produced by the Haber-Bosch process, typically using fused iron as the catalyst. However, the iron-based catalyst requires harsh reaction conditions (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) and catalysts that work under mild conditions are therefore much needed. This may become more relevant in the future because as the needs for ammonia shift from fertilizer to energy, new plants operating under different conditions (lower pressure, alternate N 2 / H 2 ratios, etc.) may become necessary.In terms of the catalyst metal, ruthenium (Ru) is one of the most promising candidate elements for ammonia synthesis under mild conditions. Ru-based catalysts on carbon [3] and MgO [4] have also been extensively reported. Regarding support materials, over the most recent past few years, a number of new studies have focused on new catalysts incorporating metal hydride or electride in the support, such as 12CaO·7Al 2 O 3 :e − (C12A7:e − ), [5] [Ca 2 N]:e − , [6] CaH 2 , [7] Ca 2 NH, [8] Y 5 Si 3 , [9] LaScSi, [10] LiH, [11] and BaH 2 , [12] where all of these catalysts show high activities and unusual mechanisms. As related materials, we have recently examined BaTiO 2.5 H 0.5 and TiH 2 as a catalyst for NH 3 synthesis under Haber-Bosch conditions (400 °C, 5 MPa). [13] Titanium, being an early transition metal, was traditionally viewed as an inactive metal for catalytic NH 3 Ammonia is an attractive energy carrier for the hydrogen economy, given its high hydrogen density and ease of liquefaction. A titanate oxyhydride has recently been demonstrated that can catalyze ammonia synthesis without Ru or Fe metal, despite titanium being regarded as an inert element. Here, the synthesis activity of ammonia is examined when Ru, Fe, and Co particles are supported onto the oxyhydride BaTiO 2.5 H 0.5 . The activity of BaTiO 2.5 H 0.5 as support is significantly higher than BaTiO 3 . For example, the activity for Fe and Co increases by a factor of 70-400, making them more active than Ru/ MgO, one conventional Ru catalyst. In terms of mechanism, for Ru, H/D isotope studies show participation of lattice hydride in the catalytic cycle, while kinetic analysis shows reduced H 2 poisoning probably due to spillover. For Fe (and Co), the presence of hydride results in significantly lower activation energy and N 2 reaction order, likely due to strong electron donation from the oxyhydride. This metal-dependent support effect is further verified by N 2 isotopic exchange experiments. These perovskite-type oxyhydrides can be easily modified in terms of A-and B-site (A = Ba, B = Ti); the high potential for compositional variation and morphologies will expand the search for efficient catalysts for ammonia synthesis.