Transition Metal Doped B12N12 Nanocage as Single Atom Catalyst for Nitrogen Reduction to Ammonia: A DFT Study

Abstract

Ammonia (NH3) is a crucial chemical used in fertilizer production and is now being recognized as a carbon-free hydrogen transporter for renewable energy applications. Despite its significance, the commercial synthesis of ammonia (NH3) relies primarily on the century-old Haber-Bosch process, which consumes large amounts of fossil fuels and contributes significantly to global CO2 emissions. As a result, establishing sustainable, low- energy routes for ammonia production under moderate settings has become a top research focus. The electrochemical nitrogen reduction reaction (eNNR) is a promising option, but its practical application is limited by slow N2 activation kinetics, low Faradaic efficiency, and competition with the hydrogen evolution reaction (HER). The TM-doped B12N12 nanocages as enhanced single atom catalyst (SACs) for effective electrochemical NH3 production are examined. In this study, density functional theory (DFT) simulations are performed to assess the structural integrity, thermodynamic stability, adsorption behavior, and electrical characteristics of different TM@B12N12 combinations. Interaction energy is calculated to investigate the stability of all transition metal doped boron nitride (B12N12) complexes, with the highest interaction energy observed for Ti@B12N12. Moreover, electronic analyses, such as Frontier Molecular Orbital (FMO), Natural Bond Orbital (NBO), Quantum Theory of Atoms in Molecules (QTAIM) and Molecular Electrostatic Potential (MEP), are used to investigate orbital interactions and to interpret shared or partially covalent interactions and charge transfer dynamics and bond characteristics during N2 adsorption and reduction. The adsorption of molecular nitrogen on the TM@B12N12 complexes exhibits negative adsorption energy, confirming the exothermic nature of N2 adsorption. Among the screened candidates, the Nickle-doped nanocage (Ni@B12N12) demonstrates outstanding catalytic performance, characterized by strong and favorable N2 adsorption, effective electron donation/ back-donation, and robust bonding properties, as proven by QTAIM parameters. The MEP and NBO investigations show a considerable charge distribution between active sites and N2 molecules, enhancing its activation. The prediction energy barrier for the potential-determining step (PDS) is 0.4258eV, indicating the catalyst’s thermodynamic feasibility for efficient ammonia synthesis. This study emphasizes that the TM@B12N12 complexes, i.e., Ni@B12N12 system, serves as a viable SAC platform for sustainable ammonia production. Atomic level insights guide the design of next generation of eNNR catalysts with higher selectivity and energy efficiency.