Laser-induced nitrogen fixation

The fixation of molecular nitrogen gas (N2) is essential for fertilizer production and natural nitrogen chemistry. The Haber–Bosch process dominates current industrial ammonia synthesis but operates at high temperatures (400–500 °C) and pressures (100–200 bar), consumes 1–2% of global annual energy, relies on methane-derived hydrogen, and emits over 300 million tons of CO2 per year (~1.5% of total). With the increasing availability of renewable electricity, solar radiation, and heat, there is interest in alternatives compatible with intermittent and decentralized energy. However, traditional H–B plants require continuous operation and cannot easily accommodate intermittency. To overcome these challenges, the authors analyze a laser-pulse driven chemical conversion system under 1.0 bar N2 that enables a one-step, solvent-free transformation of metal oxide to metal nitride, which upon hydrolysis yields ammonia. The approach aims to enable small-scale, distributed production using focused light to drive non-linear, non-equilibrium chemistry directly in bulk oxide powders, minimizing ionic diffusion limits and side reactions typical of electrochemical and photocatalytic methods.

Recent techniques compatible with intermittent renewable energy include chemical looping, electrochemistry, plasma- and photocatalysis, and mechanochemical methods. Although some reports show performance improvements, these methods still lack yield and rate for practical relevance. For example, lithium-mediated electrochemical N2 fixation achieved 150 nmol s−1 cm−2 at room temperature and 15-bar N2, below the commonly defined minimum of 900 nmol s−1 cm−2 (at 300 mA cm−2 and 90% faradaic efficiency). Further increases are challenged by ionic diffusion barriers in electrolytes and side reactions (e.g., electrolyte decomposition) at higher overpotentials. The authors position their laser-induced approach as addressing these limitations by bypassing electrolyte transport and leveraging high photon flux for direct bulk activation of metal oxides.

Preparation of metal oxide film: Approximately 3.0 mg of Li2O, MgO, Al2O3, CaO, or ZnO powder was filled into a circular hole (2.1 ± 0.1 cm diameter, 0.1 mm thickness) in a titanium sheet and dehydrated under argon at 350 °C. Laser-induced process: A CO2 laser (10.6 ± 0.03 µm, 0.12 eV; Speedy 100, Trotec) with a 2.5-inch focus lens (focal depth ≈3 mm, focus diameter d = 170 µm) was used. Pulsed mode at 1000 Hz with approximately 75 µs pulse duration was applied. Scanning speeds ranged from 0.17 to 1.36 mm s−1 and power from 10.8 to 37.3 W (power measured with Solo 2 power meter). The photon density per area per pulse was calculated as f = P/(ωπr2), where P is power, ω is laser frequency, ε is photon energy, and r is focus radius. The oxide film (on Ti) was placed in a sealed reactor with a ZnSe window, allowing IR transmission and focusing on the oxide. N2 pressure was 1.0–7.5 bar. Before irradiation, 99.999% N2 flowed at 30 mL min−1 for 10 min to establish the N2 atmosphere. For experiments at ambient conditions, inlet gas was pre-purified by sequentially passing through 0.1 M NaOH (to remove NOx), 0.1 M HCl (to remove NH3), and two drying bottles with polycarbonate and molecular sieves. Isotope labeling used 15N2 (98 at%) after argon purging; 15N2 was fed at 10 mL min−1. Control experiments used pure argon with laser treatment and N2 without laser treatment. Substrate choice: Titanium sheets were selected for high stability and melting point; formation of titanium nitride during operation enhances stability for repeated use. Product characterization: Formation of Li3N was confirmed by XRD, XPS, SEM, and EDS. Ammonia was quantified after hydrolysis of Li3N, with yields normalized to geometric area of the oxide film. 1H NMR with isotope labeling (15N2) distinguished 15NH3 (doublet with J(15N–H) = 73.6 Hz) from 14NH3. Operating parameters varied included N2 pressure (1.0–7.5 bar), laser power density (e.g., 118 kW cm−2 corresponding to 26.9 W at focus), and scanning speed.

