The main objective of this dissertation is to demonstrate two strategies for incorporating highly efficient organic corrosion inhibitors into aerospace coatings and to establish fundamental guidelines for designing such coatings. Each chapter tackles important scientific and industrial challenges related to corrosion, organic inhibitors, and coating systems, using newly developed inhibitor-loading strategies, lab-scale operando optics, electrochemistry, spectroscopic and surface techniques, and industrial techniques.

Chocolate milk, paint and snow globes are all examples of solid-liquid suspensions found in daily life. In industry, colloids are also commonly used to process solids, as liquids are typically easier to handle than powders. Given the environmental concerns surrounding the usage of organic solvents, which are often evaporated and released during the process, water-based colloids are preferred. However, not all particles have a natural affinity with water. When particles have insufficient affinity for water, the solid and liquid phases segregate. To enable such particles to still form stable water-based suspensions, their surface properties can be modified....

Polypropylene (PP) is a versatile polymer extensively used in industries such as food packaging, automotive, healthcare, and textiles. Industrially, PP is produced via gas-phase solid-catalyzed polymerization in horizontal stirred bed, vertical stirred bed, or fluidized bed reactors. These reactors operate under controlled conditions to polymerize propylene monomers into solid PP particles. Despite their widespread application, operating these reactors is challenging due to a lack of fundamental understanding and modeling capabilities, which leads to reduced production capacity and lower quality of the final product. This gap in understanding is primarily due to the scarcity of detailed experimental data, which is difficult to obtain because of the opaqueness of the flow and the rapidly evolving gas-solids distribution, necessitating non-optical measurements with high temporal resolution.

In this dissertation, a deeper understanding of granular flow behavior in these reactors was achieved through detailed experimental measurements using radiation-based imaging. Recognizing the direct link between macro-scale flow behavior and particle-scale phenomena, this research spanned both scales. Although the primary focus of this thesis is on a horizontal stirred bed, experiments were also conducted using two additional lab-scale reactor configurations: a vertical stirred bed and a fluidized bed. High-quality data on flow patterns, phase holdup, and particle dynamics were obtained using X-ray imaging and single-photon emission radioactive particle tracking. A key novelty of this research was the use of industrial-grade powders, such as polypropylene reactor powder, as encountered in horizontal stirred bed reactors. The collected data were thoroughly analyzed to identify the key parameters influencing granular flow behavior, utilizing statistical methods and visualization tools to uncover critical insights.

First, the flow behavior of polypropylene reactor powder in a laboratoryscale horizontal stirred bed reactor (HSBR) was investigated using X-ray imaging. It was observed that agitation significantly dictates overall flow behavior and phase holdup in the HSBR. Gas injection through inlet points at the bottom resulted in spouting behavior, and the gas holdup at fixed agitator positions remained highly consistent across successive revolutions. The presence of liquid was found to deteriorate the flow behavior due to liquid bridging at particle contact points, with particle size and surface morphology influencing the powders’ susceptibility to liquid.

Subsequently, a single-photon emission radioactive particle tracking method was presented, allowing the tracking of individual photon-emitting particles to evaluate the hydrodynamics of multiphase flows. This method directly utilized detected photon hit locations to reconstruct the three-dimensional position of the tracer particle, avoiding assumptions in count rate fluctuations. The tracer particle’s position was determined by finding the intersection point of three two-dimensional planes from the detectors, achieving a spatial accuracy of approximately 1 mm through a subsequent calibration experimentation procedure.

Thereafter, the method was employed to characterize the particle dynamics in the HSBR. It was found that, besides the agitator rotation speed, the flow behavior is significantly influenced by the reactor fill level. At low rotation speeds and fill levels, solids motion was primarily induced by impeller blade passage, resulting in semi-static bed motion and poor solids distribution. Increased fill levels and rotation speeds led to continuous solids motion and uniform distribution. Solids circulation, quantified by a dimensionless cycle number, increased with higher fill levels and rotation speeds. The axial dispersion coefficient ranged from 10^-6 to 10^-5 m² s^-1, increasing with rotation speed, although no conclusive relationship with fill level was observed.

