as CO2 plasmas with high dissociation degrees.

Energy-efficient, plasma-based CO2 conversion can also be obtained upon the addition of a reactive carbon bed in the post-discharge region. The reaction between solid carbon and O2 to form CO allows to both reduce the separation costs and increase the selectivity towards CO, thus, increasing the energy efficiency of the overall conversion process. In this regard, a novel 0D model to infer the mechanism underlying the performance of the carbon bed over time is developed. The model outcome indicates that gas temperature and oxygen complexes formed at the surface of solid carbon play a fundamental and interdependent role. These findings open the way towards further optimization of the coupling between plasma and carbon bed.

Experimentally, it has been demonstrated that “warm” plasmas (e.g. microwave or gliding arc plasmas) can yield very high energy efficiency for CO2 conversion, but typically only at reduced pressure. For industrial application, it will be important to realize such good energy efficiency at atmospheric pressure as well. However, recent experiments illustrate that the microwave plasma at atmospheric pressure is too close to thermal conditions to achieve a high energy efficiency. Hence, we use a comprehensive set of advanced diagnostics to characterize the plasma and the reactor performance, focusing on CO2 and CO2/CH4 microwave discharges. The results lead to a deeper understanding of the mechanism of power concentration with increasing pressure, typical of plasmas in most gases, which is of great importance for model validation and understanding of reactor performance.

Elise Vervloessem

• 20/06/2023
• 14.00 uur
• Locatie: Campus Drie Eiken, R2
• ​Online doctoraatsverdediging
• Promotoren: Annemie Bogaerts & Nathalie De Geyter
• Departement Chemie

Abstract

Synthetic fertilizers (like ammonia/NH3) are of vital importance, however, their production process does not fit in the sustainable world we are trying to achieve. A potential alternative or complementary process is plasma, a partially ionized gas made up of a wide range of species types, which facilitates atypical chemistry while being compatible with current sustainability standards. ​ ​

The aim of this thesis is to elucidate (wet) plasma-based nitrogen fixation with a focus on (1) the role of pulsing in achieving low energy consumption, (2) the role of H2O as a hydrogen source in nitrogen fixation and (3) elucidation of nitrogen fixation pathways in humid air and humid N2 plasma in a combined experimental and computational study.

The main conclusions are as follows: (1) A quasi-1D chemical kinetics model reveals that the strong temperature drop in between pulses affects the NOx production and decomposition reactions (back – and forward reactions of the Zeldovich mechanism) positively, enabling efficient use of the power put into the plasma. (2) We show that the selectivity of plasma-based NF in humid air and humid N2 can be controlled by changing the humidity in the feed gas and suggest NH3 is mainly formed in the gas phase as opposed to the liquid phase, contrary to what is suggested predominantly in literature. (3) Lastly, we identify a significant loss pathway for HNO2 and NH3, where these molecules are synthesized simultaneously, i.e. downstream from the plasma, HNOx reacts with NH3 to form NH4NOx which decomposes into N2 and H2O or precipitates. To prevent ineffective nitrogen fixation, this pathway should be considered in future works aimed at optimizing nitrogen fixation. ​ ​

The thesis closes with a point of view on future research in the field of wet plasma-based nitrogen fixation. In short, (1) it would be important to validate our results further in other plasma setups and to attempt to apply the knowledge presented in this thesis for performance enhancement, (2) when the underlying chemistry of wet plasma-based NF has been more established and the advantages and disadvantages have been mapped, we can look for synergies with other NF fields, for example plasma-electrochemistry, and (3) wet plasma-based NF is in an earlier research stage compared to dry plasma-based NF, nonetheless is it important to also focus on the technological aspects of this application.

Senne Van Alphen

• 17/03/2023
• 11 a.m.
• Venue: Campus Drie Eiken, Building O, O.01
• ​Online PhD defence
• Supervisors: Annemie Bogaerts & Rony Snyders
• Department of Chemistry

Abstract

200 years ago, humanity started the industrial revolution by discovering fossil fuels, which lead to unprecedented technological advancements. However it has become alarmingly clear that the major environmental concerns associated with fossil fuels require a short-term transition from a carbon-based energy economy to a sustainable one based on green electricity. A key step concerning this transition exists in developing electricity-driven alternatives for chemical processes that rely on fossil fuels as a raw material. A technology that is gaining increasing interest to achieve this, is plasma technology.