• Demonstration of laser-induced nitrogen fixation (LINF) converting Li2O to Li3N under N2 via multiphoton absorption and subsequent hydrolysis to NH3 at ambient conditions.
• Ammonia yield rates: Highest reported 43.3 µmol s−1 cm−2 at 7.5 bar N2 using 118 kW cm−2 (26.9 W) and 1.36 mm s−1; at 1.0 bar and 25 °C, 30.9 µmol s−1 cm−2 was achieved. These rates are roughly two orders of magnitude higher than commonly recognized minimum rates for practical electrochemical methods (equivalent to ~12.5 A cm−2 at 100% efficiency for the 43.3 µmol s−1 cm−2 case).
• Energy consumption based on light power: ~322 kWh per kg NH3, higher than industrial H–B (10–13 kWh kg−1) but competitive with lab-scale H–B (400 °C, 1 bar) and other emerging methods; scope for improvement with scaled processes, heat management, and solar pumping.
• Isotope labeling with 1.0 bar 15N2 produced exclusively 15NH3 (1H NMR doublet, J = 73.6 Hz), with a 15NH3 yield rate of 32.8 µmol s−1 cm−2. Control experiments (Ar + laser; 15N2 without laser) generated no NH3, confirming N2 origin and laser requirement.
• Mechanism: Evidence supports Li–O bond multiphoton activation leading to zero-valent Li that reacts with N2 to form Li3N. DFT indicates high-frequency phonon modes on Li2O(211) facilitate Li adatom formation (ΔE = 1.25 eV; ΔE_act = 1.31 eV), creating vacancies where N2 adsorbs exothermically (ΔE = −0.88 eV) and stretches N–N to 1.165 Å. Required bond dissociation energies rationalize higher activity of Li2O vs MgO, Al2O3, CaO, ZnO.
• Observed luminescence during operation (white under Ar; red for Li2O under N2; green for MgO) consistent with multiphoton dissociation and exothermic processes.
• Other performance: Ammonia yield of 40.3 µmol s−1 cm−2 at 200 °C; successful lithium cycling Li2O → Li3N ⇌ LiOH; photon utilization efficiency from Li2O to Li3N currently ~5%; scale-up demonstration producing 1.3 mg NH3 after 78 s of irradiation.

The findings demonstrate that focused infrared laser pulses can induce non-equilibrium, multiphoton dissociation of Li2O to generate zero-valent Li, which rapidly fixes N2 to Li3N and produces NH3 upon hydrolysis. This bypasses the interfacial transport and electrolyte limitations of electrochemical and photocatalytic approaches, enabling very high ammonia production rates at ambient pressure and temperature, and is compatible with intermittent renewable energy. Although current energy consumption is higher than industrial-scale Haber–Bosch, it is competitive with lab-scale implementations and other emerging methods. Mechanistic insights from DFT and experimental observations support a pathway involving Li adatom formation, N2 activation at vacancies, and efficient nitridation of Li at room temperature, explaining the superior performance of Li2O relative to other oxides that require higher temperatures for nitridation. The process generates substantial heat, suggesting opportunities for integrated heat management. With improvements in photon utilization, optical coupling to the oxide film, and thermal management, and by employing solar-pumped lasers, the approach could offer decentralized, small-scale ammonia production with reduced infrastructure demands compared to traditional large-scale H–B plants.

This work introduces a solvent-free, laser-induced nitrogen fixation route that converts Li2O to Li3N under N2 via multiphoton activation and yields NH3 upon hydrolysis, delivering ammonia production rates two orders of magnitude higher than other ambient-condition methods. The approach operates at mild conditions, demonstrates successful lithium cycling (Li2O → Li3N ⇌ LiOH), and shows scalability. Mechanistic and computational studies corroborate a multiphoton-driven dissociation pathway and N2 activation at Li2O surfaces. Future research should focus on increasing photon utilization efficiency (targeting ≥40%), optimizing oxide film density and optics to reduce reflection and pre-nonlinear relaxation losses, improving thermal management and heat recovery, and implementing solar-pumped laser systems to lower energy costs and enable distributed, renewable ammonia production.

• Energy consumption (~322 kWh kg−1 NH3 based on light power) remains significantly higher than industrial H–B, though improvements are anticipated with scale and heat integration.
• Photon utilization efficiency is low (~5% from Li2O to Li3N); substantial photon losses occur via reflection and dissipation as heat due to suboptimal film densities and pre-nonlinear relaxation.
• For oxides other than Li2O (e.g., MgO, Al2O3, CaO, ZnO), nitridation requires higher temperatures (>700 °C), leading to lower yields under similar conditions.
• Mechanistic aspects (thermal vs purely nonlinear dissociation contributions) are not fully resolved.
• Significant heat generation during operation necessitates secondary heat management strategies for practical deployment.