Thereafter, the fluidization behavior of Geldart B particles in a vertical stirred bed reactor was investigated using X-ray imaging, pressure drop measurements, and numerical simulations via Computational Fluid Dynamics (CFD) coupled with Discrete Element Method (DEM) and Immersed Boundary Method (IBM). The experimentally obtained minimum fluidization curve and time-averaged pressure drop showed good qualitative agreement with simulations. Visual observations indicated that increasing the agitator’s angular velocity reduced bubble size and improved bed homogeneity, as evidenced by reduced pressure fluctuations. Simulations revealed that while the impeller enhances solids agitation, a proper design study is essential, as static immersed bodies like the stirrer shaft can adversely impact solids motion.

Finally, the correlation between the fluidization behavior and flow properties of 10 commercially available cohesive powders was experimentally investigated. The fluidization quality of the powders in a laboratory-scale fluidized bed was assessed using a Fluidization Quality Index (FQI), computed by integrating gas holdup and its temporal variation acquired through X-ray imaging. Flowability was measured in a rotating drum operated at high speeds, which aerated the powder bed, a critical factor in correlating fluidization behavior with flow properties. This study established a positive correlation between cohesive powders’ flowability and fluidization quality, suggesting that fast and user-friendly flowability measurements in a rotating drum instrument can predict fluidization potential, aiding in process optimization and enhancing fluidization studies for cohesive powders.

In summary, the insights acquired from this thesis enhance the understanding of flow behavior and phase holdup in stirred bed reactors and cohesive fluidized beds. These findings can serve as a valuable foundation for designing, optimizing, and intensifying systems for the industrial-scale manufacturing of high-quality PP resins.

The severe effects of climate change, along with the rising global energy demand, have driven extensive research efforts into the development of sustainable technologies for energy generation, conversion, storage, distribution, and CO2 removal from various industrial sectors. Electrocatalysis is expected to play a pivotal role in achieving these goals, as it can utilize intermitent renewable energy sources such as wind, geothermal, hydropower and solar energy, together with CO2 directly captured from the air or from flue gas, and H2O, to store energy into chemical building blocks. Meanwhile, the catalyst is indispensable in these electrochemical conversions, as it enables the reduction of the reaction energy barrier, thereby lowering the electrochemical overpotential required to initiate reactions. Moreover, it facilitates the direction of reactions along specific pathways without itself being consumed in the process, thereby enhancing reaction rates and improving the efficiency. This thesis focuses on the electrocatalysts used for CO2 reduction and water spli􀆫ng, and uses atomic layer deposition (ALD) and molecular layer deposition (MLD) to precisely control the catalyst structure and protect the catalysts from degradation and poisoning...

In 2023, renewable energy generation reached an all-time high, with 29% of all electricity coming from renewable sources. However, electrical energy will not be able to fully replace fossil fuels, as the intermittency of renewable sources requires additional solutions to match the energy demand. Additionally, conversion to chemical bonds is required to supply chemicals for plastics, fertilizers, steel, etc. Electrolysis – particularly water and CO2 electrolysis – offer a promising solution to these problems by converting renewable electricity into fuels and chemical building blocks. Although electrolysis processes are very promising for the energy transition, their costs are currently still too high.
This thesis focusses on the role that gas bubbles have on the performance of electrolysers: the formation of gas bubbles is inevitable in most electrolysers, since the common electrolysis products (e.g. H2, O2 or CO) have a poor solubility in water. Controlling the behaviour of gas bubbles offers a pathway to lower the cell voltage or improve the mass transport, which allows operation at higher operating current densities. This could help with decreasing the costs of electrolysers, bringing them closer to competing with fossil fuel-based processes...

