Defended Ph. D. Theses

Colin O'Modhrain

• 04/09/2025
• 3 p.m.
• Venue: Campus Drie Eiken, Q.002
• ​Online PhD defence​
• Supervisor: Annemie Bogaerts
• Department of Chemistry

Abstract

Plasma-based CO₂ conversion is emerging as a promising technology in the global effort to combat climate change. While its scientific foundations are now well understood, the next step is to develop efficient, practical, and scalable systems for real-world use.

This research advances the field in four key areas: optimising post-plasma processes, integrating additional reactants and catalysts, introducing novel conversion methods, and scaling up reactor designs.

An innovative carbon bed system works in tandem with a gliding arc plasmatron reactor, using a unique insulated design to maintain uniform temperatures and capture waste heat for improved efficiency. This setup enables long-term, oxygen-free operation while achieving high conversion rates at low energy costs.

Efficiency gains continue with the integration of methane and a nickel-based catalyst in the post-plasma stage, enabling effective "dry reforming" without gas dilution. This configuration achieves the lowest reported energy cost for post-plasma dry reforming to date.

A new cyclic process transforms solid carbon—a common by-product—into a useful reactant, increasing CO₂ conversion while keeping the system clean. This self-sustaining approach is compatible with biogas compositions and can handle high methane content.

For scale-up, the research demonstrates a practical method of parallelising multiple plasma reactors within a single housing. This design combines technical and economic advantages, achieving peak performance and energy efficiency at an optimal operating point.

By combining creative engineering with fundamental scientific insight, this work delivers new pathways for turning CO₂ from a waste product into a resource, moving plasma-based conversion technology closer to widespread adoption and contributing to the transition toward a sustainable, net-zero future.

Rui An

• 15/07/2025
• 3 p.m.
• Venue: Campus Groenenborger, US.024
• ​Online PhD defence​
• Supervisor: Vera Meynen
• Department of Chemistry

Abstract

Metal oxides are being applied in many applications including chromatography, membranes, catalysis, and chemical sensors due to their excellent thermal, mechanical, and chemical stability. However, their native surface properties limit the diversity and specificity of surface interactions. Therefore, surface functionalization with organic molecules has been applied to broaden the versatility of metal oxides and enable custom-designed materials for specific applications. While organosilylation is one of the most common surface modification methods, it is primarily suitable for silica surfaces and shows limited hydrolytic stability on metal oxides. This thesis focuses on organophosphonic acid (PA) grafting, which overcomes the drawbacks of organosilylation by forming a hydrolytically stable Ti-O-P bond on titania. Moreover, unlike organosilylation, PA grafting avoids polymerization reactions and permits the use of water as a sustainable solvent.

This thesis investigates the synthesis-properties correlation of TiO2 P25 modified with alkylphosphonic acids and aminoalkylphosphonic acids, examining how synthesis conditions (particularly the pH of the PA solutions used for the synthesis) and molecular structure (chain length and amine functionality) influence surface properties. Using characterization techniques including TGA, ICP-OES, XPS, solid-state 31P-NMR, and DFT calculations, the research reveals a significant impact of the pH on the surface modification process. Alkylphosphonic acids show a more rapid decrease in modification degrees from pH 2 upwards compared to the aminoalkylphosphonic acids. Another key finding is the correlation between aminoalkyl chain length and NH2/NH3+ ratio, which could be elucidated by DFT calculations. Furthermore, the research also elucidates thermal stability differences, revealing that α-aminoalkylphosphonic acids grafted on TiO2 P25 have significantly lower P-C bond strength.

EPR studies demonstrate that PA-modified TiO2 surfaces can be reduced under vacuum at room temperature, leading to Ti-O2- radical formation. In addition, spin-probe molecules (TEMPO and 3CP) adsorbed on the TiO2 surfaces reveal surface property differences between the P25 and Hombikat M311 surfaces as well as the impact of amine groups.

