Titanium-Based Hydrides as Heterogeneous Catalysts for Ammonia Synthesis

Abstract

The problem of activating N₂ and its subsequent hydrogenation to form NH₃ has been approached from many directions. One of these approaches involves the use of transition metal hydride complexes. Recently, transition metal hydride complexes of Ti and Ta have been shown to activate N₂, but without catalytic formation of NH₃. Here, we show that at elevated temperatures (400 °C, 5 MPa), solid-state hydride-containing Ti compounds (TiH₂ and BaTiO_2.5H_0.5) form a nitride-hydride surface similar to those observed with titanium clusters, but continuously (∼7 days) form NH₃ under H₂/N₂ flow conditions to achieve a catalytic cycle, with activity (up to 2.8 mmol·g·^-1·h^-1) almost comparable to conventional supported Ru catalysts such as Cs-Ru/MgO or Ru/BaTiO₃ that we have tested. As with the homogeneous analogues, the initial presence of hydride within the catalyst is critical. A rare hydrogen-based Mars van Krevelen mechanism may be at play here. Conventional scaling rules of pure metals predict essentially no activity for Ti, making this a previously overlooked element, but our results show that by introducing hydride, the repertoire of heterogeneous catalysts can be expanded to include formerly unexamined compositions without resorting to precious metals.