Laser-induced nitrogen fixation

The Haber-Bosch (H-B) process, currently dominant in ammonia production, is energy-intensive and carbon-emitting. Its reliance on high temperatures (400–500 °C) and pressures (100–200 bar) contributes significantly to global CO2 emissions (over 300 million tons annually). The intermittent nature of renewable energy sources makes it challenging to integrate them into continuous H-B plants. Alternative methods, such as chemical looping, electrochemistry, plasma and photocatalysis, and mechanochemical methods, have emerged as potential solutions. However, they still fall short of industrial relevance in terms of yield and efficiency. This study addresses these limitations by proposing a laser-pulse driven system for ammonia synthesis, offering advantages of small-scale, distributed production, highly concentrated energy for non-equilibrium chemistry, and minimized diffusion limitations and side reactions compared to electrochemical or photocatalytic approaches.

The authors review existing literature on alternative nitrogen fixation methods, highlighting their limitations in yield and rate. Electrochemical approaches, for instance, while showing improvements, still lag behind the required minimum yield rate for practical application due to kinetic barriers and side reactions. This necessitates the exploration of novel methods like laser-induced conversion to overcome these challenges. The paper references several key studies in electrochemical and other methods highlighting the existing gap that the new method hopes to fill.

The study utilizes a laser-induced nitrogen fixation (LINF) system. Lithium oxide powder, loaded onto a titanium sheet substrate, is placed in a reactor filled with nitrogen gas (1.0–7.5 bar). A pulsed CO2 laser beam focuses on the powder, inducing multiphoton heating and thermal dissociation of Li2O. The generated zero-valent lithium spontaneously reacts with nitrogen to form lithium nitride (Li3N), confirmed through XRD, XPS, SEM, and EDS analysis. Li3N is then hydrolyzed to produce ammonia (NH3), and the resulting LiOH can be recycled. The ammonia yield rate is calculated by normalizing the amount of ammonia produced to the geometric area of the oxide film. Isotope-labeled 15N2 gas experiments using NMR spectroscopy confirm that the ammonia originates from the LINF process. The mechanism is investigated using DFT simulations, analyzing the activation of Li2O, its dissociation into zero-valent lithium, and subsequent reaction with N2. The experiments involve varying laser power and scanning speed to optimize ammonia production, along with control experiments using argon and nitrogen without laser treatment. Titanium sheets are chosen as a substrate due to their stability and high melting point, further enhanced by the formation of titanium nitride during LINF. Detailed laser parameters including laser power, scanning speed, frequency are described along with the experimental setup of the reactor and gas purification techniques used.

The LINF method achieves a remarkably high ammonia yield rate of 43.3 µmol s−1 cm−2 under 7.5 bar nitrogen and 118 kW cm−2 laser power. This is two orders of magnitude higher than the commonly recognized minimum for practical application of electrochemical methods. The energy consumption is calculated to be approximately 322 kWh kg−1 NH3, which is competitive with lab-scale H-B processes and other emerging methods. Experiments with 15N2 gas unequivocally confirm that the ammonia produced originates from nitrogen fixation. The study proposes a mechanism based on the activation of Li2O, followed by the formation of zero-valent lithium and its reaction with N2. DFT simulations support this mechanism, revealing the preferential formation of lithium adatoms on the Li2O surface as the initiation step for N2 adsorption and activation. Successful lithium cycling (Li2O → Li3N ⇌ LiOH) is demonstrated, further enhancing the process's efficiency and sustainability. Even at 200°C, a high yield of 40.3 µmol s⁻¹ cm⁻² is achieved. Scaling potential is shown with 1.3 mg of ammonia produced in 78 seconds of irradiation. Currently the photon utilization efficiency is 5%, however, improving thermal management and other factors might lead to a 40% or higher photon utilization rate, making it economically competitive with industrial ammonia production.

The high ammonia yield rate and relatively low energy consumption of the LINF method represent significant advancements over existing alternative synthesis processes. The use of a CO2 laser, readily adaptable to solar pumping, offers a pathway for decarbonizing ammonia production. The system's simplicity, mild operating conditions, and the potential for recycling lithium hydroxide contribute to its economic viability. The ability to achieve high yields at elevated temperatures also suggest the potential use of waste heat to improve the energy efficiency of the process further. The findings address the limitations of existing methods by demonstrating a fast, efficient, and potentially scalable approach for ammonia synthesis.

The LINF process offers a promising pathway towards sustainable and efficient ammonia production. Its high yield rate, potential for solar pumping, and straightforward scalability hold significant promise for industrial applications. Future research should focus on enhancing photon utilization efficiency, optimizing thermal management, and exploring the application of this technology to other chemical processes.

The current photon utilization efficiency of 5% is relatively low, indicating potential for further optimization through improvements in laser technology and thermal management. The study is primarily focused on lithium oxide; further research is needed to investigate the applicability and efficiency of this method with other metal oxides. The current small scale nature of the experiment will need to be scaled up for industrial feasibility.