This thesis provides valuable insights to fine-tune surface properties by selecting appropriate reaction conditions and PAs, advancing the development of customized materials for specific applications.

Céderic Ver Elst

• ​10/07/2025
• 4.30 p.m.
• Venue: Campus Groenenborger, V.008
• ​Online PhD defence​
• Supervisor: Bert Maes
• Department of Chemistry

Abstract

One of the major challenges of our time is the transformation of our petroleum based industry towards a more sustainable model based on other carbon sources. Indeed, the global warming combined with an ever growing world population make this matter more actual than ever. This has driven chemists to look into alternative feedstocks for the chemical industry, with non-edible biomass (e.g. lignocellulose from plant waste) being a particularly attractive candidate. In the so-called “biorefinery”, this is transformed into "platform chemicals”, which can subsequently be transformed into a broad range of other molecules with a wide variety of applications.

Levulinic acid is named one of the key biorenewable platform molecules, with its derivatives spanning from solvents and biofuel additives, to pharmaceuticals and plasticizers. It is currently produced industrially from non-edible sugars such as xylan or cellulose. However, these methods are lacking as they require corrosive acid catalysts, suffer from rather low yields and experience char formation in the form of humins. It comes as no surprise that levulinic acid is currently only produced in small scales and that there is an ongoing search for new production methods from biorenewable feedstocks.​​

In this PhD thesis, an alternative route to levulinic acid is explored, starting from another biorenewable platform chemical called muconic acid. This can be produced from lignocellulose through both chemical and biotechnological means. It was discovered that simply heating muconic acid in high-temperature pressurized water can efficiently form levulinic acid in high yields, without requiring additional reagents or complicated separations. This method was demonstrated to work on real bio-based feedstocks, such as pine wood and muconic acid produced by yeast fermentation of sugars. Furthermore, the method was successfully adapted for the direct synthesis of value-added levulinate esters.

Elizabeth Mercer

• 04/07/2025
• 3 p.m.
• Venue: Stadscampus, Building S, Klooster van de Grauwzusters, Promotiezaal
• Supervisor: Annemie Bogaerts
• Department of Chemistry

Abstract

This thesis investigates microwave-based plasma technology for carbon utilization, with applications ranging from terrestrial CO2 conversion to Martian atmosphere processing for in-situ resource utilization. The research explores how thermal management strategies and flow configurations influence the performance of microwave plasma. Key innovations include demonstrating that preheating CO2 can increase conversion at near-atmospheric pressure by influencing plasma contraction dynamics and expanding the reactive volume. The work further examines post-plasma reactive quenching with CH4 using a dual injection system in a CO2 microwave plasma, showing enhancement to CO2 conversion and comparing to conventional (admixing) dry reforming of methane. Through comprehensive spectroscopic analysis, temperature mapping, and product characterization, this work enhances fundamental understanding of microwave plasma chemistry and develops practical strategies for improving conversion. Additionally, we investigated microwave plasma conversion at near Martian atmosphere (25 mbar), demonstrating the technology's potential for space applications. The findings provide valuable insights for designing next-generation plasma reactors that can contribute to a circular carbon economy on Earth and support future human exploration of Mars.

Mathias Bal

• 02/07/2025
• 4.30 p.m.
• Venue: Campus Groenenborger, V.008
• Supervisor: Bert Maes
• Department of Chemistry

Abstract

The usage of biomass as feedstock for renewable carbon has been put forward to help addressing urgent climate change issues such as our reliance on fossil resources and the associated necessity to reduce CO2 emission. In this context of defossilization, lignin, one of the three components of abundantly available lignocellulosic biomass, has emerged as a promising biorenewable resource for aromatic platform molecules, which are currently obtained by petroleum refining. To unlock its potential, further research is required to efficiently transform the complex lignin structure into valuable product streams. Here, the big challenge is to go selectively cleave lignin into a limited number of arenes. In this respect lignin first approaches starting from lignocellulose rather than lignin are more interesting as during extraction of (hemi)cellulose from lignocellulose (e.g. in the paper industry) depolymerization and repolymerization of native lignin occurs creating a recalcitrant lignin, difficult to chemically cleave.