The electrochemical conversion of captured carbon dioxide (CO2) at low temperatures holds promise as a sustainable method for producing materials and fuel using renewable energy sources. However, technological hurdles such as mass transfer limitations and operational instability hinder its industrial application. This dissertation aims to address these challenges by exploring the use of gas-liquid Taylor flow (series of confined gaseous CO2 bubbles, which are separated from each other by liquid electrolyte and from the channel walls by a thin liquid film) in electrolysis, which can enhance mass transfer without requiring complex electrode designs, potentially improving long-term operational reliability. Additionally, a multi-scale modelling framework is introduced to evaluate electrolyser designs from an economic standpoint, aiding in the identification of bottlenecks and guiding technology development.
In Chapter 2, we propose a tubular electrolyser design operating under gas-liquid Taylor flow to overcome mass transfer limitations. By developing a numerical model, we investigate the relationship between process conditions, mass transfer, and reactor performance. Insights gained from this model allow us to derive an easy-to-use analytical relation to evaluate the impact of changes in inlet flow rates on Faradaic efficiency and current density. We find that long gaseous CO2 bubbles and low velocities enhance the current density towards CO, outperforming traditional H-cells. However, achieving performance comparable to flow-through electrolysers operated with a gas diffusion electrode (GDE) requires means to increase CO2 solubility in the liquid electrolyte, by for example increasing pressure.
Chapter 3 focuses on experimentally testing how Taylor flow influences the electrolyser performance within the established zero-gap water electrolyser concept adapted for CO2 reduction, by employing a silver gauze as the cathode. Our experimental findings reveal that Taylor flow enhances the Faradaic efficiency towards CO compared to single-phase flow, with minimal influence from gas holdup within the studied velocity range. Contrary to the tubular design, high velocities are desirable to increase the Faradaic efficiency towards CO in the rectangular flow channel. We find that further optimisation of
cathode design and fabrication is needed to fully exploit the potential of this electrolyser concept.
In Chapter 4, techno-economic aspects of electrochemical CO2 conversion are addressed, aiming to optimise operational parameters for industrial applications. A multiscale model capturing mass transfer effects over the channel length of a GDE electrolyser is integrated into an economic framework to analyse the interdependencies of key performance variables on the economic outlook. The analysis indicates that optimal current densities may differ significantly from previously reported benchmarks, emphasising the importance of multi–scale modelling for evaluating electrolyser designs under economic considerations.

Hydrogen is expected to play a vital role as an energy carrier in the future decarbonized system. Currently, a large portion of the hydrogen produced comes from the steam reforming of methane present in natural gas, which produces significant amounts of carbon dioxide emissions. However, with the growing need to reduce greenhouse gas emissions, a shift from fossil fuels to renewable energy sources is required. As a result, there is a growing emphasis on green hydrogen, i.e., hydrogen generated using renewable power. Green hydrogen has been growing at an exponential rate since 2020 and is expected to account for the majority of hydrogen production by 2050. However, a few studies have recently suggested that hydrogen may indirectly contribute to global warming. It is believed that hydrogen delays the decomposition of methane, a strong greenhouse gas, and thus extends its lifetime in the atmosphere. If green hydrogen is to be the primary fuel in the energy transition, hydrogen emissions from an electrolysis unit should be investigated.

This project focuses on identifying the sources of hydrogen emissions from an electrolysis unit. The goal is to comprehend the depth of this potential issue and investigate possible solutions. This project is carried out in collaboration with Worley, a market leader in the design, construction, and delivery of green H2 facilities. The leakage estimates for the green hydrogen alkaline electrolysis plant are based on Worley’s in-house data. Venting during startup and shutdowns when power is unavailable, as well as hydrogen crossover in the electrolyzer, have been identified as two major contributors to hydrogen emissions. Solutions such as flaring systems to combust the vented hydrogen and battery energy systems to reduce frequent shutdowns and startups are investigated. To reduce emissions from hydrogen crossover, a reactor is modeled to explore the catalytic recombination of hydrogen and oxygen. These solutions are subjected to a techno-economic analysis to determine their viability.

Flare systems and battery energy systems are both deemed feasible. In the long
run, however, installing a battery energy system would be preferable to combusting the hydrogen product. In comparison to other battery technologies such as Li-ion and lead-acid batteries, vanadium redox flow battery systems have been found to provide the maximum incentives and highest optimal capacities at the lowest overall costs. To avoid emissions from hydrogen crossover in a low-pressure alkaline electrolysis unit, the most cost-effective design involves a single-stage compression followed by a scrubber, heater, and reactor. However, this design is still costly because the annualized costs are four times greater than the costs offset by emissions reductions per year. Governments can encourage the adoption of such solutions by providing financial incentives to businesses.

CH4 and CO2 are ideal candidates in the context of C1 chemistry as alternatives to oil-based feedstocks for chemicals and fuels production, due to their abundancy, low cost and potential to develop a closed carbon cycle.

