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Study of catalyst-assisted microwave plasma for N₂ fixation

par Sadeghi, Babak
Président du jury Konstantinidis, Stephanos
Promoteur Delplancke, Marie-Paule ;SNYDERS, Rony UMONS
Publication Non publié, 2024-03-29

Thèse de doctorat

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Résumé :

Nitrogen is an essential component of all living organisms and a basic element for synthesizing many chemical products. However, N2 is chemically inert and inaccessible to most organisms in its original form. Therefore, it must be first converted into other more reactive forms like ammonia or nitrates in a process named “Nitrogen Fixation”. Nitrogen fixation (NF) with low energy consumption and minimal CO2 emissions is a significant challenge in today's technological landscape. The natural processes allowing NF are mostly lightning and the activity of some bacteria. The global population growth also increases food demands, thus the industrialization of agriculture using nitrogen-based fertilizers has driven the development of artificial NF processes. In the beginning of the 20th century, intensive research efforts led to the development of several industrial NF processes. The most developed processes are (I) Birkeland–Eyde (B-E) process, (II) Frank–Carlo (F-C) process, and (III) Haber–Bosch (H-B) process. As a result of high energy consumption, the first two processes were abandoned and quickly replaced by the H-B process. In the H-B process, nitrogen from air is combined with hydrogen during steam methane reforming under extremely high pressures (100−250 bar) and moderately high temperatures (500°C) in presence of an active catalyst (Fe or Ru–based catalysts) to yield a high proportion of ammonia. For a fully optimized and integrated H-B process, an energy efficiency as low as 0.48 MJ/mol of nitrogen is reported. Nevertheless, despite its effectiveness, this process consumes almost 1%–2% of the world’s total energy resources and emits more than 180 million tons of CO2 annually. In addition, a fully operational H-B process demands a large infrastructure that does not allow for decentralization on a small scale which makes the transportation requirement an additional disadvantage of this method. It is therefore necessary to develop a new NF process allowing for lower energy costs and presenting a lower impact on the environment. Electrochemical approaches at low pressure and temperature face technical difficulties due to the inertness of nitrogen molecules. Nitrogen oxides (NOx) are also valuable chemical feedstock because of their commercial relevance. NOx is used in the manufacturing of household cleaning agents, as intermediate in the manufacturing of nitric acid, as a flour bleaching and room temperature sterilization agent. Most of today’s synthetic NOx is made by oxidizing ammonia. However, direct synthesis of NOx requiring 3.5 times less energy than ammonia, presents a feasible alternative when employing non-thermal plasma with and without a catalyst. Thereafter, the produced NOx can even be further converted into NH3 or HNOx through additional processes like the Ostwald process, offering a lower energy-cost pathway for N-based products. Such small-scale setups, driven by renewable electricity and intrinsically CO2-free, hold potential for sustainable NF.This work, as one of the objectives of the EOS project which aims to fundamentally examine NF process in different non-thermal plasma sources, explores the formation of NOx with a catalyst-assisted microwave plasma (MWP) at low pressure and at pulse regime. A discussion about the catalysts activity provided by using absorption bands of NOx molecules via the Fourier transform infrared (FTIR) spectroscopy. The study encompasses diagnosing reactive species generated in a MWP by means of optical emission spectroscopy (OES). Furthermore, state-of-the-art material characterization techniques, including Transmission Electron Microscopy (TEM), Raman spectroscopy, and more, were used to characterize the catalysts. This comprehensive characterization of both the plasma and catalysts aims to enhance our understanding of the plasma-catalytic processes. As a result, the catalysts performance correlated to their physicochemical properties regarding NOx production to interpret the underlying mechanism. The influence of process parameters on MWP reactor performance, in terms of energy consumption and product concentration, is also discussed. The investigation highlights the importance of vibrational excitation in MW plasma for efficient NF and proposes approaches to optimize plasma parameters. The study revealed that introducing a γ-Al2O3 catalyst with MWP resulted in the synthesis of NO2 species in addition to NO production. The catalyst provided an additional reaction pathway for NO2 formation at the gas-solid interface. The calcination temperature of γ-Al2O3 affected NO2 formation, while it had little influence on NO production. Physicochemical properties of the catalysts, such as porosity and surface area, and crystal structure played a role in NO2 formation. The addition of Mo oxide on γ-Al2O3 enhanced NOx production, particularly with Mo loadings below 10 wt%, while the addition of Co oxide with low concentration worked as a promoter, enhancing NOx synthesis. Improving the energy efficiency of the system can be achieved through plasma power modulation and synchronizing power delivery with vibrational-translational (V-T) relaxation and gas residence time. Additionally, in-situ analysis of the plasma-catalyst interaction, investigating the surface chemistry of the catalyst, and pretreatment of catalysts can provide valuable insights to enhance process efficiency. In conclusion, this work provides valuable insights into catalyst-assisted plasma for NF and highlights potential strategies for optimizing the process, reducing energy consumption, and enhancing NOx production efficiency. Further research and development are essential to address the remaining challenges and make this approach economically competitive with the industrial Haber-Bosch. Enhancing plasma-catalyst interactions and integrating modeling with in-situ techniques to analysis plasma-catalyst interface are the most pronounced approach to optimizing the process.