In Chapter 1 of this thesis, a brief introduction on lignin, its most important features and current depolymerization strategies is given. Given the abundancy of aryl methyl ether functionalities in lignin and lignin-derived products (monomers, dimers and oligomers) obtained via biorefinery, their O-demethylation is important for further downstream processing. Unsurprisingly, various methods have already been reported using a wide variety of both catalytic and non-catalytic reagents, each accompanied with their own advantages and disadvantages.

Because of this, a new O-demethylation protocol, utilizing acid (HCl or zeolites) in high temperature water, was developed in Chapter 2, keeping the different aspects of Green Chemistry in high regard. The CHEM21 Green Metrics Toolkit was used to assess the green credentials of our newly developed method versus state-of-the-art routes reported in the literature. In Chapter 3, the mechanism of the developed O-demethylation reactions was further unraveled using a combination of experimental studies and operando molecular modeling.​ 

Anilines are core structures for the chemically industry. They can be obtained via nitration of arenes and reduction. In Chapter 4, a novel selective reduction of nitrobenzenes into anilines was developed as traditional protocols still impose significant shortcomings with respect to Green Chemistry. Rather than relying on a thermal process, visible light-photoinduction was used to accomplish this transformation.

Pepijn Heirman

• 01/07/2025
• 3 p.m.
• Venue: Campus Drie Eiken, Q.002
• ​Online PhD defence
• Supervisor: Annemie Bogaerts
• Department of Chemistry

Abstract

Since the discovery, almost 30 years ago, that cold atmospheric plasma (CAP) can inactivate bacteria, research in the field of plasma medicine has expanded into a wide range of possible biomedical applications. This includes research into various strategies to treat cancer, as CAP was shown later to both have the ability to kill cancer cells as well as to induce an immune response against them. While in vivo research is crucial to progress the field of plasma medicine in a clinical setting, in vitro research still lies at the basis of understanding the possible effects of plasma treatment on cells. In turn, modelling can provide fundamental insight into underlying mechanisms that are out of reach with experimental methods.​​

In this thesis, we develop a 2D-axisymmetric fluid model, to investigate the transport phenomena that occur during the treatment of a well plate with a plasma jet, as commonly done for experimental in vitro research. With this model, we investigate the physical phenomena that occur in the studied system, and study examples where a lack of uniformity in plasma medicine research, both experimental and computational, hinders the direct comparison of results. In addition, we perform molecular dynamics simulations to gain insight into the actual, biological effects that plasma treatment may induce in treated (cancer) cells. Specifically, we focus on the immunotherapeutic potential of CAP treatment by modelling the effect of CAP-induced oxidation on HLA-E and HLA-Cw4, two protein ligands that are important for the crosstalk between cancer cells and natural killer (NK) cells. By comparing our simulations to experimental results, this data demonstrates the complex chemical and biological interactions between CAP and cancer cells, with regard to NK cell recognition, and provides an interesting starting point for further research.​​

Taken together, the findings presented in this thesis provide a deeper insight into the in vitro treatment of cancer cells with a plasma jet on both a macroscopic and microscopic scale. We show various ways in which the chosen setup geometry can influence the treatment itself. It is important to keep these effects in mind as a source of variation in experiments, both when conducting experiments and when comparing them among each other. Moreover, the importance of benchmarking and comparison between computational results is underlined. Finally, we shed some light onto the immunotherapeutic potential of CAP treatment, and point toward possible paths for future research.

