(495d) Mechanocatalytic Ammonia Synthesis over Transition Metal Nitrides
AIChE Annual Meeting
Wednesday, November 10, 2021 - 1:30pm to 1:50pm
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.