(5h) Mechanocatalytic Ammonia Synthesis over TiN in Transient Microenvironments

With the growing world population, the demand for distributed fertilizer production increases. Ammonia, a fertilizer precursor, is obtained almost exclusively through the Haber-Bosch process. Burdened by two competing factors – the stable N₂ triple-bond and thermodynamic limitations at higher temperatures – harsh reaction conditions must be applied, requiring centralized plants. Hence, local production in developing regions is not feasible, limiting the agricultural output. We recently introduced a new approach for distributed, small-scale ammonia production via mechanochemistry (ACS Energy Letters 2020, 5, 3362). Mechanochemistry drives reactions through mechanical collisions, which can create transient surface sites and result in sharp, local temperature rises. Here, NH₃ was mechanocatalytically synthesized from the elements over in situ formed titanium nitride catalysts at nominally ambient reaction conditions (<50 °C, 1 atm).

Mechanochemical N₂ fixation was demonstrated with complimentary evidence from XRD and XAS, which showed a substantial transformation of Ti to TiN within 6 h of milling in N₂. The synthesized ammonia was qualitatively detected by colorimetry and mass spectrometry of the reactor effluent, and quantitatively with ion chromatography. A transient Mars-van Krevelen mechanism is proposed for mechanocatalytic NH₃ synthesis, where nitride formation and ammonia synthesis occur in distinct thermodynamic regimes. By exploiting rapidly evolving, transient microenvironments, effective catalytic activity can be increased by several orders of magnitude while also facilitating difficult chemistry at mild conditions. A preliminary technoeconomic analysis reveals the competitiveness of this approach to other novel methods for ammonia synthesis.

Expanding on this work, we aim to determine the characteristics of an optimal catalyst. Hence, we are comparing the activity of five different transition metals (Ti, Zr, Nb, Mo, V) for their steady-state reactivity toward nitridation and hydrogenation. Correlating reactivity trends with material properties will build the foundation for a rational catalyst design and an optimized catalytic system for ammonia synthesis.