Karel Weemaes

• 23/06/2025
• 4 p.m.
• Venue: Campus Groenenborger, V.008
• Supervisor: Bert Maes
• Department of Chemistry

Abstract

Our world is engaged in a battle against global warming and greenhouse gas emissions. Chemistry provides solutions across multiple areas, such as reducing dependence on fossil-based feedstocks, advancing solar cells, improving battery materials, reducing industrial waste production, and facilitating industrial and household waste recycling. A particularly important material class is polyurethane (PU), which has a significant societal impact. Polyurethanes are formed by the reaction of polyisocyanates and polyols, both of which are produced on a large industrial scale.

Despite its widespread use, isocyanates are primarily under scrutiny due to their toxicity. ‘Isocyanate asthma’ refers to a form of occupational asthma caused by exposure to isocyanates, which can lead to lung obstructions with prolonged exposure. Therefore, workers handling isocyanates are required to undergo mandatory training to minimize exposure. The ECHA even tightened the rules for usage of free isocyanates which eventually might result in a future ban.

Scientists are searching for new pathways to access these polyurethane materials without the need of going via free isocyanates. Blocked isocyanates are monomers formed by reacting an isocyanate with a blocking agent. This process involves "quenching" the reactive isocyanate group with a nucleophile to form a stable, protected isocyanate. This is safer to transport and use, and alleviates most concerns related to the usage of free isocyanates (toxicity, safety). No exposure to free isocyanates happens at room temperature, but the adduct will release the isocyanate again when the temperature transcends the deblocking temperature. This controlled release allows for immediate trapping with a nucleophile and prevents exposure to the isocyanates. However, blocked isocyanates are now industrially made by reaction of isocyanates with a blocking group.

In this thesis a novel way to access blocked isocyanates has been developed, which does not rely on the toxic phosgene or isocyanate intermediates. Moreover, using this unique approach novel blocked isocyanates were likewise made accessible. Last, these novel blocked isocyanates were investigated for organic synthesis applications.

Ivan Tsonev

• 26/05/2025
• 3 p.m.
• Venue: Campus Drie Eiken, Q.002
• Supervisor: Annemie Bogaerts
• Department of Chemistry

Abstract

Plasma-based gas conversion is a promising technology for supporting the green transition by enabling sustainable processes such as CO₂ splitting and nitrogen fixation. This thesis investigates both the fundamental and applied aspects of plasma-assisted N₂ fixation and CO₂ conversion, with the aim of improving energy efficiency and understanding key limitations. The work begins by assessing the environmental need for plasma technologies and reviewing the current state of the field. A novel computational fluid dynamics (CFD) model is developed to analyze how cathode emission mechanisms affect direct current plasma discharges, revealing arc discharges to be more effective for gas conversion than glow discharges. Experimental studies in a gliding arc reactor show that increased pressure enhances NO and NO₂ production while lowering energy costs. For CO₂ splitting, a six-temperature plasma model is introduced to capture vibrational-translational non-equilibrium effects with reduced computational demand, showing good agreement with experimental data and highlighting the role of cluster and negative ions. This model is later adapted to N₂ fixation to evaluate energy efficiency under non-equilibrium conditions. The findings suggest that while vibrational-translational non-equilibrium alone cannot reduce energy costs at atmospheric pressure, thermal NO production under chemical non-equilibrium offers a new pathway beyond thermodynamic limits.

Jian Zhu

• 21/02/2025
• 10.00 uur
• Locatie: Campus Drie Eiken, R1
• ​Online Doctoraatsverdediging​
• Promotoren: Pegie Cool & Shoubhik Das
• Departement Chemie

Abstract

With the rapid development of the economy and society, the continuous increase in energy demands and CO2 emissions are becoming critical to global warming and posing severe challenges to human life and social development. Therefore, it is urgent to close the anthropogenic carbon cycle through a chemical conversion of CO2 into value-added products and fuels by renewable energy, which can not only alleviate ever-increasing environmental pollution but also store intermittent energies into chemical bonds. Of all technologies to convert CO2 into value-added products, the electrocatalytic route appears as one of the most sustainable techniques as it can be conducted under mild conditions such as ambient pressures and temperatures, in neutral pH, and powered by renewable electricity from wind, solar, or hydro powerplants. However, CO2 is chemically stable, exhibiting a significant energy gap between its highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Therefore, a high energy for the dissociation of the C=O bond (806 kJ mol−1) is required. Moreover, the competing hydrogen evolution reaction (HER) occurs alongside CO2 reduction on transition metal catalyst is detrimental to the product selectivity and reaction kinetics. How can we mitigate those challenges?

