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.