Manipulating the Geometric and Electronic Structures of Manganese Nitrides for Ammonia Synthesis

Manganese (Mn) nitrides are important nitrogen (N) carriers for small-scale intermittent ammonia (NH3) synthesis. However, only 3 similar to 8 % of lattice N are converted into NH3. In this study, the geometric and electronic structures of well-defined Mn4N and Mn2N lattices were altered using transition metal heteroatoms (Cr, Fe, Co, Ni, Mo) to understand the driving force behind lattice N diffusion and extraction. Density Functional Theory (DFT) revealed that the binding of early hydrogenation product (NH) follows a linear relationship with lattice N over a wide range of close-packed surfaces. But, the binding of NH2 and NH3 are more sensitive to the geometric and electronic structures. Further, the chemical bonding can be quantitatively characterized with the covalency derived from Crystal Orbital Hamiltonian Population (COHP). In the Eley Rideal-Mars van Krevelan pathway, the overall NH3 formation free energy (Delta GNH3 ) and lattice N diffusion barrier (Ea ) are the respective thermodynamic and kinetic determining factors. Aided by a rate-determining step (RDS) model, Mn4N modified with Fe, Co, Ni single-atom dopants all show enhanced rates for NH3 formation.