Water (H ₂ O) microdroplets are sprayed onto a magnetic iron oxide (Fe ₃ O ₄ ) and Nafion-coated graphite mesh using compressed N ₂ or air as the nebulizing gas. The resulting splash of microdroplets enters a mass spectrometer and is found to contain ammonia (NH ₃ ). This gas–liquid–solid heterogeneous catalytic system synthesizes ammonia in 0.2 ms. The conversion rate reaches 32.9 ± 1.38 nmol s ⁻¹ cm ⁻² at room temperature without application of an external electric potential and without irradiation. Water microdroplets are the hydrogen source for N ₂ in contact with Fe ₃ O ₄ . Hydrazine (H ₂ NNH ₂ ) is also observed as a by-product and is suspected to be an intermediate in the formation of ammonia. This one-step nitrogen-fixation strategy to produce ammonia is eco-friendly and low cost, which converts widely available starting materials into a value-added product.