Titanium-Based Hydrides as Heterogeneous Catalysts for Ammonia Synthesis.

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 BaTiO2.5H0.5) 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.