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Ti2O is trigonal omega structured and crystallizes in the trigonal P-3m1 space group. The structure is two-dimensional and consists of one Ti2O sheet oriented in the (0, 0, 1) direction. Ti is bonded in a distorted T-shaped geometry to three equivalent O atoms. All Ti–O bond lengths are 2.15 Å. O is bonded to six equivalent Ti atoms to form edge-sharing OTi6 octahedra.
Ti2O crystallizes in the monoclinic P2/m space group. The structure is two-dimensional and consists of one Ti2O sheet oriented in the (0, 0, 1) direction. there are three inequivalent Ti sites. In the first Ti site, Ti is bonded in a 4-coordinate geometry to four O atoms. All Ti–O bond lengths are 2.06 Å. In the second Ti site, Ti is bonded in an L-shaped geometry to two equivalent O atoms. Both Ti–O bond lengths are 2.04 Å. In the third Ti site, Ti is bonded in a 3-coordinate geometry to three O atoms. There are a spread of Ti–O bond distances ranging from 2.13–2.54 Å. There are two inequivalent O sites. In the first O site, O is bonded to six Ti atoms to form a mixture of distorted corner, edge, and face-sharing OTi6 octahedra. The corner-sharing octahedra tilt angles range from 6–58°. In the second O site, O is bonded to six Ti atoms to form a mixture of corner, edge, and face-sharing OTi6 octahedra. The corner-sharing octahedra tilt angles range from 37–58°.
Perovskite oxyhydrides such as BaTiO 2.5 H 0.5 have been found to be able to catalyze NH3 synthesis, but the mechanism and the role of the catalyst's lattice hydrides in the catalytic reaction remain unknown. Here we employ first principles density functional theory to investigate the mechanism of ammonia synthesis and the role of lattice hydrides on a prototypical perovskite oxyhydride, BaTiO 2.5 H 0.5 (BTOH). Two mechanistic hypotheses, the distal and alternating pathways, have been tested on the Ti2O 2 termination of the BTOH (210) surface, previously determined to be the most stable surface termination under the reaction conditions considered. In the distal pathway, H atoms hydrogenate N 2 to form the *N–NH x key intermediates, followed by N–N bond breaking. In the alternating pathway, H atoms hydrogenate N 2 in an alternating fashion to form the *NH x –NH y intermediates before N–N bond breaking and formation of co-adsorbed *NH x /*NH y on the surface. We find that the subsurface hydride vacancy formed after reaction of *N 2 with the lattice hydride is key to the distal pathway, leading to surface nitride formation after breaking the *N–NH 3 bond, while the neighboring surface Ti sites are key to bridging and stabilizing the *NNH intermediate in the alternating pathway. In both pathways, desorption of NH 3 is the most uphill in energy. Our results provide important insights into the role of hydrides and surface vacancies in hydrogenation reactions over BTOH, which will be useful to guide future spectroscopic experiments such as operando IR and inelastic neutron scattering to verify the key intermediates.