Laser-induced nitrogen fixation

Abstract

For decarbonization of ammonia production in industry, alternative methods by exploiting renewable energy sources have recently been explored. Nonetheless, they still lack yield and efficiency to be industrially relevant. Here, we demonstrate an advanced approach of nitrogen fixation to synthesize ammonia at ambient conditions via laser-induced multiphoton dissociation of lithium oxide. Lithium oxide is dissociated under non-equilibrium multiphoton absorption and high temperatures under focused infrared light, and the generated zero-valent metal spontaneously fixes nitrogen and forms a lithium nitride, which upon subsequent hydrolysis generates ammonia. The highest ammonia yield rate of 30.9 micromoles per second per square centimeter is achieved at 25 °C and 1.0 bar nitrogen. This is two orders of magnitude higher than state-of-the-art ammonia synthesis at ambient conditions. The focused infrared light here is produced by a commercial simple CO₂ laser, serving as a demonstration of potentially solar pumped lasers for nitrogen fixation and other high excitation chemistry. We anticipate such laser-involved technology will bring unprecedented opportunities to realize not only local ammonia production but also other new chemistries .

Fig. 1. Schematic illustration of the laser-induced nitrogen fixation process.

Lithium oxide powder is placed on the titanium sheet, and by laser-induction, the lithium-oxygen bond is dissociated to form metallic lithium. The activated lithium reacts with nitrogen to form lithium nitride. Lithium nitride is hydrogenated into NH₃ as an energy carrier and raw materials for fertilizer production. The lithium hydroxide obtained by hydrolysis can be directly used in laser-induced cycling.

Fig. 2. Characterization of metal nitrides.

a XRD patterns of laser-induced Li₃N with the reference XRD patterns of: Li₃N (01-075-8959), Li₂O (01-086-3380), TiN (98-000-0339), Ti (04-003-5042); b N_1s region of XPS spectra of laser-induced Li₃N film: A strong N signal is observed on the surface and the N1s region displays three peaks at 396.1, 396.9, and 399.5 eV, which are assigned to N-Li, N-Ti and N-O-Ti, respectively; c Top-view SEM micrograph of Li₃N film with d correlated EDXS element maps. It is worth noting that in the LINF process, Ti (substrate) also reacts with N₂ to form TiN following a thermal nitridation process (Supplementary Fig. 28–29, Supplementary Table 3).

Fig. 3. NH₃ production from laser-induced nitrogen fixation.

a 2D plot of ammonia yield rate by using Li₂O as a precursor vs. laser power density and scanning speed at 7.5 bar; b 2D plot of the corresponding energy consumption by using Li₂O as a precursor vs. laser power density and scanning speed at 7.5 bar; c Comparison of the maximum ammonia yield of this work with other ammonia synthesis methods: Mechanochemical, Haber-Bosch, chemical looping, photochemical, plasma electrolytic, and electrochemical;^{, ,–} d The NMR data from experiments by using ¹⁵N₂ and Argon with laser treatment and ¹⁴N₂ without laser treatment.

Fig. 4. Proposed mechanism of laser-induced nitrogen fixation.

a Ammonia yield rate by using lithium oxide, magnesium oxide, aluminum oxide, calcium oxide and zinc oxide as precursor mediators: a scanning speed of 0.17 mm s⁻¹ and an N₂ pressure at 7.5 bar; b The number of photons gathered on a single Li₂O unit cell (purple ball represents lithium atom; red ball represents oxygen atom) by using laser power of 118 kW cm⁻² with a pulse (75 μs) and a laser focus diameter of 170 μm; c The phonon density of states of Li₂O (211) surface, the inset of the figure shows the atom movements corresponding to high frequency mode (843.7 cm^-1); d Nudged elastic band calculation for the generation of the adatom (Li_ad) defect in the Li₂O (211) surface. N₂ adsorption in the vacancy close to the adatom with N-N bond distance stretching to 1.165 Å (gas phase N-N = 1.115 Å). Li adatom is shown in the dark purple color, lithium atoms as purple, oxygen atoms as red, nitrogen atoms as blue. e Schematic illustration of multiphoton absorption by Li₂O as an example during the laser-induced process. After each oxygen-lithium bond absorbs at least tens of photons (the energy of each photon is 0.12 eV), the oxygen-lithium bond is dissociated, and the excited state lithium transitions to a lower energy level and emits bright light. Part of the excited state lithium is combined with nitrogen gas forms lithium nitride and emits visible red light.