Electronic Origin of Catalytic Activity of TiH2 for Ammonia Synthesis

Transition metals with a higher-lying Fermi level (a smaller work function) such as Ti can easily activate nitrogen and hydrogen reductively to form nitrides and hydrides on its surface, but ammonia synthesis does not occur. H atoms adsorbed on the surface of Ti have a negative charge, identified as H-. In order for an N-H bond to form on the Ti surface, the hydrogen atom must dump its extra charge into empty levels above the Fermi level of the surface. The high Fermi level of Ti impedes this process. Although the Fermi level of TiH2 is as high as that of Ti, TiH2 has been reported to act as an ammonia synthesis catalyst under Haber-Bosch conditions, characterized by its robust catalytic activity. To clarify the electronic aspects of the catalytic activity of TiH2, a theoretical study on the surface of TiH2 using density functional theory calculations is carried out. The negative charge of the H atom about to bond to the N atom in ammonia synthesis on the TiH2 surface is as large as that of the H atom on the Ti surface. However, since surface hydrides of TiH2 present nearby interact with the H atom in an antibonding manner and the electronic state of the H atom is destabilized, the energy required for dumping the extra charge upon the formation of the N-H bond is substantially reduced. Nitrides in the surface of TiH2, the formation of which during the reaction has been suggested experimentally, make the amount of the charge of the H atom smaller by oxidizing the Ti atoms nearby. This further reduces the activation barrier associated with the N-H bond formation, leading to a good agreement between theory and experiment.