Plasma-Catalytic Synthesis of NH3: An Experimental Investigation and Insights on H2 Storage Solutions

This work will address two aspects that are seemingly distant in terms of technological maturity, yet share a common goal: to develop and optimize the production and use of green hydrogen — that is, hydrogen produced from renewable sources — for purposes that support decarbonization, whether by storing electrical energy or by enerating essential feedstocks such as ammonia. Hydrogen has long been recognized as a critical raw material in various industrial processes, playing a central role in sectors such as refining, chemical industry, and fertilizer production. One of the main challenges associated with hydrogen exploitation is its production source: 95% of hydrogen is currently produced via steam methane reforming, according to the reaction: CH4 + 2H2O → CO2 + 4H2 In addition to CO2 emissions, this reaction also requires a catalyst, which is usually nickel-based, and operates at high temperatures between 750 and 800°C 800 °C. This process contributes to the emission of 830 million tons of CO2 globally each year. Today, the primary use of this hydrogen is ammonia production (150 million tons annually), which alone accounts for 1.3% of global CO2 emissions. For these reasons, developing effective methods to achieve zero-emission hydrogen production can greatly aid in reaching the European Union’s net-zero targets. Hydrogen can be categorized into three main types according to its production method: • Gray Hydrogen: Produced via steam methane reforming • Blue Hydrogen: Produced via steam reforming but with CO2 capture and utilization or storage technologies. • Green Hydrogen: Produced via electrolysis powered by renewable energy sources. A particularly interesting application of hydrogen is tied to the latter method, green hydrogen production. Storing large quantities of electrical energy, e.g., via batteries, poses significant challenges. This is a critical issue, given that the primary renewable energy sources we aim to rely on, such as solar and wind, are intermittent by nature. Therefore, it is essential to store excess energy generated by these sources, and one of the most promising solutions involves chemical storage through green hydrogen production. Storing large quantities of hydrogen on a large scale presents challenges and risks associated with its storage and transportation. In this context, plasma-based technologies for converting hydrogen into a molecular vector appear promising for two key reasons related to the earlier points mentioned: • They provide a method for chemically storing excess electrical energy from renewable sources, including hydrogen. • They can produce ammonia from green hydrogen, further decarbonizing the current production process. Plasma, indeed, allows for the activation of the very stable nitrogen molecule. The activation of a molecule in the plasma phase occurs through mechanisms such as excitation, dissociation, or ionization, triggered by electron collisions. The dissociation of nitrogen is usually the rate-limiting factor: to break the triple bond of molecular nitrogen an energy of 9.8 eV is needed, which is 1.8 times the energy required to dissociate CO2 into CO and atomic oxygen and 2 times the energy needed to dissociate molecular hydrogen. The introduction of plasma in the process allows for avoiding the extreme conditions required in the current ammonia production process (the Haber-Bosch process). As will be explained in much more detail later, plasma creates a non-thermodynamic equilibrium state that enables reactions to occur under milder pressure and temperature conditions, surpassing classical thermodynamic limitations. Another aspect that makes this type of technology promising for decarbonization is its inherent compatibility with renewable energy sources. They can operate on demand, responding quickly to fluctuations in energy availability, which makes them highly adaptable to the variable output of renewable sources. This thesis will address two distinct aspects. The first concerns the current industrial capability to store electrical energy through green hydrogen production, illustrated by a practical example of a project funded by the National Recovery and Resilience Plan (NRRP) and implemented by Novareti SpA in the municipality of Rovereto, in Trentino. The second focuses on fundamental research into plasma catalysis, a promising technology for renewable energy storage and hydrogen utilization. Despite its potential, plasma catalysis is still in the very early stages of development, with only a few experimental large-scale applications to date. It still requires significant effort to better understand the fundamental mechanisms involved, as evidenced by the contrasting results reported in the literature. Green hydrogen production system for district heating in Rovereto, Italy Green hydrogen production today relies on electrolyzers, systems capable of performing water electrolysis when powered by electrical energy. The growing integration of variable renewable energy (VRE) sources, such as solar and wind, poses challenges to the stability and management of the electrical grid. One such issue is the curtailment of renewable energy, which occurs when energy production exceeds demand, resulting in significant energy losses. This typically happens because renewable energy production is often higher when energy demand is low. The surplus energy that cannot be fed into the grid can instead be utilized through electrolyzers to produce hydrogen. The renewable hydrogen production plant analyzed consists of a 1 MW Proton Exchange Membrane (PEM) electrolyzer and a storage tank. The PEM electrolyzer was preferred over the more commonly used alkaline solution due to its superior performance in managing the variable output from renewable energy sources. It is also smaller and operates at a lower temperature. This thesis evaluates the practical impact of a system designed for decarbonization, focusing on various strategies for producing and utilizing hydrogen. To achieve this, a technical model has been developed using data from plant manufacturers and electricity grid suppliers. This model aims to optimize the electrolyzer’s usage and operational strategies, maximizing hydrogen production while keeping production costs as low as possible. The main goal is to evaluate the potential effects of decarbonizing Rovereto’s district heating system and integrating the hydrogen generated into the methane gas network. Experimental study on plasma catalysis Plasma catalysis is an emerging research area that aims to integrate catalysis processes with a plasma discharge. Undoubtedly, the combination of plasma and catalysis is promising in principle. In fact, the high electron temperature of the plasma generates radicals, ions, and excited states, which can open reactive pathways that are distinct from those of conventional thermal catalysis. In addition, the presence of a catalyst promotes the conversion of some feedstock into the desired product enhancing selectivity. The approach offers advantages, such as enabling high-energy chemistry at low temperatures and creating pathways inaccessible to traditional thermal catalysis. However, many criticalities still prevent understanding the mechanisms underlying plasma catalysis. The reaction pathways within plasma catalysis differ from thermal catalysis and are unique to the specific environment. In the thermal process, all the dissociation and recombination reactions occur on the catalyst’s surface. Conversely, in plasma, dissociation also occurs in the gas phase. Excited species and radicals produced in the gas phase can interact with the catalytic material. Additionally, the presence of the electric field, charged particles, and radicals can lead to different pathways compared to traditional thermal catalysis. Another aspect to take into consideration is that catalysts need support, which affects the discharge due to both chemical and physical interactions. As with the interactions between the plasma and the catalyst, those between the plasma and the support are not well known and are usually considered less relevant. Furthermore, numerous studies have shown that the addition of a catalyst does not necessarily enhance the activity of the plasma, and, in some cases, it can even worsen its performance. This behavior has been observed in various reactions, including dry reforming of methane and carbon dioxide and ammonia production, using a wide range of materials and support. These findings point out the need for further investigation in plasma catalysis. It is important to note that most studies lack a standardized methodology for preparing the catalyst or its support; often, the methods are described only vaguely. In addition, the methodologies for conducting the reference experiment (i.e., without catalytic material) are often not addressed. This lack of standardization increases the number of variables and can make it challenging to reproduce similar experiments. As mentioned earlier, the support is crucial in determining the catalytic effect. The experimental study presented in this thesis investigates how the preparation of the support affects catalytic activity. Our approach involves using a support that has undergone the entire preparation process, excluding catalyst loading, as a reference support. This choice of reference enables a fair comparison, separating the contribution of the metal from other potential effects due to changes in the support during the preparation process. Adding silver to the supports was found to increase the production of ammonia. This outcome becomes evident only when comparing the performance of silver-loaded supports to that of unloaded supports processed in the same way, demonstrating that the preparation of the support significantly affects the final results.