Verdedigde doctoraten

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.