(495d) Mechanocatalytic Ammonia Synthesis over Transition Metal Nitrides | AIChE

(495d) Mechanocatalytic Ammonia Synthesis over Transition Metal Nitrides


Tricker, A., Georgia Institute of Technology
Phillips, E. V., Georgia Institute of Technology
Buchmann, M., Technische Universität Darmstadt
Liu, Y. H., Georgia Tech
Rose, M., RWTH Aachen University
Stavitski, E., BrookHaven National Laboratory
Medford, A., Georgia Institute of Technology
Hatzell, M., Georgia Institute of Technology
Sievers, C., Georgia Institute of Technology
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.