Abstract: Low-temperature plasma has received extensive attention due to its promising application prospects in the field of air pollutants degradation and energy conversion. To fulfill the need for particular applications, constructing stable plasma sources and investigating the interaction mechanisms between plasma and substances have been hot research topics. This thesis reports the diagnosis and improvement of plasma sources, diagnosis of the active species in plasma and a modeling study of chemical kinetics processes. The main research contents are as follows: In Chapter 3, a diffuse sine AC dielectric barrier discharge (DBD) is successfully obtained by optimizing the electrode structure. It is found that using double-layer dielectric plates can limit the discharge current intensity and significantly improve the discharge uniformity. The electrical characteristics and gas temperature with different operating time show that the discharge stability is also improved by using double-layer dielectric plates. In Chapter 4, nanosecond pulses are employed to generate diffuse DBD plasmas. Three main discharge stages are distinguished by ICCD images, i.e., the streamer breakdown from the needle tip to the plate electrode, the regime transition from streamer to diffuse plasma, and the propagation of surface discharge on the plate electrode surface. The chapter reveales that in nanosecond pulsed discharges the vibrational temperature of N2 increases with the discharge duration, while the rotational temperature mainly stays constant, which means electron energy is transferred into the vibrational levels, but gas heating is not obvious during the discharge pulse. In Chapter 5, both sine AC DBD and nanosecond pulsed DBD, studied in Chapter 2 and 3, are used for formaldehyde degradation. It is found that nanosecond pulsed DBD has more homogenous characteristics, better stability, and lower plasma gas temperature. Moreover, the energy consumption of nanosecond pulsed DBD is much lower than that of AC DBD. In Chapter 6, a 0D chemical kinetics model is developed to investigate the underlying plasma chemistry of methane dry reforming in a nanosecond pulsed discharge. An overview of the dominant reaction pathways of CO2 and CH4 conversion into the major products is given. Furthermore, most of the CO2 molecules are populated into vibrational states during the pulse. Hence, the vibrational states of CO2 play an important role in its dissociation process. In general, this PhD thesis contributes to a better insight in the mechanisms of sinusoidal AC DBD and nanosecond pulsed DBD plasmas and their applications, i.e., decomposition of formaldehyde and dry reforming of methane.