Large scale utilisation of CO2 in the chemical industry is currently limited to a few applications (e.g. synthesis of urea, carboxylic acids, food industry) and generally requires high purity feedstock. Integrated processes that combine CO2 capture from diluted sources (e.g. industrial flue gases, air) and its conversion to value-added chemicals represent a solution to enhance the utilisation of CO2 and mitigate its emissions. CH4 is an abundant hydrocarbon with diversified sources ranging from fossil-based (natural gas, shale gas) to renewable ones (biomass, biogas), which can potentially substitute oil for the synthesis of valuable chemicals and fuels, including higher hydrocarbons. At the moment, however, CH4 utilisation is circumscribed to combustion for heat and energy production or energy-intensive production of H2 and syngas (H2 + CO) via steam reforming, resulting in a high carbon footprint.

In general, the thermodynamic stability of CO2 and CH4 molecules imposes severe limitations to their exploitation as chemical feedstocks, in terms of low conversion efficiencies and control on the selectivity of products. Their efficient conversion requires harsh reaction conditions (high temperatures and pressures, highly chemically reactive substances) at which the stability of the desired products is threatened, resulting in low selectivity. In this scenario, catalysis is essential to identify functional materials and develop new catalytic processes able to maximise the selective conversion of CO2 and CH4 feedstocks to value-added products.

Unsteady-state operation in catalysis is an option to overcome the thermodynamic constraints imposed by the conventional steady-state operation. Integrated CO2 capture and conversion, sorption-enhanced reactions, chemical looping combustion are examples of intrinsically unsteady-state catalytic processes that demonstrated enhanced performances compared to their steady state analogues. Moreover, the analysis of the transient catalytic behaviour developed in unsteady-state conditions leads to a deeper understanding of the catalytic processes in terms of identification of specific reactant-catalyst interactions, the steps involved in products formation and the mechanism of catalyst deactivation.

This dissertation deals with the catalytic activation of CO2 and CH4 molecules targeting at their valorisation to important chemical commodities as CO (syngas) and light hydrocarbons. Unsteady-state catalysis is explored as a means to overcome thermodynamic constraints associated to the conventional CO2 and CH4 conversion routes....

As industrialization advances rapidly in pursuit of refining the quality of human life, there has been a release of organic micropollutants (OMPs) such as pharmaceuticals into various water reserves. This has endangered the ecosystem and the possibilities of water recovery. Commonly used tertiary wastewater treatment technologies have proven to, not under all conditions, be less effective in OMP removal. Hence, Advanced Oxidation Processes (AOPs) have gained attention in the recent past due to their production of reactive oxidative species (ROS) that unselectively degrade OMPs. Photocatalysis (PC), an AOP that uses solar radiation to produce ROS, has been investigated earlier, but shows low efficiency due to fast electron-hole recombination. Photoelectrocatalysis (PEC) is a modified version that additionally uses electrical energy, thereby reducing the recombination and improving the OMP removal efficiency. Bismuth vanadate (BiVO4) photocatalyst has gained importance due to its narrow band gap of 2.4 eV and hence, efficient absorption in visible light spectrum. Thus, this research focused on investigating PEC using BiVO4 electrodes for the removal of OMPs in aqueous solutions. The BiVO4 electrodes were fabricated using dip-coating from 1 to 5 layers, and electrodeposition at -0.2 V and -0.4 V, each at durations of 2, 5, 7, 10 and 15 minutes. They were then characterized using analytical techniques to investigate their structural, optical and optoelectronic properties. Subsequently, all the BiVO4 electrodes were used for the photoelectrocatalytic degradation of Acetaminophen (ACT). The electrodes fabricated by dip-coating were shown to achieve superior degradation efficiencies of ACT of over 99% in 5 hours, due to optimum surface morphology and band gap. It was seen that their varied surface structures played an important role in improving OMP degradation, and compensated for their low average quantum efficiency at 7%, as compared to that of electrodeposition at 14%. Next, the photoelectrocatalytic degradation of multiple OMPs in a solution was studied, and it was determined that although the ROS unselectively targeted all the OMPs, some were degraded quicker than others due to their chemical structure. Scavenging experiments were also carried out that affirmed the role played by ROS in the photoelectrocatalytic degradation of the OMPs. Eventually, the degradation of OMPs in secondary wastewater treatment effluent was conducted to test its usage in purification applications. Overall, PEC using BiVO4 electrodes was found to be feasible and successful in OMP degradation, and has the potential to be developed for usage in environmental remediation strategies like water treatment and recovery.

