DIRECT SYNTHESIS OF NH3 FROM N2 BY RENEWABLE ENERGY FOR PRODUCTION OF FERTILIZERS AND AS A GREEN H2 VECTOR

Nitrogen Reduction Reaction (NRR) in mild conditions is an alternative and attractive way to produce (green) ammonia, but greatly challenging, due to the major difficulty of activating the inert dinitrogen molecule (N2), having a stable N-N triple bond. In this context, the three-year PhD research activities focused on the conversion of N2 to ammonia (NH3) by electrocatalytic-plasma routes through the development of advanced electrodes/cells to improve catalytic performance. Ammonia (NH3) is currently one of the most important industrial chemicals, serving as a vital precursor for fertilizers, and a potential carbon-free energy carrier. At present, NH3 is mainly synthesized by the Haber-Bosh (HB) process, which involves the reaction between dinitrogen (N2) and hydrogen (H2). However, this method operates at high temperature and pressure (400−600 °C, 20−40 MPa), consuming over 1% of the world’s energy supply. Unfortunately, the conventional hydrogen production routes from Steam Methane Reforming (SMR) and Water Gas Shift (WGS) processes emit a large amount of CO2, causing serious environmental damage. The European Union (EU) has set the goal of reducing greenhouse gas (GHG) emissions within 2050, and using renewable energy sources is thus a mandatory step to achieve this goal. For these reasons, many efforts have been made to mitigate the GHG emission, e.g. process electrification (with electricity coming from renewable sources), or moving to a (green) H2 economy and developing novel hydrogen or energy carriers (like ammonia). Nitrogen Reduction Reaction (NRR) in mild conditions is an alternative and attractive way to produce (green) ammonia, but greatly challenging, due to the major difficulty of activating the inert dinitrogen molecule (having a stable N-N triple bond). In this context, this PhD thesis focuses on the electrocatalytic reduction of N2 to NH3 through the development of advanced electrodes/cells to improve electrocatalytic performance. The research activities have been carried out in the framework of the PON project (Programma Operativo Nazionale – Italy), financing “Innovative Doctorates with Industrial Characterization (DOT20JCJJA)” (cycle 36th), which involves the collaboration of three institutions (University of Messina, Technical University of Eindhoven TU/e, and Casale SA). Specifically, during the period spent at the Home Institution (University of Messina, Messina - Italy), a home-made electrochemical cell operating in different configurations (gas and liquid phase, or combined gas-liquid phase) was developed and optimized. This innovative device enhances the interaction between gas nitrogen and the active surface area of the electrode through the utilization of a gas-diffusion layer (GDL) that physically separates the gas and liquid chambers. In order to fabricate the electrodes, different electrocatalysts have been synthesized and characterized, consisting of metals (Iron, Ruthenium) loaded on two different supports i.e. a-Alumina (Al2O3) and Carbon Nanotubes (CNTs), then deposited/layered over the GDL. A wide range of potentials were investigated, to evaluate the NRR conditions in relation to the competitive hydrogen evolution reaction (HER). The activity of the different metals and the role of the support have been highlighted. Many efforts were also made to improve the analytical methodology for ammonia detection and to avoid contamination. In addition, through a study in collaboration with the University of Trieste, a direct comparison between two cell setups (gas-phase approach, and gas/liquid-phase approach) has been made. For all the tests, advanced electrochemical characterization techniques such as Electrochemical Impedance Spectroscopy (EIS), Active Electrochemical Surface Area (AESA), Electrochemical Surface Area (ECSA), and Double-layer capacitance (CDL) calculations were performed, in order to support the experimental data. During a six-month period of research stay at the Eindhoven University of Technology (TU/e, Eindhoven - Netherlands) the synergy between plasma and catalysis in ammonia production was evaluated. Specifically, the synthesis of ammonia was investigated by experiments in a Dielectric Barrier Discharge (DBD) reactor by non-thermal plasma route in combination with catalysis. The different electrocatalysts mentioned before, having different chemical, physical, and electronic characteristics, were tested. Operational parameters such as feed gas ratio (N2:H2), flow rate, and power input were investigated to optimize the process. The feed gas ratio (N2: H2) was varied from 3:1 to 1:3 showing that the catalyst plays a key role in shifting the feed gas ratio to higher ammonia production. Differences between the empty reactor and the packed reactor were discussed, highlighting the internal differences between the two subgroups of catalysts, i.e., supported on AL2O3 or CNTs. Furthermore, the six-month period spent at Casale SA company (Lugano - Switzerland) was dedicated to the technical economic assessment (TEA) of current and novel processes for industrial ammonia production. The exploration of the different options for ammonia production started by examining both traditional and innovative methods. The well-established Haber-Bosh (HB) and green HB processes served as the initial references. Additionally, this research activity highlighted the potential for industrial application, albeit on a smaller scale, of emerging technologies like electrocatalysis (in aqueous, organic, and ionic liquid environments) and plasma-assisted catalysis (including thermal plasma, TP-SOEC, and non-thermal plasma, DBD). It is also considered a hybrid technique of Plasma-Electro catalysis where dinitrogen molecule is activated by plasma under NOx form, and also electrification possibilities for the HB method using magnetic induction heating. Following this, comprehensive data on plant and operating parameters were gathered to facilitate a thorough technical-economic evaluation. The main part of this work has been