Using plasmas to induce chemical reactions by selectively heating electrons in a gas has already delivered promising results for gas conversion applications like CO2 conversion and N2 fixation, but plasma reactors still require optimization to be considered industrially competitive to existing fossil fuel-based processes and emerging other electricity-based technologies. In this thesis I develop computational models to describe plasma reactors and identify key mechanisms in three different plasma reactors for three different gas conversion applications, i.e. N2 fixation, combined CO2-CH4 conversion and CO2 splitting.

I first developed models to describe a new rotating gliding arc (GA) reactor operating in two arc modes, which, as revealed by my model, are characterized by distinct plasma chemistry pathways. Subsequently, my colleague and I study the quenching effect of an effusion nozzle to this rotating GA reactor, reaching the best results to date for N2 fixation into NOx at atmospheric pressure, i.e., NOx concentrations up to 5.9%, at an energy cost down to 2.1 MJ/mol.

Afterwards, I investigate the possible improvement of N2 admixtures in plasma-based CO2 and CH4 conversion, as significant amounts of N2 are often found in industrial CO2 waste streams, and gas separations are financially costly. Through combining my models with the experiment from a fellow PhD student, we reveal that moderate amounts of N2 (i.e. around 20%) increase both the electron density and the gas temperature to yield an overall energy cost reduction of 21%.

Finally, I model quenching nozzles for plasma-based CO2 conversion in a microwave reactor, to explain the enhancements in CO2 conversion that were demonstrated in experiments. Through computational modelling I reveal that the nozzle introduces fast gas quenching resulting in the suppression of recombination reactions, which have more impact at low flow rates, where recombination is the most limiting factor in the conversion process.

Jinxin Wang

• 09/02/2023
• 2 p.m.
• Venue: Campus Drie Eiken, O.03
• Supervisors: Vera Meynen & Annemie Bogaerts
• Department of Chemistry

Abstract

The plasma-based dry reforming in a dielectric barrier discharge (DBD) reactor is important to achieve sustainable goals, but many challenges remain. For example, the conversion and energy yield of DBD reactors are relatively low, and the catalysts or packing materials used in existing studies cannot improve them, possibly due to the unsuitable properties and structures of catalysts or packing materials for plasma processes.

In order to study the effect of catalyst structure on plasma-based dry reforming, a controllable synthesis of the catalyst supports or templates was explored. In Chapter 2, an initially immiscible synthesis method was proposed to synthesize uniform silica spheres, which can replace the organic solvent-based Stöber method to successfully synthesize silica particles with the same size ranges as the original Stöber process without addition of organic solvents. Using the silica spheres as templates, 3D porous Cu and CuO catalysts with different pore sizes were synthesized in Chapter 3 to study the effect of catalyst pore size on the plasma-catalytic dry reforming. In most cases, the smaller the pore size, the higher the conversion of CH4 and CO2 due to the reaction of radicals and ions formed in the plasma. An exception are the samples synthesized from 1 μm silica, which show better performance due to the electric field enhancement for pore sizes close to the Debye length. Besides the pore size, the particle diameter of the catalyst or packing is also one of the important factors affecting the interaction between plasma and catalyst. In Chapter 4, SiO2 spheres (with or without supported metal) were used to study the effect of different support particle sizes on plasma-based dry reforming. We found that a uniform SiO2 packing improves the conversion of plasma-based dry reforming. The conversion of plasma-based dry reforming first increases and then decreases with increasing particle size, due to the balance between the promoting and hindering effect of the particle filling on the plasma discharge. Chapter 5 is to improve the design of the DBD reactor itself, in order to try to increase its low energy yield. Some stainless steel rings were put over the inner electrode rod of the DBD reactor. The presence of rings increases the local electric field, the displaced charges and the discharge fraction, and also makes the discharge more stable and with more uniform intensity. The placement of the rings improves the performance of the reactor at 30 W supplied power.

