BSr₃ Superalkali as a Promising Catalyst for Ambient Nitrogen Reduction: A Pathway toward Efficient Ammonia Synthesis

The transformation of the earth-abundant dinitrogen molecule (N₂) into ammonia (NH₃) under mild conditions remains a fundamental challenge in chemistry. The BSr₃ superatom designed in our previous work (Wiszowska N, Rogoża N, Sikorska C, Phys. Chem. Chem. Phys. (2025), 27: 23468-23486) has been used as a catalyst for the conversion of molecular nitrogen into ammonia. Owing to its low ionization energy (4.033 eV), the BSr₃ transfers electron density to adsorbed N₂, partially populating antibonding π* orbitals, lowering the triple NN bond order and initiating N-N bond elongation. Subsequent proton-coupled electron-transfer steps further elongate the N-N distance in a stepwise manner, establishing a correlation between structural distortion and the progressive weakening of the NN bond, ultimately facilitating bond cleavage. The catalytic conversion of N₂ to NH₃ catalyzed by BSr₃ proceeds via an associative distal pathway, with initial hydrogenation at the nitrogen atom coordinated to strontium atoms (N^Sr), followed by release of the first NH₃ molecule upon N-N bond rupture and subsequent hydrogenation of the second nitrogen (N^B, bound to a boron atom of the BSr₃ cluster). The release of the NH₃ molecule is the potential-determining step, with a corresponding free energy uphill of 0.28 eV. The preceding exergonic hydrogenation steps help offset the energy cost of NH₃ desorption, ensuring overall thermodynamic favorability. These findings position the BSr₃ superalkali cluster as a promising noble-metal-free catalyst for sustainable ammonia synthesis under ambient conditions.