Electrochemical Ammonia Synthesis Mediated by Titanocene Dichloride in Aqueous Electrolytes under Ambient Conditions

Under ambient conditions, the catalytic and electrocatalytic syntheses of ammonia from nitrogen and various proton sources including wet tetrahydrofuran (THF) and the protic solvents methanol and water were performed using titanocene dichloride ((η⁵-C₅H₅)₂TiCl₂, commonly abbreviated to CP₂TiCl₂) in a two-electrode cell containing 1.0 M LiCl as the electrolyte. The highest rate of ammonia synthesis, 9.5 × 10^–10 mol·cm^–2·sec^–1·M CP₂TiCl₂^–1, was achieved at −1 V in water, whereas the highest faradaic efficiency (0.95%) was achieved at −2 V in THF. On account of its lower Gibbs free energy, density functional theory calculations suggest that the nitrogen-reduction reaction catalyzed by CP₂TiCl₂ in the presence of THF, methanol, or water preferably occurs via the Cp₂TiClN₂ intermediate rather than Cp₂TiN₂N₂. Future strategies to improve both the rate of ammonia synthesis and its faradaic efficiency must consider ways of maximizing nitrogen selectivity to the catalytic active sites by controlling the transfer rates of protons and/or nitrogen