Ammonia synthesis on BaTiO2.5H0.5: computational insights into the role of hydrides

Abstract

Perovskite oxyhydrides such as BaTiO_2.5H_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_2.5H_0.5 (BTOH). Two mechanistic hypotheses, the distal and alternating pathways, have been tested on the Ti₂O₂ 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 N₂ to form the *N–NH_x key intermediates, followed by N–N bond breaking. In the alternating pathway, H atoms hydrogenate N₂ 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 *N₂ with the lattice hydride is key to the distal pathway, leading to surface nitride formation after breaking the *N–NH₃ 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 NH₃ 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.