Abstract: Sulfur hexafluoride (SF6), as a man-made gas, is widely used in power industry, semiconductor industry and metal-processing industry. However, SF6 is a greenhouse gas and its global warming potential is 23500 times that of CO2. Besides, SF6 is very stable, with a lifetime in the atmosphere for more than one thousand years. Under natural conditions, only the ultraviolet light can make it slowly decomposed. Thus, the emission of SF6 has a great threat to the environment. In recent years, with the development of our national economy, the use of SF6 increased dramatically. And 90% of the SF6 emissions come from the power industry. In the meantime, the emission of SF6 exists a ‘hysteresis effect’, as many of the SF6-gas insulation equipment will retire in next decades, the emission of SF6 may increase sharply, and this may put great pressure on the environment. Therefore, it’s necessary to make efforts in controlling and treating the SF6 emission. Among the SF6 abatement technologies, the non-thermal plasma(NTP) represented by the dielectric barrier discharge(DBD) can effectively degrade SF6 and is suitable for large-scale industry applications. However, its energy efficiency still gets room for improvement and this kind of method has a defect that it’s hard to regulate the degradation by-products. Therefore, this paper proposed the combination of the packed bed reactor and the DBD technology to form a packed DBD discharge system for SF6 degradation, so that to further improve the energy efficiency and regulate the selectivity of by-products. By experiment and simulation research, the following innovations have been achieved: (1) Based on the packed bed DBD platform, the power parameter and gas-phase parameters of SF6 degradation were studied. It was found that the discharge process was significantly enhanced with the addition of packing particles, and the discharge energy efficiency was improved. The increase of input voltage can obviously increase the degradation rate, but reduces the energy efficiency. The increase of SF6 initial concentration and gas flow rate can improve the energy efficiency, but reduce the degradation rate. Therefore, both degradation rate and energy efficiency should be considered in deciding basic experimental conditions. (2) Active gases, such as O2, H2O and NH3, could effectively promote the degradation rate of SF6, and changed the product selectivity. In our packed bed DBD system, O2 and H2O have the optimal concentration conditions, which are 2% and 1%, respectively. The addition of O2 can promote the generation of S-O-F products, and inhibit the selectivity of SO2, while the addition of H2O had the opposite effects. In addition, the synergistic degradation of NH3 and SF6 will produce solid products, such as NH3HF, NH4HF2 and elemental S. For gaseous products, the increase of NH3 will lead to the generation of SO2 in the final degradation products and inhibit the generation of S-O-F products. (3) Different kinds of packing materials have great impacts on the degradation system in the discharge parameters, degradation rate and energy efficiency, as well as the products distribution. In the experiment, we compared the degradation results in three systems: glass beads packing, γ-Al2O3 packing and no-packing system. The packing of glass beads effectively improved the discharge voltage amplitude and discharge power, while had a limited effect on the equivalent capacitance of the dielectric. Besides, γ-Al2O3 packing had little effect on voltage amplitude, but obviously increased the equivalent capacitance of the dielectric. Furthermore, the degradation rate and energy efficiency in γ-Al2O3 system was higher than that of glass bead system. For products selectivity, γ-Al2O3 system was more desirable, where S-O-F type of product selectivity was suppressed and the SO2 selectivity increased significantly. By contrast, the glass beads system hardly affected the product selectivity. This results are presumably due to the relatively high dielectric constant of γ-Al2O3 particles and γ-Al2O3 itself may act as a reactant or a catalyst participating in the degradation reactions. (4) The size and status of the packing particles also have significant effects on the degradation process. The systems packed with 1, 2 and 4mm γ-Al2O3 particles for SF6 degradation were compared, and the 2mm system had the best performance, which may because the 2mm system had a good balance between the active contact area and the gas residence time. In addition, the packing pellets suffered from a hydration process slightly reduced the discharge parameters in the γ-Al2O3 packing system and significantly reduced the degradation rate was, which may because the H2O molecules pre-occupied the active sites on the γ-Al2O3 surface and reduced the discharge process. (5) Based on density functional theory (DFT), the degradation process of SF6 in the packed bed DBD system was studied at atomic scale. It was found that the SF6 can occur a physical adsorption at AlⅢ active sites on γ-Al2O3 surface. The activation barrier for the first degradation step of SF6 on γ-Al2O3 surface is much lower than in gas phase, which proved that the SF6 molecule is activated on the γ-Al2O3 surface. In addition, the plasma may affect the γ-Al2O3 surface to generate excess electrons or external electric fields. This two effects can change the adsorbed SF6 molecules from physical adsorption to chemisorption, together with an obvious stretching of S-F bonds, indicating that the plasma surface effects prmote the activation and decomposition of SF6 molecules. Furthermore, the stepwise degradation process of SF6 on γ-Al2O3 surface were investigated. The influence of radicals produced by plasma on the degradation process was analyzed. It was found that via Eley–Rideal (ER) reactions, high-energy radicals could effectively reduce the activation barriers and promote the surface reactions. Finally, the degradation mechanism of SF6 molecules in the packed bed plasma system was summarized, which may provide a theoretical basis for the study of harmless degradation of SF6. Keywords: SF6; Packed Bed DBD; Discharge Parameters; Products Analysis; Degradation Mechanism

Abstract: abstract not available

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.

Abstract: Nowadays, the burning of fossil fuels, particularly petroleum, natural gas, and coal, meets the rising need for power and fuels for automobiles and industries. This has given rise to ecological and climate challenges. This thesis explores these issues from three distinct perspectives: (i) experimental, (ii) computational, and (iii) process simulation, with a focus on studying CO2 as an alternative and economically viable raw material. Firstly, the experimental study is focused on the synthesis, characterization, and testing of novel catalysts for electroreduction of CO2 and oxalic acid, an intermediate product of CO2. Electrocatalysts based on Cu supported by citrus (orange and lemon) peel biomass are prepared. These catalysts exhibit activity in the electrochemical reduction of CO2, emphasizing the effectiveness of biomasses, particularly orange peels, as environmentally friendly precursors for sustainable and efficient electrocatalysts. In addition, graphitic carbon nitrides/TiO2 nanotubes (g-C3N4/TiNT) composites are prepared for the electrocatalytic reduction of oxalic acid to glycolic acid, revealing superior electrocatalytic properties compared to pristine TiNT. Characterization by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electronic microscopy were performed for all the prepared electrocatalysts. Delving into the reduction of CO2 on Cu catalysts, a computational study about the synthesis of methanol on Cu(111) surface is performed by using the Vienna Ab initio Simulation Package. A systematic study is carried out to define the activation energies of the elementary reactions by using mGGA DF. Consequently, it is shown that the rate-controlling step is CH3O* hydrogenation and the formate pathway on Cu(111) proceeds through the HCOOH* intermediate. Finally, the process simulation, performed by using the software Aspen Plus 11 from AspenTech Inc., is based on the comparison of a catalytic (oxidation of ethylene glycol) and an electrocatalytic process (CO2 electroreduction chain) to synthesize glycolic acid. An economic analysis of the operational and investment costs reveals that the catalytic process is more cost-effective due to the current instability of electrocatalysts and proton exchange membranes, resulting in increased maintenance costs and, consequently, higher prices for the product.