According to the Sabatier principle, the reaction rate is determined by the interaction strength with the intermediates that are produced during the CO2 reduction process. The interaction strength depends on the electronic structure of 3d orbitals of transition metal catalysts. Therefore, high product selectivity and fast reaction kinetics are achieved by introducing defects and alloying, which can optimize the interaction strength with the intermediates. Moreover, the underlying mechanisms of the interaction between active center and heteroatom atom, N doping, and alloying in facilitating CO2 reduction were explored, which provides more in-depth insight for designing catalysts and improving product selectivity.

Yuxiang Cai

• 07/02/2025
• 13.00 uur
• Locatie: Campus Drie Eiken, O1
• ​Online Doctoraatsverdediging
• Promotoren: Annemie Bogaerts & Xin Tu (University of Liverpool)
• Departement Chemie

Abstract

Controlling carbon dioxide (CO₂) emissions and effectively utilizing it through chemical processes is a challenging issue that chemists and environmental scientists urgently need to address. Although many methods have been proposed to tackle the CO₂ problem, there is still no particularly effective chemical utilization method for this vast carbon source. This is because carbon dioxide is very stable and requires high temperatures for thermal activation.

Therefore, actively seeking new methods or supplementing with other approaches represents a new direction in carbon dioxide conversion research. Plasma technology, with its powerful activation capabilities, offers a new technological route for CO₂ conversion. Using plasma technology to convert CO₂​ into fuels and chemicals has significant application prospects. However, although the technical route of using CO₂​ and hydrogen or methane to produce syngas is feasible, the energy efficiency is still low, making industrial application difficult. The use of plasma catalysis for CO₂ hydrogenation into methanol represents a novel technological route. However, the mechanism remains unclear, resulting in a lack of systematic guidance for the design of the process.

To fully utilize the CO₂​ and convert it into value-added chemicals, while also making full use of the hydrogen and carbon resources of the co-reactant, this thesis combines experiments and simulations at different spatiotemporal scales, and conducted research in:

(1)  Understanding the mechanism of plasma-catalytic reverse water-gas shift (RWGS) reactions is important for a deeper insight into plasma-catalytic CO₂ conversion. In this thesis, perovskite catalysts with various B-site elements were successfully synthesised and evaluated in plasma-catalytic RWGS reactions. Among the tested samples, Fe-based perovskite catalyst demonstrated the best  performance, achieving 22.7% CO₂ conversion and 94.3% CO selectivity. Further improvements were observed with partial substitution of the B-site on Fe-based perovskite catalyst. The optimal catalyst, La_0.5Sr_0.5Fe_0.9Cu_0.1O₃, yielded 25.9% CO₂ conversion and 94.3% CO selectivity. The perovskite catalyst enhanced the plasma discharge characteristics, facilitating CO₂ excitation and C=O bond activation. A 0D kinetics simulation indicated CO production mainly from CO₂ dissociation. Catalyst characterization revealed that Cu substitution increased the catalyst's surface area, redox capability, and oxygen vacancies, enhancing CO₂ and H₂ adsorption and decomposition.

(2)   Targeting value-added chemicals from plasma-catalytic CO₂ conversion, a comparative study focused on catalyst supports was conducted. The Si/Al ratio of ZSM-5 significantly altered the properties of the Cu/ZSM-5 catalyst, particularly its acidity and basicity. Among all the samples, Cu/ZSM-5 with a Si/Al ratio of 38 showed the largest strong basic site percentage, which enhanced the electron-donating ability of the catalyst, promoting CO₂ adsorption. This facilitated the dissociation and activation of CO₂ molecules on the active Cu sites, further improving the catalyst activity. By combining characterisation and in-situ diagnostics, the mechanism is revealed.

