Industrial ammonia synthesis is a societally vital reaction that feeds approximately half of the global human population. Efforts have been made in exploring new catalysts that can synthesize ammonia under mild conditions, i.e., lower temperatures and pressures, for increased energy efficiency. Recent advances in materials synthesis resulted in the discovery of electride (consisting of a cation-anion pair in which the anion is a stabilized electron) and hydride-based catalysts that are capably of catalyzing ammonia synthesis at ambient pressure and temperatures below 673 K. Despite the promising performance, mechanism understanding of ammonia synthesis over these new catalysts is still lacking. In this work, we took advantage of neutron scattering for its uniquely power in detecting light elements such as hydrogen and investigated ammonia synthesis over Rh nanoparticles supported on the mayenite electride (C12A7), and Ni nanoparticles supported on BaH2. Using both in situ neutron diffraction and inelastic neutron scattering (INS), we have shown for the first time that hydride species can readily form under reaction condition (673 K and 1 atm) in C12A7. However, the bulk hydride is very stable to the reaction with N2 and thus not likely plays a role in ammonia synthesis, disapproving currently accepted mechanistic view in the literature. In the study of ammonia synthesis via an ambient pressure chemical looping process over Ni/BaH2, the structural evolution of BaH2 during N2 and H2 cycles has been successfully disclosed via in situ INS and neutron diffraction investigations coupled with first principle calculations. The studies showcased that neutron scattering has the promise of providing unique catalytic insights into the structure of catalysts, the nature of surface and bulk species, and the reaction mechanisms in hydrogen-involved reactions.

In situ neutron scattering studies of electride- and hydride-based catalysts for ammonia synthesis

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ACS Fall 2022

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