Microfluidic extraction is a promising alternative for existing extraction methods of radioisotopes due to the high surface-to-volume ratio. When assessing the compatibility of microfluidic material with the extraction system, there are three critical aspects—compatibility with organic solvents, compatibility with the radiochemistry, and, most importantly, the resistance to radiation-induced damage. Resistance to radiation-induced damage includes the primary reactions of the radiation with the matter, such as bond cleavage and the indirect damage caused by radiation due to free radicals. On the other hand, polymers are an interesting alternative as they are cheap and suitable for producing microfluidic chips. Therefore, in this experimental study, multiple interactions with polymers and quartz caused by radiation are assessed. In general, most radiation-induced modifications can be traced back to changes taking place in the structure of the material. Some of the changes have been attributed to the scission of the polymer chains, promotion of cross-linkages, breaking of covalent bonds, formation of carbon clusters, and liberation of volatile products. However, these materials must not lose their mechanical and chemical properties to maintain a well-functioning microfluidic extraction system. Atomic layer deposition (ALD) is an interesting surface modification method due to its self-limiting nature and atomic precision control. With ALD, it is possible to deposit a nano-scale layer of metal oxide that reduces surface-induced damage without changing much of its bulk properties. Furthermore, with ALD, it is possible to change the wettability of the material to make them suited for microfluidic liquid-liquid extraction. This study showed that a 40nm T iO2 thin-film was able to stabilize the surface modifications during high flux electron irradiation. A similar pattern is found between the samples coated with VALD as radiation-induced surface activation is achieved. Briding oxygen that usually is present on the surface gets replaced by carboxyl groups and increases the surface energy to increase the hydrophilicity of the surface. Furthermore, polycarbonate, quartz, and high-density polyethylene showed impressive radiation resistance up to 5MGy as Young’s modulus showed no significant difference. With this study, it is possible to use nano-coating to stabilize the radiation-induced surface activation and can be helpful for surface modifications in multiple fields.

Electrochemical reduction of CO2 using renewable energy sources is one of the promising avenues to pursue towards mitigating the emissions of the notorious CO2. However, the CO2 electrolysis in aqueous systems, due to the low solubility of CO2, are severely limited by mass transfer. State of the art review shows that a significant amount of the research is done to improve mass transfer, where a variety of electrolyser designs were studied. Despite the effort, the challenge to enhance mass transfer remains and is the focus of the present work. To improve mass transfer, an innovative concept is proposed - Taylor flow in an electrochemical flow cell. Taylor flow has been extensively studied in the literature, especially in micro channels and monolith reactors. Therefore the characteristics of the Taylor flow are known to a certain extent. But they were never tested in an electrochemical system. For that reason, a numerical investigation is carried out to assess the performance of Taylor flow on the flow cell. A simplified 2D model was formulated using a unit cell strategy and is verified based on experimental data and theoretical concepts. The effect of dissolving bubbles is also modelled using a quasi-steady-state analysis The results of the 2D model show a significant improvement in the performance, i.e. in the current densities of the electrochemical cell compared to a typical flow cell. The maximum calculated current densities increase by an order of magnitude under certain flow conditions. The dissolution studies showed that the current densities deteriorate with time. Nevertheless, the overall performance is still higher than the typical flow cell. Finally, based on the insights from the 2D model, a 1D model is suggested to estimate the current densities and dissolution rates. The present study showed promising results for using Taylor flow in an electrochemical cell. The proposed 2D model can help in aiding future modelling studies while the 1D model can give simple estimates for the experimental work.