Abstract: Reactive oxygen and nitrogen species (RONS) generated by cold atmospheric plasma (CAP) can activate discrete signaling transduction pathways or disrupt redox cellular homeostasis, depending on their concentration. This makes that CAP possesses therapeutic potential towards wound healing, cancer, and other diseases. In order to effectively use CAP in the clinic, a clear understanding of the interaction of RONS with biomolecules (lipids, proteins and nucleic acids) from the atomic to the macro scale, and their biological significance, is needed. In this work, I have therefore studied the dual role of CAP-derived RONS, i.e., (i) in the signaling pathways involved in wound healing, and (ii) in their reaction with biomolecules to cause oxidation-mediated damage. I performed computer simulations to provide fundamental insight about the occurring processes that are difficult or even impossible to obtain experimentally. Furthermore, next to computational studies, I used both 2D and 3D tissue cultures. 3D model allows proliferation in a more physiologically relevant geometry that stimulates the production of extracellular matrix proteins. I investigated the treatment of human gingival fibroblasts with low doses of CAP-generated RONS. This treatment demonstrated that it can inhibit colony formation but does not induce cell death, induce the expression of metalloprotease proteins, induce extracellular matrix degradation, and promote cell migration, which could result in enhanced wound healing. In contrast, at high concentrations, RONS can disrupt the cell membrane integrity and induce cancer cell death through oxidative stress-mediated pathways. I discovered how oxidation of the cell membrane (lipid-peroxidation) can facilitate the access of a drug (Melittin) into cancer cells, and in this way, reduce the required therapeutic dose of Melittin in melanoma and breast cancer cells (demonstrated using in vitro, in ovo and in silico approaches). Furthermore, I studied how excessive lipid-oxidation in chemoresistant pancreatic cancer cells promotes ferroptotic cell death. This was due to the stimulation of the iron-dependent Fenton reaction by targeting a redox specific signaling network. However, upon oxidative stress, cells protect themselves via a sophisticated intracellular antioxidant system that involves the regulation of glutathione/glutathione peroxidase 4 (lipid repair enzyme). Cancer cells exhibited increased levels of intracellular RONS due to their hyper metabolism, leading to high expression of anti-oxidant systems. I therefore focus on the effect of reactive species on the intracellular anti-oxidant system and corresponding DNA damages in both temozolomide-sensitive as well as temozolomide-resistant glioblastoma spheroids, in a 3-dimensional tumor model with a more complex tumor microenvironment than cell monolayers.

Abstract: This thesis provides three potential ways to enhance the nitrogen removal efficiency of mainstream partial nitritation/anammox (PN/A), a key technology to enable energy-positive sewage treatment. In Chapter 1, the typical technologies to promote nitrogen removal efficiency are summarized. In Chapters 2 and 3, the concept ‘winter bioaugmentation with stored summer surplus sludge’ is proposed. Applying that, a cost-effective sludge preservation strategy is required. Preserving PN/A biomass without cooling and redox adjustment proved to be the cost-effective strategy. The reactivation of these stored sludges was also tested in low-temperature systems (15 and 10℃). Respectively 56% and 41% of granules activity compared to pre-storage activity (after Arrhenius-based temperature correction) could be recovered within a month (41% and 32% for flocs activity). In the end, the stored AnAOB bioaugmentation was successfully validated in the lab (20℃). In Chapter 4, a return-sludge nursery concept, applying the sidestream nitritation and blending the resulting effluent with mainstream effluent to achieve an intermediate temperature and nitrogen concentrations, is proposed. That led to a 33 – 36% increase in nitrogen removal efficiency. Arrhenius’ expectations (10 ℃ higher temperature, θ = 1.09) could only explain 49-51% of the activity increase in the nursery reactor, pointing to the role of other factors, e.g., the ~400% elevated electrical conductivity (15-16%), the 56-335% higher effluent nitrogen concentrations (12-14%), and the synergy and unknown factors (20-23%). Thus, the return-sludge biostimulation approach could also enhance nitrogen efficiency in the mainstream. In Chapter 5, the N2O emissions, linked to three typical nitrite-oxidizing bacteria (NOB) suppression strategies (low dissolved oxygen (DO) level, free ammonia (FA), and free nitrous acids (FNA) treatments) were tested in a biofilm system. A low emerged DO level (~0.60 mg O2 L-1) was effective to suppress NOB activity and decrease N2O emissions, but NOB adaptation gradually appeared after 200 days. Further NOB inhibition was successfully achieved by periodical (3 hours per week) FA (~30 mg NH3-N L-1) or FNA (~3 mg HNO2-N L-1) treatments. The FA treatment promoted N2O production, while the FNA treatment had no effect. Thus, PN/A systems should be operated at relatively low DO levels with periodical FNA treatment. In Chapter 6, the major findings proposed and the main conclusions drawn in this thesis are outlined. Beyond that, the possible design of a mainstream PN/A configuration that combined all described three technologies is demonstrated. Overall, the novel insights from this thesis potential to improve nitrogen removal efficiency in the mainstream.