(495d) Mechanocatalytic Ammonia Synthesis over Transition Metal Nitrides

With the growing world population, the demand for distributed, sustainable fertilizer production increases. Ammonia (NH₃), a precursor for fertilizers, is currently obtained almost exclusively through the Haber-Bosch process. Burdened by two competing factors - the stable N₂ triple-bond and thermodynamic limitations at higher temperatures - high pressure must be applied, requiring centralized plants. With this constraint, local ammonia production is not feasible in developing regions, driving efforts for novel reaction systems. We recently introduced a new approach for ambient ammonia production (ACS Energy Letters 2020, 5, 3362). Herein, NH₃ is mechanocatalytically synthesized from N₂ and H₂ over an in situ formed titanium nitride (TiN) catalyst. All experiments were conducted in a vibratory ball mill. The stainless-steel milling vessel was neither externally heated nor pressurized.

Mechanochemical N₂ fixation was demonstrated by XRD and XAS. XRD showed amorphization and TiN formation when titanium (Ti) was milled in N₂ for 3 h. The XANES spectrum of Ti milled in N₂ was dominated by characteristic TiN features. Hydrogenating TiN by milling in H₂ led to depletion of reactive nitride, and NH₃ production halted within 1.5 h. Milling Ti in a co-feed of N₂ and H₂ resulted in continuously increasing NH₃ yields with increasing milling time, suggesting that the catalytic activity of the TiN is increasing over an extended time period.

A transient Mars-van Krevelen mechanism is proposed for mechanocatalytic NH₃ synthesis, where nitride formation and NH₃ synthesis occur in distinct thermodynamic regimes. TiN is mechanically activated to form an environment suitable for N₂ activation. Next, the TiN relaxes to conditions where NH₃ formation is thermodynamically and kinetically feasible, before returning to an inactive state. Through these transient microenvironments, difficult chemistry can be achieved at mild conditions.