(3)  For predicting the comprehensive reaction networks of plasma-catalytic CO₂ hydrogenation on Cu, the performance of a meta-generalized gradient approximation (mGGA) level density functional, rMS-RPBEl-rVV10, was evaluated and utilized. The rMS-RPBEl-rVV10 density functional closely predicted metal description, thermal dynamics, and the adsorption process without empirical corrections and excelled in predicting dissociation barriers critical for reaction networks. Also the reaction pathways on Cu(111) and Cu(211) surfaces were studied. On Cu(111), the formate and CO₂ dissociation pathways were equally favourable, with identical highest barriers, while the carboxyl path had a higher barrier. On Cu(211), the CO₂ dissociation pathway was most favourable with the lowest rate-controlling barrier. Generally, intermediates were more stable and reaction barriers lower on Cu(211). The Eley-Rideal (E-R) mechanism is discussed, the participation of plasma species significantly reduces or even eliminates energy barriers, while also providing key intermediates for fundamental reactions, leading to high selectivity and yield of CH₃OH at low temperatures and atmospheric pressures discussed in Chapter 4. This study provided valuable insights for the understanding of a comprehensive plasma-catalytic CO₂ hydrogenation mechanism.

(4)  Developing a hybrid machine learning model using limited experimental data to predict and analyse plasma-catalytic dry reforming of methane (DRM). Combining artificial neural network (ANN), support vector regression (SVR), and regression tree (RT) with genetic algorithm (GA) for optimization, the model was trained on 100 data points with four reaction parameters and four performance indicators. It achieved high predictive accuracy. The model revealed significant interactions between discharge power and total flow rate, with optimal conditions identified for maximum energy yield (0.398 mmol/kJ) and fuel production efficiency (13.2%). Despite not providing mechanistic insights, the model provided an efficient way for predicting and optimizing plasma-catalytic DRM, and also shows potential of the application in other plasma catalysis system.

Rani Vertongen

• 23/01/2025
• 17.00 uur
• Locatie: Campus Drie Eiken, Q0.02
• ​Online Doctoraatsverdediging
• Promotor: Annemie Bogaerts
• Departement Chemie

Abstract

CO2 is one of the main contributors to global warming. The best strategies to mitigate climate change are to electrify and decarbonize industry, but this cannot be achieved overnight. In the meantime, we need new technologies to deal with CO2: not only cut our carbon emissions, but also to lower the high levels of CO2 currently in the atmosphere. Carbon capture and utilization technologies are especially interesting, since they can produce value-added chemicals and fuels as new raw materials in industry to reduce our dependence on fossil sources and prevent more CO2 from entering the atmosphere. Plasma technology is especially promising thanks to its flexible and electric operation, coupling well with renewable energy sources, and its use of cheap and abundant materials in the reactor. However, the potential of plasma technology for CO2 conversion is not fully realized yet. Often, the conversion is limited, or high conversions can only be achieved at low energy efficiencies. How can we improve CO2 conversion in plasma technology? By investigating both reactor and process design, this thesis presents some encouraging answers to this question.

These experiments teach us some general insights on how to improve the conversion of CO2 in a plasma reactor. Good plasma stability can be achieved through proper reactor design, which will result in a higher energy input, yielding a higher conversion. Equally important is the design of the post-plasma zone, where effective quenching can help to improve the conversion. Furthermore, smart process design can modify the energy input by putting reactors in series and tune this technology for specific applications by adding hydrogen carriers or sorbent materials.

Overall, the reactor and process design in this thesis resulted in a higher CO2 conversion. The insights in the underlying mechanisms shine a light on future research paths, so that we can further develop plasma technology and contribute to a sustainable future.