Radionuclides are often used in the field of nuclear medicine. For some deceases the use of radionuclides is the best possible care, or even the only means of diagnosis or treatment. For these medical applications high specific activity (high activity per unit of mass) is required. Commonly, medical radionuclides are man-made. They can either be produced by neutron activation, charged particle or photon activation or by means of radionuclide generator. Furthermore, hospitals prefer an ‘on demand’ supply. A radionuclide generator is ideal. Radionuclide generators can also be used to produce high specific activity. Conventional radionuclide generators work with the principle that the mother and daughter radionuclide have different electrostatic interactions with the column material. This allows for easy elution of the daughter radionuclide. However, when working with chemical identical mother-daughter radionuclide pairs (e.g. ^177mLu / ¹⁷⁷Lu) another separation principle is required. Utilising ‘hot atom’ chemical principles such a mother-daughter pairs can be separated. ‘Hot atom’ principles describe the chemical effects that occur due to nuclear interactions or due to decay. An example of these effects is bond rupture. The effective range of those principles is rather limited, requiring thin layers. A possible technique to apply these thin layers is atomic layer deposition (ALD). ALD is commonly used in the semi-conductor industry, but can due to its versatility also be used in the field of catalysis or pharmaceutical. The advantage of using ALD is that this gas phase deposition technique allows for thin conformal coating of complex structured materials. Furthermore, the amount of material that can be deposited can easily be adapted to need because ALD is a self-limiting process. In this thesis the usefulness of ALD in combination with radionuclide production is explored. Because of the versatility of ALD it can also be used to create target materials for charged particle activation and enrichment experiments (Chapter 2). This versatility is illustrated by three case studies, namely this production of targets for ⁶⁴Cu production, the production of ¹⁷⁷Lu by means of a radionuclide generator and the production of ⁹⁹Mo using three different routes. Also described is how ALD can be used to alter the surface chemistry of high surface area materials to increase their adsorption capacity for Mo (Chapter 3). The obtained particles with an alumina coating are then tested for their adsorption capacity and compared to acid activated alumina, the current used material in ⁹⁹Mo/^99mTc-radionuclide generators. The adsorption capacity of the obtained particles is twice that of acid activated alumina and has a 99mTc elution efficiency of 55%. Furthermore, the coating of nano-particles for the development of with Lu coated particles (Chapter 4) for the preparation of a radionuclide generator is described. ALD allows for a deposition of up to 15w% Lu. Furthermore, the gamma dose received during neutron activation has an influence on the specific activity produced (Chapter 5). Using Cu(II)-phthalocyanine as a target it is shown that an increase in gamma dose during neutron activation results in an increase in Cu release and hence a decrease in specific activity obtained.

Air pollution is a worldwide prevailing issue affecting the health of human beings in urban areas requiring urgent measures to adopted. This graduation project targets on adoption of passive air remediation techniques that can be widely applied into urban areas. Within the passive technical measures available, integrating greeneries on the infrastructure and photocatalytic materials prove promising to be incorporated in a broader perspective. Photocatalytic materials one-time installation, easy to maintain and self-cleaning properties has led to the selection of this material to quantify its air purifying capability.
TiO2 coating has been used in building materials for its self-cleaning property. However, its capability to break down NOx compounds has broadened its scope for pollution abatement. Though it can be applied as a coating, the performance of the photocatalyst in urban scenarios is affected by contact wind speeds and incident UV irradiation. Also, increased surface area provides more area of photo catalytically active sites. These factors are governed by the geometrical form of the panel. Hence, the main objective of this research is to design a façade panel with enlarged surface area and surface roughness to manipulate the environmental factors to favor photocatalysis. Also, its air purification effect is quantified to understand the effectiveness of this passive strategy and the design.
The façade concept has been designed following a series of strategies to respond to wind and irradiation. The concept with optimum surface enlargement and irradiation is developed into modular panels which are materialized by choosing the material with least environmental impact. The final part of this graduation project is about applying these panels in the street canyons, a controlled urban environment and evaluating the pollution abatement. The results indicate a pollution abatement of 3.5% - 8.9% in winter and 18% - 37.5% in percentage in summer for London.

Nanoparticles have properties of interest in biology, physics, ecology, geology, chemistry, medicine, aerospace, food science, and engineering among many other fields, due to their intrinsic properties arising from their large surface area to volume ratio and small scale. Most nanoparticle applications require particle’s surface adaptations, for which numerous methods have been developed. For this purpose, the characteristics of fluidization that make it an attractive processing technique are the large gas-solid contact area, no solvent, potential scalability, and suitability for continuous processing. Nanoparticles are not fluidized individually, but rather as clusters, which formdue to the relatively large interparticle forces. As a result, fluidization dynamics is strongly linked to nanoparticle agglomeration.