Ionic conductivity and disorder in calcium and barium nitrogen hydrogen phases

Abstract

Nitrogen–hydrogen based alkali and alkaline earth metal compounds have recently received a substantial amount of attention as co-catalysts for heterogeneous mild condition ammonia synthesis (MCAS). The incorporation of these materials has been shown to result in positive reaction orders with respect to H₂, solving the issue of hydrogen poisoning, e.g., the occupation of the majority of transition metal (TM) active sites by H-adatoms due to the significantly faster kinetics of H₂ dissociation as compared to N₂. The mechanism that underlies this is thought to be the incorporation (sinking) of H-adatoms from the surface of TMs to the bulk of the N–H phases. Thus, the slower kinetics of N₂ dissociation no longer inhibit ammonia synthesis, and improvements in the kinetics dissociation for TM can be realised without consideration for which specific gases are affected (e.g., the circumventing of scaling relations). The ability to transport H-adatoms from the surface of TM is therefore of fundamental importance to the properties of the N–H co-catalyst implying that the conductivity of these species towards H and N ions, and NH_x species, is of utmost importance. As such, we investigate two N–H systems that can be prepared by reacting the respective hydrides with nitrogen resulting in nitride–hydride and imide forms for Ca and Ba, respectively. These have both been previously shown to promote ammonia synthesis and here we investigate their conductive properties, and discuss these systems in the context of activity and stability of the total system with specific focus on the rise of secondary anion species, and the presence of barium in the system.