Shahid Ullah Khan

• 19/01/2023
• 2 p.m.
• Venue: Campus Drie Eiken, Q.002
• ​Online PhD defence​
• Supervisors: Karolien De Wael & Sammy Verbruggen
• Department of Chemistry

Abstract

Singlet molecular oxygen (1O2) has continuously attracted researchers' interest because of its involvement in various processes, such as in photodynamic reactions in biological and chemical systems. 1O2 is an effective electrophile and potent oxidizing agent and can be easily generated by photosensitization via the illumination of organic dyes with visible light. As described in Chapter 1, 1O2 has gained prominence in various applications such as wastewater treatment, photodynamic therapy of cancer, organic synthesis, and recently developed 1O2-based photoelectrochemical (PEC) sensing of phenolic compounds. Phenolic compounds are a potential source of contaminants that originates from industrial effluents and waste products of chemical and pharmaceutical industries. These phenolic compounds pose severe threats to humans and aquatic life after reaching the environment. Therefore, it is imperative to develop photoactive materials that efficiently generate 1O2 and oxidize phenolic compounds and antibiotics. The existing 1O2 generating photosensitizers (PSs) include porphyrins, phthalocyanines (Pcs), subphthalocyanines (SubPcs), and other dyes such as derivatives of xanthene (e.g., Rose Bengal (RB)), and fluorinated boron-dipyrromethene (BODIPYs), and phenothiazinium dyes (e. g. Methylene Blue) which display long-lived triplet excited state and can be used in 1O2-based applications. This thesis focuses on preparing efficient hybrid materials based on newly synthesized Pcs, different surface area titanium dioxide (TiO2) and plasmonic gold nanoparticles (AuNPs) for their use in the PEC detection of phenolic compounds.

The first focus was on developing a fast amperometric method to test the photo-electrocatalytic activity of 1O2 producing PSs dissolved in MeOH based on the redox cycling of an electroactive phenolic compound, hydroquinone (HQ) (Chapter 2). This method of testing PSs does not require the accumulation of a reaction product since the amperometric signal develops near instantly when the light is on, which enables dynamic monitoring of a PSs activity at varying conditions in a single experiment. This method was crucial to measure high 1O2 quantum yield and low yield in the same experimental conditions. Moreover, the obtained results revealed a range of working parameters affecting the PEC activity of PSs.​​

The next goal was to immobilize tert-butyl substituted aluminum Pc (t-BuPcAlCl) on the solid support, which showed a high 1O2 quantum yield. However, before immobilizing Pc on a solid support such as TiO2, it is essential to know the electronic energy level of Pcs for the possible electron transfers from Pcs to TiO2. Therefore, Chapter 3 explored the (spectro)electrochemical properties of t-BuPcAlCl Pc. Next, in Chapter 4, t-BuPcAlCl Pc and other tert-butyl substituted Pcs with Zn central metal, t-BuPcZn, and its metal-free derivative t-BuPcH2 were immobilized on different surface area TiO2. The PEC activity of immobilized Pcs on TiO2 toward different phenols and antibiotics was studied, and the action mechanism was revealed and compared with sterically hindered fluorinated Pc F64PcZn.​​

In the final part of this thesis plasmonic AuNPs were introduced combined with trimethylsilane-protected acetylene functionalized ZnPc (TMSZnPc) to study the synergistic effect that boosts the overall activity toward the detection of phenols under visible light illumination (Chapter 5) . The TMSZnPc was coupled with AuNPs via a click chemistry approach. The 1O2 quantum yield of TMSZnPc improved significantly after conjugating with AuNPs, and, subsequently, the PEC activity for detecting HQ. The theoretical and experimental investigation demonstrated that the plasmonic enhancement of TMSZnPc is driven by the near-field mechanism. This shows the importance of plasmonic AuNPs with other photoactive species for their use in 1O2-based applications.​​

The fundamental knowledge obtained in this doctoral study will ultimately deepen the understanding of developing 1O2-based PEC sensors for detecting phenolic compounds and pharmaceuticals in the wastewater stream, helping to choose efficient materials and, in the last instance, a more sustainable future especially access to clean water for everyone.