Nitrogen fixation using metallic lithium nanoparticles formed by electrospray deposition

Abstract

Multiphase reduction of ambient dinitrogen (N₂) was observed during the deposition of charged or uncharged aqueous microdroplets containing a lithium salt pre-catalyst onto conducting or inert surfaces to yield mixed lithium nanoparticles. We propose that this method leverages the high electric fields and partial solvation of ions at the gas–water–solid interface to enhance reaction rates and perform normally moisture-sensitive chemistry in aqueous droplets. Ammonia (NH₃) is formed and Li⁺ is regenerated from the hydrolysis of transient lithium nitride Li₃N, constituting a complete catalytic cycle. The ammonia is captured in situ using formaldehyde to yield hexamethylenetetramine, a solid nitrogenous fuel. By measuring product formation with mass spectrometry, it was determined that 2.97 ± 0.36 μg h⁻¹ of NH₃ was produced by a single spray source when using pure N₂ sheath gas and an applied potential, though microdroplets alone appear a sufficient source of electrons in cases where an external potential is not applied; air is also an adequate source of N₂ to produce NH₃. The mixed lithium metal nanoparticles were characterized with (S)TEM, EDS, and EELS. This ammonia synthesis has implications for the formation of nitrogenous compounds in environmental, prebiotic, and traditional synthetic contexts.