devoted to understanding what the current ammonia production scenario (scenario 0) is and what the critical issues are. For this reason, three scenarios have been assumed, covering all the ammonia production needs on multiple scales. In scenario 1, we explored the possibility of using a portable device (stand-alone) for ammonia production powered only by photovoltaic panels. Scenario 2 entails a medium-sized, decentralized facility capable of utilizing various renewable sources. Scenario 3 implements magnetic induction heating for pre-existing HB plants (electrification process). Closely related to the Scenario 1, solar energy is a sustainable and abundant source of energy, obtained by converting sunlight into electricity. This process takes place through the use of photovoltaic cells, commonly known as solar panels, or through the excitation of photo-cathode or photo-anode materials. In this context, artificial leaves (ALs) are devices designed to mimic the process of photosynthesis in plants, capturing solar energy to produce chemicals with higher added value or electricity. Applicable in the case of CO2 reduction reaction (CO2RR, as will be discussed in Chapter 6), NRR, and in electrolysers for the production of green hydrogen. Coupling photovoltaic cells with electrochemical (PV-EC) cells is a strategy to store solar energy in chemical form using electrochemical processes. These approaches contribute to the search for sustainable solutions for energy production and storage, reducing dependence on nonrenewable energy sources and mitigating associated environmental impacts. Below a brief summary of the single chapters is reported. Chapter 1 is a general introduction to the global landscape of current ammonia production technologies. It begins with an introduction to how the historical problem of nitrogen fixation was approached up to the process devised by German chemists Fritz Haber and Carl Bosch. Then, the focus is shifted to emerging technologies designed to meet global emission requirements. Moreover, the ultimate goal is to provide all preliminary information as the state of the art. Chapter 2 contains a detailed explanation of the chemical and physical properties of the substrate materials used (alumina and CNTs) in both electrocatalytic and plasma catalytic applications. The type of synthesis adopted in comparison with other techniques (Impregnation, co-precipitation, Atomic Layer Deposition) and experimental details (weight, loading, volumes, weight ratios, precursors, assays, and so on) are provided. In addition, material characterizations through X-Ray diffraction (XRD), Brunauer-Emmett-Teller (BET) calculations, Scanning Electron Microscope (SEM), and Energy Dispersive X-Ray (EDX) analysis, are discussed. In Chapter 3, the electrochemical approach by unconventional techniques using heterogeneous catalysts (in aqueous environment under mild conditions, i.e. room temperature and ambient pressure) is discussed. The comparison between two chemically and physically different media, investigated by doping with metals such as Ru, Fe, or a mix of them, is reported. The results show a higher tendency of Ru-Fe/Al2O3 to catalyse the reaction due to the higher productivity (1.05 ug mgcat-1 h-1) with a Faradic efficiency of 0.5% and a current density of 294 uA cm-2 at -0.3V vs RHE. Cell design is also crucial as it affects the electrocatalytic performances. The gas-phase (electrochemical cell 1) and gas-liquid-phase (electrochemical cell 2) approaches have strengths and weaknesses related to the presence of the membrane electrode assembly (MEA), as evidenced by the rGO-MnxOy-Fe and rGO-MnxOy comparison. The latter catalysts (in collaboration with the University of Trieste) are here reported especially to emphasize the different behaviour depending on the cell configuration, as expounded in Chapter 3. The approach to plasma catalysis in Chapter 4, expresses the connubial approach between plasma and catalysis. The experimental behaviour of a dielectric barrier discharge (DBD) reactor is reported, monitoring the ammonia performance by varying several operating parameters, including the N2:H2 ratio and the flow rate, by keeping the frequency constant at 20 kHz and maintaining a constant power of 27 W. The comparison shows that the ruthenium-based catalyst doped on alumina (Ru/Al2O3) achieved the best performance of 4725.7 ppm (22.7 umol min-1) with an N2:H2 ratio of 2:1 and an energy consumption of 70.1 MJ mol-1. The value, unbalanced toward higher nitrogen contents, shifts the reaction ratio from 1:3 to 2:1. Chapter 5 reports the techno-economic evaluation of ammonia production processes. Specifically, a study related to the energy consumption - costs of all the technologies mentioned in Chapter 1, and other particular technologies found in the literature, such Thermal Plasma- Solid Oxide Electrolyser Cell (TP-SOEC) and a Hybrid Plasma-Electrocatalitic (HPE) reactor, are discussed. Then, after evaluating the scenarios, the energy consumption related to the individual reactor is compared as a boundary condition. This preliminary TEA serves to emphasize the possibility of using the technologies synergistically. Finally, Chapter 6 is dedicated to the electrochemical-photovoltaic coupling, also discussed in Chapter 5 - scenario 1, but used here for the carbon dioxide reduction reaction (CO2RR). The reaction under investigation is different from the Nitrogen Reduction Reaction (NRR) but the issues are quite similar, thus it is possible to translate these results (including the design of the PV-EC cell working as an artificial leaves) the to a potential solar-driven NRR for future purposes. Chapter 7 draws general conclusions and discuss future outlooks. This thesis aims to provide an understanding of the current global overview of technologies and for developing sustainable energy solutions. These approaches could revolutionize energy production and storage, helping to mitigate climate change and promote the transition to clean energy sources. Future implications include the possibility of a more efficient energy supply with less impact on the environment.